0001193125-18-243527 | 10-Q | XBRL Viewer

Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2018	Jul. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Trading Symbol	AVRO
Entity Registrant Name	AVROBIO, INC.
Entity Central Index Key	0001681087
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		23,936,838

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 155,015	$ 5,963
Prepaid expenses and other current assets	1,132	345
Total current assets	156,147	6,308
Property and equipment, net	2,050	349
Other assets	533	365
Total assets	158,730	7,022
Current liabilities:
Accounts payable	4,193	527
Accrued expenses and other current liabilities	5,438	2,098
Total current liabilities	9,631	2,625
Warrant to purchase redeemable convertible preferred stock		35
Derivative liability		371
Deferred rent, net of current portion	781	126
Other long-term liability		500
Total liabilities	10,412	3,657
Commitments and contingencies (Note 13)
Redeemable convertible preferred stock (Note 8)		26,500
Stockholders' equity (deficit):
Common stock, $0.0001 par value; 150,000,000 and 51,000,000 shares authorized as of June 30, 2018 and December 31, 2017, respectively; 23,936,838 and 2,581,474 shares issued as of June 30, 2018 and December 31, 2017, respectively; 23,722,049 and 2,305,173 shares outstanding as of June 30, 2018 and December 31, 2017, respectively	2
Additional paid-in capital	192,443	339
Accumulated deficit	(44,127)	(23,474)
Total stockholders' equity (deficit)	148,318	(23,135)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	$ 158,730	$ 7,022

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, authorized	150,000,000	51,000,000
Common stock, issued	23,936,838	2,581,474
Common stock, outstanding	23,722,049	2,305,173

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating expenses:
Research and development	$ 7,407	$ 1,881	$ 13,054	$ 3,315
General and administrative	2,140	661	4,281	1,271
Total operating expenses	9,547	2,542	17,335	4,586
Loss from operations	(9,547)	(2,542)	(17,335)	(4,586)
Other income (expense):
Interest income	234	11	392	15
Change in fair value of preferred stock warrant liability	(150)		(162)
Change in fair value of derivative liability	(1,042)	(3)	(1,629)	(35)
Other expense	(2)		(15)	(5)
Total other income (expense), net	(960)	8	(1,414)	(25)
Net loss	(10,507)	(2,534)	(18,749)	(4,611)
Comprehensive loss	(10,507)	(2,534)	(18,749)	(4,611)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(10,507)	(2,534)	(18,749)	(4,611)
Accretion of issuance costs on redeemable convertible preferred stock			(2,243)	(47)
Net loss attributable to common stockholders-basic and diluted	$ (10,507)	$ (2,534)	$ (20,992)	$ (4,658)
Net loss per share attributable to common stockholders - basic and diluted (Note 12)	$ (2.98)	$ (1.15)	$ (7.16)	$ (2.12)
Weighted-average number of common shares used in computing net loss per share attributable to common stockholders-basic and diluted	3,529,269	2,202,735	2,930,358	2,192,283

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net loss	$ (18,749)	$ (4,611)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	566	64
Depreciation and amortization expense	66	15
Impairment loss of property and equipment	235
Amortization of deferred offering costs	22	2
Deferred rent expense	(23)	135
Change in fair value of preferred stock warrant liability	162
Change in fair value of derivative liability	1,629	35
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(768)	(176)
Other assets	(300)	(3)
Accounts payable	3,029	(545)
Accrued expenses and other current liabilities	(98)	626
Net cash used in operating activities	(14,229)	(4,458)
Cash flows from investing activities:
Change in restricted cash	24
Purchases of property and equipment	(424)	(62)
Net cash used in investing activities	(400)	(62)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	58,258	3,468
Payments of issuance costs on debt facility		(32)
Proceeds from issuance of common shares upon completion of initial public offering, net of offering costs	105,423
Net cash provided by financing activities	163,681	3,436
Net increase (decrease) in cash and cash equivalents	149,052	(1,084)
Cash and cash equivalents at beginning of period	5,963	5,357
Cash and cash equivalents at end of period	155,015	4,273
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable	756	240
Purchases of property and equipment paid for by landlord	842
Deferred offering costs included in accrued expenses	1,241	11
Accretion of issuance costs related to redeemable convertible preferred stock	2,240	47
Issuance of warrants associated with debt facility		$ 18
Property and equipment held for sale	$ 19

Nature of the Business and Basis of Presentation	6 Months Ended
Nature of the Business and Basis of Presentation	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation AVROBIO, Inc. (the “Company” or “AVROBIO”) is a clinical stage gene therapy company focused on developing potentially curative ex vivo lentiviral gene therapies to treat rare diseases following a single dose. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including but not limited to, risks associated with completing preclinical studies and clinical trials, receiving regulatory approvals for product candidates, development by competitors of new biopharmaceutical products, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize revenue from product sales. *Basis of Presentation* The accompanying condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited annual consolidated financial statements as of and for the year ended December 31, 2017, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of June 30, 2018, and the results of its operations and its cash flows for the three and six months ended June 30, 2018 and 2017. The results for the six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2017, and the notes thereto, which are included elsewhere in the Company’s final prospectus for its initial public offering (“IPO”), filed with the Securities and Exchange Commission (“SEC”) pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended, on June 21, 2018 (the “Prospectus”). The accompanying condensed financial statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the financial statements. As of June 30, 2018, the Company’s significant accounting policies and estimates, which are detailed in the Company’s Prospectus, have not changed. *Reverse Stock Split* On June 1, 2018, the Board approved a 1-for-4.312 reverse stock split of the Company’s common stock. The reverse stock split was approved by the stockholders on June 7, 2018 and became effective on June 7, 2018. Stockholders entitled to fractional shares as a result of the reverse stock split will receive a cash payment in lieu of receiving fractional shares. All share and per share data shown in the accompanying unaudited condensed consolidated financial statements and related notes have been retroactively revised to reflect the reverse stock split. Shares of common stock underlying outstanding stock options and other equity instruments were proportionately reduced and the respective exercise prices, if applicable, were proportionately increased in accordance with the terms of the agreements governing such securities. Shares of common stock reserved for issuance upon the conversion of the Company’s Preferred Stock were proportionately reduced and the respective conversion prices were proportionately increased. *Initial Public Offering* On June 20, 2018, the Company’s registration statement on Form S-1 relating to its IPO was declared effective by the SEC. The IPO closed on June 25, 2018 and the Company issued and sold 5,247,958 common shares at a public offering price of $19.00 per share for net proceeds of $90,271 after deducting underwriting discounts and commissions of $6,980 and other offering expenses of approximately $2,459. Simultaneously, on June 25, 2018, the Company issued and sold 787,193 additional common shares, pursuant to the full exercise of the underwriters’ option to purchase additional shares, for net proceeds of $13,910 after deducting underwriting discounts and commissions of $1,047. Thus, the aggregate net proceeds to the Company from the IPO, after deducting underwriting discounts and commissions and other offering costs, were $104,182. Upon the closing of the IPO, series Seed redeemable convertible preferred stock (the “Series Seed Preferred Stock”), series A redeemable convertible preferred stock (the “Series A Preferred Stock”) and series B redeemable convertible preferred stock (the “Series B Preferred Stock”), (the Series Seed Preferred Stock, the Series A Preferred Stock and the Series B Preferred Stock are collectively referred to as the “Preferred Stock”) then outstanding converted into an aggregate of 15,320,213 shares of common stock.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Subsequent Event Considerations

The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. See Note 15.

Use of Estimates

The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, stock-based compensation expense, the valuation of equity and derivative instruments and the recoverability of the Company’s net deferred tax assets and related valuation allowance. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ materially from those estimates.

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks and amounts held in interest-bearing money market accounts. Cash equivalents are carried at cost, which approximates their fair market value.

Concentrations of Credit Risk

The Company has no significant off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and restricted cash. Periodically, the Company maintains deposits in accredited financial institutions in excess of federally insured limits. The Company deposits its cash and cash equivalents in financial institutions that it believes have high credit quality and has not experienced any losses on such accounts and does not believe it is exposed to any unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

Fair Value Measurements

Certain assets and liabilities of the Company are carried at fair value under GAAP (see Note 3). Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

•

Level 3—Unobservable inputs that are supported by little or no market activity that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

Segment Information

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer (“CEO”). The Company and the CEO view the Company’s operations and manage its business as one operating segment. All material long-lived assets of the Company reside in the United States.

Research and Development Costs

Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, facilities costs, depreciation, third-party license fees, and external costs of outside vendors engaged to conduct preclinical development activities and clinical trials as well as to manufacture research and development materials. Non-refundable prepayments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized. Such amounts are recognized as an expense as the goods are delivered or the related services are performed or until it is no longer expected that the goods will be delivered or the services rendered.

The Company has entered into various research and development related contracts with parties both inside and outside of the United States. The payments to these agreements are recorded as research and development expenses as incurred. The Company records accrued liabilities for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.

Stock-Based Compensation

For stock-based awards issued to employees and members of the Company’s board of directors (the “Board”) for their services on the Board, the Company measures the estimated fair value of the stock-based award on the date of grant and recognizes compensation expense for those awards over the requisite service period, which is generally the vesting period of the respective award. The Company issues stock-based awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. The Company has not issued any stock-based awards with performance- or market-based vesting conditions. The Company accounts for forfeitures as they occur.

Stock-based awards issued to non-employees are recorded at their fair values, and are periodically revalued as the awards vest and are recognized as expense over the related service period. For stock-based awards granted to non-employees subject to graded vesting that only contain service conditions, the Company has elected to recognize stock-based compensation expense using the straight-line recognition method.

The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s cash compensation costs are classified.

Given the absence of an active market for the Company’s common stock prior to the IPO, the Company and the Board, the members of which the Company believes have extensive business, finance, and venture capital experience, were required to estimate the fair value of the Company’s common stock at the time of each grant of a stock-based award. The Company and the Board determined the estimated fair value of the Company’s equity instruments based on a number of factors, including external market conditions affecting the biotechnology industry sector. The Company and the Board utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants’ Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its common stock. Each valuation methodology includes estimates and assumptions that require the Company’s judgment. These estimates and assumptions include a number of objective and subjective factors in determining the value of the Company’s common stock at each grant date, including: (1) prices paid for the Company’s redeemable convertible preferred stock, which the Company had sold to outside investors in arm’s-length transactions, and the rights, preferences, and privileges of the Company’s redeemable convertible preferred stock and common stock; (2) valuations performed by an independent valuation specialist; (3) the Company’s stage of development; (4) the fact that the grants of stock-based awards involved illiquid securities in a private company; and (5) the likelihood of achieving a liquidity event for the common stock underlying the stock-based awards, such as an IPO or sale of the Company, given prevailing market conditions.

The fair value of each stock option award is estimated on the date of grant using the Black-Scholes option pricing model. As there was no public market for its common stock prior to June 21, 2018, which was the first day of trading, and as the trading history of the Company’s common stock was limited through June 30, 2018, the Company determined the volatility for awards granted based on an analysis of reported data for a group of guideline companies that issued options with substantially similar terms. The expected volatility has been determined using a weighted-average of the historical volatility measures of this group of guideline companies. The Company expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. The Company has not paid, and does not anticipate paying, cash dividends on its common stock; therefore, the expected dividend yield is assumed to be zero.

See Note 10 for the assumptions used by the Company in determining the grant date fair value of stock-based awards granted, as well as a summary of the stock-based award activity under the Company’s stock-based compensation plan for the six months ended June 30, 2018.

Warrant to Purchase Preferred Stock

Prior to the IPO, the Company classified the warrant for the purchase of shares of its redeemable convertible preferred stock (see Note 7) as a liability on its consolidated balance sheets as the warrant is a free-standing financial instrument that may require the Company to transfer assets upon exercise. The preferred stock warrant liability was initially recorded at fair value upon the date of issuance and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of the warrant to purchase preferred stock are recognized as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. Changes in the fair value of the warrant to purchase preferred stock will continue to be recognized until the warrant is exercised, expires or qualifies for equity classification.

The Company utilizes the Black-Scholes option-pricing model, which incorporates assumptions and estimates, to value the warrant. The Company assesses these assumptions and estimates on a quarterly basis as additional information impacting the assumptions is obtained. Estimates and assumptions impacting the fair value measurement include the fair value per share of the underlying redeemable convertible preferred stock issuable upon exercise of the warrant, the remaining contractual term of the warrant, the risk-free interest rate, the expected dividend yield and the expected volatility of the price of the underlying redeemable convertible preferred stock.

Upon the IPO, the warrant to purchase preferred stock was converted to a warrant to purchase common stock. The carrying amount of warrant to purchase preferred stock as of the date of IPO was transferred to the account of additional paid in capital.

Comprehensive Income (Loss)

Comprehensive income (loss) is defined as the change in stockholders’ equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. Comprehensive income (loss) includes net income (loss) as well as other changes in stockholders’ (deficit) equity which includes certain changes in equity that are excluded from net income (loss). Comprehensive loss has been disclosed in the accompanying statements of operations and comprehensive loss and equals the Company’s net loss for all periods presented.

Leases

The Company categorizes leases at their inception as either operating or capital leases. On certain lease arrangements, the Company may receive rent holidays or other incentives. The Company recognizes lease costs on a straight-line basis once control of the space is achieved, without regard to deferred payment terms, such as rent holidays, that defer the commencement date of required payments or escalating payment amounts. The difference between required lease payments and rent expense has been recorded as deferred rent and other accrued expenses and other current liabilities in the accompanying consolidated balance sheets. Additionally, incentives received are treated as a reduction of costs over the term of the agreement, as they are considered an inseparable part of the lease agreement.

Net Income (Loss) per Share Attributable to Common Stockholders

Net income (loss) per share attributable to common stockholders is determined using the two-class method, which includes the weighted-average number of shares of common stock outstanding during the period and other securities that participate in dividends (a participating security). In periods of income, the redeemable convertible preferred stock would be considered participating securities because the shares include rights to participate in dividends with the common stock; however, the redeemable convertible preferred stock is not considered a participating security in periods of loss as they do not have an obligation to share in the Company’s net losses.

Under the two-class method, basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period. Diluted net income (loss) per share attributable to common stockholders is computed using the more dilutive of (1) the two-class method or (2) the if-converted method. The Company allocates net income first to the holders of Preferred Stock based on dividend rights under the Company’s certificate of incorporation and then to preferred and common stockholders based on ownership interests.

Emerging Growth Company Status

The Company is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act, or JOBS Act, and may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company may take advantage of these exemptions until the Company is no longer an “emerging growth company.” Section 107 of the JOBS Act provides that an “emerging growth company” can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected to use the extended transition period for complying with new or revised accounting standards and as a result of this election, its consolidated financial statements may not be comparable to companies that comply with public company effective dates. The Company may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of an offering or such earlier time that it is no longer an “emerging growth company”.

Recently Issued Accounting Pronouncements

In June 2018, the FASB issued ASU No. 2018-07, “Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting” which addresses accounting for issuance of all share-based payments on the same accounting model. Previously, accounting for share-based payments to employees was covered by ASC Topic 718 while accounting for such payments to non-employees was covered by ASC Subtopic 505-50. As it considered recently issued updates to ASC 718, the FASB, as part of its simplification initiatives, decided to replace ASC Subtopic 505-50 with Topic 718 as the guidance for non-employee share-based awards. Under this new guidance, both sets of awards, for employees and non-employees, will essentially follow the same model, with small variations related to determining the term assumption when valuing a non-employee award as well as a different expense attribution model for non-employee awards as opposed to employee awards. The ASU is effective for public business entities beginning in 2019 calendar years and one year later for non-public business entities. The Company is currently evaluating the impact that the adoption of ASU 2018-07 will have on its consolidated financial statements.

In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception. Part I applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within ASC Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. For public entities, the amendments in Part I of this update are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. For all other entities, the amendments in Part I of this update are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted for all entities, including adoption in an interim period. The Company is currently evaluating the impact that the adoption of ASU 2017-11 will have on its consolidated financial statements.

In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The standard is effective for annual periods beginning after December 15, 2017. Early adoption is permitted, including adoption in any interim period, for (1) public business entities for reporting periods for which financial statements have not yet been issued and (2) all other entities for reporting periods for which financial statements have not yet been made available for issuance. The Company is currently evaluating the impact that the adoption of ASU 2017-09 will have on its consolidated financial statements.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”), which requires restricted cash to be presented with cash and cash equivalents on the statement of cash flows and disclosure of how the statement of cash flows reconciles to the balance sheet if restricted cash is shown separately from cash and cash equivalents on the balance sheet. For public entities, the standard is effective for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. For all other entities, the standard is effective for annual periods beginning after December 15, 2018, including interim periods within fiscal years beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2016-18 will have on its consolidated financial statements.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”), to address diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. For public entities, the standard is effective for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. For all other entities, the standard is effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. The Company is currently evaluating the impact that the adoption of ASU 2016-15 will have on its consolidated financial statements.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today. ASU 2016-02 supersedes the previous leases standard, ASC 840, Leases . For public entities, not-for-profit entities and an employee benefit plan that files financial statements with the U.S. Securities and Exchange Commission (SEC), the standard is effective for public entities for annual periods beginning after December 15, 2018 including interim periods within those fiscal years. For all other entities, the standard is effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted.

In September 2017, the FASB issued ASU No. 2017-13, Revenue Recognition (Topic 605), Revenue from Contracts with Customers (Topic 606), Leases (Topic 840), and Leases (Topic 842) , which provides additional clarification and implementation guidance related to ASU 2016-02 and has the same effective date and transition requirements as ASU 2016-02. The Company is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements.

Fair Value of Financial Assets and Liabilities

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

3. Fair Value of Financial Assets and Liabilities

The following table presents information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicates the level of the fair value hierarchy utilized to determine such fair values as of June 30, 2018 and December 31, 2017, respectively:


	Fair Value Measurements as of June 30, 2018 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	154,549	$	—	$	—	$	154,549


	Fair Value Measurements as of December 31, 2017 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	5,684	$	—	$	—	$	5,684
Restricted cash		24		—		—		24

	$	5,708	$	—	$	—	$	5,708

Liabilities:
Derivative liability	$	—	$	—	$	371	$	371
Warrant to purchase redeemable convertible preferred stock		—		—		35		35

	$	—	$	—	$	406	$	406

During the six months ended June 30, 2018 and 2017, there were no transfers between Level 1, Level 2 and Level 3.

Valuation of the Warrant to Purchase Preferred Stock

The warrant to purchase preferred stock liability in the table above is composed of the fair value of a warrant to purchase shares of Series A Preferred Stock that was issued to a lender in connection with a loan and security agreement in 2017. The fair value of the warrant to purchase preferred stock was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy.

In order to determine the fair value of the warrant to purchase preferred stock, the Company utilized available facts and circumstances to estimate the number of shares of Series A Preferred Stock for which the warrant will ultimately be exercisable. The Company then used the Black-Scholes option-pricing model, which incorporates assumptions and estimates, to value the warrant to purchase preferred stock. Estimates and assumptions impacting the fair value measurement include the fair value of the underlying shares of Series A Preferred Stock, the remaining contractual term of the warrant, risk-free interest rate, expected dividend yield and expected volatility of the price of the underlying preferred stock. The Company determined the fair value of the underlying preferred stock based on various valuation methodologies. The Company lacks company-specific historical and implied volatility information of its stock. Therefore, it estimates its expected stock volatility based on the historical volatility of publicly traded guideline companies for a term equal to the remaining contractual term of the warrant. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrant. The Company estimated no expected dividend yield based on the fact that the Company has never paid or declared dividends and does not intend to do so in the foreseeable future. Upon the IPO, the warrant to purchase preferred stock was converted to a warrant to purchase common stock. The carrying amount of warrant to purchase preferred stock as of the date of IPO was transferred to the account of additional paid in capital.

The following table sets forth a summary of changes in the fair value of the Company’s preferred stock warrant liability for which fair value was determined by Level 3 inputs:


	Warrant Liability
Balance as of December 31, 2017	$	35
Change in fair value		162
Conversion of preferred stock warrant to common stock warrant		(197	)
Balance as of June 30, 2018	$	—

Valuation of Derivative

In January 2016, in connection with a license agreement entered into with University Health Network (“UHN”), and as part of the initial consideration for the license, the Company issued 1,161,665 shares of common stock to UHN pursuant to a stock purchase agreement (the “Stock Purchase Agreement”). The Stock Purchase Agreement contains a provision requiring the Company to make a cash payment to UHN of up to $2,000 if UHN’s fully diluted ownership is reduced within specified percentages as part of an IPO by the Company. The Company concluded the anti-dilution feature represented a derivative instrument and should be measured at fair value, with changes in fair value recognized as a gain or loss to other income (expense), net in the consolidated statements of operations and comprehensive loss. The initial fair value of the derivative of $49 was recorded as research and development expense in January 2016.

On June 30, 2017, the Company remeasured the fair value of the derivative, using current assumptions, resulting in an increase in fair value of $3 and $35, which was recorded in other expense in the accompanying condensed consolidated statements of operations and comprehensive loss for the three months and six months ended June 30, 2017, respectively.

On June 21, 2018, in connection with the Company’s IPO, the Company remeasured the fair value of the derivative to $2,000 as the Company was required to pay the dilution payment as mentioned above, which the Company paid in July 2018. An increase in fair value of $1,042 and $1,629 was recorded in other expense in the accompanying condensed consolidated statements of operations and comprehensive loss for the three and six months ended June 30, 2018, respectively.

The following table sets forth a summary of changes in the fair value of the Company’s derivative liability for which fair value is determined by Level 3 inputs:


	Derivative Liability
Balance as of December 31, 2017	$	371
Change in fair value		1,629
Transferred to accrued expenses		(2,000	)

Balance as of June 30, 2018	$	—

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Prepaid Expenses and Other Current Assets

6 Months Ended

Jun. 30, 2018

Text Block [Abstract]

Prepaid Expenses and Other Current Assets

4. Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:


	June 30, 2018		December 31, 2017
Prepaid development costs	$	762	$	163
Prepaid rent		76		122
Other current assets		294		60

	$	1,132	$	345

Property and Equipment, Net

6 Months Ended

Jun. 30, 2018

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net consisted of the following:


	June 30, 2018			December 31, 2017
Laboratory and office equipment	$	926		$	126
Leasehold improvements		1,112			240
Computer equipment and software		83			28

		2,121			394
Less: Accumulated depreciation and amortization		(71	)		(45	)

	$	2,050		$	349

Depreciation and amortization expense for the three months ended June 30, 2018 and 2017 was $45 and $13, respectively. Depreciation and amortization expense

Accrued Expenses

6 Months Ended

Jun. 30, 2018

Payables and Accruals [Abstract]

Accrued Expenses

6. Accrued Expenses

Accrued expenses and other current liabilities consisted of the following:


	June 30, 2018		December 31, 2017
Dilution payment (Note 3)	$	2,000	$	—
Compensation and benefit costs		846		794
Short-term deferred rent		171		9
Research and development costs		753		831
IPO issuance cost		693		—
Property and equipment		667		—
Other liabilities		308		464

	$	5,438	$	2,098

Loan Agreement and Warrant to Purchase Preferred Stock	6 Months Ended
Loan Agreement and Warrant to Purchase Preferred Stock	Jun. 30, 2018
Statement of Stockholders' Equity [Abstract]
Loan Agreement and Warrant to Purchase Preferred Stock	7. Loan Agreement and Warrant to Purchase Preferred Stock On June 23, 2017, the Company entered into the Loan Agreement with a lender, which provided for the issuance of term loans of up to $10,000, subject to the achievement of various development and corporate milestones. Any outstanding principal amounts under the Loan Agreement will accrue interest at a floating per annum rate equal to the greater of 1% and the “prime rate,” as defined in the Loan Agreement, minus 3%. Payments on the Loan Agreement are interest only, payable monthly in arrears, until November 1, 2018, which can be extended by six months if the third tranche is drawn. Thereafter, principal and interest amounts are repayable over a 30-month period, unless the third tranche is funded and the initial interest-only period is extended by six months, in which case principal and interest amounts are repayable over a 24-month period. As of June 30, 2018, the Company had not drawn down from the facility and $10,000 was available to the Company. The Loan Agreement contains customary indemnification obligations and customary events of default. In the event of default by the Company under the Loan Agreement, the lender would be entitled to exercise their remedies thereunder, including the right to accelerate the debt, upon which the Company may be required to repay all amounts then outstanding under the Loan Agreement or the lender may take possession of the collateral securing the Loan Agreement. No events of default had occurred through June 30, 2018. The Loan Agreement also includes certain restrictions on, among other things, the Company’s ability to incur additional indebtedness, change the name or location of its business, merge with or acquire other entities, pay dividends or make other distributions to holders of the Company’s capital stock, make certain investments, engage in transactions with affiliates, create liens, open new deposit accounts, sell assets or pay subordinated debt. In connection with the Loan Agreement, the Company agreed to issue a warrant to the lender for the purchase of up to 188,702 shares of the Company’s Series A Preferred Stock with an exercise price of $0.7949 per share. The warrant expires on June 22, 2027. The warrant is initially exercisable for the purchase of up to 28,305 shares of Series A Preferred Stock and can become exercisable for up to an additional 160,397 shares of Series A Preferred Stock based on the amounts drawn under the Loan Agreement. On the issuance date of the warrant, the Company recorded a deferred financing cost and a liability for the warrant to purchase preferred stock in the Company’s consolidated balance sheet equal to the issuance-date fair value of the warrant. Upon closing of the IPO, the carrying amount of the warrant to purchase preferred stock as of the date of IPO was transferred to the account of additional paid in capital (Note 3). The Company recognized a loss of $150 and $162 in the condensed consolidated statements of operations and comprehensive loss for the three and six months ended June 30, 2018, respectively, related to the change in fair value of the warrant.

Redeemable Convertible Preferred Stock

6 Months Ended

Jun. 30, 2018

Text Block [Abstract]

Redeemable Convertible Preferred Stock

8. Redeemable Convertible Preferred Stock

Prior to the IPO, the authorized capital stock of the Company included 63,491,857 shares of $0.0001 par value preferred stock, of which 3,333,333 shares have been designated as Series Seed Preferred Stock, 31,639,202 shares have been designated as Series A Preferred Stock and 28,519,322 shares have been designated as Series B Preferred Stock.

In January 2018, the Company issued and sold 28,519,322 shares of Series B Preferred Stock, at a price of $2.1389 per share, for total proceeds of $58,757, net of issuance costs of $2,243.

As of December 31, 2017, the Preferred Stock consisted of the following:


	December 31, 2017
	Preferred Shares Authorize		Preferred Shares Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series Seed preferred stock		3,333,333		3,333,333	$	1,500,000	$	1,500,000		806,711
Series A preferred stock		31,639,202		31,450,499		25,000,000		25,000,000		7,611,438

		34,972,535		34,783,832	$	26,500,000	$	26,500,000		8,418,149

Upon closing of the IPO, the Preferred Stock as of December 31, 2017, together with Series B Preferred Stock issued in January 2018, were converted to 15,320,213 shares of common stock. The holders of the Company’s Preferred Stock had certain voting, dividend, and redemption rights, as well as liquidation preferences and conversion privileges. All rights, preferences, and privileges associated with the preferred stock were terminated at the time of the Company’s IPO in conjunction with the conversion of all outstanding shares of Preferred Stock into shares of common stock.

Common Stock

6 Months Ended

Jun. 30, 2018

Federal Home Loan Banks [Abstract]

Common Stock

9. Common Stock

As of June 30, 2018, the authorized capital stock of the Company included 150,000,000 shares of common stock, $0.0001 par value, respectively.

At June 30, 2018 and December 31, 2017, the Company has reserved the following shares of common stock for future issuance:


	June 30, 2018		December 31, 2017
Shares reserved for Series Seed Preferred Stock outstanding		—		806,711
Shares reserved for Series A Preferred Stock outstanding		—		7,611,438
Shares reserved for issuance under the 2018 Stock Option and Incentive Plan		533,911		—
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		223,200		—
Shares reserved for vesting of restricted stock awards		214,789		276,301
Shares reserved for exercise of outstanding stock options		1,871,139		1,034,961
Shares reserved for issuance under the 2015 Stock Option and Grant Plan		—		143,717

Total shares of authorized common stock reserved for future issuance		2,843,039		9,873,128

Stock-Based Compensation

6 Months Ended

Jun. 30, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation

10. Stock-Based Compensation

Amended and Restated 2015 Stock Option and Grant Plan

The Company’s Amended and Restated 2015 Stock Option and Grant Plan, (the “2015 Plan”) provides for the Company to issue restricted stock awards and restricted stock units, or to grant incentive stock options or non-statutory stock options. Incentive stock options may be granted only to the Company’s employees including officers and members of the Board who are also employees. Restricted stock awards and restricted stock units and non-statutory stock options may be granted to employees, members of the Board, outside advisors, and consultants of the Company.

The total number of common shares that may be issued under the 2015 Plan was 2,008,564 shares. Following the IPO, no further grants will be made under 2015 plan.

Shares that expire, are terminated, surrendered or canceled under the 2015 Plan without having been fully exercised will be available for future awards under the 2018 Plan (as defined below). In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for future awards.

The 2015 Plan is administered by the Board. Equity awards granted to employees and members of the Board typically vest over four years.

2018 Stock Option and Incentive Plan

The Company’s 2018 Stock Option and Incentive Plan (the “2018 Plan”) was adopted by the Board June 1, 2018 and approved by stockholders on June 7, 2018 and became effective upon the effectiveness of the Company’s Registration Statement on Form S-1. The 2018 Plan replaced the 2015 Plan as the Board determined not to make additional awards under the 2015 Plan following the pricing of the Company’s IPO. The 2018 Plan allows the compensation committee to make equity-based and cash-based incentive awards to its officers, employees, directors and other key persons (including consultants).

The Company initially reserved 616,300 shares of its common stock for the issuance of awards under the 2018 Plan. The 2018 Plan provides that the number of shares reserved and available for issuance under the plan will automatically increase each January 1, beginning on January 1, 2019, by 4% of the outstanding number of shares of our common stock on the immediately preceding December 31, or such lesser number of shares as determined by its compensation committee. This number is subject to adjustment in the event of a stock split, stock dividend or other change in its capitalization.

The total number of common shares that may be issued under the 2018 Plan was 616,300 shares as of June 30, 2018, of which 533,911 shares remained available for future grant.

During the six months ended June 30, 2018 and 2017, the Company granted options to purchase 836,178 and 196,872 shares of common stock to employees and members of the Board, respectively.

2018 Employee Stock Purchase Plan

The Company’s 2018 Employee Stock Purchase Plan (the “ESPP”) was adopted by the Board June 1, 2018 and approved by stockholders on June 7, 2018 and became effective upon the effectiveness of the Company’s Registration Statement on Form S-1. The ESPP is intended to qualify as an “employee stock purchase plan” within the meaning of Section 423(b) of the Code. The ESPP initially reserves and authorizes the issuance of up to a total of 223,200 shares of common stock to participating employees. The ESPP provides that the number of shares reserved and available for issuance will automatically increase each January 1, beginning on January 1, 2019 and each January 1 thereafter through January 1, 2028, by the least of (i) 1% of the outstanding number of shares of our common stock on the immediately preceding December 31; (ii) 1,115,700 shares or (iii) such number of shares as determined by the ESPP administrator. The number shares reserved under the ESPP is subject in the event of a stock split, stock dividend or other change in our capitalization.

The total number of common shares that may be issued under the ESPP was 223,200 shares as of June 30, 2018, of which 223,200 shares remained available for future grant.

As of June 30, 2018, the initial purchase period under the ESPP has not yet commenced.

Stock Option Valuation

The assumptions that the Company used to determine the grant-date fair value of stock options granted to employees and members of the Board were as follows, presented on a weighted-average basis:


	Six Months Ended June 30,
	2018			2017
Expected option life (years)		6.06			6.08
Risk-free interest rate		2.73	%		1.90	%
Expected volatility		83.86	%		85.50	%
Expected dividend yield		—	%		—	%

The following table summarizes the Company’s stock option activity for the six months ended June 30, 2018:


	Number of Options		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2017		1,034,961	$	0.78		8.94	$	3,427
Granted		836,178	$	6.38		9.74
Exercised		—
Cancelled or forfeited		—

Outstanding as of June 30, 2018		1,871,139	$	3.29		9.02	$	47,228

Exercisable as of June 30, 2018		309,594	$	0.72		8.13	$	8,619
Unvested as of June 30, 2018		1,559,125	$	3.80		9.20	$	38,609

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock for those stock options that had exercise prices lower than the estimated fair value of the Company’s common stock at June 30, 2018.

The weighted-average grant-date fair value of the Company’s stock options granted during the six months ended June 30, 2018 and 2017 was $7.63 and $0.66, respectively.

Restricted Common Stock

The following table summarizes the Company’s restricted common stock activity for the six months ended June 30, 2018:


	Number of Shares			Weighted-Average Grant Date Fair Value
Issued and unvested as of December 31, 2017		276,301		$	0.42
Vested		(61,512	)		0.42
Forfeited, canceled or expired		—			—

Issued an unvested as of June 30, 2018		214,789			0.42

The total fair value of restricted common stock vested during the six months ended June 30, 2018 and 2017 was $26 and $17, respectively.

Stock-Based Compensation

Stock-based compensation expense was allocated as follows:


	Six Months Ended June 30,
	2018		2017
Research and development	$	187	$	35
General and administrative		379		29

Total stock-based compensation expense	$	566	$	64

As of June 30, 2018, total unrecognized compensation cost related to the unvested stock-based awards was $6,776, which is expected to be recognized over a weighted-average period of 3.61 years.

License Agreements	6 Months Ended
License Agreements	Jun. 30, 2018
Text Block [Abstract]
License Agreements	11. License Agreements *Agreements with UHN* Fabry License Agreement— On January 27, 2016, the Company entered into an agreement with UHN, pursuant to which UHN granted the Company an option to enter into an exclusive license under the UHN intellectual property related to Fabry disease in accordance with the pre-negotiated licensing terms. On November 4, 2016, the Company exercised its option and entered into a license agreement with UHN, pursuant to which UHN granted the Company an exclusive worldwide license under certain intellectual property rights and a non-exclusive worldwide license under certain know-how, in each case subject to certain retained rights, to develop, commercialize and sell products for use in the treatment of Fabry disease. In addition, for three years following the execution of the agreement, UHN granted the Company an exclusive option to obtain a license under certain improvements to the licensed intellectual property rights as well as an option to negotiate a license under certain other improvements. Under this agreement, the Company paid an option fee of CAD $20, an upfront license fee of CAD $75, plus the annual license maintenance fee for the first year. Thereafter, the Company is also required to pay UHN future annual license maintenance fees until the first sale of a licensed product in certain markets. The Company is also obligated to make future milestone payments in an aggregate amount of up to CAD $2,450 upon the achievement of specified milestones as well as royalties on a country-by-country basis of a low to mid-single digit percentage of annual net sales of licensed products and a lower single-digit royalty percentage in certain circumstances. Additionally, the Company has agreed to pay a low double-digit royalty percentage of all sublicensing revenue. The agreement requires the Company to meet certain performance milestones within specified timeframes. UHN may terminate the agreement if the Company fails to meet these performance milestones despite using commercially reasonable efforts and the Company is unable to reach agreement with UHN on revised timeframes. The Company’s royalty obligation expires on a licensed product-by-licensed product and country-by-country basis upon the latest to occur of the expiration or termination of the last valid claim under the licensed intellectual property rights in such country, the tenth anniversary of the first commercial sale of such licensed product in such country and the expiration of any applicable regulatory exclusivity in such country. Unless terminated earlier, the agreement expires upon the expiration of the Company’s royalty obligation for all licensed products. UHN can terminate the agreement if the Company fails to make any payments within a specified period after receiving written notice of such failure, or in the event that the Company fails to obtain or maintain insurance. Either the Company or UHN may terminate the license agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. The Company can voluntarily terminate the agreement with prior notice to UHN. For the six months ended June 30, 2018 and 2017, no upfront fee and maintenance fees were incurred related to Fabry license agreement. Interleukin 12 License Agreement— On January 27, 2016, the Company entered into an exclusive license agreement with UHN, pursuant to which UHN granted the Company a license to certain patent rights for the commercial development, manufacture, distribution and use of any products or processes resulting from development of those patent rights related to Interleukin 12. Upon execution of this agreement, the Company paid an upfront license fee of CAD $264. In addition, as part of the initial consideration for the license, the Company issued to UHN 1,161,665 shares of the Company’s common stock. The fair value of the shares issued to UHN of $480 and the upfront fee was expensed upon the execution of the agreement. In addition, the Company agreed to pay UHN up to $2,000 upon the closing of an IPO if certain criteria are met. This obligation is considered a derivative instrument and was initially recorded at fair value of $49. The Company is also required to pay UHN future annual license maintenance fees of CAD $50 on each anniversary of the effective date of the license agreement prior to expiration or termination and potential future milestone payments of up to CAD $19,275 upon the achievement of specified clinical and regulatory milestones. The Company also agreed to pay UHN royalties of a low single digit percentage of net sales of licensed products sold by the Company. If the Company grants any sublicense rights under the license agreement, the Company has agreed to pay UHN a low double-digit royalty percentage of any sublicense income received by the Company. The agreement requires the Company to meet certain due diligence requirements based upon specified milestones. The agreement expires on the later of the date the last patent rights expire in the last country or ten years from the date of first sale. UHN can terminate the agreement if the Company fails to make any payments within a specified period after receiving written notice of such failure, or in the event that the Company fails to obtain or maintain insurance. The Company can voluntarily terminate the agreement with prior notice to UHN. Either the Company or UHN may terminate the license agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. For the six months ended June 30, 2018 and 2017, the Company recorded research and development expense related to this agreement with UHN of $41 and $151 (nil for the three months ended June 30, 2018 and 2017), respectively, which consists of upfront fees and license maintenance fees and development milestone payments. *Agreement with BioMarin Pharmaceutical Inc. (“BioMarin”)* On August 31, 2017, the Company entered into a license agreement with BioMarin, pursuant to which BioMarin granted the Company an exclusive worldwide license under certain intellectual property rights owned or controlled by BioMarin to develop, commercialize and sell products for use in the treatment of Pompe disease. As consideration for this agreement, the Company paid an upfront license fee of $500 in cash and issued 233,765 shares of Series B Preferred Stock to BioMarin at the time of our Series B Preferred Stock financing in January 2018. Both the upfront cash payment of $500 and the value of the shares Series B Preferred Stock issued of $500 were recorded as research and development expense during the year ended December 31, 2017. The Company is also obligated to make future milestone payments of up to $13,000 upon the achievement of certain specified milestones and agreed to pay BioMarin royalties of a low single-digit percentage of net sales of licensed products sold by the Company or its affiliates covered by patent rights in a relevant country. No upfront fees related to the license were recorded for the six months ended June 30, 2018. Unless terminated earlier, the agreement expires upon the expiration of the Company’s royalty obligation for all licensed products throughout the world. BioMarin and the Company can terminate the agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. The Company may terminate the agreement at will upon written notice to BioMarin. BioMarin has the right to terminate the agreement upon the Company’s bankruptcy or insolvency, or in the event of any challenge or opposition to the licensed patent rights or related actions brought by the Company or its affiliates or sublicensees, or if the Company, its affiliates or sublicensees knowingly assist a third-party in challenging or otherwise opposing the licensed patent rights, except as required under a court order or subpoena. *Agreement with GenStem Therapeutics, Inc. (“GenStem”)* On October 2, 2017, the Company entered into a license agreement with GenStem, pursuant to which GenStem granted the Company an exclusive worldwide license, subject to certain retained rights, under certain intellectual property rights owned or controlled by GenStem to develop, commercialize and sell products for use in the treatment of cystinosis. Under this agreement, the Company paid an upfront license fee of $1,000 and is required to make payments upon completion of certain milestones up to an aggregate of $16,000. The Company also agreed to pay GenStem a tiered mid to high single-digit royalty percentage on annual net sales of licensed products as well as a low double-digit percentage of sublicense income received from certain third party licensees. The Company’s royalty obligation expires on a licensed product-by-licensed product and country-by-country basis on the eleventh anniversary of the first commercial sale of such licensed product in such country or the expiration of the last valid claim under the licensed patent rights covering such licensed product in such country, whichever is later. Unless terminated earlier, the agreement expires upon the expiration of the Company’s royalty obligation for all licensed products throughout the world. GenStem and the Company can terminate the agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. The Company may terminate the agreement at will upon the specified prior written notice to GenStem. No upfront fees related to the license were recorded for the six months ended June 30, 2018. *Agreement with Lund University Rights Holders* On November 17, 2016, the Company entered into a license agreement with affiliates of Lund University, along with certain other relevant rights holders that may be added from time to time, pursuant to which such rights holders granted to the Company an exclusive worldwide license, subject to certain retained rights, under certain intellectual property rights to develop, commercialize and sell products in any and all uses relevant to Gaucher disease. As consideration for the license, the Company is required to make payments in connection with the achievement of certain milestones up to an aggregate of $550. The agreement expires on the latest of (i) the twentieth anniversary of the end of a certain research project the Company is funding pursuant to an agreement with Lund University, (ii) the expiration of the term of any patent filed on the licensed rights that covers a licensed product, (iii) the expiration of any applicable marketing exclusivity right and (iv) such time that neither the Company nor any sublicensees, partners or contractors are commercializing a licensed product. Either the Company or the rights holders acting together may terminate the license agreement if the other such party commits a material breach and fails to cure such breach within a certain period of time, or if the other party enters into liquidation, becomes insolvent, or enters into composition or statutory reorganization proceedings. No upfront fees related to the license were recorded for the six months ended June 30, 2018 and 2017.

Net Loss per Share

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

Net Loss per Share

12. Net Loss per Share

For purposes of the diluted net loss per share calculation, stock options, unvested restricted stock, the warrant to purchase shares of Series A Preferred Stock and Preferred Stock were considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, the weighted-average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same.

The following potentially dilutive common stock equivalents, presented based on amounts outstanding at each period end, were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:


	Six Months Ended June 30,
	2018		2017
Options to purchase common stock		1,871,139		796,311
Restricted common stock		214,789		368,062
Redeemable convertible preferred stock (as converted to common stock)		—		4,155,742
Warrants to purchase redeemable convertible preferred stock (as converted to common stock)		—		6,850
Warrants to purchase common stock		6,850		—

Commitments and Contingencies	6 Months Ended
Commitments and Contingencies	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies *Lease Agreements* On January 12, 2018, the Company entered into a lease agreement for office space located in Cambridge, Massachusetts. The lease agreement expires in January 2023, with a landlord who is an affiliate of the landlord of the Company’s prior lease facility. The annual lease payments are subject to a 3% increase each year. The Company recognizes rent expense on a straight-line basis over the lease period and has recorded deferred rent for rent expense incurred but not yet paid. The Company received a tenant incentive allowance of $842 in 2018. Such incentive allowance is being amortized as a reduction of rent expense on a straight-line basis over the lease period. In accordance with the lease agreement, the Company is required to maintain a security deposit of $209, which was recorded in other assets. In contemplation of this agreement, the Company terminated its prior lease agreement. *Legal Proceedings* The Company, from time to time, may be party to litigation arising in the ordinary course of business. The Company was not subject to any material legal proceedings during the six months ended June 30, 2018 and 2017, and, to the best of its knowledge, no material legal proceedings are currently pending or threatened. *Other* The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at June 30, 2018 and December 31, 2017, or royalties on future sales of specified products. No milestone or royalty payments under these agreements are expected to be payable in the immediate future. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company agrees to indemnify, hold harmless, and to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners, in connection with any U.S. patent or any copyright or other intellectual property infringement claim by any third-party with respect to the Company’s products. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Related Party Transactions	6 Months Ended
Related Party Transactions	Jun. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	14. Related Party Transactions *UHN* In connection with the Company’s entry into a license agreement with UHN on January 27, 2016, the Company issued UHN 1,161,665 shares of its common stock. As a result of the issuance of common stock, UHN owned 4.64% and 9.65% of the Company’s fully diluted equity as of June 30, 2018 and December 31, 2017, respectively. Upon the closing of the IPO, as UHN’s fully-diluted percentage ownership of the Company was reduced within a range of specified percentages, the Company is obligated to pay UHN an amount up to $2,000, which was recorded in accrued expenses as of June 30, 2018. See Note 3 for further discussion on the accounting treatment for this provision. During the six months ended June 30, 2018 and 2017, the Company recognized $41 and $151 (nil for three months ended June 30, 2018 and 2017), respectively, of research and development expense related to the license agreements with UHN. For the six months ended June 30, 2018 and 2017 the Company recorded expenses of $40 and $48 ($2 and $24 for the three months ended June 30, 2018 and 2017), respectively, related to consulting services provided by an entity affiliated with an officer of the Company and a member of the Board. The entity is also a shareholder of the Company and owned 0.48% and 1.56% of the Company’s fully diluted equity as of June 30, 2018 and 2017, respectively. *Others* For the three and six months ended June 30, 2017, the Company recorded expenses of $15 and $15, respectively, related to services provided by an entity affiliated with a member of the Board and the use of office space. The lease was terminated in February 2017 and the services were no longer being provided as of June 30, 2017.

Subsequent Events	6 Months Ended
Subsequent Events	Jun. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	15. Subsequent Events On July 13, 2018, the lender of the Loan Agreement exercised its common stock warrants to purchase 6,850 shares of common stock on a net basis, resulting in the issuance of 6,091 shares of common stock.

Nature of the Business and Basis of Presentation (Policies)

6 Months Ended

Jun. 30, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Basis of presentation

Basis of Presentation

The accompanying condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB).

The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited annual consolidated financial statements as of and for the year ended December 31, 2017, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of June 30, 2018, and the results of its operations and its cash flows for the three and six months ended June 30, 2018 and 2017.

The results for the six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2017, and the notes thereto, which are included elsewhere in the Company’s final prospectus for its initial public offering (“IPO”), filed with the Securities and Exchange Commission (“SEC”) pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended, on June 21, 2018 (the “Prospectus”).

The accompanying condensed financial statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the financial statements. As of June 30, 2018, the Company’s significant accounting policies and estimates, which are detailed in the Company’s Prospectus, have not changed.

Reverse Stock Split

Reverse Stock Split

On June 1, 2018, the Board approved a 1-for-4.312 reverse stock split of the Company’s common stock. The reverse stock split was approved by the stockholders on June 7, 2018 and became effective on June 7, 2018. Stockholders entitled to fractional shares as a result of the reverse stock split will receive a cash payment in lieu of receiving fractional shares. All share and per share data shown in the accompanying unaudited condensed consolidated financial statements and related notes have been retroactively revised to reflect the reverse stock split. Shares of common stock underlying outstanding stock options and other equity instruments were proportionately reduced and the respective exercise prices, if applicable, were proportionately increased in accordance with the terms of the agreements governing such securities. Shares of common stock reserved for issuance upon the conversion of the Company’s Preferred Stock were proportionately reduced and the respective conversion prices were proportionately increased.

Subsequent Event Considerations

Subsequent Event Considerations

Use of Estimates

Use of Estimates

Cash and Cash Equivalents

Cash and Cash Equivalents

Concentrations of Credit Risk

Concentrations of Credit Risk

Fair Value Measurements

Fair Value Measurements

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Segment Information

Segment Information

Research and Development Costs

Research and Development Costs

Stock-Based Compensation

Stock-Based Compensation

Warrant to Purchase Preferred Stock

Warrant to Purchase Preferred Stock

Comprehensive Income (Loss)

Comprehensive Income (Loss)

Leases

Leases

Net Income (Loss) per Share Attributable to Common Stockholders

Net Income (Loss) per Share Attributable to Common Stockholders

Emerging Growth Company Status

Emerging Growth Company Status

Recently Issued Accounting Pronouncements

Recently Issued Accounting Pronouncements

Fair Value of Financial Assets and Liabilities (Tables)

6 Months Ended

Jun. 30, 2018

Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis


	Fair Value Measurements as of June 30, 2018 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	154,549	$	—	$	—	$	154,549


	Fair Value Measurements as of December 31, 2017 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	5,684	$	—	$	—	$	5,684
Restricted cash		24		—		—		24

	$	5,708	$	—	$	—	$	5,708

Liabilities:
Derivative liability	$	—	$	—	$	371	$	371
Warrant to purchase redeemable convertible preferred stock		—		—		35		35

	$	—	$	—	$	406	$	406

Warrants to Purchase Redeemable Convertible Preferred Stock [Member]

Summary of Changes in Fair Value Determined by Level 3 Inputs

The following table sets forth a summary of changes in the fair value of the Company’s preferred stock warrant liability for which fair value was determined by Level 3 inputs:


	Warrant Liability
Balance as of December 31, 2017	$	35
Change in fair value		162
Conversion of preferred stock warrant to common stock warrant		(197	)
Balance as of June 30, 2018	$	—

Derivative Liabilities [Member]

Summary of Changes in Fair Value Determined by Level 3 Inputs

The following table sets forth a summary of changes in the fair value of the Company’s derivative liability for which fair value is determined by Level 3 inputs:


	Derivative Liability
Balance as of December 31, 2017	$	371
Change in fair value		1,629
Transferred to accrued expenses		(2,000	)

Balance as of June 30, 2018	$	—

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Prepaid Expenses and Other Current Assets (Tables)

6 Months Ended

Jun. 30, 2018

Text Block [Abstract]

Summary of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:


	June 30, 2018		December 31, 2017
Prepaid development costs	$	762	$	163
Prepaid rent		76		122
Other current assets		294		60

	$	1,132	$	345

Property and Equipment, Net (Tables)

6 Months Ended

Jun. 30, 2018

Property, Plant and Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following:


	June 30, 2018			December 31, 2017
Laboratory and office equipment	$	926		$	126
Leasehold improvements		1,112			240
Computer equipment and software		83			28

		2,121			394
Less: Accumulated depreciation and amortization		(71	)		(45	)

	$	2,050		$	349

Accrued Expenses (Tables)

6 Months Ended

Jun. 30, 2018

Payables and Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:


	June 30, 2018		December 31, 2017
Dilution payment (Note 3)	$	2,000	$	—
Compensation and benefit costs		846		794
Short-term deferred rent		171		9
Research and development costs		753		831
IPO issuance cost		693		—
Property and equipment		667		—
Other liabilities		308		464

	$	5,438	$	2,098

Redeemable Convertible Preferred Stock (Tables)

6 Months Ended

Jun. 30, 2018

Text Block [Abstract]

Summary of Preferred Stock

As of December 31, 2017, the Preferred Stock consisted of the following:


	December 31, 2017
	Preferred Shares Authorize		Preferred Shares Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series Seed preferred stock		3,333,333		3,333,333	$	1,500,000	$	1,500,000		806,711
Series A preferred stock		31,639,202		31,450,499		25,000,000		25,000,000		7,611,438

		34,972,535		34,783,832	$	26,500,000	$	26,500,000		8,418,149

Common Stock (Tables)

6 Months Ended

Jun. 30, 2018

Federal Home Loan Banks [Abstract]

Summary of Common Stock Reserved for Future Issuance

At June 30, 2018 and December 31, 2017, the Company has reserved the following shares of common stock for future issuance:


	June 30, 2018		December 31, 2017
Shares reserved for Series Seed Preferred Stock outstanding		—		806,711
Shares reserved for Series A Preferred Stock outstanding		—		7,611,438
Shares reserved for issuance under the 2018 Stock Option and Incentive Plan		533,911		—
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		223,200		—
Shares reserved for vesting of restricted stock awards		214,789		276,301
Shares reserved for exercise of outstanding stock options		1,871,139		1,034,961
Shares reserved for issuance under the 2015 Stock Option and Grant Plan		—		143,717

Total shares of authorized common stock reserved for future issuance		2,843,039		9,873,128

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

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+ Details

Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Assumptions that Used to Determine Grant-Date Fair Value of Stock Options Granted to Employees and Members of Board

The assumptions that the Company used to determine the grant-date fair value of stock options granted to employees and members of the Board were as follows, presented on a weighted-average basis:


	Six Months Ended June 30,
	2018			2017
Expected option life (years)		6.06			6.08
Risk-free interest rate		2.73	%		1.90	%
Expected volatility		83.86	%		85.50	%
Expected dividend yield		—	%		—	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the six months ended June 30, 2018:


	Number of Options		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2017		1,034,961	$	0.78		8.94	$	3,427
Granted		836,178	$	6.38		9.74
Exercised		—
Cancelled or forfeited		—

Outstanding as of June 30, 2018		1,871,139	$	3.29		9.02	$	47,228

Exercisable as of June 30, 2018		309,594	$	0.72		8.13	$	8,619
Unvested as of June 30, 2018		1,559,125	$	3.80		9.20	$	38,609

Summary of Restricted Stock Unit Activity

The following table summarizes the Company’s restricted common stock activity for the six months ended June 30, 2018:


	Number of Shares			Weighted-Average Grant Date Fair Value
Issued and unvested as of December 31, 2017		276,301		$	0.42
Vested		(61,512	)		0.42
Forfeited, canceled or expired		—			—

Issued an unvested as of June 30, 2018		214,789			0.42

Schedule of Stock-based Compensation Expense

Stock-based compensation expense was allocated as follows:


	Six Months Ended June 30,
	2018		2017
Research and development	$	187	$	35
General and administrative		379		29

Total stock-based compensation expense	$	566	$	64

Net Loss per Share (Tables)

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

Schedule of Dilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share


	Six Months Ended June 30,
	2018		2017
Options to purchase common stock		1,871,139		796,311
Restricted common stock		214,789		368,062
Redeemable convertible preferred stock (as converted to common stock)		—		4,155,742
Warrants to purchase redeemable convertible preferred stock (as converted to common stock)		—		6,850
Warrants to purchase common stock		6,850		—

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands				6 Months Ended
	Jun. 25, 2018 USD ($) shares	Jun. 20, 2018 USD ($) $ / shares shares	Jun. 01, 2018	Jun. 30, 2018 USD ($) shares
Nature Of Organization And Operations [Line Items]
Reverse stock split ratio			0.2420135
Net proceeds from IPO, after deducting underwriting discounts and commissions and other offering expenses				$ 105,423
IPO [Member]
Nature Of Organization And Operations [Line Items]
Common shares issued and sold in initial public offering \| shares	787,193	5,247,958
Public offering price \| $ / shares		$ 19.00
Net proceeds from IPO, after deducting underwriting discounts and commissions and other offering expenses	$ 13,910	$ 90,271		$ 104,182
Underwriting discounts and commissions	$ 1,047	6,980
Initial public offering expenses		$ 2,459
Preferred stock converted into common stock \| shares				15,320,213

Summary of Significant Accounting Policies - Additional Information (Detail)	Jun. 30, 2018
Expected Dividend Yield [Member]
Significant Of Accounting Policies [Line Items]
Expected cash dividend yield	0

Fair Value of Financial Assets and Liabilities - Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Assets Fair Value Disclosure
Assets Fair Value Disclosure		$ 5,708
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		406
Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 154,549	5,684
Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure		24
Derivative Liabilities [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		371
Warrants to Purchase Redeemable Convertible Preferred Stock [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		35
Level 1 [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure		5,708
Level 1 [Member] \| Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 154,549	5,684
Level 1 [Member] \| Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure		24
Level 3 [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		406
Level 3 [Member] \| Derivative Liabilities [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		371
Level 3 [Member] \| Warrants to Purchase Redeemable Convertible Preferred Stock [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		$ 35

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended
	Jan. 27, 2016	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jul. 31, 2018
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Fair value input transfer between levels		$ 0	$ 0	$ 0	$ 0
Increase in fair value of derivative		1,042,000	$ 3,000	1,629,000	$ 35,000
Dilution payment		$ 2,000,000		$ 2,000,000
IPO [Member] \| Subsequent Event [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Dilution payment						$ 2,000,000
License Agreement [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Provision for maximum cash payment	$ 2,000,000
License Agreement [Member] \| Research and Development Expense [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Initial fair value of the derivative	$ 49,000
License Agreement [Member] \| Stock Purchase Agreement [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Common stock issued for initial consideration for license	1,161,665

Fair Value of Financial Assets and Liabilities - Summary of Changes in Fair Value of Preferred Stock Warrant Liability for Fair Value Determined by Level 3 Inputs (Detail) - Level 3 [Member] - Warrants to Purchase Redeemable Convertible Preferred Stock [Member] $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 35
Change in fair value	162
Conversion of preferred stock warrant to common stock warrant	$ (197)

Fair Value of Financial Assets and Liabilities - Summary of Changes in Fair Value of Derivative Liability for Fair Value Determined by Level 3 Inputs (Detail) - Level 3 [Member] - Derivative Liabilities [Member] $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 371
Change in fair value	1,629
Transferred to accrued expenses	$ (2,000)

Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid development costs	$ 762	$ 163
Prepaid rent	76	122
Other current assets	294	60
Total	$ 1,132	$ 345

Property and Equipment, Net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 2,121	$ 394
Less: Accumulated depreciation and amortization	(71)	(45)
Property plant and equipment, net	2,050	349
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	926	126
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	1,112	240
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 83	$ 28

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 45	$ 13	$ 66	$ 15

Accrued Expenses - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accrued Liabilities, Current [Abstract]
Dilution payment (Note 3)	$ 2,000
Compensation and benefit costs	846	$ 794
Short-term deferred rent	171	9
Research and development costs	753	831
IPO issuance cost	693
Property and equipment	667
Other liabilities	308	464
Accrued expenses	$ 5,438	$ 2,098

Loan Agreement and Warrant to Purchase Preferred Stock - Additional Information (Detail) - USD ($)		3 Months Ended	6 Months Ended
	Jun. 23, 2017	Jun. 30, 2018	Jun. 30, 2018
Class of Warrant or Right [Line Items]
Maximum issuance of term loans	$ 10,000,000
Available facility		$ 10,000,000	$ 10,000,000
Loss related to change in fair value of warrant		$ 150,000	$ 162,000
Prime Rate [Member]
Class of Warrant or Right [Line Items]
Floating interest rate per annum	3.00%
Series A Preferred Stock [Member]
Class of Warrant or Right [Line Items]
Warrant to purchase shares	188,702
Warrant exercise price	$ 0.7949
Warrant expiration date		Jun. 22, 2027	Jun. 22, 2027
Shares purchase upon exercise of warrant	28,305
Additional shares purchase upon exercise of warrant	160,397
Minimum [Member]
Class of Warrant or Right [Line Items]
Floating interest rate per annum	1.00%

Redeemable Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended
	Jan. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 20, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		63,491,857			34,972,535
Redeemable convertible preferred stock, par value		$ 0.0001
Redeemable convertible preferred stock, shares issued and sold					34,783,832
Proceeds from issuance of redeemable convertible preferred stock		$ 58,258	$ 3,468
IPO [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, sales price per share				$ 19.00
Preferred stock converted into common stock		15,320,213
Series Seed Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		3,333,333			3,333,333
Redeemable convertible preferred stock, shares issued and sold					3,333,333
Series A Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		31,639,202			31,639,202
Redeemable convertible preferred stock, shares issued and sold					31,450,499
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		28,519,322
Redeemable convertible preferred stock, shares issued and sold	28,519,322
Redeemable convertible preferred stock, sales price per share	$ 2.1389
Proceeds from issuance of redeemable convertible preferred stock	$ 58,757
Redeemable convertible preferred stock, net of issuance costs	$ 2,243

Redeemable Convertible Preferred Stock - Summary of Preferred Stock (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Preferred Shares Authorized	63,491,857	34,972,535
Preferred Shares Issued		34,783,832
Preferred Shares Outstanding		34,783,832
Carrying Value		$ 26,500,000
Liquidation Preference		$ 26,500,000
Common Stock Issuable Upon Conversion		8,418,149
Series Seed Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	3,333,333	3,333,333
Preferred Shares Issued		3,333,333
Preferred Shares Outstanding		3,333,333
Carrying Value		$ 1,500,000
Liquidation Preference		$ 1,500,000
Common Stock Issuable Upon Conversion		806,711
Series A Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	31,639,202	31,639,202
Preferred Shares Issued		31,450,499
Preferred Shares Outstanding		31,450,499
Carrying Value		$ 25,000,000
Liquidation Preference		$ 25,000,000
Common Stock Issuable Upon Conversion		7,611,438

Common Stock - Additional Information (Detail) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Equity [Abstract]
Common stock, shares authorized	150,000,000	51,000,000
Common stock, par value	$ 0.0001	$ 0.0001

Common Stock - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Jun. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	2,843,039	9,873,128
Series Seed Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		806,711
Series A Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		7,611,438
2018 Stock Option and Incentive Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	533,911
2018 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	223,200
Restricted Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	214,789	276,301
Employee Stock Option [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,871,139	1,034,961
2015 Stock Option and Grant Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		143,717

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options granted	836,178
Options Granted, Weighted-average grant-date fair value	$ 7.63	$ 0.66
Total fair value of restricted common stock vested	$ 26	$ 17
Unrecognized stock-based compensation expenses	$ 6,776
Unrecognized stock-based compensation expense, period for recognition	3 years 7 months 10 days
2018 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares that may be issued under the plan	616,300
Common shares available for future grant	533,911
Number of shares of common stock outstanding, increase, percentage	4.00%
2015 Stock Option and Grant Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares that may be issued under the plan	2,008,564
Common shares available for future grant	0
Option vesting period	4 years
Number of options granted	836,178	196,872
2018 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares that may be issued under the plan	223,200
Common shares available for future grant	223,200
Employee Stock Purchase Plan, description	The ESPP provides that the number of shares reserved and available for issuance will automatically increase each January 1, beginning on January 1, 2019 and each January 1 thereafter through January 1, 2028, by the least of (i) 1% of the outstanding number of shares of our common stock on the immediately preceding December 31; (ii) 1,115,700 shares or (iii) such number of shares as determined by the ESPP administrator.
2018 Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares that may be issued under the plan	223,200
Number of shares of common stock outstanding, increase, percentage	1.00%
Number of common shares that may increase under the plan	1,115,700

Stock-Based Compensation - Summary of Assumptions that Used to Determine Grant-Date Fair Value of Stock Options Granted to Employees and Members of Board (Detail)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected option life (years)	6 years 22 days	6 years 29 days
Risk-free interest rate	2.73%	1.90%
Expected volatility	83.86%	85.50%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of options, Outstanding beginning balance	1,034,961
Number of options, Granted	836,178
Number of options, Exercised	0
Number of options, Cancelled or forfeited	0
Number of options, Outstanding ending balance	1,871,139	1,034,961
Number of options, Exercisable	309,594
Number of options, Unvested	1,559,125
Weighted average exercise price, Outstanding beginning balance	$ 0.78
Weighted average exercise price, Granted	6.38
Weighted average exercise price, Exercised	0
Weighted average exercise price, Cancelled or forfeited	0
Weighted average exercise price, Outstanding ending balance	3.29	$ 0.78
Weighted average exercise price, Exercisable	0.72
Weighted average exercise price, Unvested	$ 3.80
Weighted average remaining contractual term, Granted	9 years 8 months 26 days
Weighted average remaining contractual term, Outstanding balance	9 years 7 days	8 years 11 months 8 days
Weighted average remaining contractual term, Exercisable	8 years 1 month 16 days
Weighted average remaining contractual term, Unvested	9 years 2 months 12 days
Aggregate intrinsic value, Outstanding balance	$ 47,228	$ 3,427
Aggregate intrinsic value, Exercisable	8,619
Aggregate intrinsic value, Unvested	$ 38,609

Stock-Based Compensation - Summary of Restricted Stock Unit Activity (Detail)	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of shares, Issued and unvested beginning balance	276,301
Number of shares, Vested	(61,512)
Number of shares, Forfeited, cancelled or expired	0
Number of shares, Issued and unvested ending balance	214,789
Weighted average grant date fair value, Issued and unvested, beginning balance \| $ / shares	$ 0.42
Weighted average grant date fair value, Vested \| $ / shares	0.42
Weighted average grant date fair value, Issued and unvested, ending balance \| $ / shares	$ 0.42

Stock-Based Compensation - Schedule of Stock-based Compensation Expense (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock based compensation expense	$ 566	$ 64
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock based compensation expense	187	35
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock based compensation expense	$ 379	$ 29

License Agreements - Additional Information (Detail) $ in Thousands					1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Oct. 02, 2017 USD ($)	Nov. 17, 2016 USD ($)	Jan. 27, 2016 USD ($) shares	Jan. 27, 2016 CAD ($)	Aug. 31, 2017 USD ($) shares	Jun. 30, 2018 USD ($) shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) shares	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) shares	Jan. 27, 2016 CAD ($) shares
License Agreement [Line Items]
Upfront license fee								$ 0	$ 0
Upfront fee								0	0
Annual maintenance fees								$ 0	0
Common stock, shares issued \| shares						23,936,838		23,936,838		2,581,474
Research and development						$ 7,407,000	$ 1,881,000	$ 13,054,000	3,315,000
UHN Agreement [Member] \| Fabry License Agreement [Member]
License Agreement [Line Items]
Option fee											$ 20
Upfront license fee				$ 75
Milestone payments				2,450
UHN Agreement [Member] \| Interleukin 12 License Agreement [Member]
License Agreement [Line Items]
Upfront license fee				264
Milestone payments				19,275
Annual maintenance fees				$ 50
Common stock, shares issued \| shares			1,161,665								1,161,665
Fair value of shares issued			$ 480
Payments upon closing of an initial public offering			2,000,000
Fair value of derivative liability			$ 49,000
Research and development						$ 0	$ 0	41,000	$ 151,000
BioMarin Pharmaceutical Inc [Member]
License Agreement [Line Items]
Upfront license fee					$ 500,000			0
Milestone payments								13,000,000
Research and development										$ 500,000
Upfront cash payment										$ 500,000
GenStem Therapeutics Inc [Member]
License Agreement [Line Items]
Upfront license fee	$ 1,000,000							$ 0
Milestone payments	$ 16,000,000
Lund University Rights Holders Agreement [Member]
License Agreement [Line Items]
Milestone payments		$ 550,000
Series B Preferred Stock [Member] \| BioMarin Pharmaceutical Inc [Member]
License Agreement [Line Items]
Preferred stock issued \| shares					233,765

Net Loss per Share - Schedule of Dilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Detail) - shares	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	1,871,139	796,311
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	214,789	368,062
Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount		6,850
Common Stock Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	6,850
Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount		4,155,742

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands		6 Months Ended
	Jan. 12, 2018	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Lease agreement expiration date	Jan. 31, 2023	Feb. 28, 2017
Percentage of annual increase in rent	3.00%
Tenant incentive allowance	$ 842
Security deposit in connection with lease	$ 209

Related Party Transactions - Additional information (Detail) - USD ($)			3 Months Ended		6 Months Ended
	Jan. 12, 2018	Jan. 27, 2016	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Research and development expense related to license agreements			$ 7,407,000	$ 1,881,000	$ 13,054,000	$ 3,315,000
Lease termination date	Jan. 31, 2023				Feb. 28, 2017
Officers and Board Members [Member] \| Consulting Services Agreement [Member]
Related Party Transaction [Line Items]
Issuance of common stock ownership percentage			0.48%	1.56%	0.48%	1.56%
Expense related to related party			$ 2,000	$ 24,000	$ 40,000	$ 48,000
Board Member [Member]
Related Party Transaction [Line Items]
Expense related to related party				15,000		15,000
License Agreement [Member]
Related Party Transaction [Line Items]
Provision for maximum cash payment		$ 2,000,000
Research and development expense related to license agreements			$ 0	$ 0	41,000	$ 151,000
License Agreement [Member] \| Accrued Expenses [Member]
Related Party Transaction [Line Items]
Provision for maximum cash payment					$ 2,000,000
License Agreement [Member] \| Stock Purchase Agreement [Member]
Related Party Transaction [Line Items]
Common stock issued for initial consideration for license		1,161,665
Issuance of common stock ownership percentage			4.64%		4.64%		9.65%

Subsequent Events - Additional Information (Detail) - shares	Jul. 13, 2018	Jun. 30, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Warrants to purchase common stock		150,000,000	51,000,000
Warrants to purchase common stock, issued		23,936,838	2,581,474
Warrant [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Warrants to purchase common stock	6,850
Warrants to purchase common stock, issued	6,091