0001564590-20-050874 | 10-Q | XBRL Viewer

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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2020	Nov. 01, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Trading Symbol	SLDB
Entity Registrant Name	Solid Biosciences Inc.
Entity Current Reporting Status	Yes
Entity Central Index Key	0001707502
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		60,449,341
Entity File Number	001-38360
Entity Tax Identification Number	90-0943402
Entity Address, Address Line One	141 Portland Street, Fifth Floor
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	337-4680
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock $0.001 par value per share
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 24,797	$ 76,043
Available-for-sale-securities		7,481
Prepaid expenses and other current assets	2,365	2,778
Total current assets	27,162	86,302
Property and equipment, net	8,869	11,645
Operating lease, right-of-use assets	3,951	4,988
Other non-current assets	209	209
Restricted cash	327	327
Total assets	40,518	103,471
Current liabilities:
Accounts payable	4,083	7,124
Accrued expenses	8,550	9,178
Operating lease liabilities	1,930	1,736
Finance lease liabilities	202	186
Other current liabilities		52
Total current liabilities	14,765	18,276
Operating lease liabilities, excluding current portion	2,943	4,414
Finance lease liabilities, excluding current portion	579	733
Total liabilities	18,287	23,423
Commitments and contingencies (Note 11)
Preferred stock, $0.001 par value; 10,000,000 shares authorized at September 30, 2020 and December 31, 2019; no shares issued and outstanding at September 30, 2020 and December 31, 2019
Common stock, $0.001 par value; 300,000,000 shares authorized at September 30, 2020 and December 31, 2019; 46,178,519 shares issued and outstanding at September 30, 2020 and 45,987,571 shares issued and outstanding at December 31, 2019; 2,295,699 pre-funded warrants outstanding at September 30, 2020 and December 31, 2019	48	48
Additional paid-in capital	405,389	396,278
Accumulated other comprehensive income		1
Accumulated deficit	(383,206)	(316,279)
Total stockholders’ equity	22,231	80,048
Total liabilities and stockholders’ equity	$ 40,518	$ 103,471

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	46,178,519	45,987,571
Common stock, shares outstanding	46,178,519	45,987,571
Pre-funded warrants outstanding	2,295,699	2,295,699

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development	16,045	22,792	49,158	67,671
General and administrative	5,181	6,925	15,957	19,317
Restructuring charges			1,944
Total operating expenses	21,226	29,717	67,059	86,988
Loss from operations	(21,226)	(29,717)	(67,059)	(86,988)
Other income (expense):
Interest (expense) income	(20)	406	131	1,281
Other income		56	1	345
Total other income (expense), net	(20)	462	132	1,626
Net loss	$ (21,246)	$ (29,255)	$ (66,927)	$ (85,362)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.44)	$ (0.67)	$ (1.39)	$ (2.26)
Weighted average shares of common stock outstanding, basic and diluted	48,295,468	43,467,618	48,172,686	37,727,640

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (21,246)	$ (29,255)	$ (66,927)	$ (85,362)
Other comprehensive loss:
Unrealized (loss) gain on available-for-sale securities		(5)	(1)	3
Comprehensive loss	$ (21,246)	$ (29,260)	$ (66,928)	$ (85,359)

- Definition

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income. Excludes changes in equity resulting from investments by owners and distributions to owners.

+ References + Details

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

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+ Details

Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2018	$ 125,183	$ 35	$ 324,209	$ (5)	$ (199,056)
Beginning balance, units at Dec. 31, 2018		35,432,460
Equity-based compensation	10,803		10,803
Sale of common stock, net of issuance costs	47,246	$ 11	47,235
Sale of common stock, net of issuance costs, shares		10,607,525
Sale of pre-funded warrants	10,652	$ 2	10,650
Sale of pre-funded warrants, shares		2,295,699
Net loss	(85,362)				(85,362)
Repurchase of shares of common stock, shares		(41,041)
Unrealized gain (loss) on available for sale securities	3			3
Ending balance at Sep. 30, 2019	108,525	$ 48	392,897	(2)	(284,418)
Ending balance, units at Sep. 30, 2019		48,294,643
Beginning balance at Jun. 30, 2019	76,495	$ 35	331,620	3	(255,163)
Beginning balance, units at Jun. 30, 2019		35,398,894
Equity-based compensation	3,392		3,392
Sale of common stock, net of issuance costs	47,246	$ 11	47,235
Sale of common stock, net of issuance costs, shares		10,607,525
Sale of pre-funded warrants	10,652	$ 2	10,650
Sale of pre-funded warrants, shares		2,295,699
Net loss	(29,255)				(29,255)
Repurchase of shares of common stock, shares		(7,475)
Unrealized gain (loss) on available for sale securities	(5)			(5)
Ending balance at Sep. 30, 2019	108,525	$ 48	392,897	(2)	(284,418)
Ending balance, units at Sep. 30, 2019		48,294,643
Beginning balance at Dec. 31, 2019	80,048	$ 48	396,278	1	(316,279)
Beginning balance, units at Dec. 31, 2019		48,283,270
Equity-based compensation	9,111		9,111
Net loss	(66,927)				(66,927)
Vesting of restricted stock, units		284,700
Forfeiture of restricted stock awards units		(93,752)
Unrealized gain (loss) on available for sale securities	(1)			$ (1)
Ending balance at Sep. 30, 2020	22,231	$ 48	405,389		(383,206)
Ending balance, units at Sep. 30, 2020		48,474,218
Beginning balance at Jun. 30, 2020	40,713	$ 48	402,625		(361,960)
Beginning balance, units at Jun. 30, 2020		48,364,092
Equity-based compensation	2,764		2,764
Net loss	(21,246)				(21,246)
Vesting of restricted stock, units		162,725
Forfeiture of restricted stock awards units		(52,599)
Ending balance at Sep. 30, 2020	$ 22,231	$ 48	$ 405,389		$ (383,206)
Ending balance, units at Sep. 30, 2020		48,474,218

Condensed Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Statement Of Stockholders Equity [Abstract]
Issuance costs	$ 2,079	$ 2,079

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (66,927)	$ (85,362)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of (discount)/premium on available-for-sale securities	(20)	(246)
Equity-based compensation expense	9,111	10,803
Depreciation expense	3,126	2,018
Loss on sale of property and equipment		2
Changes in operating assets and liabilities:
Prepaid expenses and other current and non-current assets	1,450	(1,106)
Accounts payable	(2,602)	8
Accrued expenses and other current and non-current liabilities	(2,044)	2,414
Net cash used in operating activities	(57,906)	(71,469)
Cash flows from investing activities:
Purchases of property and equipment	(839)	(3,742)
Proceeds from sale and maturities of available-for-sale securities	7,900	56,899
Purchases of available-for-sale securities	(401)	(31,496)
Net cash provided by investing activities	6,660	21,661
Cash flows from financing activities:
Proceeds from issuance of common stock		49,325
Proceeds from issuance of warrants		10,652
Payment of offering costs		(1,815)
Net cash provided by financing activities		58,162
Net (decrease)/increase in cash, cash equivalents and restricted cash	(51,246)	8,354
Cash, cash equivalents, and restricted cash at beginning of period	76,370	86,693
Cash, cash equivalents, and restricted cash at end of period	$ 25,124	95,047
Supplemental disclosure of non-cash investing and financing activities:
Operating lease liabilities arising from obtaining right-of-use asset		1,629
Property and equipment included in accounts payable and accruals		51
Offering costs included in accounts payable and accruals		$ 264

Nature of the Business and Basis of Presentation

9 Months Ended

Sep. 30, 2020

Accounting Policies [Abstract]

Nature of the Business and Basis of Presentation

1. Nature of the Business and Basis of Presentation

Nature of Business

Solid Biosciences Inc. was organized in March 2013 under the name SOLID Ventures Management, LLC. In October 2013, the company changed its name to Solid Ventures, LLC and in June 2015, the company changed its name to Solid Biosciences, LLC. The company operated as a Delaware limited liability company under the name Solid Biosciences, LLC until immediately prior to the effectiveness of its registration statement on Form S-1 on January 25, 2018, at which time it completed a statutory corporate conversion into a Delaware corporation (the “Corporate Conversion”) and changed its name to Solid Biosciences Inc. (the “Company”). In addition, entities formed solely for the purpose of holding membership interests in the Company’s limited liability company were merged with and into the Company. As a result of the Corporate Conversion, all of the Series 1 and 2 Senior Preferred, Junior Preferred Units, Series A, B, C and D Common Units of Solid Biosciences, LLC converted into shares of common stock of Solid Biosciences Inc. on a one for 0.8485 basis and all of the unit holders of Solid Biosciences, LLC became holders of common stock of Solid Biosciences Inc.

The Company’s mission is to cure Duchenne muscular dystrophy (“Duchenne”), a genetic muscle-wasting disease predominantly affecting boys. It is caused by mutations in the dystrophin gene, which result in the absence or near-absence of dystrophin protein. Dystrophin protein works to strengthen muscle fibers and protect them from daily wear and tear. Without functioning dystrophin and certain associated proteins, muscles suffer excessive damage from normal daily activities and are unable to regenerate, leading to the build-up of fibrotic, or scar, and fat tissue. The Company’s lead product candidate, SGT-001, is a gene transfer candidate under investigation for its ability to drive functional dystrophin protein expression in patients’ muscles and improve the course of the disease. SGT-001 has been granted Rare Pediatric Disease Designation and Fast Track in the United States and Orphan Drug Designations in both the United States and European Union. The Company filed an Investigational New Drug application (“IND”) in September 2017 and initiated a Phase I/II clinical trial for SGT-001 in the United States during the fourth quarter of 2017, which is called IGNITE DMD. In November 2019, IGNITE DMD was placed on clinical hold by the U.S. Food and Drug Administration (“FDA”). In April 2020, the Company submitted a response to the FDA that included changes to the clinical protocol designed to enhance patient safety, as well as information related to improvements to its manufacturing process. The FDA responded by maintaining the clinical hold and requesting further data and analyses relating to this manufacturing process. In June 2020, the Company submitted a response to the FDA that provided data related to manufacturing process improvements. In July 2020, the Company announced that the FDA responded by maintaining the clinical hold and requesting further manufacturing information and updated safety and efficacy data for all patients dosed in the trial, as well as providing direction on the total viral load to be administered per patient. In October 2020, the Company announced that the FDA lifted the clinical hold placed on the Company’s IGNITE DMD Phase I/II clinical trial.

The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on licenses, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical studies and clinical trials and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance and reporting capabilities.

The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from, among others, other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, partners and consultants.

Liquidity

The accompanying condensed consolidated financial statements have been prepared on a basis that assumes the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the ordinary course of business. Through September 30, 2020, the Company has funded its operations primarily with the proceeds from the sale of redeemable preferred units and member units as well as the sale of common stock and prefunded warrants to purchase shares of its common stock in private placements and the sale of common stock in its initial public offering.

In accordance with Accounting Standards Codification (“ASC”) 205-40, Going Concern, the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date the financial statements are issued. As of September 30, 2020, the Company had an accumulated deficit of $383,206. During the three and nine months ended September 30, 2020, the Company incurred a net loss of $21,246 and $66,927, respectively, and the Company used $57,906 of cash in operations for the nine months ended September 30, 2020. The Company expects to continue to generate operating losses in the foreseeable future. Based upon its current operating plan, the Company expects that its cash and cash equivalents of $24,797 as of September 30, 2020, together with the proceeds of $40,000 received in October 2020 from the sale of shares of common stock to Ultragenyx Pharmaceutical, Inc. (“Ultragenyx”) as well as the net proceeds of $23,209 received from the sale of shares of common stock in October 2020 pursuant to a sales agreement, dated March 13, 2019, between the Company and Jefferies LLC (the “ATM Sales Agreement”) will be sufficient to fund its operating expenses and capital requirements into the second half of 2021.

In accordance with the requirements of ASC 205-40, the Company determined that there is substantial doubt about the Company’s ability to continue as a going concern within twelve months of the issuance date of these financial statements. The financial statements do not include any adjustments that might result from the outcome of this uncertainty. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all, nor is it considered probable under the accounting standards. As such, under the requirements of ASC 205-40, management may not consider the potential for future capital raises or management plans to reduce costs that are not considered probable in its assessment of the Company’s ability to meet its obligations for the next twelve months. If the Company is unable to obtain funding, the Company would be forced to delay, reduce or eliminate some or all of its research and development programs, preclinical and clinical testing or commercialization efforts, which could adversely affect its business prospects.

The accompanying condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying condensed consolidated financial statements include the accounts of Solid Biosciences Inc. and its wholly owned or controlled subsidiaries. All intercompany accounts and transactions have been eliminated.

In the opinion of management, the Company’s accompanying unaudited condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair statement of the Company’s financial statements for interim periods in accordance with GAAP. The information included in this quarterly report on Form 10-Q should be read in conjunction with the Company’s consolidated financial statements and the accompanying notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from the Company’s audited financial statements but does not include all disclosures required by GAAP. The results of operations for the three and nine months ended September 30, 2020 are not necessarily indicative of the operating results for the full year or for any other subsequent interim period.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2020

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

The Company’s accounting policies are described in the “Notes to Consolidated Financial Statements” in its Annual Report on Form 10-K for the year ended December 31, 2019 and updated, as necessary, in this report.

Use of Estimates

The preparation of the Company’s condensed consolidated financial statements in conformity with GAAP requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the recognition of research and development expenses and equity-based compensation. 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. The full extent to which the COVID-19 pandemic will directly or indirectly impact the Company’s business, results of operations and financial condition, including clinical trials and employee-related amounts, will depend on future developments that are highly uncertain, including new information that may emerge concerning COVID-19 and the actions taken to contain it or treat its impact. The Company has made estimates of the impact of COVID-19 within its financial statements and there may be changes to those estimates in future periods. Actual results could differ from the Company’s estimates.

Cash Equivalents

The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents.

Restricted Cash

The Company held restricted cash of $327 in separate restricted bank accounts as security deposits for leases of the Company’s facilities as of September 30, 2020 and December 31, 2019. The Company has included restricted cash of $327 as a non-current asset as of September 30, 2020 and December 31, 2019. A reconciliation of the amounts of cash and cash equivalents and restricted cash from the cash flow statement to the balance sheet is as follows:

	September 30, 2020		December 31, 2019
Cash and cash equivalents as presented on balance sheet	$	24,797	$	76,043
Restricted cash, non-current, as presented on balance sheet		327		327
Cash and cash equivalents and restricted cash as presented on cash flow statement	$	25,124	$	76,370

Segment Data

The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing treatments through gene therapy and other means for patients with Duchenne. All of the Company’s tangible assets are held in the United States.

Recently Adopted Accounting Pronouncements

In August 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement. This new standard modifies certain disclosure requirements on fair value measurements. This new standard became effective for the Company on January 1, 2020. The adoption of this new standard did not have a material impact on the Company’s disclosures.

Recently Issued Accounting Pronouncements

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (‘‘ASU 2016-13’’), which requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. ASU 2016-13 replaces the existing incurred loss impairment model with an expected loss model. It also eliminates the concept of other-than-temporary impairment and requires credit losses related to available-for-sale debt securities to be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. These changes may result in earlier recognition of credit losses. In November 2018, the FASB issued ASU No. 2018-19, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, which narrowed the scope and changed the effective date for non-public entities for ASU 2016-13. The FASB subsequently issued supplemental guidance within ASU No. 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief (‘‘ASU 2019-05’’). ASU 2019-05 provides an option to irrevocably elect the fair value option for certain financial assets previously measured at amortized cost basis. For public entities that are Securities and Exchange Commission filers, excluding entities eligible to be smaller reporting companies, ASU 2016-13 is effective for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. For all other entities, ASU 2016-13 is effective for annual periods beginning after December 15, 2022, including interim periods within those fiscal years. Early adoption is permitted. This standard will be effective for the Company on January 1, 2023. The Company is currently evaluating the potential impact that this standard may have on its financial statements and related disclosures.

Fair Value of Financial Assets and Liabilities

9 Months Ended

Sep. 30, 2020

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

3. Fair Value of Financial Assets and Liabilities

The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values:

	Fair Value Measurements as of September 30, 2020 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	—	$	14,462	$	—	$	14,462
	$	—	$	14,462	$	—	$	14,462

	Fair Value Measurements as of December 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	—	$	50,037	$	—	$	50,037
Available-for-sale securities		—		7,481		—		7,481
	$	—	$	57,518	$	—	$	57,518

As of September 30, 2020, there were no available-for-sale securities. As of December 31, 2019, the fair values of the Company’s available-for-sale debt securities were determined using Level 2 inputs. At December 31, 2019, the Company’s portfolio of available-for-sale securities consisted of corporate bond securities and commercial paper. During the nine months ended September 30, 2020 and the year ended December 31, 2019, there were no transfers between Level 1, Level 2 and Level 3.

The fair value of the Company’s cash, restricted cash, accounts payable, and accrued expenses and other current liabilities approximate their carrying value due to their short-term maturities.

Available-for-Sale Securities

9 Months Ended

Sep. 30, 2020

Investments Debt And Equity Securities [Abstract]

Available-for-Sale Securities

4. Available-for-Sale Securities

As of September 30, 2020, there were no available-for-sale securities. As of December 31, 2019, the fair value of available-for-sale securities by type of security was as follows:

	December 31, 2019
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Fair Value
Investments:
Corporate bond securities	$	1,502	$	1	$	—	$	1,503
Commercial paper		5,978		—		—		5,978
	$	7,480	$	1	$	—	$	7,481

The estimated fair value and amortized cost of the Company’s available-for-sale securities by contractual maturity are summarized as follows:

	December 31, 2019
	Amortized Cost		Fair Value
Due in one year or less	$	7,480	$	7,481
Total available-for-sale securities	$	7,480	$	7,481

The weighted average maturity of the Company’s available-for-sale securities as of December 31, 2019 was approximately 0.2 years.

Property and Equipment

9 Months Ended

Sep. 30, 2020

Property Plant And Equipment [Abstract]

Property and Equipment

5. Property and Equipment

Property and equipment consists of the following:

	September 30, 2020		December 31, 2019
Furniture and fixtures	$	203	$	203
Laboratory equipment		9,616		9,425
Leasehold improvements		4,713		4,686
Computer equipment		436		428
Computer software		553		372
Construction in process		1,265		1,322
		16,786		16,436
Less accumulated depreciation		7,917		4,791
	$	8,869	$	11,645

Depreciation expense was $766 and $3,126 for the three and nine months ended September 30, 2020, respectively, and $764 and $2,018 for the three and nine months ended September 30, 2019, respectively.

Prepaid Expenses and Other Current Assets

9 Months Ended

Sep. 30, 2020

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

Prepaid Expenses and Other Current Assets

6. Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consist of the following:

	September 30, 2020		December 31, 2019
Prepaid research and development expenses	$	587	$	1,290
Prepaid expenses and other assets		1,778		1,488
	$	2,365	$	2,778

Accrued Expenses and Other Current Liabilities

9 Months Ended

Sep. 30, 2020

Payables And Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

7. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consist of the following:

	September 30, 2020		December 31, 2019
Accrued research and development	$	3,099	$	3,742
Accrued compensation		3,961		3,583
Accrued other		1,490		1,853
	$	8,550	$	9,178

Stockholders' Equity

9 Months Ended

Sep. 30, 2020

Equity [Abstract]

Stockholders' Equity

8. Stockholders’ Equity

On July 30, 2019, the Company issued and sold in a private placement (i) 10,607,525 shares of its common stock at a price per share of $4.65 and (ii) 2,295,699 pre-funded warrants to purchase shares of its common stock at a price per warrant of $4.64. Each pre-funded warrant is exercisable for one share of common stock at an exercise price of $0.01 and the pre-funded warrants have no expiration date. The Company received gross proceeds from the private placement of $59,977, before deducting offering costs of $2,079.

No warrants were exercised during the three and nine months ended September 30, 2020 and 2019.

Equity-Based Compensation

9 Months Ended

Sep. 30, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity-Based Compensation

9. Equity-Based Compensation

In connection with the closing of the Company’s initial public offering, the board of directors and stockholders approved the 2018 Omnibus Incentive Plan, which provides for the reservation of 5,001,000 shares of common stock for equity awards. On June 16, 2020, the Company’s stockholders approved the 2020 Equity Incentive Plan (“2020 Plan”) which consists of (i) 3,000,000 shares of common stock and (ii) additional shares of common stock (up to 4,879,025) as is equal to (i) the number of shares reserved under the Company’s 2018 Omnibus Incentive Plan (“2018 Plan”) that remain available for grant under the 2018 Plan as of immediately prior to

the date the 2020 Plan was approved by the Company’s stockholders and (ii) the number of shares subject to awards granted under the 2018 Plan which awards expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right.

During the three and nine months ended September 30, 2020, the Company granted options for the purchase of 41,440 and 1,439,356 shares of common stock, respectively. During the three and nine months ended September 30, 2020, the Company granted 0 and 1,393,720 restricted stock units, respectively.

The Company recorded equity-based compensation expense related to all of its share-based awards to employees and non-employees in the following captions within its condensed consolidated statements of operations:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Research and development	$	1,524	$	2,070	$	4,379	$	6,003
General and administrative		1,240		1,322		3,881		4,800
Restructuring		—		—		851		—
Total	$	2,764	$	3,392	$	9,111	$	10,803

Income Taxes

9 Months Ended

Sep. 30, 2020

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

During the three and nine months ended September 30, 2020 and 2019, the Company recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits and orphan drug credits generated in each year due to its uncertainty of realizing a benefit from those items. The Company has provided a valuation allowance for the full amount of its net deferred tax assets because, at September 30, 2020 and December 31, 2019, it was more likely than not that any future benefit from deductible temporary differences and net operating loss and tax credit carryforwards would not be realized.

As of September 30, 2020, and December 31, 2019, the Company had not recorded any amounts for unrecognized tax benefits. The Company files income tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending income tax examinations. The Company’s C-Corporation tax years beginning with the year ended December 31, 2018 are open under statute. Any tax credit or net operating loss carryforward can be adjusted in future periods after the respective year of generation’s statute of limitation has closed.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Commitments and Contingencies

9 Months Ended

Sep. 30, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

Legal Proceedings

The Company may periodically become subject to legal proceedings and claims arising in connection with ongoing business activities, including claims or disputes related to patents that have been issued or that are pending in the field of research on which the Company is focused. The Company is not aware of any material claims as of September 30, 2020.

Net Loss per Share

9 Months Ended

Sep. 30, 2020

Earnings Per Share [Abstract]

Net Loss per Share

12. Net Loss per Share

Basic and diluted net loss per share attributable to common stockholders were calculated as follows:

The numerator for basic and diluted net loss per share attributable to common stockholders is as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2020			2019			2020			2019
Net loss attributable to common stockholders	$	(21,246	)	$	(29,255	)	$	(66,927	)	$	(85,362	)

The denominator is as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Weighted average shares of common stock outstanding, basic and diluted		45,999,769		41,945,470		45,876,987		37,214,682
Weighted average shares of pre-funded warrants to purchase common stock		2,295,699		1,522,148		2,295,699		512,958
Total		48,295,468		43,467,618		48,172,686		37,727,640

Net loss per share attributable to common stockholders, basic and diluted is as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2020			2019			2020			2019
Net loss per share attributable to common stockholders	$	(0.44	)	$	(0.67	)	$	(1.39	)	$	(2.26	)

The following potential common stock equivalents, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect for the three and nine months ended September 30:

	2020		2019
Options to purchase shares of common stock		3,324,885		2,662,162
Unvested shares of common stock		102,753		430,683
Unvested restricted stock units		1,254,545		264,650
		4,682,183		3,357,495

Restructuring

9 Months Ended

Sep. 30, 2020

Restructuring And Related Activities [Abstract]

Restructuring

13. Restructuring

In January 2020, the Company’s board of directors approved a restructuring plan to reduce operating costs and better align the Company’s workforce with the needs of its business following the Company’s November 2019 announcement that the SGT-001 IGNITE DMD trial was placed on clinical hold by the U.S. Food and Drug Administration.

Under the restructuring plan, the Company made changes to its management team and reduced headcount by approximately 30 percent. Affected employees were eligible to receive severance payments and outplacement services in connection with the restructuring plan. During the three and nine months ended September 30, 2020, the Company recorded aggregate restructuring charges of $0 and $1,944, respectively, related to severance payments and other employee-related costs. The Company does not expect to incur any additional significant costs associated with this restructuring. During the three and nine months ended September 30, 2020, $246 and $1,671, respectively, of the estimated restructuring charges were paid. The Company expects the remaining accrued restructuring costs of $273 will be paid in the next three months.

The following table shows the total amount expected to be incurred and the liability related to the 2020 restructuring for the three and nine months ended September 30, 2020:

	One-Time Employee Termination Benefits
Accrued restructuring costs as of December 31, 2019	$	-
Restructuring charges incurred during the period		1,944
Amounts paid during the period		(963	)
Accrued restructuring costs as of March 31, 2020	$	981
Amounts paid during the period		(462	)
Accrued restructuring costs as of June 30, 2020	$	519
Amounts paid during the period		(246	)
Accrued restructuring costs as of September 30, 2020	$	273

Subsequent Events

9 Months Ended

Sep. 30, 2020

Subsequent Events [Abstract]

Subsequent Events

14. Subsequent Events

Ultragenyx Collaboration Agreement

On October 22, 2020 the Company entered into a collaboration and license agreement (the “Collaboration Agreement”) with Ultragenyx, to focus on the development and commercialization of new gene therapies for Duchenne. The Company granted Ultragenyx an exclusive worldwide license for any pharmaceutical product that expresses the Company’s proprietary microdystrophin construct from AAV8 and variants thereof in clade E for the treatment of Duchenne and other diseases resulting from the lack of functional dystrophin. The Company will conduct certain activities agreed to by the parties with respect to the development of licensed products. Ultragenyx will reimburse the Company for personnel and out-of-pocket costs that the Company incurs in conducting such development activities. Otherwise, Ultragenyx has decision-making authority with respect to the development, manufacturing and commercialization of licensed products. The Company retains exclusive rights to all other uses of its microdystrophin proteins, including under its existing SGT-001 program.

In connection with the execution of the Collaboration Agreement, Ultragenyx and the Company also entered into a stock purchase agreement , pursuant to which the Company issued and sold 7,825,797 shares of its common stock to Ultragenyx at a price of $5.1113 per share, which represented a 33% premium to the volume weighted average price of the Company’s common stock for the 10 trading days prior to the closing date, for an aggregate purchase price of approximately $40,000.

Ultragenyx also agreed to pay up to $255,000 in cumulative milestone payments per product upon achievement of specified milestone events, and tiered royalties on worldwide net sales at low double digit to mid-teens percentages. Upon achievement of proof-of-concept, the Company has the right to opt-in to co-fund collaboration programs in return for participation in a profit share or increased royalty payments.

At-the-Market Offering

In October 2020, the Company sold 6,302,632 shares of common stock pursuant to the ATM Sales Agreement resulting in net proceeds to the Company of $23,209.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2020

Accounting Policies [Abstract]

Use of Estimates

Cash Equivalents

The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents.

Restricted Cash

	September 30, 2020		December 31, 2019
Cash and cash equivalents as presented on balance sheet	$	24,797	$	76,043
Restricted cash, non-current, as presented on balance sheet		327		327
Cash and cash equivalents and restricted cash as presented on cash flow statement	$	25,124	$	76,370

Segment Data

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

- Definition

Disclosure of accounting policy for segment reporting.

+ References + Details

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2020

Accounting Policies [Abstract]

Reconciliation of Amounts From Cash Flow Statement to Balance Sheet

A reconciliation of the amounts of cash and cash equivalents and restricted cash from the cash flow statement to the balance sheet is as follows:

	September 30, 2020		December 31, 2019
Cash and cash equivalents as presented on balance sheet	$	24,797	$	76,043
Restricted cash, non-current, as presented on balance sheet		327		327
Cash and cash equivalents and restricted cash as presented on cash flow statement	$	25,124	$	76,370

Fair Value of Financial Assets and Liabilities (Tables)

9 Months Ended

Sep. 30, 2020

Fair Value Disclosures [Abstract]

Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis and Indicate Level of Fair Value Hierarchy Utilized

	Fair Value Measurements as of September 30, 2020 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	—	$	14,462	$	—	$	14,462
	$	—	$	14,462	$	—	$	14,462

	Fair Value Measurements as of December 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	—	$	50,037	$	—	$	50,037
Available-for-sale securities		—		7,481		—		7,481
	$	—	$	57,518	$	—	$	57,518

Available-for-Sale Securities (Tables)

9 Months Ended

Sep. 30, 2020

Investments Debt And Equity Securities [Abstract]

Schedule of Fair Value of Available-for-Sale Securities by Type of Security

As of September 30, 2020, there were no available-for-sale securities. As of December 31, 2019, the fair value of available-for-sale securities by type of security was as follows:

	December 31, 2019
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Fair Value
Investments:
Corporate bond securities	$	1,502	$	1	$	—	$	1,503
Commercial paper		5,978		—		—		5,978
	$	7,480	$	1	$	—	$	7,481

Summary Estimated Fair Value and Amortized Cost of Available-for-Sale Securities by Contractual Maturity

The estimated fair value and amortized cost of the Company’s available-for-sale securities by contractual maturity are summarized as follows:

	December 31, 2019
	Amortized Cost		Fair Value
Due in one year or less	$	7,480	$	7,481
Total available-for-sale securities	$	7,480	$	7,481

Property and Equipment (Tables)

9 Months Ended

Sep. 30, 2020

Property Plant And Equipment [Abstract]

Summary of Property and Equipment

Property and equipment consists of the following:

	September 30, 2020		December 31, 2019
Furniture and fixtures	$	203	$	203
Laboratory equipment		9,616		9,425
Leasehold improvements		4,713		4,686
Computer equipment		436		428
Computer software		553		372
Construction in process		1,265		1,322
		16,786		16,436
Less accumulated depreciation		7,917		4,791
	$	8,869	$	11,645

Prepaid Expenses and Other Current Assets (Tables)

9 Months Ended

Sep. 30, 2020

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consist of the following:

	September 30, 2020		December 31, 2019
Prepaid research and development expenses	$	587	$	1,290
Prepaid expenses and other assets		1,778		1,488
	$	2,365	$	2,778

Accrued Expenses and Other Current Liabilities (Tables)

9 Months Ended

Sep. 30, 2020

Payables And Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consist of the following:

	September 30, 2020		December 31, 2019
Accrued research and development	$	3,099	$	3,742
Accrued compensation		3,961		3,583
Accrued other		1,490		1,853
	$	8,550	$	9,178

Equity-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Equity-based Compensation Expense

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Research and development	$	1,524	$	2,070	$	4,379	$	6,003
General and administrative		1,240		1,322		3,881		4,800
Restructuring		—		—		851		—
Total	$	2,764	$	3,392	$	9,111	$	10,803

Net Loss per Share (Tables)

9 Months Ended

Sep. 30, 2020

Earnings Per Share [Abstract]

Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders

The numerator for basic and diluted net loss per share attributable to common stockholders is as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2020			2019			2020			2019
Net loss attributable to common stockholders	$	(21,246	)	$	(29,255	)	$	(66,927	)	$	(85,362	)

The denominator is as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Weighted average shares of common stock outstanding, basic and diluted		45,999,769		41,945,470		45,876,987		37,214,682
Weighted average shares of pre-funded warrants to purchase common stock		2,295,699		1,522,148		2,295,699		512,958
Total		48,295,468		43,467,618		48,172,686		37,727,640

Net loss per share attributable to common stockholders, basic and diluted is as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2020			2019			2020			2019
Net loss per share attributable to common stockholders	$	(0.44	)	$	(0.67	)	$	(1.39	)	$	(2.26	)

Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders

	2020		2019
Options to purchase shares of common stock		3,324,885		2,662,162
Unvested shares of common stock		102,753		430,683
Unvested restricted stock units		1,254,545		264,650
		4,682,183		3,357,495

Restructuring (Tables)

9 Months Ended

Sep. 30, 2020

Restructuring And Related Activities [Abstract]

Schedule of Total Amount Expected to be Incurred and Liability Related to Restructuring

The following table shows the total amount expected to be incurred and the liability related to the 2020 restructuring for the three and nine months ended September 30, 2020:

	One-Time Employee Termination Benefits
Accrued restructuring costs as of December 31, 2019	$	-
Restructuring charges incurred during the period		1,944
Amounts paid during the period		(963	)
Accrued restructuring costs as of March 31, 2020	$	981
Amounts paid during the period		(462	)
Accrued restructuring costs as of June 30, 2020	$	519
Amounts paid during the period		(246	)
Accrued restructuring costs as of September 30, 2020	$	273

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 22, 2020	Oct. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jan. 25, 2018
Nature Of Business And Basis Of Presentation [Line Items]
Net loss			$ 21,246	$ 29,255	$ 66,927	$ 85,362
Accumulated deficit			383,206		383,206		$ 316,279
Net cash used in operating activities					57,906	71,469
Cash and cash equivalents			$ 24,797		$ 24,797		$ 76,043
Proceeds from issuance of common stock						$ 49,325
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement \| Ultragenyx \| Subsequent Event
Nature Of Business And Basis Of Presentation [Line Items]
Proceeds from issuance of common stock	$ 40,000	$ 40,000
ATM Sales Agreement [Member] \| Subsequent Event
Nature Of Business And Basis Of Presentation [Line Items]
Proceeds from issuance of common stock		23,209
ATM Sales Agreement [Member] \| Jeffries L L C \| Subsequent Event
Nature Of Business And Basis Of Presentation [Line Items]
Proceeds from issuance of common stock		$ 23,209
Corporate Conversion [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock, conversion ratio								84.85%

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Restricted cash deposit	$ 327	$ 327
Restricted cash noncurrent	$ 327	$ 327
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, immaterial effect	true
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201813Member

- Definition

Indicates amendment to accounting standards.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 65
-Paragraph 2
-Subparagraph (f)(2)
-URI http://asc.fasb.org/extlink&oid=121370832&loc=SL117420844-207641

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 65
-Paragraph 2
-Subparagraph (g)(1)
-URI http://asc.fasb.org/extlink&oid=121370832&loc=SL117420844-207641

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 65
-Paragraph 15
-Subparagraph (f)(2)
-URI http://asc.fasb.org/extlink&oid=121322162&loc=SL121327923-165333

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 10
-Section 65
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121646688&loc=SL121648383-210437

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 65
-Paragraph 15
-Subparagraph (f)(1)
-URI http://asc.fasb.org/extlink&oid=121322162&loc=SL121327923-165333

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 65
-Paragraph 2
-Subparagraph (g)(2)(i)
-URI http://asc.fasb.org/extlink&oid=121370832&loc=SL117420844-207641

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 65
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Summary of Significant Accounting Policies - Reconciliation of Amounts From Cash Flow Statement to Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Reconciliation Of Beginning And End Of Period Amounts From Cash Flow Statement To Balance Sheet [Abstract]
Cash and cash equivalents as presented on balance sheet	$ 24,797	$ 76,043
Restricted cash, non-current, as presented on balance sheet	327	327
Cash and cash equivalents and restricted cash as presented on cash flow statement	$ 25,124	$ 76,370	$ 95,047	$ 86,693

Fair Value of Financial Assets and Liabilities - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis and Indicate Level of Fair Value Hierarchy Utilized (Detail) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Assets:
Cash equivalents	$ 14,462,000	$ 50,037,000
Available-for-sale securities	0	7,481,000
Assets Fair Value Disclosure	14,462,000	57,518,000
Level 2 [Member]
Assets:
Cash equivalents	14,462,000	50,037,000
Available-for-sale securities		7,481,000
Assets Fair Value Disclosure	$ 14,462,000	$ 57,518,000

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Available-for-sale securities	$ 0	$ 7,481,000
Fair value, assets, level 1 to level 2 transfers, amount	0	0
Fair value, assets, level 2 to level 1 transfers, amount	0	0
Fair value, assets, transfers into level 3, amount	0	0
Fair value, assets, transfers out of level 3, amount	$ 0	$ 0

Available-for-Sale Securities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2020
Investments Debt And Equity Securities [Abstract]
Available-for-sale securities	$ 7,481,000	$ 0
Available-for-sale securities weighted average maturity period	2 months 12 days

Available-for-Sale Securities - Schedule of Fair Value of Available-for-Sale Securities by Type of Security (Detail) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		$ 7,480,000
Gross Unrealized Gain		1,000
Fair Value	$ 0	7,481,000
Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,502,000
Gross Unrealized Gain		1,000
Fair Value		1,503,000
Commercial Paper [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		5,978,000
Fair Value		$ 5,978,000

Available-for-Sale Securities - Summary Estimated Fair Value and Amortized Cost of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Due in one year or less, amortized cost		$ 7,480,000
Amortized Cost		7,480,000
Due in one year or less, fair value		7,481,000
Total available-for-sale securities, fair value	$ 0	$ 7,481,000

Property and Equipment - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 16,786	$ 16,436
Less accumulated depreciation	7,917	4,791
Property and equipment, net	8,869	11,645
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	203	203
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	9,616	9,425
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,713	4,686
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	436	428
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	553	372
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,265	$ 1,322

Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Property Plant And Equipment [Abstract]
Depreciation expense	$ 766	$ 764	$ 3,126	$ 2,018

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid research and development expenses	$ 587	$ 1,290
Prepaid expenses and other assets	1,778	1,488
Prepaid expenses and other current assets	$ 2,365	$ 2,778

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	$ 8,550	$ 9,178
Accrued Research and Development [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	3,099	3,742
Accrued Compensation [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	3,961	3,583
Accrued Other [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	$ 1,490	$ 1,853

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	9 Months Ended
	Jul. 30, 2019	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Class Of Stock [Line Items]
Gross proceeds from private placement	$ 59,977
Offering costs				$ 1,815
Private Placement [Member]
Class Of Stock [Line Items]
Offering costs	$ 2,079
Common Stock [Member] \| Private Placement [Member]
Class Of Stock [Line Items]
Stock issued and sold	10,607,525
Stock price per share	$ 4.65
Pre-Funded Warrants [Member]
Class Of Stock [Line Items]
Warrants exercised		0	0
Pre-Funded Warrants [Member] \| Private Placement [Member]
Class Of Stock [Line Items]
Warrants to purchase common stock	2,295,699
Warrants price per share	$ 4.64
Warrants exercisable for common stock	1
Exercise price of warrant	$ 0.01

Equity-Based Compensation - Additional Information (Detail) - shares		3 Months Ended	9 Months Ended
	Jun. 16, 2020	Sep. 30, 2020	Sep. 30, 2020
2018 Omnibus Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for common stock for equity awards		5,001,000	5,001,000
Common stock purchased		41,440	1,439,356
2018 Omnibus Incentive Plan [Member] \| Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted		0	1,393,720
2020 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for common stock for equity awards	3,000,000
2020 Equity Incentive Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized for common stock for equity awards	4,879,025

Equity-Based Compensation - Schedule of Equity-based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 2,764	$ 3,392	$ 9,111	$ 10,803
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	1,524	2,070	4,379	6,003
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 1,240	$ 1,322	3,881	$ 4,800
Restructuring [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense			$ 851

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ 0	$ 0	$ 0	$ 0
Unrecognized tax benefits	$ 0		$ 0		$ 0

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References + Details

Net Loss per Share - Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share Basic [Line Items]
Net loss attributable to common stockholders	$ (21,246)	$ (29,255)	$ (66,927)	$ (85,362)
Weighted average shares of common stock outstanding, basic and diluted	48,295,468	43,467,618	48,172,686	37,727,640
Net loss per share attributable to common stockholders	$ (0.44)	$ (0.67)	$ (1.39)	$ (2.26)
Common Stock [Member]
Earnings Per Share Basic [Line Items]
Weighted average shares of common stock outstanding, basic and diluted	45,999,769	41,945,470	45,876,987	37,214,682
Pre-Funded Warrants [Member]
Earnings Per Share Basic [Line Items]
Weighted average shares of common stock outstanding, basic and diluted	2,295,699	1,522,148	2,295,699	512,958

- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	4,682,183	3,357,495	4,682,183	3,357,495
Options to Purchase Shares of Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	3,324,885	2,662,162	3,324,885	2,662,162
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	1,254,545	264,650	1,254,545	264,650
Unvested Shares of Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	102,753	430,683	102,753	430,683

Restructuring- Additional Information (Detail) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)
Restructuring Costs [Abstract]
Number of headcount reduced, percentage		30.00%
Restructuring charges related to severance payment and other employee-related costs	$ 0	$ 1,944
Restructuring charges were paid	246	1,671
Restructuring costs	$ 273	$ 273

Restructuring - Schedule of Total Amount Expected to be Incurred and Liability Related to Restructuring (Detail) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2020
Restructuring Costs [Line Items]
Restructuring charges incurred during the period				$ 1,944
Amounts paid during the period	$ (246)			(1,671)
Accrued restructuring costs as of March 31, 2020	273			273
One-Time Employee Termination Benefits [Member]
Restructuring Costs [Line Items]
Accrued restructuring costs as of December 31, 2019	519	$ 981
Restructuring charges incurred during the period			$ 1,944
Amounts paid during the period	(246)	(462)	(963)
Accrued restructuring costs as of March 31, 2020	$ 273	$ 519	$ 981	$ 273

Subsequent Events - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	9 Months Ended
	Oct. 22, 2020	Oct. 21, 2020	Oct. 31, 2020	Sep. 30, 2019
Subsequent Event [Line Items]
Proceeds from issuance of common stock				$ 49,325
Subsequent Event \| ATM Sales Agreement [Member]
Subsequent Event [Line Items]
Stock issued and sold			6,302,632
Proceeds from issuance of common stock			$ 23,209
Subsequent Event \| Ultragenyx \| Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Subsequent Event [Line Items]
Stock issued and sold	7,825,797
Share price	$ 5.1113
Percentage of premium on weighted average price of 10 trading days.		33.00%
Proceeds from issuance of common stock	$ 40,000		$ 40,000
Cumulative Milestone Payments per Collaboration Agreement	$ 255,000