Cabaletta Bio (CABA) 10-Q 5 Dec 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Dec. 05, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Trading Symbol	CABA
Entity Registrant Name	CABALETTA BIO, INC.
Entity Current Reporting Status	No
Entity Central Index Key	0001759138
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		24,034,022
Entity File Number	001-39103
Entity Tax Identification Number	82-1685768
Entity Address, Address Line One	2929 Arch Street
Entity Address, Address Line Two	Suite 600
Entity Address, City or Town	Philadelphia
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	19104
City Area Code	267
Local Phone Number	759-3100
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, par value $0.00001 per share
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 71,041	$ 33,017
Prepaid expenses and other current assets	708	977
Total current assets	71,749	33,994
Property and equipment, net	540
Deferred offering costs	2,563	180
Deposits	91
Total Assets	74,943	34,174
Current liabilities:
Accounts payable	1,154	603
Accrued and other current liabilities	1,910	340
Total current liabilities	3,064	943
Commitments and Contingencies (see Note 6)
Convertible preferred stock:
Series A, A-1, A-2 and B convertible preferred stock, $0.00001 par value: 20,762,168 and 12,393,497 shares authorized as of September 30, 2019 and December 31, 2018, respectively; 19,356,835 and 12,393,047 shares issued and outstanding at September 30, 2019 and December 31, 2018, respectively; aggregate liquidation preference of $94,475 at September 30, 2019.	97,954	43,921
Stockholders’ deficit:
Common stock, $0.00001 par value: 29,000,000 and 21,147,115 shares authorized as of September 30, 2019 and December 31, 2018, respectively; 3,848,320 shares issued and outstanding at both December 31, 2018 and September 30, 2019	0	0
Additional paid-in capital	1,481	1,762
Accumulated deficit	(27,556)	(12,452)
Total stockholders’ deficit	(26,075)	(10,690)
Total liabilities, convertible preferred stock and stockholders’ deficit	$ 74,943	$ 34,174

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Convertible preferred stock, outstanding shares	19,356,835	12,393,047
Convertible preferred stock, aggregate liquidation preference	$ 94,475
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, authorized shares	29,000,000	21,147,115
Common stock, issued shares	3,848,320	3,848,320
Common stock, outstanding shares	3,848,320	3,848,320
Preferred Stock
Convertible preferred stock , par value	$ 0.00001	$ 0.00001
Convertible preferred stock, authorized shares	20,762,168	12,393,497
Convertible preferred stock, issued shares	19,356,835	12,393,047
Convertible preferred stock, outstanding shares	19,356,835	12,393,047

Condensed Statements of Operati

Condensed Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating expenses:
Research and development	$ 3,220	$ 2,589	$ 8,645	$ 3,228
General and administrative	1,811	491	4,178	900
Total operating expenses	5,031	3,080	12,823	4,128
Loss from operations	(5,031)	(3,080)	(12,823)	(4,128)
Other income:
Interest income	381	48	1,283	75
Fair value adjustments on convertible notes		(6,244)		(6,244)
Net loss	(4,650)	(9,276)	(11,540)	(10,297)
Deemed dividend			(5,326)
Net loss attributable to common stockholders	$ (4,650)	$ (9,276)	$ (16,866)	$ (10,297)
Net loss per share, basic and diluted	$ (2.17)	$ (9.47)	$ (9.34)	$ (5.44)
Weighted-average number of shares used in computing net loss per share, basic and diluted	2,142,958	979,069	1,806,494	1,891,456

Condensed Statements of Convert

Condensed Statements of Convertible Preferred Stock and Stockholders' Deficit (Unaudited) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ (249)			$ 1	$ (250)
Beginning balance, Shares at Dec. 31, 2017			3,333,332
Issuance of common stock, Shares			33,670
Stock-based compensation	131			131
Net loss	(1,021)				(1,021)
Ending balance at Jun. 30, 2018	(1,139)			132	(1,271)
Ending balance, Shares at Jun. 30, 2018			3,367,002
Beginning balance at Dec. 31, 2017	(249)			1	(250)
Beginning balance, Shares at Dec. 31, 2017			3,333,332
Net loss	$ (10,297)
Ending balance, Shares at Sep. 30, 2018	0
Ending balance at Sep. 30, 2018	$ (10,220)			327	(10,547)
Ending balance, Shares at Sep. 30, 2018			3,367,002
Beginning balance at Jun. 30, 2018	(1,139)			132	(1,271)
Beginning balance, Shares at Jun. 30, 2018			3,367,002
Stock-based compensation	195			195
Net loss	$ (9,276)				(9,276)
Ending balance, Shares at Sep. 30, 2018	0
Ending balance at Sep. 30, 2018	$ (10,220)			327	(10,547)
Ending balance, Shares at Sep. 30, 2018			3,367,002
Beginning balance at Dec. 31, 2018	$ 43,921	$ 43,921
Beginning balance, Shares at Dec. 31, 2018	12,393,047	12,393,047
Beginning balance at Dec. 31, 2018	$ (10,690)			1,762	(12,452)
Beginning balance, Shares at Dec. 31, 2018	3,848,320		3,848,320
Issuance of convertible preferred stock, net of issuance costs		$ 48,707
Issuance of convertible preferred stock, net of issuance cost, Shares		6,963,788
Exchange of convertible preferred stock, including deemed dividend	$ (5,326)	$ 5,326		(2,674)	(2,652)
Stock-based compensation	912			912
Net loss	(6,890)				(6,890)
Ending balance at Jun. 30, 2019		$ 97,954
Ending balance, Shares at Jun. 30, 2019		19,356,835
Ending balance at Jun. 30, 2019	(21,994)				(21,994)
Ending balance, Shares at Jun. 30, 2019			3,848,320
Beginning balance at Dec. 31, 2018	$ 43,921	$ 43,921
Beginning balance, Shares at Dec. 31, 2018	12,393,047	12,393,047
Beginning balance at Dec. 31, 2018	$ (10,690)			1,762	(12,452)
Beginning balance, Shares at Dec. 31, 2018	3,848,320		3,848,320
Issuance of convertible preferred stock, net of issuance costs	$ 50,000
Net loss	(11,540)
Ending balance at Sep. 30, 2019	$ 97,954	$ 97,954
Ending balance, Shares at Sep. 30, 2019	19,356,835	19,356,835
Ending balance at Sep. 30, 2019	$ (26,075)			1,481	(27,556)
Ending balance, Shares at Sep. 30, 2019	3,848,320		3,848,320
Beginning balance at Jun. 30, 2019		$ 97,954
Beginning balance, Shares at Jun. 30, 2019		19,356,835
Beginning balance at Jun. 30, 2019	$ (21,994)				(21,994)
Beginning balance, Shares at Jun. 30, 2019			3,848,320
Reclassification (see Note 1)				912	(912)
Stock-based compensation	569			569
Net loss	(4,650)				(4,650)
Ending balance at Sep. 30, 2019	$ 97,954	$ 97,954
Ending balance, Shares at Sep. 30, 2019	19,356,835	19,356,835
Ending balance at Sep. 30, 2019	$ (26,075)			$ 1,481	$ (27,556)
Ending balance, Shares at Sep. 30, 2019	3,848,320		3,848,320

Condensed Statements of Conve_2

Condensed Statements of Convertible Preferred Stock and Stockholders' Deficit (Unaudited) (Parenthetical) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Convertible Preferred Stock
Issuance costs	$ 1,293

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (11,540)	$ (10,297)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	1,481	326
Change in fair value of convertible notes		6,244
Obligation to issue common stock for research and development		1,155
Depreciation	53
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	449	(220)
Deposits	(91)
Accounts payable	552	(140)
Accrued and other current liabilities	934	179
Net cash used in operating activities	(8,162)	(2,753)
Cash flows from investing activities:
Purchases of property and equipment	(420)
Net cash used in investing activities	(420)
Cash flows from financing activities:
Proceeds from issuance of convertible notes		12,535
Proceeds from issuance of convertible preferred stock	50,000
Issuance costs of convertible preferred stock	(1,293)
Payments of deferred offering costs	(2,101)
Net cash provided by financing activities	46,606	12,535
Net increase in cash and cash equivalents	38,024	9,782
Cash and cash equivalents—beginning of period	33,017	1
Cash and cash equivalents—end of period	71,041	9,783
Supplemental disclosures of non-cash financing activities:
Exchange of convertible preferred stock, including deemed dividend	10,090
Deferred offering costs in accrued expenses at end of period	462	$ 43
Property and equipment in accrued expenses at end of period	$ 174

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Basis of Presentation	1. Basis of Presentation Cabaletta Bio, Inc. (the Company or Cabaletta) was incorporated in April 2017 in the State of Delaware as Tycho Therapeutics, Inc. and, in August 2018, changed its name to Cabaletta Bio, Inc. The Company is headquartered in Pennsylvania. Cabaletta is a clinical-stage biotechnology company focused on the discovery and development of T cell therapies for B cell-mediated autoimmune diseases. Principal operations commenced in April 2018, when the Company executed sponsored research agreements with the Trustees of the University of Pennsylvania (Penn). On October 16, 2019, the Company effected a 1-for-1.5 reverse split of the Company’s issued and outstanding shares of common stock, par value $0.00001 per share (Common Stock). Upon the effectiveness of the reverse stock split: (i) all shares of outstanding Common Stock were adjusted; (ii) the conversion price of the Series A, Series A-1, Series A-2 and Series B preferred stock (collectively, the Preferred Shares) was adjusted; (iii) the number of shares of Common Stock for which each outstanding option to purchase Common Stock is exercisable was adjusted; and (iv) the exercise price of each outstanding option to purchase Common Stock was adjusted. All of the outstanding Common Stock share numbers (including shares of Common Stock subject to the Company’s options and as converted for the outstanding convertible preferred stock shares), share prices, exercise prices and per share amounts contained in the condensed financial statements have been retroactively adjusted in the condensed financial statements to reflect this reverse stock split for all periods presented. The par value per share and the authorized number of shares of Common Stock and convertible preferred stock were not adjusted as a result of the reverse stock split. On October 29, 2019, the Company completed its initial public offering (IPO) of 6,800,000 shares of Common Stock at an offering price of $11.00 per share. The Company received net proceeds of $66,164 after deducting underwriting discounts, commissions and estimated offering expenses. In connection with the IPO, the Company’s outstanding shares of convertible preferred stock were automatically converted into 12,904,534 shares of Common Stock. After the IPO, the Company had a total of 23,552,854 outstanding shares of Common Stock. In November 2019, the underwriters partially exercised their option and purchased an additional 475,501 shares of Common Stock resulting in net proceeds to the Company of approximately $4,864, after deducting underwriting discounts and commissions. The financial statements as of September 30, 2019, including share and per share amounts, do not include the effects of the IPO. Risks and Uncertainties The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. As a result, the Company is unable to predict the timing or amount of increased expenses or when or if the Company will be able to achieve or maintain profitability. Further, the Company is currently dependent on Penn for much of its preclinical research, clinical research and development activities, and expects to be dependent upon Penn for initial manufacturing activities (Note 6). Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. Even if the Company is able to generate revenues from the sale of its product candidates, if approved, it may not become profitable. If the Company fails to become profitable or is unable to sustain profitability on a continuing basis, then it may be unable to continue its operations at planned levels and be forced to reduce its operations. Liquidity The Company expects that its operating losses and negative cash flows will continue for the foreseeable future. As of the issuance date of these unaudited condensed financial statements as of and for the nine months ended September 30, 2019, the Company expects that the $71,028 net proceeds from its IPO and the partial exercise of the underwriters’ option to purchase additional shares together with its cash and cash equivalents as of September 30, 2019 will be sufficient to fund its operating expenses and capital expenditure requirements through at least the first quarter of 2022.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Unaudited Interim Financial Information The accompanying unaudited interim financial statements have been prepared in conformity with generally accepted accounting principles (GAAP) and the applicable rules and regulations of the Securities and Exchange Commission (SEC) regarding interim financial reporting. Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification and Accounting Standards Updates (ASU) of the Financial Accounting Standards Board (FASB). As permitted under these rules, certain footnotes and other financial information that are normally required by GAAP have been condensed or omitted. In the opinion of management, the accompanying unaudited interim financial statements include all normal and recurring adjustments (which consist primarily of accruals and estimates that impact the financial statements) considered necessary to present fairly the Company’s financial position as of September 30, 2019 and the results of its operations and its cash flows for the nine months ended September 30, 2019 and 2018. The results for the nine months ended September 30, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019, any other interim periods, or any future year or period. The balance sheet as of December 31, 2018 included herein was derived from the audited financial statements as of that date. The unaudited interim financial statements, presented herein, do not contain the required disclosures under GAAP for annual financial statements. These unaudited financial statements should be read in conjunction with the Company’s audited financial statements, which are included in the Company’s prospectus dated October 24, 2019, that forms a part of the Company’s Registration Statements on Form S-1 (File Nos. 333-234017 and 333-234315), as filed with the SEC pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include, but are not limited to, the fair value of common stock, stock-based compensation, the valuation allowance on the Company’s deferred tax assets, and the fair value of convertible debt. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates. Deferred Offering Costs The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ deficit as a reduction of additional paid-in capital generated as a result of the offering. Should the in-process equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the statements of operations. Fair Value Measurement Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability 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. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2—Inputs (other than quoted prices included in Level 1) that are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active. Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. Reclassification A reclassification of $912 has been made increasing additional paid-in capital and increasing accumulated deficit in the three month period ended September 30, 2019 with respect to the exchange of convertible preferred stock recorded in January 2019. The reclassification had no impact to the Statements of Operations, including Net Income and Earnings Per Share, Total Assets and Total Equity. Related Party Transactions The Company engaged a firm controlled by a former executive (until February 2019) of the Company for professional services related to accounting, finance and other administrative functions. For the three month periods ended September 30, 2019 and 2018, the costs incurred under this arrangement totaled $180 and $30, respectively. For the nine month periods ended September 30, 2019 and 2018, the costs incurred under this arrangement totaled $445 and $41, respectively. These amounts were recorded as general and administrative expense in the accompanying statements of operations. As of September 30, 2019, amounts owed under this arrangement totaled $47 and are included in accounts payable in the accompanying balance sheets. The Company engaged the services of its current Chief Executive Officer and President prior to his employment in October 2018. For the three and nine month periods ended September 30, 2018, the costs incurred under this arrangement totaled $103 and $168, respectively, which were recorded as general and administrative expense in the accompanying statements of operations. Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the JOBS Act). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act, until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases In July 2017, the FASB issued ASU 2017-11, Earnings Per Share 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 Distinguishing Liabilities from Equity

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following tables present financial information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: September 30, 2019 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial assets Cash equivalents: Money market funds $ 71,041 $ 71,041 $ — $ — Total $ 71,041 $ 71,041 $ — $ — December 31, 2018 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial assets Cash equivalents: Money market funds $ 33,017 $ 33,017 $ — $ — Total $ 33,017 $ 33,017 $ — $ — The fair value of the convertible notes (Note 5) is a recurring Level 3 fair value measurement. Below is a rollforward of the fair value of the convertible notes for the nine months ended September 30, 2018: Convertible Notes Fair value at December 31, 2017 $ — Fair value upon the May 2018 closing 12,535 Change in fair value at September 30, 2018 6,244 Fair value at September 30, 2018 $ 18,779 There were no transfers among Level categories in the periods presented.

Accrued and Other Current Liabi

Accrued and Other Current Liabilities	9 Months Ended
	Sep. 30, 2019
Payables And Accruals [Abstract]
Accrued and Other Current Liabilities	4. Accrued and Other Current Liabilities Accrued and other current liabilities consist of the following: September 30, 2019 December 31, 2018 Research and development services $ 458 $ 181 General and administrative services 202 36 Accrued offering costs 462 — Compensation 631 121 Other 157 2 $ 1,910 $ 340

Convertible Notes

Convertible Notes	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Convertible Notes	5. Convertible Notes In May 2018, the Company issued convertible notes (the Notes) with aggregate proceeds to the Company in an initial closing of $12,535, including $5,000 issued to Penn. The Notes carried a stated interest rate of 7.5% per annum. All unpaid principal, together with the then accrued interest, for the Notes was due and payable at the earlier of May 4, 2021 or upon an event of default. The terms of the Notes provided for an additional milestone-based closing of $12,567 upon the achievement of certain Company-specific events. The Notes contained a number of provisions addressing automatic and optional conversion, events of default and prepayment provisions. The Notes were amended in September 2018 to adjust the terms of the automatic and optional conversion provisions. In October 2018, the Notes were amended again to reduce the qualified financing threshold, make a qualified financing a milestone event, revise the structure of a milestone-based closing and reallocate milestone closing purchase rights to new purchasers and the existing noteholders. On the same day, immediately following the amendment of the Notes, the Company completed a qualified financing, issuing 3,146,551 shares of Series A convertible preferred stock (Series A Preferred) for gross proceeds of $12,744 (Note 7). At this time, the Company issued 4,553,452 shares of Series A-1 convertible preferred stock (Series A-1 Preferred) in connection with the milestone-based closing resulting in $12,567 of proceeds ($2.76 per share) and the Notes together with interest accrued thereon ($409) were converted into 2,819,267 shares of Series A-1 Preferred and 1,873,777 shares of Series A-2 convertible preferred stock (Series A-2 Preferred), reflecting a conversion price per share of $2.76. On issuance, the Company elected to account for the Notes at fair value with any changes in fair value being recognized through the statements of operations until the Notes settled. In this connection, the Company’s policy is to report a single non-operating income/(expense) line item to record fair value adjustments on convertible notes and does not report interest expense as a separate line item in the statements of operations. On issuance, total debt issuance costs of $53 were expensed and recognized as general and administrative expense in the accompanying statements of operations. On issuance, the fair value of the Notes was determined to be equal to $12,535, which is the principal amount of the Notes. The fair value of the Notes upon settlement in October 2018 was determined based on the fair value of the Series A-1 Preferred and Series A-2 Preferred issued, which was determined to be $3.39 per share of Series A-1 Preferred and Series A-2 Preferred, using an option pricing method (OPM) framework and utilized the backsolve method for inferring and allocating the equity value predicated on the capital raise that transpired just prior to the valuation date. This method was selected as the Company concluded that the contemporaneous financing transaction was an arm’s-length transaction. Application of the OPM back-solve method involves making assumptions for the expected time to liquidity, volatility and risk-free rate and then solving for the value of equity such that value for the most recent financing equals the amount paid. The OPM allocation of total equity value was determined with reference to a recent financing transaction and the Company assumed a 71% volatility rate, a 1.3-year estimated term and a probability weighted average discount for lack of marketability of 35%. The fair value of the Notes as of September 30, 2018 was determined to be the same as that on settlement in October 2018 based on management’s determination of no material changes to the assumptions underlying the determination of the fair value of the Notes. For the nine months ended September 30, 2018, the Company recognized $6,244 in the accompanying statement of operations as other expense—fair value adjustments on the convertible notes.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Operating Lease Agreement In February 2019, the Company entered into an operating lease agreement for new office space in Philadelphia, Pennsylvania. The lease term commenced in May 2019 and will expire in July 2022. The initial annual base rent is $261, and such amount will increase by 2% annually on each anniversary of the commencement date. Rent expense related to this lease agreement recognized in the accompanying statements of operations was $67 for the three months ended September 30, 2019 and $110 for the nine months ended September 30, 2019. As of September 30, 2019, the future minimum payments for operating leases are as follows: October 1, 2019 to December 31, 2019 $ 65 2020 263 2021 268 2022 158 Thereafter — $ 754 Research Service Agreement In August 2018, the Company entered into a research service agreement with the Children’s Hospital of Philadelphia (CHOP) for the manufacturing of preclinical study and clinical trial material. Research and development expense related to this research service agreement with CHOP recognized in the accompanying statements of operations was $0 and $162 for the three months ended September 30, 2019 and 2018, respectively, and $201 and $162 for the nine months ended September 30, 2019 and 2018, respectively. There were no amounts due under the research service agreement with the Children’s Hospital of Philadelphia as of September 30, 2019. License Agreement with the Trustees of the University of Pennsylvania In August 2018, the Company entered into a license agreement with Penn (the Penn Agreement) and activated the license in October 2018 pursuant to which the Company obtained (a) a non-exclusive, non-sublicensable worldwide license to certain of Penn’s intellectual property to conduct research, product development, clinical trials, cell manufacturing and other activities, and (b) an exclusive, worldwide, royalty-bearing right and license, with a right to sublicense, on a target-by-target basis, under certain of Penn’s intellectual property to make, use, sell, offer for sale, import, and otherwise commercialize products for the treatment of autoimmune and alloimmune diseases. In July 2019, the Penn Agreement was amended and restated to include CHOP as a party to the agreement. Unless earlier terminated, the Penn Agreement expires on the expiration or abandonment or other termination of the last valid claim in Penn’s intellectual property licensed by the Company. The Company may terminate the Penn Agreement at any time for convenience upon 60 days’ written notice. In the event of an uncured, material breach, Penn may terminate the Penn Agreement upon 60 days’ written notice. Under the terms of the Penn Agreement, the Company is obligated to pay $2,000 annually for three years beginning August 2018 for funding to the laboratories of each of Drs. Milone and Payne (see Sponsored Research Agreements—Penn There were no amounts due under the Penn Agreement as of September 30, 2019. Sponsored Research Agreements Penn The Company has sponsored research agreements with two faculty members at Penn, who are also scientific co-founders of the Company and members of the Company’s scientific advisory board. Under the agreements, the Company has committed to funding a defined research plan for three years through April 2021. The total $8,500 under the two agreements satisfies the Company’s annual obligation under the Penn Agreement (see License Agreement with the Trustees of the University of Pennsylvania The Regents of the University of California In October 2018, the Company executed a research agreement with The Regents of the University of California. Under the agreement, the Company has committed to funding scientific research through October 2020 in an immaterial amount. There were no amounts due under this research agreement with The Regents of the University of California as of September 30, 2019. Master Translational Research Services Agreement In October 2018, the Company entered into a service agreement (the Services Agreement) with Penn for additional research and development services from various laboratories within Penn. The research and development activities will be detailed in Penn organization-specific addenda to be separately executed. In October and November 2018, the Company executed three project addenda under this agreement for research and development work. The three projects commenced in 2019 and are anticipated to conclude in 2019. In January 2019, the Company executed two project addenda under the Services Agreement for manufacturing and preclinical work. The two projects are commenced in 2019. The manufacturing addendum is expected to occur through September 2020. The preclinical addenda concluded in the nine months ended September 30, 2019. In July 2019, the Company executed four project addenda under the Services Agreement for manufacture of the Company’s clinical supply of DGS3-CAART, the Company’s lead product candidate, for its Phase 1 clinical trial. Pursuant to the addendum, the Company will pay Penn a fee for dedicated resources as well as the cost to manufacture the clinical supply. Research and development expense related to executed addenda under the master translational research service agreement with Penn recognized in the accompanying statements of operations in the three months ended September 30, 2019 was $286 and for the nine months ended September 30, 2019 and 2018 was $1,970 and $0, respectively. Amounts due under the master translational research service agreement with Penn were $324 as of September 30, 2019. Subscription and Technology Transfer Agreement In July 2019, the Company entered into a subscription and technology transfer agreement pursuant to which the Company will pay Penn an upfront subscription fee and a nominal non-refundable royalty on the net sales of products, a portion of which will be credited toward milestone payments and royalties, respectively, under the Amended License Agreement. Technology transfer activities will be at the Company’s cost and subject to agreement as to the technology to be transferred. Under this agreement, the Company recognized $250 to research and development expense for the three and nine month periods ended September 30, 2019. Alliance Management In July 2019, the Company entered into an alliance agreement with Penn pursuant to which the Company will pay Penn a nominal annual fee in order for Penn to provide an adequate and consistent level of support to the services that it provides to the Company.

Convertible Preferred Stock

Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2019
Temporary Equity Disclosure [Abstract]
Convertible Preferred Stock	7. Convertible Preferred Stock The Company has issued Series A Preferred, Series A-1 Preferred, Series A-2 Preferred, and Series B convertible preferred stock (Series B Preferred) (collectively, the Convertible Preferred Stock). The Company classifies Convertible Preferred Stock outside of stockholders’ deficit because the shares contain deemed liquidation rights that are a contingent redemption feature not solely within the control of the Company. There was no Convertible Preferred Stock issued during the nine months ended September 30, 2018 or outstanding as of September 30, 2018. The following table summarizes the Company’s Convertible Preferred Stock issued in the nine months ended September 30, 2019 and outstanding as of September 30, 2019: Series A Preferred Series A-1 Preferred Series A-2 Preferred Series B Preferred Total Convertible Preferred Stock Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Balance—December 31, 2018 3,146,551 $ 12,575 7,372,719 $ 24,994 1,873,777 $ 6,352 — $ — 12,393,047 $ 43,921 Issuance — — — — — — 6,963,788 50,000 6,963,788 50,000 Exchange, including deemed dividend — — — — (1,405,332 ) (4,764 ) 1,405,332 10,090 — 5,326 Issuance costs — — — — — — — (1,293 ) — (1,293 ) Balance—September 30, 2019 3,146,551 $ 12,575 7,372,719 $ 24,994 468,445 $ 1,588 8,369,120 $ 58,797 19,356,835 $ 97,954 Non-Voting Common Stock Election In October 2019, certain holders of the Company’s Convertible Preferred Stock elected to have such shares convert into 6,409,519 shares of non-voting Common Stock following the closing of the Company’s proposed initial public offering. The non-voting shares of Common Stock shall have the same rights and preferences as the Common Stock but shall be non-voting.

2018 Stock Option and Grant Pla

2018 Stock Option and Grant Plan	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
2018 Stock Option and Grant Plan	8. 2018 Stock Option and Grant Plan In September 2018, the Company adopted the 2018 stock option and grant plan (the 2018 Plan), which provides for the Company to sell or issue common stock, or other stock-based awards, to employees, members of the board of directors and consultants of the Company. The 2018 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than 10 years. The Company generally grants stock-based awards with service conditions only (service-based awards), although there has been one grant with performance conditions. Stock options granted under the 2018 Plan generally vest over three to four years. There were 2,580,453 shares reserved under the 2018 Plan for the future issuance of equity awards and, as of September 30, 2019, 638,042 shares were available for grant. There were no awards in the nine months ended September 30, 2018. A summary of the stock option activity under the 2018 Plan is presented below: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (years) Aggregate Intrinsic Value Outstanding as of January 1, 2019 971,353 $ 1.01 — $ — Granted 988,058 7.11 Forfeited (17,000 ) 1.01 Outstanding as of September 30, 2019 1,942,411 4.11 9.2 10,547 Options Exercisable at September 30, 2019 108,378 2.36 6.2 778 The aggregate intrinsic value of options granted is calculated as the difference between the exercise price of the options and the estimated fair value of the Company’s common stock. The weighted average grant-date fair value of stock options granted during the nine months ended September 30, 2019 was $4.77. The aggregate grant-date fair value of options vested during the nine months ended September 30, 2019 was $276. The Company uses the Black-Scholes option pricing method to calculate the grant-date fair value of an award. The fair values of options granted during the nine months ended September 30, 2019 were calculated using the following assumptions: risk-free interest rate of 1.39%—2.59% percent; expected term of 0.3—6.1 years; expected volatility of 70.3%—75.5% percent; and no expected dividend. Stock-based Compensation The Company recognizes stock-based compensation with respect to equity awards under the 2018 Plan and with respect to vesting common stock issued to founders. The Company has recorded stock-based compensation in the accompanying statements of operations as follows: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Research and development $ 365 $ 151 $ 962 $ 252 General and administrative 204 44 519 74 Total $ 569 $ 195 $ 1,481 $ 326 As of September 30, 2019, there was $5,650 of unrecognized compensation cost related to unvested option awards, including $527 with respect to one grant with performance-based vesting terms, which is expected to be recognized over a weighted-average period of 2.8 years. As of September 30, 2019, there was $1,088 of unrecognized compensation cost related to unvested stock awards granted to several Company founders, which is expected to be recognized over a weighted-average period of 1.6 years.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The Company did not record an income tax benefit in its statements of operations for the nine months ended September 30, 2019 and 2018 as it is more likely than not that the Company will not recognize the federal and state deferred tax benefits generated by its losses. The Company has provided a valuation allowance for the full amount of its net deferred tax assets and liabilities as of December 31, 2018 and September 30, 2019, as management has determined it is more likely than not that any future benefit from deductible temporary differences and net operating loss and tax credit carryforwards would not be realized. The Company has not recorded any amounts for unrecognized tax benefits as of December 31, 2018 or September 30, 2019.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	10. Net Loss Per Share The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share, as their effect is anti-dilutive: As of September 30, 2019 2018 Convertible Preferred Stock 19,356,835 — Stock options to purchase common stock 1,942,411 — Non-vested common stock 1,619,772 2,386,858 22,919,018 2,386,858

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2019
Subsequent Events [Abstract]
Subsequent Events	11. Subsequent Events The Company has evaluated subsequent events through the issuance date of these interim financial statements. Refer to Note 1 regarding the completion of the Company’s IPO on October 29, 2019. In connection with the IPO, the Company secured directors and officers liability insurance effective as of October 24, 2019 for an annual premium of $2,300. 2019 Stock Option and Incentive Plan The 2019 Stock Option and Incentive Plan (2019 Plan) was approved by the Company’s board of directors on October 14, 2019, and became effective on October 23, 2019. The 2019 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights to the Company’s officers, employees, directors and consultants. The number of shares initially reserved for issuance under the 2019 Plan is 2,342,288, which shall be cumulatively increased on January 1, 2020 and each January 1 thereafter by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors. 2019 Employee Stock Purchase Plan The 2019 Employee Stock Purchase Plan (2019 ESPP) was approved by the Company’s board of directors on October 14, 2019, and became effective on October 23, 2019. A total of 234,229 shares of common stock were initially reserved for issuance under the 2019 ESPP, which shall be cumulatively increased on January 1, 2020 and each January 1 thereafter by 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Unaudited Interim Financial Information	Unaudited Interim Financial Information The accompanying unaudited interim financial statements have been prepared in conformity with generally accepted accounting principles (GAAP) and the applicable rules and regulations of the Securities and Exchange Commission (SEC) regarding interim financial reporting. Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification and Accounting Standards Updates (ASU) of the Financial Accounting Standards Board (FASB). As permitted under these rules, certain footnotes and other financial information that are normally required by GAAP have been condensed or omitted. In the opinion of management, the accompanying unaudited interim financial statements include all normal and recurring adjustments (which consist primarily of accruals and estimates that impact the financial statements) considered necessary to present fairly the Company’s financial position as of September 30, 2019 and the results of its operations and its cash flows for the nine months ended September 30, 2019 and 2018. The results for the nine months ended September 30, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019, any other interim periods, or any future year or period. The balance sheet as of December 31, 2018 included herein was derived from the audited financial statements as of that date. The unaudited interim financial statements, presented herein, do not contain the required disclosures under GAAP for annual financial statements. These unaudited financial statements should be read in conjunction with the Company’s audited financial statements, which are included in the Company’s prospectus dated October 24, 2019, that forms a part of the Company’s Registration Statements on Form S-1 (File Nos. 333-234017 and 333-234315), as filed with the SEC pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include, but are not limited to, the fair value of common stock, stock-based compensation, the valuation allowance on the Company’s deferred tax assets, and the fair value of convertible debt. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ deficit as a reduction of additional paid-in capital generated as a result of the offering. Should the in-process equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the statements of operations.
Fair Value Measurement	Fair Value Measurement Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability 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. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2—Inputs (other than quoted prices included in Level 1) that are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active. Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.
Reclassification	Reclassification A reclassification of $912 has been made increasing additional paid-in capital and increasing accumulated deficit in the three month period ended September 30, 2019 with respect to the exchange of convertible preferred stock recorded in January 2019. The reclassification had no impact to the Statements of Operations, including Net Income and Earnings Per Share, Total Assets and Total Equity.
Related Party Transactions	Related Party Transactions The Company engaged a firm controlled by a former executive (until February 2019) of the Company for professional services related to accounting, finance and other administrative functions. For the three month periods ended September 30, 2019 and 2018, the costs incurred under this arrangement totaled $180 and $30, respectively. For the nine month periods ended September 30, 2019 and 2018, the costs incurred under this arrangement totaled $445 and $41, respectively. These amounts were recorded as general and administrative expense in the accompanying statements of operations. As of September 30, 2019, amounts owed under this arrangement totaled $47 and are included in accounts payable in the accompanying balance sheets. The Company engaged the services of its current Chief Executive Officer and President prior to his employment in October 2018. For the three and nine month periods ended September 30, 2018, the costs incurred under this arrangement totaled $103 and $168, respectively, which were recorded as general and administrative expense in the accompanying statements of operations.
Emerging Growth Company Status	Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the JOBS Act). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act, until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases In July 2017, the FASB issued ASU 2017-11, Earnings Per Share 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 Distinguishing Liabilities from Equity

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Financial Assets Measured at Fair Value on Recurring Basis	The following tables present financial information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: September 30, 2019 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial assets Cash equivalents: Money market funds $ 71,041 $ 71,041 $ — $ — Total $ 71,041 $ 71,041 $ — $ — December 31, 2018 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial assets Cash equivalents: Money market funds $ 33,017 $ 33,017 $ — $ — Total $ 33,017 $ 33,017 $ — $ —
Summary of Rollforward of Fair Value of Convertible Notes	The fair value of the convertible notes (Note 5) is a recurring Level 3 fair value measurement. Below is a rollforward of the fair value of the convertible notes for the nine months ended September 30, 2018: Convertible Notes Fair value at December 31, 2017 $ — Fair value upon the May 2018 closing 12,535 Change in fair value at September 30, 2018 6,244 Fair value at September 30, 2018 $ 18,779

Accrued and Other Current Lia_2

Accrued and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2019
Payables And Accruals [Abstract]
Schedule of Accrued and Other Current Liabilities	Accrued and other current liabilities consist of the following: September 30, 2019 December 31, 2018 Research and development services $ 458 $ 181 General and administrative services 202 36 Accrued offering costs 462 — Compensation 631 121 Other 157 2 $ 1,910 $ 340

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Payments for Operating Leases	As of September 30, 2019, the future minimum payments for operating leases are as follows: October 1, 2019 to December 31, 2019 $ 65 2020 263 2021 268 2022 158 Thereafter — $ 754

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	9 Months Ended
	Sep. 30, 2019
Partners Capital [Abstract]
Summary of Convertible Preferred Stock	The following table summarizes the Company’s Convertible Preferred Stock issued in the nine months ended September 30, 2019 and outstanding as of September 30, 2019: Series A Preferred Series A-1 Preferred Series A-2 Preferred Series B Preferred Total Convertible Preferred Stock Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Balance—December 31, 2018 3,146,551 $ 12,575 7,372,719 $ 24,994 1,873,777 $ 6,352 — $ — 12,393,047 $ 43,921 Issuance — — — — — — 6,963,788 50,000 6,963,788 50,000 Exchange, including deemed dividend — — — — (1,405,332 ) (4,764 ) 1,405,332 10,090 — 5,326 Issuance costs — — — — — — — (1,293 ) — (1,293 ) Balance—September 30, 2019 3,146,551 $ 12,575 7,372,719 $ 24,994 468,445 $ 1,588 8,369,120 $ 58,797 19,356,835 $ 97,954

2018 Stock Option and Grant P_2

2018 Stock Option and Grant Plan (Tables)	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	A summary of the stock option activity under the 2018 Plan is presented below: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (years) Aggregate Intrinsic Value Outstanding as of January 1, 2019 971,353 $ 1.01 — $ — Granted 988,058 7.11 Forfeited (17,000 ) 1.01 Outstanding as of September 30, 2019 1,942,411 4.11 9.2 10,547 Options Exercisable at September 30, 2019 108,378 2.36 6.2 778
Summary of Stock-Based Compensation	The Company recognizes stock-based compensation with respect to equity awards under the 2018 Plan and with respect to vesting common stock issued to founders. The Company has recorded stock-based compensation in the accompanying statements of operations as follows: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Research and development $ 365 $ 151 $ 962 $ 252 General and administrative 204 44 519 74 Total $ 569 $ 195 $ 1,481 $ 326

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Schedule of Antidilutive Shares Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share, as their effect is anti-dilutive: As of September 30, 2019 2018 Convertible Preferred Stock 19,356,835 — Stock options to purchase common stock 1,942,411 — Non-vested common stock 1,619,772 2,386,858 22,919,018 2,386,858

Basis of Presentation - Additio

Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands	Oct. 29, 2019USD ($)$ / sharesshares	Oct. 16, 2019$ / shares	Nov. 30, 2019USD ($)shares	Sep. 30, 2019USD ($)$ / sharesshares	Oct. 30, 2019shares	Dec. 31, 2018$ / sharesshares
Basis Of Presentation [Line Items]
Reverse stock split description				1-for-1.5 reverse split
Common stock, par value | $ / shares				$ 0.00001		$ 0.00001
Net proceeds from initial public offering | $				$ 71,028
Common stock, outstanding shares				3,848,320		3,848,320
Subsequent Event
Basis Of Presentation [Line Items]
Reverse stock split ratio		0.67
Common stock, par value | $ / shares		$ 0.00001
Net proceeds from initial public offering | $			$ 4,864
Common stock, outstanding shares					23,552,854
Number of additional common stock purchased by underwriters upon partial exercise of option			475,501
Subsequent Event | Initial Public Offering
Basis Of Presentation [Line Items]
Issuance of common stock, shares	6,800,000
Common stock issued, per share | $ / shares	$ 11
Net proceeds from initial public offering | $	$ 66,164
Conversion of convertible preferred stock to common stock	12,904,534

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Summary of Significant Accounting Policies [Line Items]
Increase in additional paid in capital from reclassification	$ 912
Increase in accumulated deficit from reclassification	912
Amounts due to related party	47		$ 47
Chief Executive Officer and President
Summary of Significant Accounting Policies [Line Items]
Costs incurred under the arrangement recorded as general and administrative expense		$ 103		$ 168
Professional Services
Summary of Significant Accounting Policies [Line Items]
Costs incurred under the arrangement recorded as general and administrative expense	$ 180	$ 30	$ 445	$ 41

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash equivalents:
Total	$ 71,041	$ 33,017
Quoted Prices in Active Markets for Identical Assets (Level 1)
Cash equivalents:
Total	71,041	33,017
Money Market Funds
Cash equivalents:
Total	71,041	33,017
Money Market Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Cash equivalents:
Total	$ 71,041	$ 33,017

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Rollforward of Fair Value of Convertible Notes (Details) - Convertible Notes - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	May 31, 2018	Sep. 30, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value upon the May 2018 closing	$ 12,535
Change in fair value at September 30, 2018		$ 6,244
Significant Unobservable Inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value upon the May 2018 closing		12,535
Change in fair value at September 30, 2018		6,244
Fair value at September 30, 2018		$ 18,779

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair value, assets, level 1 to level 2 transfers, amount	$ 0	$ 0	$ 0
Fair value, assets, level 2 to level 1 transfers, amount	0	0	0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers, net	$ 0	$ 0	$ 0

Accrued and Other Current Lia_3

Accrued and Other Current Liabilities - Schedule of Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Research and development services	$ 458	$ 181
General and administrative services	202	36
Accrued offering costs	462
Compensation	631	121
Other	157	2
Accrued and other current liabilities	$ 1,910	$ 340

Convertible Notes - Additional

Convertible Notes - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		9 Months Ended
	Oct. 31, 2018	May 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Proceeds from convertible preferred stock			$ 50,000
Convertible preferred stock issued				0
Series A Convertible Preferred Stock
Debt Instrument [Line Items]
Proceeds from convertible preferred stock	$ 12,744
Convertible preferred stock issued	3,146,551
Series A-1 Convertible Preferred Stock
Debt Instrument [Line Items]
Proceeds from convertible preferred stock	$ 12,567
Convertible preferred stock issued	4,553,452
Conversion price per share	$ 2.76
Accrued interest	$ 409
Conversion of preferred stock to convertible preferred stock	2,819,267
Series A-2 Convertible Preferred Stock
Debt Instrument [Line Items]
Conversion price per share	$ 2.76
Conversion of preferred stock to convertible preferred stock	1,873,777
Convertible Notes
Debt Instrument [Line Items]
Proceeds from issuance of debt		$ 12,535
Debt instrument, stated interest rate		7.50%
Proceeds from convertible preferred stock		$ 12,567
Fair value of convertible notes issued		12,535
Fair value, debt instrument volatility rate	71.00%
Fair value, debt instrument term	1 year 3 months 18 days
Fair value, debt instrument weighted average discount rate for lack of marketability	35.00%
Fair value adjustments on the convertible notes				$ 6,244
Convertible Notes | General and Administrative Expense
Debt Instrument [Line Items]
Debt issuance costs		53
Convertible Notes | Series A-1 Convertible Preferred Stock
Debt Instrument [Line Items]
Share price	$ 3.39
Convertible Notes | Series A-2 Convertible Preferred Stock
Debt Instrument [Line Items]
Share price	$ 3.39
Penn | Convertible Notes
Debt Instrument [Line Items]
Proceeds from issuance of debt		$ 5,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2018	Sep. 30, 2019USD ($)Faculty	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)Faculty	Sep. 30, 2018USD ($)
Commitments and Contingencies [Line Items]
Research and development expense		$ 3,220,000	$ 2,589,000	$ 8,645,000	$ 3,228,000
Operating Lease Agreement
Commitments and Contingencies [Line Items]
Operating lease, description				The lease term commenced in May 2019 and will expire in July 2022.
Lease commencement period				2019-05
Lease expiration period				2022-07
Initial annual base rent				$ 261,000
Percentage of annual increase of base rent				2.00%
Rent expense		67,000		$ 110,000
Research Service Agreement | Children’s Hospital of Philadelphia
Commitments and Contingencies [Line Items]
Research and development expense		0	162,000	201,000	162,000
Amounts due to agreement		0		0
Penn Agreement
Commitments and Contingencies [Line Items]
Amounts due to agreement		0		0
Obligation to be paid		2,000,000		$ 2,000,000
Payment term				3 years
Payment commencement date				2018-08
License maintenance fee				$ 10,000
Total milestone payments				20,000,000
Sponsored Research Agreements | Penn
Commitments and Contingencies [Line Items]
Research and development expense		710,000	$ 710,000	2,131,000	1,247,000
Obligation to be paid		$ 8,500,000		$ 8,500,000
Number of faculty member | Faculty		2		2
Funding term				3 years
Funding term end date				2021-04
Sponsored Research Agreements | Penn | Prepaid Expenses and Other Current Assets
Commitments and Contingencies [Line Items]
Advance payments		$ 174,000		$ 174,000
Sponsored Research Agreements | The Regents of the University of California
Commitments and Contingencies [Line Items]
Obligation to be paid		0		0
Funding term end date	2020-10
Master Translational Research Services Agreement
Commitments and Contingencies [Line Items]
Research and development expense		286,000		1,970,000	$ 0
Obligation to be paid		324,000		324,000
Subscription and Technology Transfer Agreement
Commitments and Contingencies [Line Items]
Research and development expense		$ 250,000		$ 250,000

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Payments for Operating Leases (Details) $ in Thousands	Sep. 30, 2019USD ($)
Commitments And Contingencies Disclosure [Abstract]
October 1, 2019 to December 31, 2019	$ 65
2020	263
2021	268
2022	158
Total	$ 754

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Details) - shares	1 Months Ended
	Oct. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Temporary Equity [Line Items]
Convertible preferred stock, issued shares				0
Convertible preferred stock, outstanding shares		19,356,835	12,393,047	0
Subsequent Event
Temporary Equity [Line Items]
Convertible preferred stock convert into non-voting common stock	6,409,519

Convertible Preferred Stock - S

Convertible Preferred Stock - Summary of Convertible Preferred Stock (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019USD ($)shares
Temporary Equity [Line Items]
Beginning balance	$ 43,921
Beginning balance, Shares | shares	12,393,047
Issuance of convertible preferred stock, net of issuance costs	$ 50,000
Issuance, Shares | shares	6,963,788
Exchange, including deemed dividend	$ 5,326
Issuance costs	(1,293)
Ending balance	$ 97,954
Ending balance, Shares | shares	19,356,835
Series A Preferred
Temporary Equity [Line Items]
Beginning balance	$ 12,575
Beginning balance, Shares | shares	3,146,551
Ending balance	$ 12,575
Ending balance, Shares | shares	3,146,551
Series A-1 Preferred
Temporary Equity [Line Items]
Beginning balance	$ 24,994
Beginning balance, Shares | shares	7,372,719
Ending balance	$ 24,994
Ending balance, Shares | shares	7,372,719
Series A-2 Preferred
Temporary Equity [Line Items]
Beginning balance	$ 6,352
Beginning balance, Shares | shares	1,873,777
Exchange, including deemed dividend	$ (4,764)
Exchange, including deemed dividend, Shares | shares	(1,405,332)
Ending balance	$ 1,588
Ending balance, Shares | shares	468,445
Series B Preferred
Temporary Equity [Line Items]
Issuance of convertible preferred stock, net of issuance costs	$ 50,000
Issuance, Shares | shares	6,963,788
Exchange, including deemed dividend	$ 10,090
Exchange, including deemed dividend, Shares | shares	1,405,332
Issuance costs	$ (1,293)
Ending balance	$ 58,797
Ending balance, Shares | shares	8,369,120

2018 Stock Option and Grant P_3

2018 Stock Option and Grant Plan - Additional Information (Details) - 2018 Plan - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation cost related to unvested stock-based awards		$ 5,650
Unrecognized compensation cost, weighted average period for recognition		2 years 9 months 18 days
Founders
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation cost related to unvested stock-based awards		$ 1,088
Unrecognized compensation cost, weighted average period for recognition		1 year 7 months 6 days
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance of equity awards		2,580,453
Share-based compensation, number of shares available for grant		638,042
Number of awards granted		988,058	0
Weighted average grant-date fair value of stock options granted		$ 4.77
Aggregate grant-date fair value of options vested		$ 276
Risk-free interest rate, minimum		1.39%
Risk-free interest rate, maximum		2.59%
Expected volatility, minimum		70.30%
Expected volatility, maximum		75.50%
Expected dividend		0.00%
Stock Options | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of exercise price per share of fair market value	100.00%
Share-based compensation, vesting period	3 years
Expected term		3 months 18 days
Stock Options | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Term of stock options after grant date	10 years
Share-based compensation, vesting period	4 years
Expected term		6 years 1 month 6 days
Performance-Based Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation cost related to unvested stock-based awards, other than options		$ 527

2018 Stock Option and Grant P_4

2018 Stock Option and Grant Plan - Summary of Stock Option Activity (Details) - Stock Options - 2018 Plan - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Number of Shares
Number of Shares, Beginning Balance	971,353
Number of Shares, Granted	988,058	0
Number of Shares, Forfeited	(17,000)
Number of Shares, Ending Balance	1,942,411
Number of Shares, Options Exercisable	108,378
Weighted Average Exercise Price
Weighted Average Exercise Price, Beginning Balance	$ 1.01
Weighted Average Exercise Price, Granted	7.11
Weighted Average Exercise Price, Forfeited	1.01
Weighted Average Exercise Price, Ending Balance	4.11
Weighted Average Exercise Price, Options Exercisable	$ 2.36
Weighted Average Remaining Contractual Term (years)
Term (years)	9 years 2 months 12 days
Weighted Average Remaining Contractual Term (years), Options Exercisable	6 years 2 months 12 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Ending Balance	$ 10,547
Aggregate Intrinsic Value, Options Exercisable	$ 778

2018 Stock Option and Grant P_5

2018 Stock Option and Grant Plan - Summary of Stock-Based Compensation (Details) - Stock Options - 2018 Plan - Founders - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Stock-based compensation	$ 569	$ 195	$ 1,481	$ 326
Research and Development
Stock-based compensation	365	151	962	252
General and Administrative Expense
Stock-based compensation	$ 204	$ 44	$ 519	$ 74

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income tax benefit	$ 0	$ 0
Unrecognized tax benefits	$ 0		$ 0

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Antidilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive shares have been excluded from calculation of diluted net loss per share	22,919,018	2,386,858
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive shares have been excluded from calculation of diluted net loss per share	19,356,835
Stock Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive shares have been excluded from calculation of diluted net loss per share	1,942,411
Non-vested Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive shares have been excluded from calculation of diluted net loss per share	1,619,772	2,386,858

Subsequent Events -Additional I

Subsequent Events -Additional Information - (Details) - Subsequent Event - USD ($) $ in Thousands	Oct. 24, 2019	Oct. 23, 2019
Directors & Officers Liability Insurance
Subsequent Event [Line Items]
Annual insurance premium	$ 2,300
2019 Plan
Subsequent Event [Line Items]
Shares reserved for issuance		2,342,288
Increase in share reserved percentage		4.00%
2019 ESPP
Subsequent Event [Line Items]
Shares reserved for issuance		234,229
Increase in share reserved percentage		1.00%