Deferred revenue, current portion

22,567

26,724

Income tax payable

13,374

13,339

50,765

52,713

Total current liabilities

96,217

97,908

Deferred revenue, net of current portion

197,754

225,267

Operating lease liabilities - long term

27,008

Other long-term liabilities

963

3,050

Total liabilities

321,942

326,225

Commitments and contingencies

Stockholders' equity:

Common stock, $0.00001 par value; 75,000,000 shares authorized; 45,157,652 and 45,083,209 shares issued and outstanding at March 31, 2019 and December 31, 2018, respectively

Additional paid-in capital

451,613

445,956

Accumulated other comprehensive income (loss)

(93)

Accumulated deficit

(329,116)

(314,981)

Total stockholders' equity

122,571

130,883

Total liabilities and stockholders' equity

444,513

457,108

Convertible Preferred Stock

Stockholders' equity:

Convertible preferred stock, $0.00001 par value; 10,000,000 shares authorized and no shares issued and outstanding at March 31, 2019 and December 31, 2018.

[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	45,157,652	45,083,209
Common stock, shares outstanding	45,157,652	45,083,209
Convertible Preferred Stock
Convertible Preferred stock, par value	$ 0.00001	$ 0.00001
Convertible Preferred stock, shares authorized	10,000,000	10,000,000
Convertible Preferred stock, shares issued	0	0
Convertible Preferred stock, shares outstanding	0	0

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Revenues	$ 29,485	$ 14,184
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 36,376	$ 22,458
General and administrative	9,674	7,356
Total operating expenses	46,050	29,814
Loss from operations	(16,565)	(15,630)
Interest income	2,496	1,375
Other expense	(61)	(140)
Loss before income taxes	(14,130)	(14,395)
Provision for (benefit from) income taxes	(6)	1,098
Net loss	$ (14,124)	$ (15,493)
Net loss per share, basic and diluted	$ (0.31)	$ (0.40)
Shares used to compute net loss per share, basic and diluted	45,122,456	38,647,878
Other comprehensive income (loss):
Changes in unrealized gain (loss) on short-term investments, net of tax	$ 155	$ (134)
Impact of adoption of new accounting pronouncement	11
Comprehensive loss	$ (13,958)	$ (15,627)

STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($)
$ in Thousands

Total

Common Stock

Additional Paid-in Capital

Accumulated Other Comprehensive Income/(Loss)

Accumulated Deficit

Beginning balance at Dec. 31, 2017

$ 69,896

$ 1

$ 289,454

$ (94)

$ (219,465)

Beginning balance, shares at Dec. 31, 2017

38,478,560

Impact of adoption of new accounting pronouncement | ASC 606

(10,912)

Exercise of stock options

2,602

Exercise of stock options, shares

425,139

Stock-based compensation

3,688

Other comprehensive income

(134)

Net loss

(15,493)

Ending balance at Mar. 31, 2018

49,647

$ 1

295,744

(228)

(245,870)

Ending balance, shares at Mar. 31, 2018

38,903,699

Beginning balance at Dec. 31, 2018

130,883

^[1]

$ 1

445,956

(93)

(314,981)

Beginning balance, shares at Dec. 31, 2018

45,083,209

Impact of adoption of new accounting pronouncement | ASU 2018-02

11,000

(11)

Exercise of stock options

$ 465

465

Exercise of stock options, shares

74,443

Stock-based compensation

$ 5,192

5,192

Other comprehensive income

155

Net loss

(14,124)

Ending balance at Mar. 31, 2019

$ 122,571

$ 1

$ 451,613

$ 73

$ (329,116)

Ending balance, shares at Mar. 31, 2019

45,157,652

[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (14,124)	$ (15,493)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of intangible assets	36	36
Depreciation and amortization	617	376
Accretion of discounts on investments	(713)	(261)
Stock-based compensation expense	5,192	3,688
Changes in operating assets and liabilities
Accounts receivable	53	10,058
Prepaid expenses and other current assets	814	(1,122)
Other assets		(20)
Accounts payable	5,162	1,115
Accrued liabilities, income tax payable and other long-term liabilities	(6,605)	1,145
Deferred revenue	(29,461)	(14,134)
Net cash used in operating activities	(39,029)	(14,612)
Cash flows from investing activities:
Purchases of property and equipment	(1,835)	(751)
Purchases of short-term investments	(62,317)	(48,132)
Maturities of short-term investments	45,546	25,500
Net cash used in investing activities	(18,606)	(23,383)
Cash flows from financing activities:
Proceeds from exercise of stock options	465	2,602
Net cash provided by financing activities	465	2,602
Net decrease in cash, cash equivalents and restricted cash	(57,170)	(35,393)
Cash, cash equivalents and restricted cash, beginning of period	248,494	178,465
Cash, cash equivalents and restricted cash, end of period	191,324	143,072
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	$ 245	$ 391

Description of the Business

3 Months Ended

Mar. 31, 2019

Disclosure Text Block [Abstract]

Description of the Business

1. Description of the Business

CytomX Therapeutics, Inc. (the “Company”) is a clinical-stage, oncology-focused biopharmaceutical company with a vision of transforming lives with safer, more effective therapeutics. The Company is pioneering a novel class of investigational antibody therapeutics, based on its Probody™ therapeutic technology platform for the treatment of cancer. The Probody therapeutic approach is designed to more specifically target antibody therapeutics to the tumor microenvironment and minimize drug activity in healthy tissue and in circulation. The Company is located in South San Francisco, California and was incorporated in the state of Delaware in September 2010.

Basis of Presentation and Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Basis of Presentation and Summary of Significant Accounting Policies

2. Basis of Presentation and Summary of Significant Accounting Policies

Basis of Presentation

The accompanying interim condensed financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) regarding interim financial reporting.

Unaudited Interim Financial Information

The accompanying interim condensed financial statements and related disclosures are unaudited, have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair statement of the results of operations for the periods presented.

The condensed balance sheet data as of December 31, 2018 was derived from audited financial statements, but does not include all disclosures required by U.S. GAAP. The condensed results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the full year or for any other future year or interim period. The accompanying condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 filed with the SEC.

Use of Estimates

The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates.

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents.

Restricted Cash

Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed balance sheets that sum to the total of the amounts shown in the condensed statements of cash flows.

	March 31, 2019		December 31, 2018		March 31, 2018		December 31, 2017
	(in thousands)
Cash and cash equivalents	$	190,407	$	247,577	$	142,155	$	177,548
Restricted cash - non-current assets		917		917		917		917
Total	$	191,324	$	248,494	$	143,072	$	178,465

Comprehensive Income (Loss)

Comprehensive income (loss) represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s unrealized gains and losses on short-term investments represent the only component of other comprehensive income (loss) that is excluded from the reported net loss.

Revenue Recognition

The Company’s revenues are primarily derived through its license, research, development and commercialization agreements. The terms of these types of agreements may include (i) licenses for the Company’s technology or programs, (ii) research and development services, and (iii) services or obligations in connection with participation in research or steering committees. Payments to the Company under these arrangements typically include one or more of the following: nonrefundable upfront and license fees, research funding, milestone and other contingent payments to the Company for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products. The Company assesses whether the promises in its arrangements with customers are considered as distinct performance obligations that should be accounted for separately. Judgment is required to determine whether the license to the Company’s intellectual property is distinct from the research and development services or participation on steering committees.

The Company’s collaboration and license agreements may include contingent payments related to specified research, development and regulatory milestones. Such milestone payments are typically payable under the collaborations when the collaboration partner claims or selects a target, or initiates or advances a covered product candidate in preclinical or clinical development; upon submission for marketing approval of a covered product with regulatory authorities; or upon receipt of actual marketing approvals of a covered product or for additional indications. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At each reporting date, the Company re-evaluates whether the related milestones are considered probable of being reached and estimates the amount to be included in the transaction price by using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price in such period of determination.

The Company’s collaboration and license agreements may also include contingent payments related to sales-based milestones. Sales-based milestones are typically payable when annual sales of a covered product reach specified levels. Sales-based milestones are recognized at the later of when the associated performance obligation has been satisfied or when the sales occur. Unlike other contingency payments, such as regulatory milestones, sales-based milestones are not included in the transaction price based on estimates at the inception of the contract, but rather, are included when the sales or usage occur.

The transaction price in each arrangement is allocated to the identified performance obligations based on the relative standalone selling price (“SSP”) of each distinct performance obligation, which requires judgment. In instances where SSP is not directly observable, such as when a license or service is not sold separately, SSP is determined using information that may include market conditions and other observable inputs. Due to the early stage of the Company’s licensed technology, the license of such technology is typically combined with research and development services and steering committee participation as one performance obligation. In the event that the Company receives non-cash consideration such as consideration in the form of a research license and research support services from the counterparty, the transaction price of a non-monetary exchange that has commercial substance is estimated based on the fair value of the non-cash consideration received, which may be determined through a valuation analysis.

In certain cases, the Company’s performance creates an asset that does not have an alternative use to the Company and the Company has an enforceable right to payment at all times for performance completed to date. In these cases, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

AbbVie Ireland Unlimited Company (“AbbVie”), one of the Company’s collaboration partners, entered into a license agreement with Seattle Genetics, Inc. (“SGEN”) to license certain intellectual property rights. As part of the Company’s collaboration agreement with AbbVie, the Company pays SGEN sublicense fees. These sublicense fees are treated as reductions to the transaction price and combined with the performance obligation to which they relate.

Contract Balances

Customer payments are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the Company satisfies its performance obligations under the arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional.

Research and Development Expenses

The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and includes these costs in accrued liabilities in the balance sheets and within research and development expense in the statements of operations. These costs are a significant component of the Company’s research and development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations.

Research and development expenses include costs directly attributable to the conduct of research and development programs, including the cost of salaries, payroll taxes, employee benefits, materials, supplies, depreciation on and maintenance of research equipment, the cost of services provided by outside contractors, and the allocated portions of facility costs, such as rent, utilities, insurance, repairs and maintenance, depreciation, and general support services. All costs associated with research and development are expensed as incurred.

In January, 2019, the Company acquired certain technology know-how that is complementary to the Company’s proprietary Probody technology, for $5.0 million. This technology will be utilized with certain of the Company’s discovery stage projects, and accordingly the $5.0 million is recorded as research and development expense for the three months ended March 31, 2019.

Stock-based Compensation

The Company recognizes compensation costs related to stock options granted to employees based on the estimated fair value of the awards on the date of grant. The Company records forfeitures as they are incurred. The Company estimates the grant date fair value, and the resulting stock-based compensation expense, using the Black-Scholes option-pricing model. The grant date fair value of stock-based awards is expensed on a straight-line basis over the period during which the employee is required to provide service in exchange for the award (generally the vesting period).

The Company estimates the fair value of its stock-based awards using the Black-Scholes option-pricing model, which requires the input of assumptions. The assumptions are as follows:

•

Expected term. The expected term of stock options represents the period that the stock options are expected to remain outstanding and is based on vesting terms, exercise term and contractual lives of the options. The expected term of the ESPP shares is equal to the six-month look-back period.

•

Expected volatility. The expected stock price volatility for the Company’s stock options was derived from the average historical volatilities of the Company’s stock price and the stock price of several comparable publicly traded companies within the biotechnology and pharmaceutical industry. The Company will continue to apply this process until a sufficient amount of historical information on the Company’s own stock price becomes available.

•

Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury whose term was consistent with expected term of the Company’s stock options.

•

Dividend yield. The expected dividend is assumed to be zero as the Company has never paid dividends and have no current plans to pay any dividends on our common stock.

Leases

The Company determines if an arrangement is or contains a lease at inception. Operating leases are recorded as operating lease right-of-use (“ROU”) assets and operating lease liabilities in the Company’s balance sheet. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses an implicit rate when readily available, or its incremental borrowing rate based on the information available at lease commencement date in determining the present value of lease payments. The operating lease ROU assets also include any lease prepayments made and reduced by lease incentives. The Company’s lease terms may include options to extend the lease when it is reasonably certain that such option will be exercised. Lease expenses are recognized on a straight-line basis over the lease term. The Company’s operating lease arrangement includes lease and non-lease components which are generally accounted for separately. See Note 9, Leases

Adopted Accounting Pronouncements

Leases

The Company adopted the Accounting Standard Update (“ASU”) No. 2016-02, Leases (Topic 842) on January 1, 2019, using the modified retrospective approach. This new standard amends the guidance for the accounting and disclosure of leases and requires that lessees recognize on the balance sheet the assets and liabilities that arise from leases, including leases classified as operating leases under current GAAP, and disclose qualitative and quantitative information about leasing arrangements. In July 2018, the Financial Accounting Standard Board (“FASB”) further amended this standard to allow for a new transition method that provides the option to use the effective date as the date of initial application. The Company has elected such option and accordingly the comparative periods are not recast.

The Company has also elected the package of practical expedients permitted under the new standard, or ASC 842. Accordingly, the Company continues to account for its existing operating leases as operating leases under the new guidance, without reassessing (a) whether the contracts contain a lease under ASC 842, (b) whether classification of the operating leases would be different in accordance with ASC 842, or (c) whether the unamortized initial direct costs before transition adjustments would have met the definition of initial direct costs in ASC 842 at lease commencement. In addition, the Company also elected the short-term lease practical expedients allowed under the standard. As a result of the adoption of ASC 842, the Company recorded a right-of-use asset of $28.0 million and a lease liability $30.1 million on January 1, 2019. This standard does not have material impact on the Company’s results of operations or cash flows. See Note 9, Leases.

In March 2019, the FASB issued ASU No. 2019-01, Leases (Topic 842): Codification Improvements. The amendments in this ASU provides clarifications to: 1) determination of the fair value of the underlying asset by lessors that are not manufacturers or dealers; 2) presentation on statement of cash flows for sales-type and direct financing leases specifically for lessors that are depository and lending institutions; 3) certain exceptions to the transition disclosures requirements related to Topic 250, Accounting Changes and Error Corrections upon adoption of Topic 842. The Company does not expect the transition guidance in (1) and (2) above to have a material impact on its financial statements. Also, the adoption of transition guidance (3) did not have a material impact on its current period financial statements.

Reclassification of Certain Tax Effects

In February 2018, the FASB issued ASU No. 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. The amendments in this standard allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. The Company adopted this standard on January 1, 2019. There was no material impact on its financial statement upon adoption of this ASU and the Company recorded an increase in other comprehensive income of $11,000 against its retained earnings upon the adoption.

Nonemployee Share-Based Payment

In June 2018, the FASB issued ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. The amendments in this ASU expand the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. The Company adopted this standard on January 1, 2019. There was no material impact on its financial statements upon the adoption of this ASU.

Recent 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. The new standard changes the impairment model for most financial assets and certain other instruments. Under the new standard, entities holding financial assets and net investment in leases that are not accounted for at fair value through net income are to be presented at the net amount expected to be collected. An allowance for credit losses will be a valuation account that will be deducted from the amortized cost basis of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. The new standard will be effective for the Company on January 1, 2020. The Company is currently evaluating the impact of this new guidance.

In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. The new standard simplifies the measurement of goodwill by eliminating the Step 2 impairment test. Step 2 measures a goodwill impairment loss by comparing the implied fair value of a reporting unit's goodwill with the carrying amount of that goodwill. The new guidance requires an entity to compare the fair value of a reporting unit with its carrying amount and recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value. Additionally, an entity should consider income tax effects from any tax-deductible goodwill on the carrying amount of the reporting unit when measuring the goodwill impairment loss, if applicable. The new guidance becomes effective for goodwill impairment tests in fiscal years beginning after December 15, 2019, though early adoption is permitted. The Company is currently evaluating the impact of this new guidance.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820). The amendments in this ASU modify the disclosure requirements on fair value measurements in Topic 820, Fair Value Measurement. Various disclosure requirements have been removed, including the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, the policy for timing of transfers between levels, the valuation processes for Level 3 fair value measurements held at the end of the reporting period. The ASU also modified various disclosure requirements and added some disclosure requirements for Level 3 fair value measurements. The amendments in this ASU are effective for the Company on January 1, 2020. The additional disclosures on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty should be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption. All other amendments should be applied retrospectively to all periods presented upon their effective date. An entity is permitted to early adopt any removed or modified disclosures upon issuance of this ASU and delay adoption of the additional disclosures until their effective date. The Company does not expect the adoption of this ASU will have a material impact on its financial statements.

In August 2018, the FASB issued ASU No. 2018-15, Intangibles-Goodwill and Other- Internal-Use Software (Subtopic 350-40). The amendments in this ASU on the accounting for implementation, setup and other upfront costs (collectively “implementation costs”) apply to entities that are a customer in a hosting arrangement. The amendments under this ASU align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. Accordingly, the amendments in this ASU require an entity in a hosting arrangement that is a service contract to follow the guidance in Subtopic 350-40 to determine which implementation costs to expense. They also require an entity to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. This ASU is effective for the Company on January 1, 2020. The Company is currently evaluating the impact of this new guidance.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the interaction between Topic 808 and Topic 606. The amendments in this ASU targeted improvements to generally accepted accounting principles for collaborative arrangements by clarifying that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. In addition, unit-of-account guidance in Topic 808 was aligned with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606. The ASU is effective for the Company on January 1, 2020, and interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact of this new guidance.

Net Loss Per Share

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Net Loss Per Share

3. Net Loss Per Share

Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period. Diluted net loss per share attributable to common stockholders is calculated using the weighted-average number of common shares outstanding, plus potential dilutive common stock during the period. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since the effect of the potentially dilutive securities is anti-dilutive.

The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share for the periods presented, because including them would have been anti-dilutive:

	Three Months Ended
	March 31,
	2019		2018
Options to purchase common stock		8,962,799		7,199,679
Total		8,962,799		7,199,679

Fair Value Measurements and Short-Term Investments

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements and Short-Term Investments

4. Fair Value Measurements and Short-Term Investments

In accordance with Accounting Standards Codification (“ASC”) 820-10, Fair Value Measurements and Disclosures, the Company determines the fair value of financial and non-financial assets and liabilities using the fair value hierarchy, which establishes three levels of inputs that may be used to measure fair value, as follows:

•

Level I: Inputs which include quoted prices in active markets for identical assets and liabilities.

•

Level II: Inputs other than Level I that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

•

Level III: Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

The carrying amounts of the Company’s financial instruments, including restricted cash, accounts receivable, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. The Company’s financial instruments consist of Level I assets which consist primarily of highly liquid money market funds, some of which are included in restricted cash, and U.S. government bonds that are included in short-term investments.

The following tables set forth the fair value of the Company’s short-term investments subject to fair value measurements on a recurring basis and the level of inputs used in such measurements (in thousands):

		March 31, 2019
	Valuation Hierarchy	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Aggregate Fair Value
		(in thousands)
Assets
Money market funds	Level I	$	175,738	$	—	$	—	$	175,738
Restricted cash (money market funds)	Level I		917		—		—		917
U.S. Government bonds	Level I		206,058		131		—		206,189
Total		$	382,713	$	131	$	—	$	382,844

As of March 31, 2019, no securities have contractual maturities of longer than one year.

		December 31, 2018
	Valuation Hierarchy	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
		(in thousands)
Assets
Money market funds	Level I	$	226,979	$	—	$	—		$	226,979
Restricted cash (money market funds)	Level I		917		—		—			917
U.S. Government bonds	Level I		188,616		—		(66	)		188,550
Total		$	416,512	$	—	$	(66	)	$	416,446

- Definition

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

+ References + Details

Accrued Liabilities

3 Months Ended

Mar. 31, 2019

Accrued Liabilities Current [Abstract]

Accrued Liabilities

5. Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	March 31,		December 31,
	2019		2018
	(in thousands)
Research and clinical expenses	$	14,613	$	18,520
Payroll and related expenses		3,469		6,585
Legal and professional expenses		1,393		830
Operating lease liabilities - short term		2,544		-
Other accrued expenses		548		789
Total	$	22,567	$	26,724

Research and Collaboration Agreements

3 Months Ended

Mar. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Research and Collaboration Agreements

6. Research and Collaboration Agreements

The following table summarizes the revenue by collaboration partners:

	Three Months Ended March 31,
	2019		2018
	(in thousands)
AbbVie	$	2,948	$	2,671
Amgen		1,035		1,255
BMS		25,502		8,168
ImmunoGen		-		735
Pfizer		-		1,355
Total Revenue	$	29,485	$	14,184

AbbVie Ireland Unlimited Company

In April 2016, the Company and AbbVie entered into two agreements, a CD71 Co-Development and Licensing Agreement (the “CD71 Agreement”) and a Discovery Collaboration and Licensing Agreement (the “Discovery Agreement” and together with the CD71 Agreement the “AbbVie Agreements”). Under the terms of the CD71 Agreement, the Company and AbbVie will co-develop a Probody Drug Conjugates (“PDC”) against CD71, with the Company responsible for pre-clinical and early clinical development. AbbVie will be responsible for later development and commercialization, with global late-stage development costs shared between the two companies. The Company will assume 35% of the net profits or net losses related to later development unless it opts-out. If the Company opts-out from participation of co-development of the CD71 PDC, AbbVie will have sole right and responsibility for the further development, manufacturing and commercialization of such CD71 PDC.

Under the CD71 Agreement, the Company received an upfront payment of $20.0 million in April 2016, and is eligible to receive up to $470.0 million in development, regulatory and commercial milestone payments, a 35% profit split on U.S. sales, and royalties on ex-U.S. sales in the high teens to low twenties if the Company participates in the co-development of the CD71 Licensed Product subject to a reversion to a royalty on U.S. sales, and reduction in royalties on ex-U.S. sales, if the Company opts-out from the co-development of the CD71 PDC. The Company’s share of later stage co-development costs for each CD71 PDC are capped, provided that AbbVie may offset the Company’s co-development cost above the capped amounts from future payments such as milestone payments and royalties. In July 2017, the Company received a milestone payment of $14.0 million (net of payment of an associated sublicense fee of $1.0 million to SGEN under the Seattle Genetics Agreement) from AbbVie for achieving certain milestones required to be met to begin GLP toxicology studies under the CD71 Agreement. In May 2018, the United States Food and Drug Administration (“FDA”) cleared the IND application for CX-2029, the PDC targeting CD71. As a result, the Company achieved the IND success criteria under the CD71 Agreement and received a $21.0 million milestone payment (net of the payment of an associated sublicense fee of $4.0 million to SGEN). The Company commenced enrollment of our Phase 1/2 clinical trial and dosed the first patient in a clinical trial at the end of the second quarter of 2018.

Under the terms of the Discovery Agreement, AbbVie receives exclusive worldwide rights to develop and commercialize PDCs against up to two targets, one of which was selected in March 2017. The Company shall perform research services to discover the Probody therapeutics and create PDCs for the nominated collaboration targets. From that point, AbbVie shall have sole right and responsibility for development and commercialization of products comprising or containing such PDCs (“Discovery Licensed Products”).

Under the Discovery Agreement, the Company received an upfront payment of $10.0 million in April 2016 and may receive an additional payment upon the selection by AbbVie of the second target and the satisfaction of certain success criteria under the CD71 Agreement. AbbVie has not selected the second target, but the success criteria under the CD71 Agreement were met in September 2016. The Company is also eligible to receive up to $275.0 million in target nomination, development, regulatory and commercial milestone payments and royalties in the high single to low teens from commercial sales of any resulting PDCs.

The Company has determined that the CD71 Agreement and Discovery Agreement with AbbVie should be combined and evaluated as a single arrangement in determining revenue recognition, because both agreements were concurrently negotiated and executed.

The Company identified the following performance obligations at the inception of the AbbVie Agreements:

(1) the research, development and commercialization license for CD71 Probody therapeutic,

(2) the research services related to CD71 Probody therapeutic,

(3) the obligation to participate in the CD71 Agreement joint research committee,

(4) the research services related to the first discovery target

(5) the research, development and commercialization license for the first discovery target, and

(6) the obligation to participate in the Discovery Agreement joint research committee.

The Company concluded that, at the inception of the agreement, AbbVie’s option for the second discovery target is not a material right and is therefore not a performance obligation.

The Company determined that the research, development and commercialization licenses for CD71 and discovery targets are not distinct from the Company’s respective research services and expertise. The Company considered factors such as novelty of the Probody therapeutic and PDC technology and lack of other parties’ expertise in this space, the Company’s rights to technology relating to a proprietary platform to enable the Probody therapeutic development and AbbVie’s contractual obligation to use the Company’s research services. The Company determined that the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee were a combined performance obligation and were distinct from the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee. Therefore, the Company concluded that there are two distinct performance obligations:

(1)

the CD71 Agreement performance obligation consisting of the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee, and

(2)

the Discovery Agreement performance obligation consisting of the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee.

The total transaction price upon adoption of ASC 606 on January 1, 2018 of $39.8 million consists of $30.0 million in upfront payments, $14.0 million milestone payment received (net of the payment of an associated sublicense fee of $1.0 million to SGEN) less $4.2 million of estimated sublicense fees. The upfront payments under the AbbVie Agreements are allocated between the two performance obligations based on the estimated relative standalone selling prices. The $30.0 million of upfront payments is allocated $20.0 million to the CD71 Agreement, with the remaining $10.0 million allocated to the Discovery Agreement. The $14.0 million milestone payment received (net of the payment of an associated sublicense fee of $1.0 million to SGEN) and estimated sublicense fees of $4.2 million are allocated to the CD71 Agreement performance obligation as they are directly related to the development of the CD71 Probody therapeutic.

In May 2018, the Company earned a $21.0 million milestone payment (net of the payment of an associated sublicense fee of $4.0 million to SGEN). The $21.0 million milestone payment was included as part of the transaction price in May 2018 and a revenue adjustment of $9.9 million was recognized in the second quarter of 2018 reflecting the percentage completed to-date on the project related to this milestone. The Company determined that the remaining potential milestone payments are probable of significant revenue reversal as their achievement is highly dependent on factors outside the Company’s control. Therefore, these payments have been fully constrained and were not included in the transaction price as of March 31, 2019.

The Company is obligated to make sublicense payments under the sublicense agreement with the Regents of the University of California, acting through its Santa Barbara campus (“UCSB”), as amended, equal to 5% of certain upfront and milestone payments owed to or received by the Company. As of March 31, 2019 and December 31, 2018, the Company recorded a liability of $1.0 million and $1.1 million, respectively, representing the sublicense fee payable to UCSB under the CD71 Agreement. The $1.0 million liability on March 31, 2019 was paid in April 2019 after the Company and UCSB entered into Amendment No.3 to the Exclusive License Agreement. See Note 11, Subsequent Event.

Of the $39.8 million total transaction price, the Company recognized the initial transaction price of $29.8 million allocated to the CD71 Agreement performance obligation using a cost-based input measure. In applying the cost-based input method, revenue is recognized based on actual full-time employee (“FTE”) hours incurred as a percentage of total estimated FTE hours as the Company completes its combined performance obligation over the five-year service period. As the Discovery Agreement performance obligation represents an obligation to continuously make the Company’s Probody therapeutic technology platform available to AbbVie, the initial transaction price of $10.0 million allocated to this performance obligation is recognized over the common measure of progress for the entire performance obligation over the estimated research service period of five years.

The Company recognized revenue of $2.9 million and $2.7 million for the three months ended March 31, 2019 and 2018, respectively. related to the AbbVie Agreements. As of March 31, 2019 and December 31, 2018, deferred revenue related to the CD71 Agreement performance obligation was $20.7 million and $23.2 million, respectively, and deferred revenue related to the Discovery Agreement performance obligation was $4.2 million and $4.7 million, respectively. As of both March 31, 2019 and December 31, 2018, no amount was due from AbbVie under the CD71 and Discovery Agreements.

Amgen, Inc.

On September 29, 2017, the Company and Amgen, Inc. (“Amgen”) entered into a Collaboration and License Agreement (the “Amgen Agreement”). Pursuant to the Amgen Agreement, the Company received an upfront payment of $40.0 million in October 2017. Concurrent with the entry into the Amgen Agreement, the Company and Amgen entered into a Share Purchase Agreement (the “Purchase Agreement”) pursuant to which Amgen purchased 1,156,069 shares of the Company’s common stock at a price of $17.30 per share (calculated based on a 20-day volume-weighted average price), for total proceeds of $20.0 million, which the Company received on October 6, 2017, the closing date of the transaction. The Company estimated a premium on the stock sold to Amgen of $0.5 million, which takes into account a discount due to the lack of marketability resulting from the six-month lockup period.

Under the terms of the Amgen Agreement, the Company and Amgen will co-develop a Probody T-cell engaging bi-specific therapeutic targeting EGFR (“EGFR Products”). The Company is responsible for early-stage development of EGFR Products and all related costs up to certain pre-set costs and certain limits based on clinical study size. Amgen will be responsible for late-stage development, commercialization, and all related costs of EGFR Products. Following early-stage development, the Company will have the right to elect to participate financially in the global co-development of EGFR Products with Amgen, during which the Company would bear certain of the worldwide development costs for EGFR Products and Amgen would bear the rest of such costs (the “EGFR Co-Development Option”). If the Company exercises its EGFR Co-Development Option, the Company will share in somewhat less than 50% of the profit and losses from sales of such EGFR Products in the U.S., subject to certain caps, offsets, and deferrals. If the Company chooses not to exercise its EGFR Co-Development Option, the Company will not bear any costs of later stage development. The Company is eligible to receive up to $455.0 million in development, regulatory, and commercial milestone payments for EGFR Products, and royalties in the low-double-digit to mid-teen percentage of worldwide commercial sales, provided that if the Company exercises its EGFR Co-Development option, it shall receive a profit and loss split of sales in the United States and royalties in the low-double-digit to mid-teen percentage of commercial sales outside of the United States.

Amgen also has the right to select a total of up to three targets, including the two additional targets discussed below. The Company and Amgen collaborate in the research and development of Probody T-cell engaging bi-specifics products directed against such targets. Amgen has selected one such target (the “Amgen Other Product”). If Amgen exercises its option within a specified period of time, it can select two such additional targets (the “Amgen Option Products” and, together with the Amgen Other Product, the “Amgen Products”). Except with respect to preclinical activities to be conducted by CytomX, Amgen will be responsible, at its expense, for the development, manufacture, and commercialization of all Amgen Products. If Amgen exercises all of its options and advances all three of the Amgen Products, CytomX is eligible to receive up to $950.0 million in upfront, development, regulatory, and commercial milestones and tiered high single-digit to low-teen percentage royalties. The Company concluded that, at the inception of the agreement, Amgen’s option to select the two additional targets is not a material right and does not represent a performance obligation of the agreement.

At the initiation of the collaboration, CytomX had the option to select, from programs specified in the Amgen Agreement, an existing pre-clinical stage T-cell engaging bispecific product from the Amgen pre-clinical pipeline. In March 2018, CytomX selected the program. CytomX is responsible, at its expense, for converting this program to a Probody T-cell engaging bispecific product, and thereafter, will be responsible for development, manufacturing, and commercialization of the product (“CytomX Product”). Amgen is eligible to receive up to $203.0 million in development, regulatory, and commercial milestone payments for the CytomX Product, and tiered mid-single digit to low double-digit percentage royalties.

The Company considered the criteria for combining contracts in ASC 606 and determined that the Amgen Agreement and the Purchase Agreement should be combined into one contract. The Company accounted for the Amgen Agreement based on the fair values of the assets and services exchanged. The Company identified the following performance obligations at the inception of the Amgen Agreement:

(1) the research, development and commercialization license,

(2) the research and development services for the EGFR Products and the Amgen Other Product, and

(3) the obligation to participate in the joint steering committee (“JSC”) and the joint research committee (“JRC”).

The Company determined that research, development and commercialization license and the participation in the JSC and JRC are not distinct from the research and development services and therefore those performance obligations were combined into one combined performance obligation. The Amgen Other Products are accounted for as a separate performance obligation from the EGFR Products as the nature of the services being performed is not the same and the value that Amgen can derive from one program is not dependent on the success of the other.

Concurrent with the execution of the Amgen Agreement, the Company entered into a sublicense agreement whereby the Company granted Amgen a sublicense of its rights to one patent family that it co-owns with UCSB, that is exclusively licensed to the Company under the UCSB Agreement covering Probody antibodies and other pro-proteins in the fields of therapeutics, in vivo diagnostics and prophylactics. This sublicense was incremental to the patents, patent applications and know-how covering T-cell engaging bispecific Probody molecules that were developed and owned by the Company and licensed to Amgen. Under the UCSB Agreement, as amended, the Company is obligated to make a sublicense payment to UCSB equal to up to 7.5% of certain upfront and milestone payments owed to or received by the Company. As of March 31, 2019 and December 31, 2018, the Company recorded a liability of $1.9 million and $2.1 million, respectively, representing the sublicense fee payable to UCSB. The $1.9 million liability on March 31, 2019 was paid in April 2019 after the Company and UCSB entered into the Amendment No.3 to the Exclusive License Agreement. See Note 11, Subsequent Event.

The total transaction price of $51.2 million, consisting of the $40.0 million upfront payment, an estimated fair value of $10.7 million for the CytomX Product and $0.5 million of premium on the sale of the Company’s common stock, was allocated between the two performance obligations based on the relative standalone selling price of each performance obligation. To determine the standalone selling price, the Company used the discounted cash flow method by calculating risk-adjusted net present values of estimated cash flows. The Company determined that the remaining potential milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company’s control. As a result, these payments were fully constrained and were not included in the transaction price as of March 31, 2019.

Of the $51.2 million total transaction price, the Company allocated $46.4 million to the EGFR Products performance obligation and $4.8 million to the Amgen Other Product performance obligations. The transaction price of the EGFR Product performance obligation was recognized using a cost-based input measure. In applying the cost-based input method of revenue recognition, the Company uses actual FTE hours incurred relative to estimated total FTE hours expected to be incurred for the combined performance obligation over the research service period. In the fourth quarter of 2018, the JSC discontinued work on two molecules that did not meet required research criteria. Pre-clinical evaluation of additional molecules is ongoing as part of the candidate identification phase of the EGFR project, and as a result, there was a change in estimate of the projected costs and research service period to seven years during the fourth quarter of 2018.

The $4.8 million transaction price allocated to the Amgen Other Product performance obligation represents an obligation to continuously make the Probody therapeutic technology platform available to Amgen, which is recognized over the common measure of progress for the entire performance obligation over the estimated research service period of six years.

The Company recognized revenue of $1.0 million and $1.3 million for the three months ended March 31, 2019 and 2018, respectively, related to the Amgen Agreement. As of March 31, 2019 and December 31, 2018, deferred revenue related to the EGFR Products performance obligation was $39.8 million and $40.7 million, respectively. As of March 31, 2019 and December 31, 2018 deferred revenue related to the Amgen Other Products performance obligation was $3.6 million and $3.8 million, respectively. As of March 31, 2019 and December 31 2018, no amount was due from Amgen under the Amgen Agreement.

Bristol-Myers Squibb Company

On May 23, 2014, the Company and Bristol-Myers Squibb Company (“BMS”) entered into a Collaboration and License Agreement (the “BMS Agreement”) to discover and develop compounds for use in human therapeutics aimed at multiple immuno-oncology targets using the Company’s Probody therapeutic technology. The effective date of the BMS Agreement was July 7, 2014.

Under the terms of the BMS Agreement, the Company granted BMS exclusive worldwide rights to develop and commercialize Probody therapeutics for up to four oncology targets. BMS had additional rights to substitute up to two collaboration targets within three years of the effective date of the BMS Agreement. These rights expired in May 2017. Each collaboration target had a two-year research term and the two additional targets had to be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target could be extended in one year increments up to three times.

Pursuant to the BMS Agreement, the financial consideration from BMS was comprised of an upfront payment of $50.0 million and the Company was initially entitled to receive contingent payments of up to an aggregate of $1,217.0 million as follows: (i) up to $25.0 million for additional targets; (ii) up to $114.0 million in development milestone payments per research target program or up to $456.0 million if the maximum of four research targets are selected; (iii) up to $124.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program or up to $496.0 million if the maximum of four research targets are selected, and (iv) up to $60.0 million in sales milestones payments per research target program or up to $240.0 million if maximum of four research targets are selected. The Company is entitled to royalty payments in the mid-single digits to low double-digit percentages from potential future sales. The Company also receives research and development service fees based on a prescribed FTE rate that is capped.

The Company identified the following performance obligations at the inception of the BMS Agreement:

(1) the exclusive research, development and commercialization license,

(2) the research and development services and

(3) the obligation to participate in the joint research committee.

The Company determined that the license, the Company’s research services and expertise related to the development of the product candidates should be combined with the research services and participation in the joint research committee as one combined performance obligation. The Company concluded that, at the inception of the agreement, BMS’ options for the third and fourth targets were not material rights and not performance obligations. As such, each option was accounted for as a separate arrangement upon exercise. Additionally, the Company considered whether the services performed for each target should be considered as separate performance obligations and concluded that all targets should be accounted for as one combined performance obligation. The Company received an upfront payment of $50.0 million from BMS in July 2014. In January and December 2016, BMS selected the third and fourth targets, respectively, and paid the Company $10.0 million and $15.0 million, respectively, pursuant to the terms of the BMS Agreement. In December 2016, BMS selected a clinical candidate pursuant to the BMS Agreement, which triggered a $2.0 million pre-clinical milestone payment to the Company. In November 2017, the Company recognized a $10.0 million milestone payment from BMS upon approval of the investigational new drug application for the CTLA-4-directed Probody therapeutic.

On March 17, 2017, the Company and BMS entered into Amendment Number 1 to Extend Collaboration and License Agreement (the “Amendment”). The Amendment grants BMS exclusive worldwide rights to develop and commercialize Probody therapeutics for up to six additional oncology targets and two non-oncology targets. The effective date of the Amendment was April 25, 2017 (“Amendment Effective Date”). Under the terms of the Amendment, the Company continues to collaborate with BMS to discover and conduct preclinical development of Probody therapeutics against targets selected by BMS under the terms of the Amendment.

Pursuant to the Amendment, the financial consideration from BMS is comprised of an upfront payment of $200.0 million and the Company is eligible to receive up to an aggregate of $3,586.0 million as follows:

(i)

up to $116.0 million in development milestone payments per target or up to $928.0 million if the maximum of eight targets are selected for the first product modality;

(ii)

up to $124.0 million in milestone payments for the first commercial sale in various territories for up to three indications per target program or up to $992.0 million if the maximum of eight targets are selected for the first product modality;

(iii)

up to $60.0 million in sales milestone payments per target or up to $480.0 million if maximum of eight targets are selected for the first product modality; and

(iv)

up to $56.3 million in development milestone payments or up to $450.0 million if the maximum of eight targets are selected for the second product modality;

(v)

up to $62.0 million in milestone payments for the first commercial sale in various territories for up to three indications per target program or up to $496.0 million if the maximum of eight targets are selected for the second product modality;

(vi)

up to $30.0 million in sales milestone payments per target or up to $240.0 million if maximum of eight targets are selected for the second product modality.

The Company is also entitled to tiered mid-single to low double-digit percentage royalties from potential future sales. The Amendment does not change the term of the BMS’s royalty obligation under the BMS Agreement. BMS’s royalty obligation continues on a licensed-product by licensed-product basis until the later of (i) the expiration of the last claim of the licensed patents covering the licensed products in the country, (ii) the twelfth anniversary of the first commercial sale of a licensed product in a country, or (iii) the expiration of any applicable regulatory, pediatric, orphan drug or data exclusivity with respect to such product.

The initial transaction price is $272.8 million consisting of the upfront fees of $250.0 million, research and development service fees of $10.8 million and milestone payments received to date of $12.0 million. The Company determined that the remaining potential milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company’s control. Therefore, these payments were fully constrained and were not included in the transaction price as of March 31, 2019. The BMS Agreement represents an obligation to continuously make the Probody therapeutic technology platform available to BMS. Therefore, the initial transaction price is recognized over the estimated research service period, which ends on April 25, 2025.

During the first quarter of 2019, BMS terminated pre-clinical activities on three of the first four collaboration targets selected under the BMS Agreement. The first and second targets under the BMS Agreement were combined into a single performance obligation. The Company determined that termination of pre-clinical activities on the second target does not impact the Company’s continuing obligation to BMS for the first target, CTLA-4, as it is still being actively developed by BMS. Therefore, the Company concluded that it will continue to amortize the related deferred revenue over the original performance period. The Company has determined that upon the termination of pre-clinical activities on the third and the fourth collaboration targets selected by BMS in January and December of 2016, respectively, under the BMS Agreement, it has no further obligations and is no longer eligible to receive any further proceeds from milestones, royalties or research and development fees for such targets. As a result, the Company accelerated recognition of all of the related deferred revenue of the third and the fourth targets upon the effective date of termination and recognized $17.4 million in the first quarter of 2019.

The Company recognized revenue of $25.5 million and $8.2 million for the three months ended March 31, 2019 and 2018, respectively. As of March 31, 2019 and December 31, 2018, deferred revenue relating to the BMS Agreement was $180.2 million and $205.6 million, respectively. The amount due from BMS under the BMS Agreement was $44,000 and $0.1 million as of March 31, 2019 and December 31, 2018, respectively.

ImmunoGen, Inc.

In January 2014, the Company and ImmunoGen, Inc. (“ImmunoGen”) entered into the Research Collaboration Agreement (the “ImmunoGen Research Agreement”). The ImmunoGen Research Agreement provided the Company with the right to use ImmunoGen’s Antibody Drug Conjugate (“ADC”) technology in combination with the Company’s Probody therapeutic technology to create a PDC directed at one specified target under a research license, and to subsequently obtain an exclusive, worldwide development and commercialization license to use ImmunoGen’s ADC technology to develop and commercialize such PDCs. The Company made no upfront cash payment in connection with the execution of the agreement. Instead, the Company provided ImmunoGen with the rights to CytomX’s Probody therapeutic technology to create PDCs directed at two targets under the ImmunoGen Research Agreement and to subsequently obtain exclusive, worldwide development and commercialization licenses to develop and commercialize such PDCs. In February 2016, the Company exercised its option to obtain a development and commercialization license for CX-2009 pursuant to the terms of the ImmunoGen Research Agreement (the “CX-2009 License”). In February 2017, ImmunoGen exercised its first option to obtain a development and commercialization license for one of the two targets. Substitution rights for this first target selection program terminated in February 2017 and ImmunoGen discontinued the program in July 2017. The Company recognized the remaining deferred revenue related to the discontinued program upon the termination of the program. ImmunoGen exercised its second option to obtain a development and commercialization license pursuant to the ImmunoGen Research Agreement (the “ImmunoGen 2017 License”) for a target in December 2017 and continues research work on this program.

Under the terms of the ImmunoGen Research Agreement, both the Company and ImmunoGen performed research activities on behalf of the other party for no monetary consideration through January 2018 and the arrangement was extended to June 2018, as discussed below. Each party is solely responsible for the development, manufacturing and commercialization of any products resulting from the exclusive development and commercialization license obtained by such party under the agreement.

In consideration for the ImmunoGen 2017 License, the Company is entitled to receive up to $30.0 million in development and regulatory milestone payments, up to $50.0 million in sales milestone payments and royalties in the mid-single digit percentage on the commercial sales of any resulting product. For the CX-2009 License, the Company is obligated to pay ImmunoGen up to $60.0 million in development and regulatory milestone payments and up to $100.0 million in sales milestone payments and royalties in the mid to high single digits percentage on the commercial sales of any resulting product. In August 2017, the Company made a milestone payment of $1.0 million to ImmunoGen for the first patient dosing with CX-2009. No milestone payments have been accrued to the Company under the ImmunoGen 2017 License.

The Company accounted for the ImmunoGen Research Agreement based on the fair value of the assets and services exchanged. The Company identified the following performance obligations at the inception of the ImmunoGen Research Agreement:

(1) the research license,

(2) the research services,

(3) the obligation to participate in the joint research committee,

(4) the exclusive research, development and commercialization license and

(5) the obligation to provide future technology improvements, when available.

The Company determined that the research license, the research services, the participation in the joint steering committee, and the technology improvements are not distinct from the development and commercialization license and therefore those performance obligations were combined into one combined performance obligation. The Company considered factors such as the limited economic benefits to ImmunoGen if the development and commercialization license was not obtained and the lack of sublicensing rights in the research license.

The estimated total fair value of the consideration of $13.2 million was recorded as deferred revenue at the inception of the ImmunoGen Research Agreement. In December 2017, the Company entered into the ImmunoGen 2017 License arrangement and extended the Company’s obligation to provide research services under the ImmunoGen Research Agreement to June 30, 2018. The fair value of the consideration for the combined performance obligation was recognized as revenue over the research period that ended on June 30, 2018. As of March 31, 2019 and December 31, 2018, neither company has further research obligations under the ImmunoGen Research Agreement.

The estimated total fair value of assets and services received was also $13.2 million, of which $12.7 million was allocated to the licenses received and was charged to research and development expense, with the remaining amount of $0.5 million allocated to the research services, joint research committee participation and technology improvements, which was expensed over the period of services provided.

The Company recognized revenue of $0 and $0.7 million for this target for the three months ended March 31, 2019 and 2018, respectively. As of March 31, 2019 and December 31, 2018, there was no deferred revenue relating to the ImmunoGen Research Agreement. As of both March 31, 2019 and December 31, 2018, no amount was due from ImmunoGen under the ImmunoGen Research Agreement.

Pfizer Inc.

In May 2013, the Company and Pfizer Inc. (“Pfizer”) entered into a Research Collaboration, Option and License Agreement (the “Pfizer Agreement”) to collaborate on the discovery and preclinical research activities related to Probody therapeutics, and PDCs for research project targets nominated by Pfizer. Pfizer nominated two research targets in 2013 and, pursuant to the Pfizer Agreement, had the option of nominating two additional research targets. In December 2014, Pfizer selected an additional research target and paid the Company $1.5 million. The option to select a fourth target lapsed in May 2016. Pfizer discontinued the epidermal growth factor receptor (“EGFR”) program and decided to terminate the remaining two targets in February and March 2018. In March 2018, Pfizer terminated the Pfizer Agreement. As such, the Company had no further performance obligations under this agreement following the termination of the Pfizer Agreement and recognized the remaining deferred revenue of $1.1 million during the first quarter of 2018.

Pursuant to the Pfizer Agreement, the Company received an upfront payment of $6.0 million and research and development service fees based on a prescribed FTE rate per year that is capped. The Company identified the following performance obligations at the inception of the Pfizer Agreement: (1) the research license, (2) the research services and (3) the obligation to participate in the joint research committee. The Company determined that the research license was not distinct from the research services and participation in the joint research committee due to the specialized nature of the research services to be provided by the Company, and accordingly, this deliverable was combined with the research services and participation in the joint research committee as a combined performance obligation. The Company concluded that, at the inception of the agreement, Pfizer’s options to obtain an exclusive development and commercialization license for each research project target did not represent a material right and were not performance obligations.

As the combined performance obligation represented an obligation to continuously make the Probody therapeutic technology platform available to Pfizer, the initial transaction price was recognized over the common measure of progress for the entire performance obligation over the estimated research service period of five and a half years.

The Company recognized revenue of $0 and $1.4 million for the three months ended March 31, 2019 and 2018, respectively. As of March 31, 2019 and December 31, 2018, there was no deferred revenue relating to the Pfizer Agreement. As of both March 31, 2019 and December 31, 2018, the amount due from Pfizer under the Pfizer Agreement was $0.

Contract Liabilities

The following table presents changes in the Company’s total contract liabilities during the three months ended March 31, 2019:

	Balance at Beginning of Period		Additions		Deductions			Balance at End of Period
	(in thousands)
Contract liabilities:
Deferred revenue	$	277,980	$	—	$	(29,461	)	$	248,519

There were no additions to deferred revenue during the first quarter of 2019. Deductions of $29.5 million represents revenue recognized, including the accelerated recognition of deferred revenue of $17.4 million due to termination of certain targets by BMS, in the three months ended March 31, 2019 that was included in the contract liability balance at the beginning of the period.

The Company estimates that the $248.5 million of deferred revenue related to the following contracts as of March 31, 2019 to be recognized as revenue as set forth below. However, the timing of revenue recognition could differ from the estimates depending on facts and circumstances impacting the various contracts, including progress of research and development, resources assigned to the contracts by the Company or its collaboration partners, or other factors outside of the Company’s control.

•

The $20.7 million of deferred revenue related to the CD71 Agreement as of March 31, 2019 is expected to be recognized based on actual FTE effort and program progress until approximately April 2021.

•

The $4.2 million of deferred revenue related to the Discovery Agreement as of March 31, 2019 is expected to be recognized ratably until approximately April 2021.

•

The $39.8 million of deferred revenue related to the Amgen EGFR Products as of March 31, 2019 is expected to be recognized based on actual FTE effort and program progress until approximately September 2024.

•

The $3.6 million of deferred revenue related to the Amgen Other Products as of March 31, 2019 is expected to be recognized ratably until approximately September 2023.

•

The $180.2 million of deferred revenue related to the BMS Agreement as of March 31, 2019 is expected to be recognized ratably until approximately April 2025.

UCSB License Agreement

3 Months Ended

Mar. 31, 2019

Research And Development [Abstract]

UCSB License Agreement

7. UCSB License Agreement

The Company has an exclusive, worldwide license agreement with UCSB (the “UCSB Agreement”), relating to the use of certain patents and technology relating to its core technology, including its therapeutic antibodies, and to certain patent rights the Company co-owns with UCSB covering Probody antibodies and other pro-proteins.

Pursuant to the UCSB Agreement, the Company is obligated to (i) make royalty payments to UCSB on net sales of its products covered under the agreement, subject to annual minimum amounts, (ii) make milestone payments to UCSB upon the occurrence of certain events, (iii) make a milestone payment to UCSB upon occurrence of an IPO or change of control, and (iv) reimburse UCSB for prosecution and maintenance of the licensed patents. If the Company sublicenses its rights under the UCSB Agreement, it is obligated to pay UCSB a percentage of the total sublicense revenue received, which total amount would be first reduced by the aggregate amount of certain research and development related expenses incurred by the Company and other permitted deductions. As part of the UCSB Agreement, the Company has annual minimum royalty obligations of $150,000 under the terms of certain exclusive licensed patent rights. The royalty obligations are cancellable any time by giving notice to the licensor, with the termination being effective 60 days after giving notice.

In 2013, the Company amended the UCSB Agreement to reduce certain amounts due to UCSB upon receipt by the Company of upfront payments, milestone payments and royalties from sublicensees. In exchange for this amendment, the Company issued to UCSB 157,332 shares of common stock. The UCSB Agreement, as amended, will remain in effect until the expiration or abandonment of the last to expire of the licensed patents.

During the three months ended March 31, 2019 and 2018, the Company did not incur any material sublicense fee under the provisions of the UCSB Agreement. In April 2019, the Company entered into Amendment No.3 to the UCSB Agreement (the “Amendment No.3”) with UCSB to, among other things, clarify and adjust the sublicense terms of this UCSB Agreement. As a result of the Amendment No.3, the Company is required to pay an additional annual maintenance fee of $750,000 from April 2019 through 2031. See Note 11, Subsequent Event.

Stock-Based Compensation

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

8. Stock-Based Compensation

Stock Options

Activities under the Company’s stock option plans for the three months ended March 31, 2019 were as follows:

	Options Outstanding
	Number of Options			Weighted- Average Exercise Price Per Share
Balances at December 31, 2018		7,803,773		$	12.622
Options granted		2,103,883			14.736
Options exercised		(74,443	)		6.240
Option forfeited/expired		(61,025	)		17.894
Balances at March 31, 2019		9,772,188			13.093
Options exercisable at March 31, 2019		4,854,438		$	8.558

Stock-based Compensation

Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows:

	Three Months Ended
	March 31,
	2019		2018
	(in thousands)
Stock-based compensation expense:
Research and development	$	2,642	$	1,808
General and administrative		2,550		1,880
Total stock-based compensation expense	$	5,192	$	3,688

Leases

3 Months Ended

Mar. 31, 2019

Leases [Abstract]

Leases

9. Leases

Operating Lease

On December 10, 2015, the Company entered into a lease (the “2016 Lease”) with HCP Oyster Point III LLC (the “Landlord”) to lease approximately 76,000 rentable square feet of office and laboratory space located in South San Francisco, California for the Company’s new corporate headquarters.

The term of the Lease commenced on October 1, 2016. The 2016 Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the 2016 Lease.

The Lease provided for annual base rent of approximately $3.1 million in the first year of the lease term. The annual base rent for the second twelve months was approximately $4.3 million, which will increase on an annual basis beginning from the 25^th month to approximately $5.5 million for the tenth year of the lease. The Company utilized the full amount of the one-time improvement allowance of $12.6 million, of which $2.3 million is recoverable by the landlord through an increase rent which continues through the expiration of the initial lease term.

In addition, the Company obtained a standby letter of credit (the “Letter of Credit”) in an amount of approximately $0.9 million, which may be drawn by the Landlord to be applied for certain purposes upon the Company’s breach of any provisions under the 2016 Lease. The Company has recorded the $0.9 million Letter of Credit in restricted cash as non-current on its balance sheet at March 31, 2019 and December 31, 2018, respectively.

Rent expense is recognized on a straight-line basis over the term of the lease and accordingly the Company records the difference between cash rent payments and the recognition of rent expense against the operating lease ROU asset. Rent expense for both the three months ended March 31, 2019 and 2018 was $1.3 million.

Supplemental information related to leases are as follows:

	Three Months Ended
	March 31, 2019
	(in thousands)
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$	1,205

	March 31, 2019
	(in thousands)
Supplemental balance sheet information related to leases:
Operating lease right-of-use assets	$	27,404
Current operating lease liabilities		2,544
Non-current operating lease liabilities		27,008
Total operating lease liabilities	$	29,552

Weighted-average remaining lease term (in years)
Operating lease		7.61
Weighted-average discount rate
Operating lease		8.25	%

	March 31, 2019
	(in thousands)
Maturity of operating lease liabilities
2019	$	4,888
2020		5,025
2021		5,165
2022		5,309
2023 and beyond		19,763
Total lease payments		40,150
Less imputed interest		(10,598	)
Present value of lease liabilities	$	29,552

Income Tax Expense

3 Months Ended

Mar. 31, 2019

Income Tax Disclosure [Abstract]

Income Tax Expense

10. Income Tax Expense

The Company recorded an income tax benefit of $6,000 and income tax expense of $1.1 million during the three months ended March 31, 2019 and 2018, respectively. The income tax benefit for the three months ended March 31, 2019 was as a result of an unrealized gain on the available for sale securities recorded in other comprehensive income during the period. The income tax expense during the three months ended March 31, 2018 was generated as a result of a temporary difference in the recognition of revenue under tax and U.S. GAAP authoritative guidance, primarily due to revenue recognition for tax purposes in 2018 of the upfront payments received pursuant to the BMS Amendment entered into in March 2017. The Company’s effective tax rate was 0.04% and (7.63)% for the three months ended March 31, 2019 and 2018, respectively. The Company maintains a full valuation allowance against its net deferred tax assets due to the Company’s history of losses as of March 31, 2019.

Subsequent Event

3 Months Ended

Mar. 31, 2019

Subsequent Events [Abstract]

Subsequent Event

11. Subsequent Event

On April 2, 2019, the Company, entered into Amendment No.3 to the UCSB Agreement with UCSB to adjust and clarify certain sublicense terms (the “Amendment No.3”). In connection with the entry into the Amendment No.3, the Company issued to UCSB 150,000 shares of CytomX common stock, pursuant to a Securities Issuance Agreement, dated April 2, 2019, by and between CytomX and UCSB.

Under the terms of the Amendment No.3, the Company and UCSB agreed to modify the Company’s obligation to pay UCSB a percentage of sublicense revenues to be earned in the future on existing and new collaboration agreements. In particular, the Amendment No.3 adjusts the definition of sublicense revenues, which includes up-front fees, sublicense maintenance fees, milestone payments or other sublicense revenues (other than royalties) and modifies the Company’s payment obligation for pre-clinical stage sublicenses going forward by potentially lowering the sublicense revenue payment to UCSB based on the contribution of intellectual property from each party. In exchange, the Company agreed to make an upfront payment of $1,000,000 as well as additional annual license maintenance fees of $750,000 through 2031.

Basis of Presentation and Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Basis of Presentation

Unaudited Interim Financial Information

Use of Estimates

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents.

Restricted Cash

Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015.

	March 31, 2019		December 31, 2018		March 31, 2018		December 31, 2017
	(in thousands)
Cash and cash equivalents	$	190,407	$	247,577	$	142,155	$	177,548
Restricted cash - non-current assets		917		917		917		917
Total	$	191,324	$	248,494	$	143,072	$	178,465

Comprehensive Income (Loss)

Revenue Recognition

Contract Balances

Research and Development Expenses

Stock-based Compensation

The Company estimates the fair value of its stock-based awards using the Black-Scholes option-pricing model, which requires the input of assumptions. The assumptions are as follows:

•

Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury whose term was consistent with expected term of the Company’s stock options.

•

Dividend yield. The expected dividend is assumed to be zero as the Company has never paid dividends and have no current plans to pay any dividends on our common stock.

Leases

Adopted Accounting Pronouncements and Recent Accounting Pronouncements

Adopted Accounting Pronouncements

Leases

Reclassification of Certain Tax Effects

Nonemployee Share-Based Payment

Recent Accounting Pronouncements

Basis of Presentation and Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Schedule of Cash, Cash Equivalents, and Restricted Cash

	March 31, 2019		December 31, 2018		March 31, 2018		December 31, 2017
	(in thousands)
Cash and cash equivalents	$	190,407	$	247,577	$	142,155	$	177,548
Restricted cash - non-current assets		917		917		917		917
Total	$	191,324	$	248,494	$	143,072	$	178,465

Net Loss Per Share (Tables)

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share

	Three Months Ended
	March 31,
	2019		2018
Options to purchase common stock		8,962,799		7,199,679
Total		8,962,799		7,199,679

Fair Value Measurements and Short-Term Investments (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis

		March 31, 2019
	Valuation Hierarchy	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Aggregate Fair Value
		(in thousands)
Assets
Money market funds	Level I	$	175,738	$	—	$	—	$	175,738
Restricted cash (money market funds)	Level I		917		—		—		917
U.S. Government bonds	Level I		206,058		131		—		206,189
Total		$	382,713	$	131	$	—	$	382,844

		December 31, 2018
	Valuation Hierarchy	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
		(in thousands)
Assets
Money market funds	Level I	$	226,979	$	—	$	—		$	226,979
Restricted cash (money market funds)	Level I		917		—		—			917
U.S. Government bonds	Level I		188,616		—		(66	)		188,550
Total		$	416,512	$	—	$	(66	)	$	416,446

Accrued Liabilities (Tables)

3 Months Ended

Mar. 31, 2019

Accrued Liabilities Current [Abstract]

Schedule of Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	March 31,		December 31,
	2019		2018
	(in thousands)
Research and clinical expenses	$	14,613	$	18,520
Payroll and related expenses		3,469		6,585
Legal and professional expenses		1,393		830
Operating lease liabilities - short term		2,544		-
Other accrued expenses		548		789
Total	$	22,567	$	26,724

Research and Collaboration Agreements (Tables)

3 Months Ended

Mar. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Schedule of Revenue by Collaboration Partners

The following table summarizes the revenue by collaboration partners:

	Three Months Ended March 31,
	2019		2018
	(in thousands)
AbbVie	$	2,948	$	2,671
Amgen		1,035		1,255
BMS		25,502		8,168
ImmunoGen		-		735
Pfizer		-		1,355
Total Revenue	$	29,485	$	14,184

Summary of Contract Liabilities

The following table presents changes in the Company’s total contract liabilities during the three months ended March 31, 2019:

	Balance at Beginning of Period		Additions		Deductions			Balance at End of Period
	(in thousands)
Contract liabilities:
Deferred revenue	$	277,980	$	—	$	(29,461	)	$	248,519

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Activities Under Company's Stock Option Plans

Activities under the Company’s stock option plans for the three months ended March 31, 2019 were as follows:

	Options Outstanding
	Number of Options			Weighted- Average Exercise Price Per Share
Balances at December 31, 2018		7,803,773		$	12.622
Options granted		2,103,883			14.736
Options exercised		(74,443	)		6.240
Option forfeited/expired		(61,025	)		17.894
Balances at March 31, 2019		9,772,188			13.093
Options exercisable at March 31, 2019		4,854,438		$	8.558

Total Stock-based Compensation Recognized

Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows:

	Three Months Ended
	March 31,
	2019		2018
	(in thousands)
Stock-based compensation expense:
Research and development	$	2,642	$	1,808
General and administrative		2,550		1,880
Total stock-based compensation expense	$	5,192	$	3,688

Leases (Tables)

3 Months Ended

Mar. 31, 2019

Leases [Abstract]

Summary of Supplemental Information Related to Leases

Supplemental information related to leases are as follows:

	Three Months Ended
	March 31, 2019
	(in thousands)
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$	1,205

	March 31, 2019
	(in thousands)
Supplemental balance sheet information related to leases:
Operating lease right-of-use assets	$	27,404
Current operating lease liabilities		2,544
Non-current operating lease liabilities		27,008
Total operating lease liabilities	$	29,552

Weighted-average remaining lease term (in years)
Operating lease		7.61
Weighted-average discount rate
Operating lease		8.25	%

	March 31, 2019
	(in thousands)
Maturity of operating lease liabilities
2019	$	4,888
2020		5,025
2021		5,165
2022		5,309
2023 and beyond		19,763
Total lease payments		40,150
Less imputed interest		(10,598	)
Present value of lease liabilities	$	29,552

Basis of Presentation and Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($)
$ in Thousands

Mar. 31, 2019

Dec. 31, 2018

Mar. 31, 2018

Dec. 31, 2017

Cash And Cash Equivalents [Abstract]

Cash and cash equivalents

$ 190,407

$ 247,577

^[1]

$ 142,155

$ 177,548

Restricted cash

917

^[1]

917

Total

$ 191,324

$ 248,494

$ 143,072

$ 178,465

[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2019	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019
Summary Of Significant Accounting Policies [Line Items]
Research and development		$ 36,376	$ 22,458
Right-of-use asset		27,404
Lease liability		29,552
ASC 842
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset				$ 28,000
Lease liability				$ 30,100
ASU 2018-02
Summary Of Significant Accounting Policies [Line Items]
Increase in other comprehensive income		11,000
Probody technology
Summary Of Significant Accounting Policies [Line Items]
Acquired, technology know-how	$ 5,000
Research and development		$ 5,000

Net Loss Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	8,962,799	7,199,679
Options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	8,962,799	7,199,679

Fair Value Measurements and Short-Term Investments - Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 382,713	$ 416,512
Gross Unrealized Holding Gains	131
Gross Unrealized Holding Losses		(66)
Aggregate Fair Value	382,844	416,446
Level I \| Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	175,738	226,979
Aggregate Fair Value	175,738	226,979
Level I \| Restricted cash (money market funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	917	917
Aggregate Fair Value	917	917
Level I \| U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	206,058	188,616
Gross Unrealized Holding Gains	131
Gross Unrealized Holding Losses		(66)
Aggregate Fair Value	$ 206,189	$ 188,550

Fair Value Measurements and Short-Term Investments - Additional Information (Details)	Mar. 31, 2019 USD ($)
Fair Value Disclosures [Abstract]
Contractual maturities of longer than one year	$ 0

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($)
$ in Thousands

Mar. 31, 2019

Dec. 31, 2018

Accrued Liabilities Current [Abstract]

Research and clinical expenses

$ 14,613

$ 18,520

Payroll and related expenses

3,469

6,585

Legal and professional expenses

1,393

830

Operating lease liabilities - short term

2,544

Other accrued expenses

548

789

Total

$ 22,567

$ 26,724

^[1]

[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

Research and Collaboration Agreements - Schedule of Revenue by Collaboration Partners (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 29,485	$ 14,184
AbbVie
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	2,948	2,671
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	1,035	1,255
BMS
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 25,502	8,168
ImmunoGen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		735
Pfizer
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		$ 1,355

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details)

1 Months Ended

3 Months Ended

Apr. 02, 2019

USD ($)

May 31, 2018

USD ($)

Jul. 31, 2017

USD ($)

Apr. 30, 2016

USD ($)

Agreement

Target

AccountingUnit

Mar. 31, 2019

USD ($)

Jun. 30, 2018

USD ($)

Mar. 31, 2018

USD ($)

Dec. 31, 2018

USD ($)

Mar. 31, 2017

Target

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

$ 22,567,000

$ 26,724,000

^[1]

$ 29,485,000

$ 14,184,000

us-gaap:LicenseAndServiceMember

Seattle Genetics Agreement

$ 248,519,000

277,980,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

AbbVie Ireland Unlimited Company

$ 4,000,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue adjustment of milestone payment recognized

21,000,000

$ 9,900,000

$ 2,900,000

$ 2,700,000

AbbVie Ireland Unlimited Company | ASC 606

us-gaap:LicenseAndServiceMember

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

$ 30,000,000

14,000,000

Estimated sublicense fees

39,800,000

$ 4,200,000

Number of accounting units | AccountingUnit

AbbVie Ireland Unlimited Company | Collaborative Arrangement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Number of collaboration agreements | Agreement

Percentage of net profits or net losses related to development costs

35.00%

Amount due from agreement

$ 0

AbbVie Ireland Unlimited Company | CD71 Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

$ 20,000,000

21,000,000

$ 14,000,000

$ 1,000,000

AbbVie Ireland Unlimited Company | CD71 Agreement | UC Regents

20,700,000

23,200,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

AbbVie Ireland Unlimited Company | CD71 Agreement | UC Regents | Subsequent Event

$ 1,000,000

1,100,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payments for sublicense fee payable

$ 1,000,000

AbbVie Ireland Unlimited Company | CD71 Agreement | ASC 606

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

20,000,000

Estimated sublicense fees

29,800,000

4,200,000

AbbVie Ireland Unlimited Company | CD71 Agreement | U.S

$ 14,000,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Profit split on U.S. sales and royalties on ex U.S. sales

35.00%

AbbVie Ireland Unlimited Company | CD71 Agreement | Maximum

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent payments receivable upon achieving development, regulatory and commercial milestones

$ 470,000,000

AbbVie Ireland Unlimited Company | Seattle Genetics Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

AbbVie Ireland Unlimited Company | Seattle Genetics Agreement | ASC 606

$ 4,000,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

AbbVie Ireland Unlimited Company | Discovery Agreement

1,000,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Number of targets selected | Target

10,000,000

Revenue recognition upon performance obligation

10,000,000

Estimated research service period

5 years

AbbVie Ireland Unlimited Company | Discovery Agreement | ASC 606

$ 4,200,000

$ 4,700,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

AbbVie Ireland Unlimited Company | Discovery Agreement | Maximum

$ 10,000,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Number of targets | Target

AbbVie Ireland Unlimited Company | Discovery Agreement | Maximum | Development, Regulatory and Commercial Milestone Payments

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 275,000,000

AbbVie Ireland Unlimited Company | Sublicense Agreement | UC Regents

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Sublicense payment percentage

5.00%

[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details 1)	Mar. 31, 2019
AbbVie Ireland Unlimited Company \| CD71 Agreement \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-04-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognition upon performance obligation, service period	5 years

Research and Collaboration Agreements - Amgen, Inc - Additional Information (Details)

1 Months Ended

3 Months Ended

Apr. 02, 2019

USD ($)

Sep. 29, 2017

USD ($)

Target

Oct. 31, 2017

USD ($)

$ / shares

Mar. 31, 2019

USD ($)

Dec. 31, 2018

USD ($)

Mar. 31, 2018

USD ($)

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

$ 22,567,000

$ 26,724,000

^[1]

$ 29,485,000

$ 14,184,000

us-gaap:LicenseAndServiceMember

Amgen Inc | EGFR Products

$ 248,519,000

$ 277,980,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Amgen Inc | Amgen Other Products

39,800,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Collaboration and License Agreement | Amgen Inc

3,600,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Common stock, shares issuable under agreement | shares

$ 40,000,000

1,156,069

Common stock, shares issuable under agreement, price per share | $ / shares

$ 17.30

Common stock, value of shares issued in connection with agreement

$ 20,000,000

Period used to calculate weighted average price per share

20 days

Lock-up period for share disposal

6 months

Estimated premium on the stock sold

$ 500,000

Number of targets selected | Target

Number of additional collaboration target | Target

Portion of transaction price allocated to premium on sale of common stock

$ 51,200,000

500,000

Estimated fair value of products

10,700,000

$ 1,000,000

$ 1,300,000

Amount due from agreement

us-gaap:LicenseAndServiceMember

$ 0

Collaboration and License Agreement | Amgen Inc | EGFR Products

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Estimated projected costs and research service period

46,400,000

7 years

Collaboration and License Agreement | Amgen Inc | Amgen Other Products

39,800,000

$ 40,700,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognition upon performance obligation

4,800,000

$ 4,800,000

Estimated research service period

6 years

Collaboration and License Agreement | Amgen Inc | Maximum

$ 3,600,000

3,800,000

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 950,000,000

Number of targets | Target

Contingent payments payable

$ 203,000,000

Collaboration and License Agreement | Amgen Inc | Maximum | EGFR Products

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 455,000

Percentage share of profit and losses

50.00%

Sublicense Agreement | Amgen Inc | UC Regents

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]