Five Prime Therapeutics (FPRX) 10-Q 12 May 14 2014 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2014	5-May-14
Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	31-Mar-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q1	'
Trading Symbol	'FPRX	'
Entity Registrant Name	'FIVE PRIME THERAPEUTICS INC	'
Entity Central Index Key	'0001175505	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Non-accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	21,438,386

Condensed_Balance_Sheets

Condensed Balance Sheets (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$11,680	$8,161
Marketable securities	117,250	67,561
Receivable from collaborative partners	20,120	296
Prepaid and other current assets	1,154	1,640
Total current assets	150,204	77,658
Property and equipment, net	3,649	3,744
Other long-term assets	453	389
Total assets	154,306	81,791
Liabilities and stockholders' equity	'	'
Accounts payable	347	348
Accrued personnel-related expenses	1,765	2,957
Other accrued liabilities	3,210	2,056
Deferred revenue, current portion	10,983	7,913
Deferred rent, current portion	569	549
Total current liabilities	16,874	13,823
Deferred revenue, long-term portion	25,503	7,123
Deferred rent, long-term portion	1,988	2,146
Other long-term liabilities	617	673
Commitments	'	'
Common stock	21	17
Preferred stock	'	'
Additional paid-in capital	269,502	209,580
Accumulated other comprehensive income	19	3
Accumulated deficit	-160,218	-151,574
Total stockholders' equity	109,324	58,026
Total liabilities and stockholders' equity	$154,306	$81,791

Condensed_Statements_of_Operat

Condensed Statements of Operations (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Income Statement [Abstract]	'	'
Collaboration revenue	$3,546	$2,975
Operating expenses:	'	'
Research and development	8,926	7,930
General and administrative	3,280	2,392
Total operating expenses	12,206	10,322
Loss from operations	-8,660	-7,347
Interest income	36	15
Other income (expense), net	-20	285
Net loss	($8,644)	($7,047)
Basic and diluted net loss per common share	($0.46)	($5.73)
Shares used to compute basic and diluted net loss per common share	18,841	1,229

Condensed_Statements_of_Compre

Condensed Statements of Comprehensive Loss (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]	'	'
Net loss	($8,644)	($7,047)
Other comprehensive loss:	'	'
Net unrealized gain (loss) on marketable securities	16	-2
Comprehensive loss	($8,628)	($7,049)

Condensed_Statement_of_Cash_Fl

Condensed Statement of Cash Flows (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Operating activities	'	'
Net loss	($8,644)	($7,047)
Adjustments to reconcile net loss to net cash used in operating activities:	'	'
Depreciation and amortization	391	425
Stock-based compensation expense	636	624
Amortization of premium on marketable securities	211	102
Revaluation of preferred stock warrant liability	'	-285
Changes in operating assets and liabilities:	'	'
Receivable from collaborative partners	9	370
Prepaid, other current assets, and other long-term assets	59	-2
Accounts payable	-1	-374
Accrued personnel-related expenses	-1,192	-1,085
Deferred revenue	1,617	6,444
Deferred rent	-138	63
Other accrued liabilities and other long-term liabilities	1,461	644
Net cash used in operating activities	-5,591	-121
Investing activities	'	'
Purchases of marketable securities	-68,384	-3,006
Maturities of marketable securities	18,500	9,875
Purchases of property and equipment	-296	-293
Net cash (used in) provided by investing activities	-50,180	6,576
Financing activities	'	'
Proceeds from public offering of common stock, net	40,104	'
Proceeds from the sale of common stock to collaborative partner	18,639	'
Proceeds from exercise of stock options	547	16
Payments under capital lease obligation	'	-4
Net cash provided by financing activities	59,290	12
Net increase in cash and cash equivalents	3,519	6,467
Cash and cash equivalents at beginning of period	8,161	11,391
Cash and cash equivalents at end of period	$11,680	$17,858

Description_of_Business

Description of Business	3 Months Ended
	Mar. 31, 2014
Accounting Policies [Abstract]	'
Description of Business	'
	1. Description of Business
	Five Prime Therapeutics, Inc. (we, us, our, or the Company) is a clinical-stage biotechnology company focused on discovering and developing novel protein therapeutics. Protein therapeutics are antibodies or drugs developed from extracellular proteins or protein fragments that block disease processes, including cancer and inflammatory diseases. We were incorporated in December 2001 in Delaware. Our operations are based in South San Francisco, California and we operate in one segment.
	Unaudited Interim Financial Information
	The accompanying financial information as of March 31, 2014 is unaudited. The Condensed Financial Statements included in this report reflect all adjustments (consisting only of normal recurring adjustments) that our management considers necessary for the fair statement of the results of operations for the interim periods covered and of the financial condition of the Company at the date of the interim balance sheet. The December 31, 2013 Condensed Balance Sheet was derived from audited financial statements, but does not include all disclosures required by generally accepted accounting principles in the United States of America, or GAAP. The results for interim periods are not necessarily indicative of the results for the entire year or any other interim period. The accompanying Condensed Financial Statements and related financial information 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, 2013 filed with the U.S. Securities and Exchange Commission.
	Follow-on Public Offering
	In February 2014, we completed a public offering of 3,450,000 shares of our common stock, or our Follow-on Public Offering, which included shares we issued pursuant to our underwriters’ exercise of their over-allotment option. We received net proceeds of $40.1 million, after underwriting discounts, commissions and estimated offering expenses, from the Follow-on Public Offering.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2014
Accounting Policies [Abstract]	'
Summary of Significant Accounting Policies	'
	2. Summary of Significant Accounting Policies
	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions about future events that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements as well as reported amounts of revenue and expenses during the reporting period. Actual results could differ materially from those estimates.
	Reclassifications
	We have reclassified certain prior period amounts to conform to the current period presentation. We reclassified certain liabilities, primarily those related to unbilled receipts, from accounts payable to other accrued liabilities on the balance sheets, and made related conforming reclassifications on the statement of cash flows.
	Fair Value of Financial Instruments
	We determine the fair value of financial and nonfinancial assets and liabilities using the fair value hierarchy, which describes three levels of inputs that may be used to measure fair value, as follows:
	Level 1—Quoted prices in active markets for identical assets or liabilities;
	Level 2—Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities, quoted prices for identical or similar assets or liabilities 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; and
	Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.
	We determine the fair value of Level 1 assets using quoted prices in active markets for identical assets. We review trading activity and pricing for Level 2 investments as of each measurement date. Level 2 inputs, obtained from various third-party data providers, represent quoted prices for similar assets in active markets, were derived from observable market data, or, if not directly observable, were derived from or corroborated by other observable market data.
	In certain cases where there is limited activity or less transparency around inputs to valuation, we classify securities as Level 3 within the valuation hierarchy. As of March 31, 2013, our Level 3 liability consists of a preferred stock warrant liability that we measured at estimated fair value.
	The following table summarizes, for assets and the liability recorded at fair value, the respective fair values and the classifications by level of input within the fair value hierarchy defined above (in thousands):
		MARCH 31, 2014
					BASIS OF FAIR VALUE
					MEASUREMENTS
		TOTAL			LEVEL 1		LEVEL 2		LEVEL 3
	Assets
	Money market funds	$	8,533		$	8,533	$	—	$	—
	U.S. Treasury securities		82,876			82,876		—		—
	U.S. government agency securities		34,374			—		34,374		—
	Total cash equivalents and marketable securities	$	125,783		$	91,409	$	34,374	$	—
		DECEMBER 31, 2013
					BASIS OF FAIR VALUE
					MEASUREMENTS
		TOTAL			LEVEL 1		LEVEL 2		LEVEL 3
	Assets
	Money market funds	$	6,456		$	6,456	$	—	$	—
	U.S. Treasury securities		18,852			18,852		—		—
	U.S. government agency securities		48,709			—		48,709		—
	Total cash equivalents and marketable securities	$	74,017		$	25,308	$	48,709	$	—
	Prior to our initial public offering in September 2013, or our IPO, we had outstanding warrants which we classified as a liability and remeasured to fair value each reporting period. We measured the estimated fair value of the preferred stock warrant liability using the Black-Scholes option-pricing model. Inputs used to determine estimated fair value include the estimated fair value of the underlying stock at the valuation measurement date, the remaining contractual term of the warrants, risk-free interest rates, expected dividends, and the expected volatility of the price of the underlying stock. In connection with the completion of our IPO in September 2013, substantially all of the warrants were automatically net exercised for a total of 4,376 shares of common stock, pursuant to the terms of the warrants.
	The change in the estimated fair value of the preferred stock warrant liability is summarized below (in thousands):
		THREE MONTHS
		ENDED
		MARCH 31, 2013
	Balance, beginning	$	563
	Change in fair value recorded in other income, net		(285	)
	Balance, ending	$	278
	As of March 31, 2013, the fair value of the above warrants was determined using the following assumptions:
	Risk-free interest rate		0.1-0.2	%
	Estimated term (years)		1.9
	Volatility		85	%
	Net Loss Per Share of Common Stock
	We compute basic net loss per common share dividing net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period. We did not include potentially dilutive securities consisting of stock options, the preferred stock warrants, the common stock warrant and convertible preferred stock in the diluted net loss per common share calculations for all periods presented, because the inclusion of such shares would have had an antidilutive effect. The convertible preferred stock contains certain participation rights.
	For the three months ended March 31, 2014 and 2013, respectively, we excluded the following securities from the calculation of diluted net loss per share as the effect would have been antidilutive (in thousands):
		MARCH 31,
		2014			2013
	Convertible preferred stock		—			9,929
	Options to purchase common stock		2,213			2,638
	Warrants to purchase convertible preferred stock		—			84
			2,213			12,651

Cash_Equivalents_and_Marketabl

Cash Equivalents and Marketable Securities	3 Months Ended
	Mar. 31, 2014
Cash And Cash Equivalents [Abstract]	'
Cash Equivalents and Marketable Securities	'
	3. Cash Equivalents and Marketable Securities
	The following is a summary of our cash equivalents and marketable securities (in thousands):
		MARCH 31, 2014
		AMORTIZED			UNREALIZED		UNREALIZED			ESTIMATED
		COST BASIS			GAINS		LOSSES			FAIR VALUE
		(unaudited)
	Money market funds	$	8,533		$	—	$	—		$	8,533
	U.S. Treasury securities		82,862			15		(1	)		82,876
	U.S. government agency securities		34,369			7		(2	)		34,374
			125,764			22		(3	)		125,783
	Less: cash equivalents		(8,533	)		—		—			(8,533	)
	Total marketable securities	$	117,231		$	22	$	(3	)	$	117,250
		DECEMBER 31, 2013
		AMORTIZED			UNREALIZED		UNREALIZED			ESTIMATED
		COST BASIS			GAINS		LOSSES			FAIR VALUE
	Money market funds	$	6,456		$	—	$	—		$	6,456
	U.S. Treasury securities		18,848			4		—			18,852
	U.S. government agency securities		48,709			3		(3	)		48,709
			74,013			7		(3	)		74,017
	Less: cash equivalents		(6,456	)		—		—			(6,456	)
	Total marketable securities	$	67,557		$	7	$	(3	)	$	67,561
	As of March 31, 2014, the amortized cost and estimated fair value of our available-for-sale securities by contractual maturity are shown below (in thousands):
		Amortized			Estimated
		Cost			Fair Value
	Debt securities maturing:
	In one year or less	$	95,121		$	95,131
	In one to two years		30,643			30,652
	Total marketable securities	$	125,764		$	125,783
	We determined that the gross unrealized losses of $3,000 on our marketable securities as of March 31, 2014 were temporary in nature. We currently do not intend to sell these securities prior to maturity and do not consider these investments to be other-than-temporarily impaired at March 31, 2014. There were no sales of available-for-sale securities in any of the periods presented.

Equity_Incentive_Plans

Equity Incentive Plans	3 Months Ended
	Mar. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'
Equity Incentive Plans	'
	4. Equity Incentive Plans
	The following table summarizes option activity under our stock plans and related information:
		OPTIONS OUTSTANDING
		NUMBER			WEIGHTED-
		OF SHARES			AVERAGE
					EXERCISE
					PRICE
					PER SHARE
	Balance at December 31, 2013		2,236,997		$	6.09
	Options granted		8,520		$	14.36
	Options exercised		(133,348	)	$	5.26
	Options forfeited		(17,897	)	$	7.2
	Balance at March 31, 2014		2,094,272		$	6.17
	Options exercisable		1,254,272		$	5.63
	As of March 31, 2014, there were 4,149,532 shares available for future issuance under our 2013 Omnibus Incentive Plan.
	As of March 31, 2014, options to purchase 2,069,087 shares of common stock were outstanding, which are fully vested or expected to vest with a weighted-average exercise price of $6.16 per share and a weighted-average remaining contractual term of 7.1 years. As of March 31, 2014, the weighted-average remaining contractual term for options exercisable was 6.2 years.
	Stock-Based Compensation
	We calculated employee stock-based compensation expense based on awards ultimately expected to vest reduced by estimated forfeitures. We estimate forfeitures at the time of grant and revise forfeitures, if necessary, in subsequent periods if actual forfeitures differ from those estimates. Total stock-based compensation expense recognized was as follows (in thousands):
		Three Months Ended
		March 31,
		2014			2013
	Research and development	$	360		$	212
	General and administrative		276			412
	Total	$	636		$	624
	In February 2013, we amended stock options held by our former CEO to extend the post-termination exercise period for the former CEO’s outstanding vested options from 18 months to 20 months, which resulted in additional incremental stock-based compensation of $157,000 in the first quarter of 2013.
	We estimated the fair value of each stock option using the Black-Scholes option-pricing model based on the date of grant of such stock option with the following assumptions:
		Three Months Ended
		March 31,
		2014			2013
	Expected term (years)		6.1		5.0-6.1
	Expected volatility		85	%	85%
	Risk-free interest rate		1.8	%	0.8-1.1%
	Expected dividend yield		0	%	0%
	As of March 31, 2014, we had $3.7 million of total unrecognized compensation expense related to nonvested employee and director stock options that we expect to recognize over a weighted-average period of 2.6 years.

Collaborative_Research_and_Dev

Collaborative Research and Development Agreements	3 Months Ended
	Mar. 31, 2014
Organization Consolidation And Presentation Of Financial Statements [Abstract]	'
Collaborative Research and Development Agreements	'
	5. Collaborative Research and Development Agreements
	Bristol-Myers Squibb Company
	On March 14, 2014, we entered into a research collaboration and license agreement, or immuno-oncology collaboration, with Bristol-Myers Squibb Company, or BMS, to carry out a research program to (i) discover novel interacting proteins in two undisclosed immune checkpoint pathways, which we refer to as the checkpoint pathways, using our target discovery platform; (ii) further the understanding of target biology with respect to targets in these checkpoint pathways; and (iii) discover and pre-clinically develop compounds suitable for development for human therapeutic uses against targets in these checkpoint pathways. Under the immuno-oncology collaboration, we granted BMS an exclusive, worldwide license to research, develop and commercialize products directed towards certain targets in the checkpoint pathways. BMS will have an option to take exclusive licenses to additional targets we may identify in these checkpoint pathways during the course of the immuno-oncology collaboration. We received an upfront payment of $20.0 million from BMS in April 2014 in connection with our entry into the immuno-oncology collaboration and expect to receive $9.5 million in research funding over the course of the three-year research term based on the research activities currently planned under the research plan. BMS may extend the research term for two additional one-year periods on a year-by-year basis, during which extensions we would be obligated to perform additional services as agreed to with BMS and BMS would be obligated to pay us research funding with respect to such services.
	We applied the Financial Accounting Standards Board Accounting Standards Update, or ASU, No. 2009-13, Multiple-Deliverable Revenue Arrangements, in evaluating the appropriate accounting for the immuno-oncology collaboration. In accordance with this guidance, we concluded that we should account for the immuno-oncology collaboration as a single unit of accounting and recognize the immuno-oncology collaboration consideration in the same manner as the final deliverable, which is research service. The $20.0 million upfront payment was recorded as deferred revenue and is being recognized over the five-year research period under the collaboration. In addition, BMS agreed to pay us $9.5 million of research funding over the initial three year research program term. We did not receive any research funding during the three months ended March 31, 2014 related to research being performed under the immuno-oncology collaboration.
	We are eligible to receive certain contingent payments with respect to each target subject to the immuno-oncology collaboration and royalties on sales of products related to such targets, if any.
	In accordance with ASU No. 2010-17, we determined that the remaining contingent payments under the immuno-oncology collaboration do not constitute milestone payments and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments under the collaboration do not meet the definition of a milestone under ASU 2010-17 because the achievement of these events solely depends on BMS’s performance. Any revenue from these contingent payments would be subject to an allocation of arrangement consideration and would be recognized over any remaining period of performance obligations, if any, relating to the collaboration. If we have no remaining performance obligations under the immuno-oncology collaboration at the time the contingent payment is triggered, we would recognize the contingent payment as revenue in full upon the triggering event.
	In connection with the immuno-oncology collaboration, BMS purchased 994,352 shares of our common stock at a price per share of $21.16, for an aggregate purchase price of $21.0 million. We determined that the purchase price of $21.16 per share exceeded the fair value of our common stock by $2.4 million and, therefore, recorded the $2.4 million as deferred revenue, which we will recognize in the same manner as the $20.0 million up-front payment.
	Total revenue recognized under the immuno-oncology collaboration was $0.2 million for the three months ended March 31, 2014. As of March 31, 2014, we had deferred revenue relating to the immuno-oncology collaboration of $22.2 million. As of March 31, 2014, the receivable from BMS under the immuno-oncology collaboration was $20.0 million and was paid in April 2014.
	The immuno-oncology collaboration will terminate upon the expiration of all payment obligations under the collaboration. In addition, BMS may terminate the immuno-oncology collaboration in its entirety or on a collaboration target-by-collaboration target basis at any time with advance written notice, and either party may terminate the collaboration in its entirety, or on a collaboration target-by-collaboration target basis with written notice for the other party’s material breach if such party fails to cure the breach or immediately upon certain insolvency events.

Subsequent_Events

Subsequent Events	3 Months Ended
	Mar. 31, 2014
Subsequent Events [Abstract]	'
Subsequent Events	'
	6. Subsequent Events
	In April 2014, we amended our research collaboration and license agreement, referred to as the respiratory diseases collaboration, with Glaxo Group Limited, or GSK UK, that we originally entered into in April 2012 to identify new therapeutic approaches to treat refractory asthma and chronic obstructive pulmonary disease function. Pursuant to the respiratory diseases collaboration, GSK UK has an option to elect to include additional screening assays under the research plan. The amendment allows GSK UK to terminate any additional screening assay it elects under the research plan within six months of so electing. Concurrent with the amendment, GSK UK exercised its option and expanded the research plan to include two additional screening assays. In connection with GSK UK’s exercise of its option, we are entitled to receive up to $1.0 million in additional research funding in 16 equal quarterly payments for each additional screening assay, for a total of up to $2.0 million in additional research funding for both additional screening assays in the event that GSK UK does not terminate either additional screening assay by October 2014.

Description_of_Business_Polici

Description of Business (Policies)	3 Months Ended
	Mar. 31, 2014
Accounting Policies [Abstract]	'
Unaudited Interim Financial Information	'
	Unaudited Interim Financial Information
	The accompanying financial information as of March 31, 2014 is unaudited. The Condensed Financial Statements included in this report reflect all adjustments (consisting only of normal recurring adjustments) that our management considers necessary for the fair statement of the results of operations for the interim periods covered and of the financial condition of the Company at the date of the interim balance sheet. The December 31, 2013 Condensed Balance Sheet was derived from audited financial statements, but does not include all disclosures required by generally accepted accounting principles in the United States of America, or GAAP. The results for interim periods are not necessarily indicative of the results for the entire year or any other interim period. The accompanying Condensed Financial Statements and related financial information 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, 2013 filed with the U.S. Securities and Exchange Commission.
Use of Estimates	'
	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions about future events that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements as well as reported amounts of revenue and expenses during the reporting period. Actual results could differ materially from those estimates.
Reclassifications	'
	Reclassifications
	We have reclassified certain prior period amounts to conform to the current period presentation. We reclassified certain liabilities, primarily those related to unbilled receipts, from accounts payable to other accrued liabilities on the balance sheets, and made related conforming reclassifications on the statement of cash flows.
Fair Value of Financial Instruments	'
	Fair Value of Financial Instruments
	We determine the fair value of financial and nonfinancial assets and liabilities using the fair value hierarchy, which describes three levels of inputs that may be used to measure fair value, as follows:
	Level 1—Quoted prices in active markets for identical assets or liabilities;
	Level 2—Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities, quoted prices for identical or similar assets or liabilities 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; and
	Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.
	We determine the fair value of Level 1 assets using quoted prices in active markets for identical assets. We review trading activity and pricing for Level 2 investments as of each measurement date. Level 2 inputs, obtained from various third-party data providers, represent quoted prices for similar assets in active markets, were derived from observable market data, or, if not directly observable, were derived from or corroborated by other observable market data.
	In certain cases where there is limited activity or less transparency around inputs to valuation, we classify securities as Level 3 within the valuation hierarchy. As of March 31, 2013, our Level 3 liability consists of a preferred stock warrant liability that we measured at estimated fair value.
	The following table summarizes, for assets and the liability recorded at fair value, the respective fair values and the classifications by level of input within the fair value hierarchy defined above (in thousands):
		MARCH 31, 2014
					BASIS OF FAIR VALUE
					MEASUREMENTS
		TOTAL			LEVEL 1		LEVEL 2		LEVEL 3
	Assets
	Money market funds	$	8,533		$	8,533	$	—	$	—
	U.S. Treasury securities		82,876			82,876		—		—
	U.S. government agency securities		34,374			—		34,374		—
	Total cash equivalents and marketable securities	$	125,783		$	91,409	$	34,374	$	—
		DECEMBER 31, 2013
					BASIS OF FAIR VALUE
					MEASUREMENTS
		TOTAL			LEVEL 1		LEVEL 2		LEVEL 3
	Assets
	Money market funds	$	6,456		$	6,456	$	—	$	—
	U.S. Treasury securities		18,852			18,852		—		—
	U.S. government agency securities		48,709			—		48,709		—
	Total cash equivalents and marketable securities	$	74,017		$	25,308	$	48,709	$	—
	Prior to our initial public offering in September 2013, or our IPO, we had outstanding warrants which we classified as a liability and remeasured to fair value each reporting period. We measured the estimated fair value of the preferred stock warrant liability using the Black-Scholes option-pricing model. Inputs used to determine estimated fair value include the estimated fair value of the underlying stock at the valuation measurement date, the remaining contractual term of the warrants, risk-free interest rates, expected dividends, and the expected volatility of the price of the underlying stock. In connection with the completion of our IPO in September 2013, substantially all of the warrants were automatically net exercised for a total of 4,376 shares of common stock, pursuant to the terms of the warrants.
	The change in the estimated fair value of the preferred stock warrant liability is summarized below (in thousands):
		THREE MONTHS
		ENDED
		MARCH 31, 2013
	Balance, beginning	$	563
	Change in fair value recorded in other income, net		(285	)
	Balance, ending	$	278
	As of March 31, 2013, the fair value of the above warrants was determined using the following assumptions:
	Risk-free interest rate		0.1-0.2	%
	Estimated term (years)		1.9
	Volatility		85	%
Net Loss Per Share of Common Stock	'
	Net Loss Per Share of Common Stock
	We compute basic net loss per common share dividing net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period. We did not include potentially dilutive securities consisting of stock options, the preferred stock warrants, the common stock warrant and convertible preferred stock in the diluted net loss per common share calculations for all periods presented, because the inclusion of such shares would have had an antidilutive effect. The convertible preferred stock contains certain participation rights.
	For the three months ended March 31, 2014 and 2013, respectively, we excluded the following securities from the calculation of diluted net loss per share as the effect would have been antidilutive (in thousands):
		MARCH 31,
		2014			2013
	Convertible preferred stock		—			9,929
	Options to purchase common stock		2,213			2,638
	Warrants to purchase convertible preferred stock		—			84
			2,213			12,651

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2014
Accounting Policies [Abstract]	'
Summary of Assets and Liabilities Recorded at Fair Value	'
	The following table summarizes, for assets and the liability recorded at fair value, the respective fair values and the classifications by level of input within the fair value hierarchy defined above (in thousands):
		MARCH 31, 2014
					BASIS OF FAIR VALUE
					MEASUREMENTS
		TOTAL			LEVEL 1		LEVEL 2		LEVEL 3
	Assets
	Money market funds	$	8,533		$	8,533	$	—	$	—
	U.S. Treasury securities		82,876			82,876		—		—
	U.S. government agency securities		34,374			—		34,374		—
	Total cash equivalents and marketable securities	$	125,783		$	91,409	$	34,374	$	—
		DECEMBER 31, 2013
					BASIS OF FAIR VALUE
					MEASUREMENTS
		TOTAL			LEVEL 1		LEVEL 2		LEVEL 3
	Assets
	Money market funds	$	6,456		$	6,456	$	—	$	—
	U.S. Treasury securities		18,852			18,852		—		—
	U.S. government agency securities		48,709			—		48,709		—
	Total cash equivalents and marketable securities	$	74,017		$	25,308	$	48,709	$	—
Change in Estimated Fair Value of Preferred Stock Warrant Liability	'
	The change in the estimated fair value of the preferred stock warrant liability is summarized below (in thousands):
		THREE MONTHS
		ENDED
		MARCH 31, 2013
	Balance, beginning	$	563
	Change in fair value recorded in other income, net		(285	)
	Balance, ending	$	278
Fair Value of Warrants Assumptions	'
	As of March 31, 2013, the fair value of the above warrants was determined using the following assumptions:
	Risk-free interest rate		0.1-0.2	%
	Estimated term (years)		1.9
	Volatility		85	%
Securities Excluded from Calculation of Diluted Net Loss per Share	'
	For the three months ended March 31, 2014 and 2013, respectively, we excluded the following securities from the calculation of diluted net loss per share as the effect would have been antidilutive (in thousands):
		MARCH 31,
		2014			2013
	Convertible preferred stock		—			9,929
	Options to purchase common stock		2,213			2,638
	Warrants to purchase convertible preferred stock		—			84
			2,213			12,651

Cash_Equivalents_and_Marketabl1

Cash Equivalents and Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2014
Cash And Cash Equivalents [Abstract]	'
Summary of Cash Equivalents and Marketable Securities	'
	The following is a summary of our cash equivalents and marketable securities (in thousands):
		MARCH 31, 2014
		AMORTIZED			UNREALIZED		UNREALIZED			ESTIMATED
		COST BASIS			GAINS		LOSSES			FAIR VALUE
		(unaudited)
	Money market funds	$	8,533		$	—	$	—		$	8,533
	U.S. Treasury securities		82,862			15		(1	)		82,876
	U.S. government agency securities		34,369			7		(2	)		34,374
			125,764			22		(3	)		125,783
	Less: cash equivalents		(8,533	)		—		—			(8,533	)
	Total marketable securities	$	117,231		$	22	$	(3	)	$	117,250
		DECEMBER 31, 2013
		AMORTIZED			UNREALIZED		UNREALIZED			ESTIMATED
		COST BASIS			GAINS		LOSSES			FAIR VALUE
	Money market funds	$	6,456		$	—	$	—		$	6,456
	U.S. Treasury securities		18,848			4		—			18,852
	U.S. government agency securities		48,709			3		(3	)		48,709
			74,013			7		(3	)		74,017
	Less: cash equivalents		(6,456	)		—		—			(6,456	)
	Total marketable securities	$	67,557		$	7	$	(3	)	$	67,561
Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Securities by Contractual Maturity	'
	As of March 31, 2014, the amortized cost and estimated fair value of our available-for-sale securities by contractual maturity are shown below (in thousands):
		Amortized			Estimated
		Cost			Fair Value
	Debt securities maturing:
	In one year or less	$	95,121		$	95,131
	In one to two years		30,643			30,652
	Total marketable securities	$	125,764		$	125,783

Equity_Incentive_Plans_Tables

Equity Incentive Plans (Tables)	3 Months Ended
	Mar. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'
Schedule of Option Activity under Stock Plans and Related Information	'
	The following table summarizes option activity under our stock plans and related information:
		OPTIONS OUTSTANDING
		NUMBER			WEIGHTED-
		OF SHARES			AVERAGE
					EXERCISE
					PRICE
					PER SHARE
	Balance at December 31, 2013		2,236,997		$	6.09
	Options granted		8,520		$	14.36
	Options exercised		(133,348	)	$	5.26
	Options forfeited		(17,897	)	$	7.2
	Balance at March 31, 2014		2,094,272		$	6.17
	Options exercisable		1,254,272		$	5.63
Schedule of Stock-Based Compensation Expenses Recognized	'
	Total stock-based compensation expense recognized was as follows (in thousands):
		Three Months Ended
		March 31,
		2014			2013
	Research and development	$	360		$	212
	General and administrative		276			412
	Total	$	636		$	624
Schedule of Stock Option Assumptions	'
	We estimated the fair value of each stock option using the Black-Scholes option-pricing model based on the date of grant of such stock option with the following assumptions:
		Three Months Ended
		March 31,
		2014			2013
	Expected term (years)		6.1		5.0-6.1
	Expected volatility		85	%	85%
	Risk-free interest rate		1.8	%	0.8-1.1%
	Expected dividend yield		0	%	0%

Description_of_Business_Additi

Description of Business - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Feb. 28, 2014	Mar. 31, 2014
		Segment
Regulatory Assets [Abstract]	'	'
Number of operating segments	'	1
Number of common stock issued through Public Offering	3,450,000	'
Net proceeds after underwriting discounts, commissions and offering expenses	'	$40,104

Summary_of_Significant_Account2

Summary of Significant Accounting Policies - Summary of Assets and Liabilities Recorded at Fair Value (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Money market funds [Member]	'	'
Assets	'	'
Assets	$8,533	$6,456
U.S. Treasury securities [Member]	'	'
Assets	'	'
Assets	82,876	18,852
U.S. government agency securities [Member]	'	'
Assets	'	'
Assets	34,374	48,709
Total cash equivalents and marketable securities [Member]	'	'
Assets	'	'
Assets	125,783	74,017
Level 1 [Member] | Money market funds [Member]	'	'
Assets	'	'
Assets	8,533	6,456
Level 1 [Member] | U.S. Treasury securities [Member]	'	'
Assets	'	'
Assets	82,876	18,852
Level 1 [Member] | U.S. government agency securities [Member]	'	'
Assets	'	'
Assets	'	'
Level 1 [Member] | Total cash equivalents and marketable securities [Member]	'	'
Assets	'	'
Assets	91,409	25,308
Level 2 [Member] | Money market funds [Member]	'	'
Assets	'	'
Assets	'	'
Level 2 [Member] | U.S. Treasury securities [Member]	'	'
Assets	'	'
Assets	'	'
Level 2 [Member] | U.S. government agency securities [Member]	'	'
Assets	'	'
Assets	34,374	48,709
Level 2 [Member] | Total cash equivalents and marketable securities [Member]	'	'
Assets	'	'
Assets	34,374	48,709
Level 3 [Member] | Money market funds [Member]	'	'
Assets	'	'
Assets	'	'
Level 3 [Member] | U.S. Treasury securities [Member]	'	'
Assets	'	'
Assets	'	'
Level 3 [Member] | U.S. government agency securities [Member]	'	'
Assets	'	'
Assets	'	'
Level 3 [Member] | Total cash equivalents and marketable securities [Member]	'	'
Assets	'	'
Assets	'	'

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Net number of warrants exercised	4,376

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Change in Estimated Fair Value of Preferred Stock Warrant Liability (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2013
Fair Value Disclosures [Abstract]	'
Balance, beginning of year	$563
Change in fair value recorded in other income, net	-285
Balance, end of year	$278

Summary_of_Significant_Account5

Summary of Significant Accounting Policies - Fair Value of Warrants Assumptions (Detail)	3 Months Ended
	Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'
Estimated term (years)	'1 year 10 months 24 days
Volatility	85.00%
Minimum [Member]	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'
Risk-free interest rate	0.10%
Maximum [Member]	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'
Risk-free interest rate	0.20%

Summary_of_Significant_Account6

Summary of Significant Accounting Policies - Securities Excluded from Calculation of Diluted Net Loss per Share (Detail)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'
Securities excluded from calculation of diluted net loss per share	2,213	12,651
Convertible preferred stock [Member]	'	'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'
Securities excluded from calculation of diluted net loss per share	'	9,929
Options to purchase common stock [Member]	'	'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'
Securities excluded from calculation of diluted net loss per share	2,213	2,638
Warrants to purchase convertible preferred stock [Member]	'	'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'
Securities excluded from calculation of diluted net loss per share	'	84

Cash_Equivalents_and_Marketabl2

Cash Equivalents and Marketable Securities - Summary of Cash Equivalents and Marketable Securities (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
AMORTIZED COST BASIS	$117,231	$67,557
UNREALIZED GAINS	22	7
UNREALIZED LOSSES	-3	-3
ESTIMATED FAIR VALUE	117,250	67,561
Money market funds [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
AMORTIZED COST BASIS	8,533	6,456
UNREALIZED GAINS	'	'
UNREALIZED LOSSES	'	'
ESTIMATED FAIR VALUE	8,533	6,456
U.S. Treasury securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
AMORTIZED COST BASIS	82,862	18,848
UNREALIZED GAINS	15	4
UNREALIZED LOSSES	-1	'
ESTIMATED FAIR VALUE	82,876	18,852
U.S. government agency securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
AMORTIZED COST BASIS	34,369	48,709
UNREALIZED GAINS	7	3
UNREALIZED LOSSES	-2	-3
ESTIMATED FAIR VALUE	34,374	48,709
Marketable securities including cash equivalents [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
AMORTIZED COST BASIS	125,764	74,013
UNREALIZED GAINS	22	7
UNREALIZED LOSSES	-3	-3
ESTIMATED FAIR VALUE	125,783	74,017
Cash equivalents [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
AMORTIZED COST BASIS	-8,533	-6,456
UNREALIZED GAINS	'	'
UNREALIZED LOSSES	'	'
ESTIMATED FAIR VALUE	($8,533)	($6,456)

Cash_Equivalents_and_Marketabl3

Cash Equivalents and Marketable Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) (USD $)	Mar. 31, 2014
In Thousands, unless otherwise specified
Debt securities maturing:	'
Amortized Cost, In one year or less	$95,121
Amortized Cost, In one to two years	30,643
Total marketable securities, Amortized Cost	125,764
Estimated Fair Value, In one year or less	95,131
Estimated Fair Value, In one to two years	30,652
Total marketable securities, Estimated Fair Value	$125,783

Cash_Equivalents_and_Marketabl4

Cash Equivalents and Marketable Securities - Additional Information (Detail) (Marketable securities [Member], USD $)	3 Months Ended
	Mar. 31, 2014
Marketable securities [Member]	'
Schedule of Available-for-sale Securities [Line Items]	'
Gross unrealized losses on marketable securities	$3,000
Sales of available-for-sale securities	$0

Equity_Incentive_Plans_Schedul

Equity Incentive Plans - Schedule of Option Activity under Stock Plans and Related Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'
NUMBER OF SHARES, Beginning balance	2,236,997
NUMBER OF SHARES, Options granted	8,520
NUMBER OF SHARES, Options exercised	-133,348
NUMBER OF SHARES, Options forfeited	-17,897
NUMBER OF SHARES, Ending balance	2,094,272
NUMBER OF SHARES, Options exercisable	1,254,272
WEIGHTED-AVERAGE EXERCISE PRICE PER SHARE, Options beginning balance	$6.09
WEIGHTED-AVERAGE EXERCISE PRICE PER SHARE, Options granted	$14.36
WEIGHTED-AVERAGE EXERCISE PRICE PER SHARE, Options exercised	$5.26
WEIGHTED-AVERAGE EXERCISE PRICE PER SHARE, Options forfeited	$7.20
WEIGHTED-AVERAGE EXERCISE PRICE PER SHARE, Options ending balance	$6.17
WEIGHTED-AVERAGE EXERCISE PRICE PER SHARE, Options exercisable	$5.63

Equity_Incentive_Plans_Additio

Equity Incentive Plans - Additional Information (Detail) (USD $)	3 Months Ended	1 Months Ended
	Mar. 31, 2014	Feb. 28, 2013	Feb. 28, 2013	Feb. 28, 2013	Mar. 31, 2014
		Incremental stock-based compensation expenses [Member]	Incremental stock-based compensation expenses [Member]	Incremental stock-based compensation expenses [Member]	2013 Omnibus Incentive Plan [Member]
			Minimum [Member]	Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'	'
Shares available for future issuance	'	'	'	'	4,149,532
Stock options to purchase common stock outstanding, vested	2,069,087	'	'	'	'
Stock options to purchase common stock outstanding, weighted-average exercise price	$6.16	'	'	'	'
Options outstanding, weighted-average remaining contractual term	'7 years 1 month 6 days	'	'	'	'
Weighted-average remaining contractual term options exercisable	'6 years 2 months 12 days	'	'	'	'
Stock option exercise period	'	'	'18 months	'20 months	'
Incremental stock-based compensation expense	'	$157,000	'	'	'
Total unrecognized compensation expense related to nonvested employee and director stock options	$3,700,000	'	'	'	'
Unrecognized compensation expense expected to recognize, weighted-average period	'2 years 7 months 6 days	'	'	'	'

Equity_Incentive_Plans_Schedul1

Equity Incentive Plans - Schedule of Stock-Based Compensation Expenses Recognized (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]	'	'
Stock-based compensation expense recognized	$636	$624
Research and development [Member]	'	'
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]	'	'
Stock-based compensation expense recognized	360	212
General and administrative [Member]	'	'
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]	'	'
Stock-based compensation expense recognized	$276	$412

Equity_Incentive_Plans_Schedul2

Equity Incentive Plans - Schedule of Stock Option Assumptions (Detail) (Options [Member])	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2013
		Minimum [Member]	Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Expected term (years)	'6 years 1 month 6 days	'5 years	'6 years 1 month 6 days
Expected volatility	85.00%	'	85.00%
Risk-free interest rate	1.80%	0.80%	1.10%
Expected dividend yield	0.00%	'	0.00%

Collaborative_Research_and_Dev1

Collaborative Research and Development Agreements - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	0 Months Ended	3 Months Ended		1 Months Ended
	Feb. 28, 2014	Mar. 31, 2014	Mar. 14, 2014	Mar. 31, 2014	Mar. 31, 2014	Apr. 30, 2014
			Bristol-Myers Squibb Company [Member]	Bristol-Myers Squibb Company [Member]	Bristol-Myers Squibb Company [Member]	Bristol-Myers Squibb Company [Member]
					Immuno-oncology collaboration [Member]	Subsequent Events [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'	'	'
Upfront payment	'	'	'	'	'	$20,000,000
Research funding received	'	'	9,500,000	'	0	'
Research agreement term	'	'	'3 years	'	'	'
Research agreement additional term	'	'	'2 years	'	'	'
Research agreement additional term description	'	'	'BMS may extend the research term for two additional one-year periods on a year-by-year basis, during which extensions we would be obligated to perform additional services as agreed to with BMS and BMS would be obligated to pay us research funding with respect to such services.	'	'	'
Deferred revenue recognition research period	'	'	'	'5 years	'	'
Number of common stock issued through public offering	3,450,000	'	'	'	994,352	'
Price per share	'	'	'	'	$21.16	'
Aggregate purchase price	'	18,639,000	'	'	21,000,000	'
Deferred revenue recognized under collaboration arrangement	'	'	'	2,400,000	22,200,000	'
Recognized revenue under collaboration arrangement	'	'	'	'	$200,000	'

Subsequent_Events_Additional_I

Subsequent Events - Additional Information (Detail) (GSK UK [Member], Subsequent Events [Member], USD $)	1 Months Ended
	Apr. 30, 2014
Business And Summary Of Significant Accounting Policies [Line Items]	'
Additional research funding received	$2,000,000
16 Equal quarterly payments [Member]	'
Business And Summary Of Significant Accounting Policies [Line Items]	'
Additional research funding received	$1,000,000