Arena Pharmaceuticals (ARNA) 10-Q 9 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2017	May 05, 2017
Document And Entity Information [Abstract]
Entity Registrant Name	ARENA PHARMACEUTICALS INC
Trading Symbol	ARNA
Entity Central Index Key	1,080,709
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		317,435,572

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 79,529	$ 90,712
Accounts receivable	1,684	20,162
Inventory	7,055	6,708
Prepaid expenses and other current assets	4,069	2,307
Total current assets	92,337	119,889
Land, property and equipment, net	42,512	43,828
Intangibles, net	2,103	2,357
Other non-current assets	2,808	2,936
Total assets	139,760	169,010
Current liabilities:
Accounts payable and other accrued liabilities	8,579	12,116
Accrued clinical and preclinical study fees	4,429	3,883
Payable to Eisai		9,074
Current portion of deferred revenues	33,170	35,288
Current portion of lease financing obligations	3,662	3,518
Total current liabilities	49,840	63,879
Other long-term liabilities	809	821
Deferred revenues, less current portion	1,667	2,167
Lease financing obligations, less current portion	60,788	61,748
Commitments and contingencies
Equity:
Common stock	24	24
Additional paid-in capital	1,449,712	1,441,715
Accumulated other comprehensive loss	(2,295)	(3,099)
Accumulated deficit	(1,420,832)	(1,398,736)
Total equity attributable to stockholders of Arena	26,609	39,904
Equity attributable to noncontrolling interest in consolidated variable interest entity	47	491
Total equity	26,656	40,395
Total liabilities and equity	$ 139,760	$ 169,010

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenues
Net product sales	$ 2,711	$ 3,518
Toll manufacturing	718	1,023
Total revenues	6,624	9,847
Operating Costs and Expenses
Cost of product sales	2,532	2,428
Cost of toll manufacturing	919	1,188
Research and development	15,511	18,502
General and administrative	8,164	6,924
Total operating costs and expenses	27,126	29,042
Loss from operations	(20,502)	(19,195)
Interest and Other Income (Expense)
Interest income	34	88
Interest expense	(1,570)	(1,679)
Other	(459)	(762)
Total interest and other expense, net	(1,995)	(2,353)
Net loss	(22,497)	(21,548)
Less net loss attributable to noncontrolling interest in consolidated variable interest entity	444
Net loss attributable to stockholders of Arena	$ (22,053)	$ (21,548)
Net loss attributable to stockholders of Arena per share:
Basic	$ (0.09)	$ (0.09)
Diluted	$ (0.09)	$ (0.09)
Shares used in calculating net loss attributable to stockholders of Arena per share:
Basic	244,822	242,876
Diluted	244,822	242,876
Comprehensive Loss:
Net loss	$ (22,497)	$ (21,548)
Foreign currency translation gain (loss)	804	2,591
Comprehensive loss	(21,693)	(18,957)
Less comprehensive loss attributable to noncontrolling interest in consolidated variable interest entity	444
Comprehensive loss attributable to stockholders of Arena	(21,249)	(18,957)
Eisai
Revenues
Other collaboration revenue	1,535	3,226
Other
Revenues
Other collaboration revenue	$ 1,660	$ 2,080

Condensed Consolidated Stateme4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating Activities:
Net loss	$ (22,497)	$ (21,548)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,531	2,356
Amortization of intangibles	300	67
Share-based compensation	1,838	2,809
Amortization of prepaid financing costs	34	34
Gain on disposal of property and equipment		(135)
Changes in operating assets and liabilities:
Accounts receivable	18,823	(1,067)
Inventory	(216)	973
Prepaid expenses and other assets	(908)	(1,112)
Payables and accrued liabilities	(12,337)	(966)
Deferred revenues	(2,813)	986
Deferred rent	(12)	24
Net cash used in operating activities	(16,257)	(17,579)
Investing Activities:
Purchases of property and equipment	(49)	(247)
Proceeds from sale of property and equipment		135
Other non-current assets	94
Net cash provided by (used in) investing activities	45	(112)
Financing Activities:
Principal payments on lease financing obligations	(816)	(687)
Proceeds from issuance of common stock, net	5,267	127
Net cash provided by (used in) financing activities	4,451	(560)
Effect of exchange rate changes on cash	578	1,600
Net decrease in cash and cash equivalents	(11,183)	(16,651)
Cash and cash equivalents at beginning of period	90,712	156,184
Cash and cash equivalents at end of period	$ 79,529	$ 139,533

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	1. Basis of Presentation The accompanying unaudited condensed consolidated financial statements of Arena Pharmaceuticals, Inc. should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2016, as filed with the Securities and Exchange Commission, or SEC, from which we derived our condensed consolidated balance sheet as of December 31, 2016. The accompanying condensed consolidated financial statements have been prepared in accordance with US generally accepted accounting principles, or GAAP, for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, since they are interim statements, the accompanying condensed consolidated financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, that are, in the opinion of our management, necessary to a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year. The accompanying consolidated financial statements include the balances and activity of our wholly owned subsidiaries and Beacon Discovery, Inc., or Beacon, a variable interest entity in which we have the controlling financial interest (see Note 13). The equity attributable to the noncontrolling interest in Beacon is presented as a separate component from the equity attributable to stockholders of Arena in the equity section of the condensed consolidated balance sheets. The results of operations and comprehensive loss attributable to the noncontrolling interest in Beacon are presented as separate components from the results of operations and comprehensive loss attributable to the stockholders of Arena in the condensed consolidated statements of operations and comprehensive loss. Liquidity. As of March 31, 2017, we had cash and cash equivalents of approximately $79.5 million. Subsequent to March 31, 2017, we raised approximately $75.5 million of net proceeds from sales of our common stock (see Note 7). We believe our cash and cash equivalents will be sufficient to fund our operations for at least the next 12 months. It will require substantial cash to achieve our objectives of discovering, developing and commercializing drugs, and this process typically takes many years and potentially several hundreds of millions of dollars for an individual drug. We may not have adequate available cash, or assets that could be readily turned into cash, to meet these objectives in the long term. We will need to obtain significant funds under our existing collaborations, under new collaboration, licensing or other commercial agreements for one or more of our drug candidates and programs or patent portfolios, or from other potential sources of liquidity, which may include the sale of equity, issuance of debt or other transactions. Recent Accounting Pronouncements Revenue Recognition. In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, Revenue from Contracts with Customers The new guidance allows for two methods of adoption: (a) “full retrospective” adoption, meaning the standard is applied to all periods presented, or (b) “modified retrospective” adoption, meaning the cumulative effect of applying the new guidance is recognized as an adjustment to the opening retained earnings balance for the year of implementation. We plan to adopt the new revenue standard effective January 1, 2018, on a modified retrospective method with the cumulative effect of the change reflected in retained earnings as of January 1, 2018. We have continued to monitor FASB activity to assess certain interpretative issues and the associated implementation of the new standard. We are in the process of reviewing our revenue arrangements, which we expect to include product sales, manufacturing support payments, royalty payments, other collaboration payments and toll manufacturing, and are not yet able to estimate the anticipated impact to our consolidated financial statements from the implementation of the new standard as we continue to interpret the principles of the new standard. Other. In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-02, Leases In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting. Use of Estimates. The preparation of financial statements in accordance with GAAP requires our management to make estimates and assumptions that affect the reported amounts (including assets, liabilities, revenues and expenses) and related disclosures. The amounts reported could differ under different estimates and assumptions.

Fair Value Disclosures

Fair Value Disclosures	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Disclosures	2. Fair Value Disclosures We measure our financial assets and liabilities at fair value, which is defined as the exit price, or the amount that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. We use the following three-level valuation hierarchy that maximizes the use of observable inputs and minimizes the use of unobservable inputs to value our financial assets and liabilities: Level 1 - Observable inputs such as unadjusted quoted prices in active markets for identical instruments. Level 2 - Quoted prices for similar instruments in active markets or inputs that are observable for the asset or liability, either directly or indirectly. Level 3 - Significant unobservable inputs based on our assumptions. The following tables present our valuation hierarchy for our financial assets and liabilities that are measured at fair value on a recurring basis, in thousands: Fair Value Measurements at March 31, 2017 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds 1 $ 21,405 $ 21,405 $ — $ — Fair Value Measurements at December 31, 2016 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds 1 $ 46,371 $ 46,371 $ — $ — (1) Included in cash and cash equivalents on our condensed consolidated balance sheets.

Inventory

Inventory	3 Months Ended
	Mar. 31, 2017
Inventory Disclosure [Abstract]
Inventory	3. Inventory Inventory consisted of the following, in thousands: March 31, December 31, 2017 2016 Raw materials $ 2,882 $ 2,553 Work in process 3,955 3,943 Finished goods 218 212 Total inventory $ 7,055 $ 6,708

Land, Property and Equipment

Land, Property and Equipment	3 Months Ended
	Mar. 31, 2017
Property Plant And Equipment [Abstract]
Land, Property and Equipment	4. Land, Property and Equipment Land, property and equipment consisted of the following, in thousands: March 31, December 31, 2017 2016 Cost $ 108,574 $ 108,356 Less accumulated depreciation and amortization (66,062 ) (64,528 ) Land, property and equipment, net $ 42,512 $ 43,828

Accounts Payable and Other Accr

Accounts Payable and Other Accrued Liabilities	3 Months Ended
	Mar. 31, 2017
Payables And Accruals [Abstract]
Accounts Payable and Other Accrued Liabilities	5. Accounts Payable and Other Accrued Liabilities Accounts payable and other accrued liabilities consisted of the following, in thousands: March 31, December 31, 2017 2016 Accounts payable $ 3,340 $ 5,977 Accrued compensation 3,625 4,820 Other accrued liabilities 1,614 1,319 Total accounts payable and other accrued liabilities $ 8,579 $ 12,116

Collaborations

Collaborations	3 Months Ended
	Mar. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	6. Collaborations Please refer to our Annual Report on Form 10-K for the year ended December 31, 2016, for additional information regarding the collaborations described below. Eisai. In July 2010, we granted Eisai exclusive commercialization rights for lorcaserin solely in the United States and its territories and possessions. In May 2012, we and Eisai entered into the first amended and restated agreement, which expanded Eisai’s exclusive commercialization rights to include most of North and South America. In November 2013, we and Eisai entered into the second amended and restated agreement, or Second Amended Agreement, which expanded Eisai’s exclusive commercialization rights for lorcaserin to all of the countries in the world, except for South Korea, Taiwan, Australia, New Zealand and Israel. On December 28, 2016, we and Eisai amended and restated the terms of the Second Amended Agreement by entering into the Eisai Agreement, which was determined to be a material modification of the Second Amended Agreement. Under the Eisai Agreement, we identified the following significant deliverables to Eisai which each qualify as a separate unit of accounting: • An exclusive royalty-bearing license or transfer of intellectual property, or License, to commercialize lorcaserin world-wide relating to certain patents, regulatory approvals, samples, records, know-how related to lorcaserin, trademarks and domain names related to the lorcaserin brand names. We also assigned to Eisai our rights under the commercial lorcaserin distribution agreements with Ildong for South Korea, CYB for Taiwan and Teva for Israel. This is collectively referred to as the License Deliverable. • A manufacturing and supply commitment for two years commencing December 28, 2016, or Manufacturing and Supply Commitment Deliverable. • Bulk inventory and precursor material for manufacturing lorcaserin, or Inventory Deliverable. Royalty payments. Pursuant to the Eisai Agreement, we are eligible to receive royalty payments from Eisai based on the global net sales of lorcaserin. The royalty rates are as follows: • 9.5% on annual net sales less than or equal to $175.0 million • 13.5% on annual net sales greater than $175.0 million but less than or equal to $500.0 million • 18.5% of annual net sales greater than $500.0 million Manufacturing and supply commitment and inventory purchase. We manufacture lorcaserin at our facility in Zofingen, Switzerland. Under the Eisai Agreement, we have agreed to manufacture and supply, and Eisai has agreed to purchase from us, all of Eisai’s requirements (or specified minimum quantities if such quantities are greater than Eisai’s requirements), subject to certain exceptions, for lorcaserin for development and commercial use for an initial two-year period. The initial period may be extended by Eisai for an additional six months upon payment of an extension fee of CHF 2.0 million. Eisai will pay us agreed upon prices to deliver finished drug product during this time. Additionally, Eisai has agreed to pay up to CHF 13.0 million in manufacturing support payments during the initial two-year period supply period, and pay up to CHF 6.0 million in manufacturing support payments during the six-month extension period, if the extension option is exercised by Eisai. On December 28, 2016, Eisai paid us $10.0 million to acquire our entire inventory of bulk lorcaserin and the precursor materials for manufacturing lorcaserin. This payment was included in the arrangement consideration allocated to the units of accounting under the Eisai Agreement. We expect this inventory will remain at our Zofingen, Switzerland facility for us to use to manufacture finished drug product in order to meet Eisai’s requirements during the initial two-year period and, if applicable, the six-month extension period. The inventory that is not expected to be used to manufacture finished drug product will be physically transferred to Eisai upon the earlier of Eisai’s request to transfer or the end of the manufacturing and supply commitment period. Allocation of Eisai Agreement arrangement consideration to the units of accounting. The total arrangement consideration of $115.6 million primarily consists of (i) the December 28, 2016, balances of deferred revenues from the upfront payments received under the prior Eisai agreements and the distribution agreements with Ildong, CYB and Teva, which were assigned to Eisai; (ii) the $10.0 million payment received from Eisai on December 28, 2016; and (iii) the product purchase payments and manufacturing support payments we expect to receive from Eisai for the initial two-year manufacturing and supply commitment period. All of the deliverables were determined to have standalone value and to meet the criteria to be accounted for as separate units of accounting. Factors considered in the determination included, among other things, for the license, the manufacturing experience and capabilities of Eisai and their sublicense rights, and for the remaining deliverables the fact that they are not proprietary and can be provided by other vendors. The total arrangement consideration was allocated to the units of accounting on the basis of their relative estimated selling prices as follows: • $64.0 million was allocated to the License Deliverable. As the License Deliverable was delivered on December 28, 2016, this amount was recognized as revenue in 2016. • $30.8 million was allocated to the Inventory Deliverable. Title to this entire inventory passed to Eisai on December 28, 2016. However, none of this inventory was physically transferred from the manufacturing facility, and there is no fixed schedule for delivery given some will be delivered on a continuous basis as we perform under the manufacturing commitment while the rest will be physically transferred to Eisai upon request by Eisai or upon the end of the manufacturing and supply commitment period. Also, the risks of ownership for this inventory have not been fully passed to Eisai as we will continue to have financial responsibility for loss, damage or destruction which occurs while in our possession. Therefore, none of the arrangement consideration allocated to this deliverable was recognized as revenue and none of the carrying value of this inventory was recognized as cost of product sales for the year ended December 31, 2016. For the three months ended March 31, 2017, we recognized revenue from net product sales related to the Inventory Deliverable of $2.2 million and cost of product sales of $0.3 million related to this inventory. • $20.8 million was allocated to the Manufacturing and Supply Commitment Deliverable. This deliverable will be provided over 2017 and 2018 as product is shipped to Eisai. For the three months ended March 31, 2017, we recognized $2.1 million as revenue for the arrangement consideration allocated to this deliverable, of which $0.5 million is classified as net product sales and $1.6 million of manufacturing support payments is classified as other Eisai collaboration revenue. The condensed consolidated balance sheet at March 31, 2017, includes deferred revenues of $29.4 million relating to the Eisai Agreement (primarily comprised of the deferred portion of the previously received upfront payments and the $10.0 million payment received from Eisai on December 28, 2016). Included in our ending inventory balance at March 31, 2017 of $7.1 million is $4.1 million related to the carrying value of the remaining product on-hand under the Inventory Deliverable. These balances are expected to be recognized in subsequent periods as this inventory is used in the manufacture and supply of lorcaserin to Eisai over the commitment period. Axovant Sciences Ltd. We and Axovant Sciences, Ltd., or Axovant, have an exclusive agreement, or Axovant Agreement, under which Axovant has exclusive worldwide rights to develop and commercialize nelotanserin, subject to regulatory approval. We also provide certain services and will manufacture and sell nelotanserin to Axovant. Under the Axovant Agreement, we received an upfront payment of $4.0 million in May 2015, which was recorded as deferred revenues and is being recognized as revenue ratably over approximately five years, which is the period in which we expect to provide services under the arrangement. We will receive payments from sales of nelotanserin under the Axovant Agreement and are eligible to receive purchase price adjustment payments based on Axovant’s annual net product sales. We are eligible to receive up to an aggregate of $41.5 million in success milestones in case of full development and regulatory success of nelotanserin. Of these payments, two development milestones totaling $4.0 million are substantive and four regulatory milestones totaling $37.5 million are substantive. For the three months ended March 31, 2017 and 2016, we recorded revenue of $0.4 million and $0.6 million, respectively related to the Axovant Agreement. Boehringer Ingelheim International GmbH. We and Boehringer Ingelheim GmbH, or Boehringer Ingelheim, have an exclusive agreement, or Boehringer Ingelheim Agreement, to conduct joint research to identify drug candidates targeting an undisclosed G protein-coupled receptor, or GPCR, that belongs to the group of orphan central nervous system, or CNS, receptors. In part consideration of the rights to our intellectual property necessary or useful to conduct the joint research under the Boehringer Ingelheim Agreement, we received from Boehringer Ingelheim an upfront payment of $7.5 million in January 2016, less $1.2 million of withholding taxes which was refunded to us in October 2016. Revenues from this upfront payment were deferred, as we determined that the exclusive rights did not have standalone value without our ongoing participation in the joint research, and are being recognized ratably as revenues over the period in which we expect the services to be rendered, which is approximately two years. Under the Boehringer Ingelheim Agreement, we are eligible to receive up to an aggregate of $251.0 million in success milestones in case of full commercial success of multiple drug products. Of these payments, three development milestones totaling $7.0 million are substantive, three development $30.0 million are non-substantive, nine regulatory milestones totaling $84.0 million are non-substantive and four commercial milestones totaling $130.0 million are non-substantive. For the three months ended March 31, 2017 and 2016, we recorded revenue of $1.2 million and $1.3 million, respectively related to the Boehringer Ingelheim Agreement.

Stockholders' Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2017
Stockholders Equity Note [Abstract]
Stockholders' Equity	7. Stockholders’ Equity On January 4, 2017, we entered into an Equity Distribution Agreement, or the ATM, with Citigroup Global Markets, Inc., or the Sales Agent, under which we may offer and sell common stock having an aggregate offering price of up to $50.0 million from time to time though our Sales Agent. We will pay the Sales Agent a commission of up to 3% of the gross proceeds. We have also agreed to reimburse the Sales Agent for certain specified expenses not to exceed $50,000. Sales of the shares under the ATM were made in transactions that are deemed to be “at-the-market” offerings as defined in Rule 415 under the Securities Act of 1933, as amended, including sales made by means of ordinary brokers’ transactions, including on the NASDAQ Stock Market. sold 4,264,970 shares of million . Subsequent to March 31, 2017, we sold an additional 625,262 shares of ATM 0.9 million On April 17, 2017, we entered into an underwriting agreement with Citigroup Global Markets Inc. and Leerink Partners LLC, for the sale of 60,000,000 shares of our common stock at the public offering price of $1.15 per share, less underwriter discounts and commissions of 6% of the per share offering price. The agreement also allowed for the purchase of up to an additional 9,000,000 shares at the option of the underwriters, which has been exercised. On April 21, 2017, we completed the sale of an aggregate of 69,000,000 shares of our common stock under the underwritten public offering. Net proceeds from the offering were approximately $74.6 million after deducting underwriting discounts and commissions, and offering expenses payable by us. We intend to use the net proceeds from this offering for the clinical and preclinical development of drug candidates, for general corporate purposes, including working capital and costs associated with manufacturing services, and for capital expenditures.

Share-based Compensation

Share-based Compensation	3 Months Ended
	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based Compensation	8. Share-based Compensation We recognized share-based compensation expense as follows, in thousands: Three months ended March 31, 2017 2016 Cost of product sales $ 51 $ 20 Research and development 379 1,763 General and administrative 1,408 1,026 Total share-based compensation expense $ 1,838 $ 2,809 Total share-based compensation expense capitalized into inventory $ — $ 37 The following table summarizes our stock option activity during the three months ended March 31, 2017, in thousands (except per share data): Options Weighted- Average Exercise Price Outstanding at January 1, 2017 25,200 $ 3.03 Granted 15,325 1.49 Exercised (15 ) 1.28 Forfeited/cancelled/expired (1,076 ) 9.00 Outstanding at March 31, 2017 39,434 $ 2.27 The following table summarizes activity with respect to our time-based restricted stock unit awards, or RSUs, during the three months ended March 31, 2017, in thousands (except per share data): RSUs Weighted- Average Grant-Date Fair Value Unvested at January 1, 2017 25 $ 4.26 Granted — Vested — Forfeited/cancelled — Unvested at March 31, 2017 25 $ 4.26 During the three months ended March 31, 2017, the remaining Total Stockholder Return, or TSR, performance restricted stock unit, or PRSU, awards that we granted to our executive officers in March 2014 were forfeited without any earnout based on the TSR of our common stock relative to the TSR of the NASDAQ Biotechnology Index over the three-year performance period that began on March 1, 2014. In the aggregate, the target number of shares of common stock that could have been earned under the PRSUs granted in March 2014 was 695,000. Of the target number of shares of 745,000 for PRSUs granted in March 2015, 355,556 have been cancelled due to management changes. All other PRSUs granted in March 2015 were outstanding and unvested at March 31, 2017.

Concentrations of Credit Risk a

Concentrations of Credit Risk and Major Customers	3 Months Ended
	Mar. 31, 2017
Risks And Uncertainties [Abstract]
Concentrations of Credit Risk and Major Customers	9. Concentrations of Credit Risk and Major Customers Financial instruments, which potentially subject us to concentrations of credit risk, consist primarily of cash and cash equivalents. We limit our exposure to credit loss by holding our cash primarily in US dollars or, from time to time, placing our cash and investments in US government, agency and government-sponsored enterprise obligations and in corporate debt instruments that are rated investment grade, in accordance with an investment policy approved by our Board of Directors. The United States and South Korea are the only jurisdictions for which BELVIQ has been commercially sold. We also produce drug products for Siegfried AG, or Siegfried, and, to a lesser extent, another third party under toll manufacturing agreements. Percentages of our total revenues are as follows: Three months ended March 31, 2017 2016 Eisai Agreement (See Note 6) 64.1 % 57.7 % Boehringer Ingelheim Agreement (See Note 6) 18.2 % 13.6 % Toll manufacturing agreements 10.8 % 10.4 % Axovant Agreement (See Note 6) 6.6 % 6.0 % Other collaboration agreements 0.3 % 12.3 % Total percentage of revenues 100.0 % 100.0 %

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Net Loss Per Share	10. Net Loss Per Share We calculate basic and diluted net loss attributable to stockholders of Arena per share using the weighted-average number of shares of common stock outstanding during the period. Since we are in a net loss position, in addition to excluding potentially dilutive out-of-the money securities, we exclude from our calculation of diluted net loss attributable to stockholders of Arena per share all potentially dilutive in-the-money (i) stock options, (ii) RSUs, (iii) PRSUs and (iv) unvested restricted stock in our deferred compensation plan, and our diluted net loss per share is the same as our basic net loss per share. The following table presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net loss attributable to stockholders of Arena per share, in thousands: Three months ended March 31, 2017 2016 Stock options 31,643 19,091 RSUs and unvested restricted stock 88 314 Total 31,731 19,405 Because the market conditions for the PRSUs were not satisfied at March 31, 2017, or March 31, 2016, such securities are excluded from the table above.

Legal Proceedings

Legal Proceedings	3 Months Ended
	Mar. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Legal Proceedings	11. Legal Proceedings Beginning on September 20, 2010, a number of complaints were filed in the US District Court for the Southern District of California, or District Court, against us and certain of our current and former employees and directors on behalf of certain purchasers of our common stock. The complaints were brought as purported stockholder class actions, and, in general, include allegations that we and certain of our current and former employees and directors violated federal securities laws by making materially false and misleading statements regarding our BELVIQ program, thereby artificially inflating the price of our common stock. The plaintiffs sought unspecified monetary damages and other relief. On August 8, 2011, the District Court consolidated the actions and appointed a lead plaintiff and lead counsel. On November 1, 2011, the lead plaintiff filed a consolidated amended complaint. On March 28, 2013, the District Court dismissed the consolidated amended complaint without prejudice. On May 13, 2013, the lead plaintiff filed a second consolidated amended complaint. On November 5, 2013, the District Court dismissed the second consolidated amended complaint without prejudice as to all parties except for Robert E. Hoffman, who was dismissed from the action with prejudice. On November 27, 2013, the lead plaintiff filed a motion for leave to amend the second consolidated amended complaint. On March 20, 2014, the District Court denied plaintiff’s motion and dismissed the second consolidated amended complaint with prejudice. On April 18, 2014, the lead plaintiff filed a notice of appeal, and on August 27, 2014, the lead plaintiff filed his appellate brief in the US Court of Appeals for the Ninth Circuit, or Ninth Circuit. On October 24, 2014, we filed our answering brief in response to the lead plaintiff’s appeal. On December 5, 2014, the lead plaintiff filed his reply brief. A panel of the Ninth Circuit heard oral argument on the appeal on May 4, 2016. On October 26, 2016, the Ninth Circuit panel reversed the District Court’s dismissal of the second consolidated amended complaint and remanded the case back to the District Court for further proceedings. On January 25, 2017, the District Court permitted us to submit a renewed motion to dismiss the second consolidated amended complaint. On February 2, 2017, we filed the renewed motion to dismiss. On February 23, 2017, the lead plaintiff filed his opposition, and on March 2, 2017, we filed our reply. On April 28, 2017, the District Court denied our renewed motion to dismiss. On September 30, 2016, we and Eisai Inc. filed a patent infringement lawsuit against Lupin Limited and Lupin Pharmaceuticals, Inc. (collectively, Lupin) in the U.S. District Court for the District of Delaware. The lawsuit relates to a “Paragraph IV certification” notification that we and Eisai Inc. received regarding an abbreviated new drug application, or ANDA, submitted to the FDA by Lupin requesting approval to engage in the commercial manufacture, use, importation, offer for sale or sale of a generic version of BELVIQ ® ® Waxman On March 6, 2017, we and Eisai Inc. filed a patent infringement lawsuit against Teva Pharmaceuticals USA, Inc. and Teva Pharmaceutical Industries Ltd. (collectively, Teva) in the U.S. District Court for the District of Delaware. The lawsuit also relates to a “Paragraph IV certification” notification that we and Eisai Inc. received regarding an ANDA submitted to the FDA by Teva requesting approval to engage in the commercial manufacture, use, importation, offer for sale or sale of a generic version of BELVIQ XR ® release ®

Appointment of Chief Medical Of

Appointment of Chief Medical Officer	3 Months Ended
	Mar. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Appointment of Chief Medical Officer	12. Appointment of Chief Medical Officer In March 2017, our Board of Directors appointed Preston Klassen, M.D., M.H.S as our Executive Vice President, Research and Development and Chief Medical Officer. In connection with his appointment, our Board’s Compensation Committee approved an inducement stock option grant to Dr. Klassen to purchase 1,293,500 shares of our common stock under our 2013 Long-Term Incentive Plan, as amended. The nonstatutory stock options have a seven-year term and will vest over 4 years, with 25% of the shares subject to vesting one year after grant and the remainder of the shares vesting monthly over the following three years in equal installments, subject to his continued service through the applicable vesting dates and possible acceleration in specified circumstances.

Beacon Discovery, Inc.

Beacon Discovery, Inc.	3 Months Ended
	Mar. 31, 2017
Variable Interest Entity Disclosure [Abstract]
Beacon Discovery, Inc.	13. Beacon Discovery, Inc. On September 1, 2016, we entered into a As Beacon would not be able to finance its activities without the financial support we are providing pursuant to these agreements, Beacon is a variable interest entity. Arena does not own any equity in Beacon; however, as these agreements provide us the controlling financial interest in Beacon, we consolidate Beacon’s balances and activity within our condensed consolidated financial statements. The noncontrolling interest attributable to Beacon presented on our condensed consolidated financial statements is comprised of Beacon’s equity ownership interests as we do not own any voting interest in Beacon. The following table presents a reconciliation of the equity attributable to the stockholders of Arena and the equity attributable to Beacon, in thousands: Equity Attributable to Stockholders of Arena Equity Attributable to Noncontrolling Interest in Consolidated Variable Interest Entity Total Equity Balance at January 1, 2017 $ 39,904 $ 491 $ 40,395 Net loss (22,053 ) (444 ) (22,497 ) Translation gain 804 — 804 Other 7,954 — 7,954 Balance at March 31, 2017 $ 26,609 $ 47 $ 26,656 The following table presents the assets and liabilities of Beacon which are included in our condensed consolidated balance sheet at March 31, 2017, in thousands. The assets include only those assets that can be used to settle obligations of Beacon. The liabilities include third party liabilities of Beacon. As of March 31, 2017, Beacon had no creditors with recourse to the general credit of Arena. The assets and liabilities exclude intercompany balances that eliminate in consolidation: Assets of Beacon that can only be used to settle obligations of Beacon Cash and cash equivalents $ 306 Accounts receivable 10 Prepaid expense and other current assets 14 Land, property and equipment, net 617 Total assets of Beacon that can only be used to settle obligations of Beacon $ 947 Liabilities of Beacon for which creditors do not have recourse to the general credit of Arena Accounts payable and other accrued liabilities $ 135 Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena $ 135

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	14. Subsequent Events See Note 7 regarding the sale of shares of our common stock and Note 11 for the update to our legal proceedings which occurred subsequent to March 31, 2017.

Basis of Presentation (Policies

Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements of Arena Pharmaceuticals, Inc. should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2016, as filed with the Securities and Exchange Commission, or SEC, from which we derived our condensed consolidated balance sheet as of December 31, 2016. The accompanying condensed consolidated financial statements have been prepared in accordance with US generally accepted accounting principles, or GAAP, for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, since they are interim statements, the accompanying condensed consolidated financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, that are, in the opinion of our management, necessary to a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year. The accompanying consolidated financial statements include the balances and activity of our wholly owned subsidiaries and Beacon Discovery, Inc., or Beacon, a variable interest entity in which we have the controlling financial interest (see Note 13). The equity attributable to the noncontrolling interest in Beacon is presented as a separate component from the equity attributable to stockholders of Arena in the equity section of the condensed consolidated balance sheets. The results of operations and comprehensive loss attributable to the noncontrolling interest in Beacon are presented as separate components from the results of operations and comprehensive loss attributable to the stockholders of Arena in the condensed consolidated statements of operations and comprehensive loss.
Liquidity	Liquidity. As of March 31, 2017, we had cash and cash equivalents of approximately $79.5 million. Subsequent to March 31, 2017, we raised approximately $75.5 million of net proceeds from sales of our common stock (see Note 7). We believe our cash and cash equivalents will be sufficient to fund our operations for at least the next 12 months. It will require substantial cash to achieve our objectives of discovering, developing and commercializing drugs, and this process typically takes many years and potentially several hundreds of millions of dollars for an individual drug. We may not have adequate available cash, or assets that could be readily turned into cash, to meet these objectives in the long term. We will need to obtain significant funds under our existing collaborations, under new collaboration, licensing or other commercial agreements for one or more of our drug candidates and programs or patent portfolios, or from other potential sources of liquidity, which may include the sale of equity, issuance of debt or other transactions.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Revenue Recognition. In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, Revenue from Contracts with Customers The new guidance allows for two methods of adoption: (a) “full retrospective” adoption, meaning the standard is applied to all periods presented, or (b) “modified retrospective” adoption, meaning the cumulative effect of applying the new guidance is recognized as an adjustment to the opening retained earnings balance for the year of implementation. We plan to adopt the new revenue standard effective January 1, 2018, on a modified retrospective method with the cumulative effect of the change reflected in retained earnings as of January 1, 2018. We have continued to monitor FASB activity to assess certain interpretative issues and the associated implementation of the new standard. We are in the process of reviewing our revenue arrangements, which we expect to include product sales, manufacturing support payments, royalty payments, other collaboration payments and toll manufacturing, and are not yet able to estimate the anticipated impact to our consolidated financial statements from the implementation of the new standard as we continue to interpret the principles of the new standard. Other. In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-02, Leases In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting.
Use of Estimates	Use of Estimates. The preparation of financial statements in accordance with GAAP requires our management to make estimates and assumptions that affect the reported amounts (including assets, liabilities, revenues and expenses) and related disclosures. The amounts reported could differ under different estimates and assumptions.
Fair Value Disclosures	We measure our financial assets and liabilities at fair value, which is defined as the exit price, or the amount that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. We use the following three-level valuation hierarchy that maximizes the use of observable inputs and minimizes the use of unobservable inputs to value our financial assets and liabilities: Level 1 - Observable inputs such as unadjusted quoted prices in active markets for identical instruments. Level 2 - Quoted prices for similar instruments in active markets or inputs that are observable for the asset or liability, either directly or indirectly. Level 3 - Significant unobservable inputs based on our assumptions.
Concentration of Credit Risk and Major Customers	Financial instruments, which potentially subject us to concentrations of credit risk, consist primarily of cash and cash equivalents. We limit our exposure to credit loss by holding our cash primarily in US dollars or, from time to time, placing our cash and investments in US government, agency and government-sponsored enterprise obligations and in corporate debt instruments that are rated investment grade, in accordance with an investment policy approved by our Board of Directors. The United States and South Korea are the only jurisdictions for which BELVIQ has been commercially sold. We also produce drug products for Siegfried AG, or Siegfried, and, to a lesser extent, another third party under toll manufacturing agreements. Percentages of our total revenues are as follows: Three months ended March 31, 2017 2016 Eisai Agreement (See Note 6) 64.1 % 57.7 % Boehringer Ingelheim Agreement (See Note 6) 18.2 % 13.6 % Toll manufacturing agreements 10.8 % 10.4 % Axovant Agreement (See Note 6) 6.6 % 6.0 % Other collaboration agreements 0.3 % 12.3 % Total percentage of revenues 100.0 % 100.0 %
Net Loss Per Share	We calculate basic and diluted net loss attributable to stockholders of Arena per share using the weighted-average number of shares of common stock outstanding during the period. Since we are in a net loss position, in addition to excluding potentially dilutive out-of-the money securities, we exclude from our calculation of diluted net loss attributable to stockholders of Arena per share all potentially dilutive in-the-money (i) stock options, (ii) RSUs, (iii) PRSUs and (iv) unvested restricted stock in our deferred compensation plan, and our diluted net loss per share is the same as our basic net loss per share.

Fair Value Disclosures (Tables)

Fair Value Disclosures (Tables)	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present our valuation hierarchy for our financial assets and liabilities that are measured at fair value on a recurring basis, in thousands: Fair Value Measurements at March 31, 2017 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds 1 $ 21,405 $ 21,405 $ — $ — Fair Value Measurements at December 31, 2016 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds 1 $ 46,371 $ 46,371 $ — $ — (1) Included in cash and cash equivalents on our condensed consolidated balance sheets.

Inventory (Tables)

Inventory (Tables)	3 Months Ended
	Mar. 31, 2017
Inventory Disclosure [Abstract]
Components of Inventory	Inventory consisted of the following, in thousands: March 31, December 31, 2017 2016 Raw materials $ 2,882 $ 2,553 Work in process 3,955 3,943 Finished goods 218 212 Total inventory $ 7,055 $ 6,708

Land, Property and Equipment (T

Land, Property and Equipment (Tables)	3 Months Ended
	Mar. 31, 2017
Property Plant And Equipment [Abstract]
Land, Property and Equipment	Land, property and equipment consisted of the following, in thousands: March 31, December 31, 2017 2016 Cost $ 108,574 $ 108,356 Less accumulated depreciation and amortization (66,062 ) (64,528 ) Land, property and equipment, net $ 42,512 $ 43,828

Accounts Payable and Other Ac23

Accounts Payable and Other Accrued Liabilities (Tables)	3 Months Ended
	Mar. 31, 2017
Payables And Accruals [Abstract]
Accounts Payable and Other Accrued Liabilities	Accounts payable and other accrued liabilities consisted of the following, in thousands: March 31, December 31, 2017 2016 Accounts payable $ 3,340 $ 5,977 Accrued compensation 3,625 4,820 Other accrued liabilities 1,614 1,319 Total accounts payable and other accrued liabilities $ 8,579 $ 12,116

Share-based Compensation (Table

Share-based Compensation (Tables)	3 Months Ended
	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share Based Compensation Expense	We recognized share-based compensation expense as follows, in thousands: Three months ended March 31, 2017 2016 Cost of product sales $ 51 $ 20 Research and development 379 1,763 General and administrative 1,408 1,026 Total share-based compensation expense $ 1,838 $ 2,809 Total share-based compensation expense capitalized into inventory $ — $ 37
Summary of Stock Option Activity	The following table summarizes our stock option activity during the three months ended March 31, 2017, in thousands (except per share data): Options Weighted- Average Exercise Price Outstanding at January 1, 2017 25,200 $ 3.03 Granted 15,325 1.49 Exercised (15 ) 1.28 Forfeited/cancelled/expired (1,076 ) 9.00 Outstanding at March 31, 2017 39,434 $ 2.27
Summary of Restricted Stock Unit Awards Activity	The following table summarizes activity with respect to our time-based restricted stock unit awards, or RSUs, during the three months ended March 31, 2017, in thousands (except per share data): RSUs Weighted- Average Grant-Date Fair Value Unvested at January 1, 2017 25 $ 4.26 Granted — Vested — Forfeited/cancelled — Unvested at March 31, 2017 25 $ 4.26

Concentrations of Credit Risk25

Concentrations of Credit Risk and Major Customers (Tables)	3 Months Ended
	Mar. 31, 2017
Sales Revenue, Net
Concentration Risk [Line Items]
Percentages of Total Revenues	Percentages of our total revenues are as follows: Three months ended March 31, 2017 2016 Eisai Agreement (See Note 6) 64.1 % 57.7 % Boehringer Ingelheim Agreement (See Note 6) 18.2 % 13.6 % Toll manufacturing agreements 10.8 % 10.4 % Axovant Agreement (See Note 6) 6.6 % 6.0 % Other collaboration agreements 0.3 % 12.3 % Total percentage of revenues 100.0 % 100.0 %

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss Attributable to Stockholders of Arena Per Share	The following table presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net loss attributable to stockholders of Arena per share, in thousands: Three months ended March 31, 2017 2016 Stock options 31,643 19,091 RSUs and unvested restricted stock 88 314 Total 31,731 19,405

Beacon Discovery, Inc. (Tables)

Beacon Discovery, Inc. (Tables)	3 Months Ended
	Mar. 31, 2017
Variable Interest Entity [Line Items]
Reconciliation of Equity Attributable to Stockholders of Arena and Equity Attributable to Beacon	The following table presents a reconciliation of the equity attributable to the stockholders of Arena and the equity attributable to Beacon, in thousands: Equity Attributable to Stockholders of Arena Equity Attributable to Noncontrolling Interest in Consolidated Variable Interest Entity Total Equity Balance at January 1, 2017 $ 39,904 $ 491 $ 40,395 Net loss (22,053 ) (444 ) (22,497 ) Translation gain 804 — 804 Other 7,954 — 7,954 Balance at March 31, 2017 $ 26,609 $ 47 $ 26,656
Beacon Discovery, Inc.
Variable Interest Entity [Line Items]
Summary of Assets and Liabilities Included in Condensed Consolidated Balance Sheet	The following table presents the assets and liabilities of Beacon which are included in our condensed consolidated balance sheet at March 31, 2017, in thousands. The assets include only those assets that can be used to settle obligations of Beacon. The liabilities include third party liabilities of Beacon. As of March 31, 2017, Beacon had no creditors with recourse to the general credit of Arena. The assets and liabilities exclude intercompany balances that eliminate in consolidation: Assets of Beacon that can only be used to settle obligations of Beacon Cash and cash equivalents $ 306 Accounts receivable 10 Prepaid expense and other current assets 14 Land, property and equipment, net 617 Total assets of Beacon that can only be used to settle obligations of Beacon $ 947 Liabilities of Beacon for which creditors do not have recourse to the general credit of Arena Accounts payable and other accrued liabilities $ 135 Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena $ 135

Basis of Presentation - Additio

Basis of Presentation - Additional Information (Detail) - USD ($)	May 09, 2017	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Subsidiary Sale Of Stock [Line Items]
Cash and cash equivalents		$ 79,529,000	$ 139,533,000	$ 90,712,000	$ 156,184,000
Net proceeds from sales of common stock		5,267,000	$ 127,000
Accounting Standards Update 2016-09
Subsidiary Sale Of Stock [Line Items]
Cumulative-effect adjustment to increase in accumulated deficit and additional paid-in capital		$ 43,000
Subsequent Event
Subsidiary Sale Of Stock [Line Items]
Net proceeds from sales of common stock	$ 75,500,000

Fair Value Disclosures - Financ

Fair Value Disclosures - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets:
Money market funds	$ 21,405	$ 46,371
Quoted Prices in Active Markets (Level 1)
Assets:
Money market funds	$ 21,405	$ 46,371

Inventory - Components of Inven

Inventory - Components of Inventory (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Inventory Disclosure [Abstract]
Raw materials	$ 2,882	$ 2,553
Work in process	3,955	3,943
Finished goods	218	212
Total inventory	$ 7,055	$ 6,708

Land, Property and Equipment -

Land, Property and Equipment - Land, Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Cost	$ 108,574	$ 108,356
Less accumulated depreciation and amortization	(66,062)	(64,528)
Land, property and equipment, net	$ 42,512	$ 43,828

Accounts Payable and Other Ac32

Accounts Payable and Other Accrued Liabilities - Accounts Payable and Other Accrued Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accounts payable	$ 3,340	$ 5,977
Accrued compensation	3,625	4,820
Other accrued liabilities	1,614	1,319
Total accounts payable and other accrued liabilities	$ 8,579	$ 12,116

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Dec. 28, 2016USD ($)	Oct. 31, 2016USD ($)	Jan. 31, 2016USD ($)	May 31, 2015USD ($)	Mar. 31, 2017USD ($)Milestone	Mar. 31, 2017CHF (SFr)Milestone	Mar. 31, 2016USD ($)	Dec. 31, 2016USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total arrangement consideration					$ 115,600,000
Payment received from Eisai	$ 10,000,000
Cost of product sales					2,532,000		$ 2,428,000
Net product sales					2,711,000		3,518,000
Inventory					$ 7,055,000			$ 6,708,000
Eisai | 9.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate on annual net sales					9.50%	9.50%
Eisai | 9.5% Royalty Rate | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount					$ 175,000,000
Eisai | 13.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate on annual net sales					13.50%	13.50%
Eisai | 13.5% Royalty Rate | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount					$ 500,000,000
Eisai | 13.5% Royalty Rate | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount					$ 175,000,000
Eisai | 18.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate on annual net sales					18.50%	18.50%
Eisai | 18.5% Royalty Rate | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount					$ 500,000,000
Axovant
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments				$ 4,000,000
Collaborative agreement revenue recognition period				5 years
Additional milestone payments on achievement					$ 41,500,000
Number of development milestone | Milestone					2	2
Number of regulatory milestone | Milestone					4	4
Revenue recorded					$ 400,000		600,000
Boehringer Ingelheim
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments			$ 7,500,000
Collaborative agreement revenue recognition period			2 years
Additional milestone payments on achievement					$ 251,000,000
Number of development milestone | Milestone					3	3
Number of regulatory milestone | Milestone					9	9
Revenue recorded					$ 1,200,000		$ 1,300,000
Refund of withholding taxes		$ 1,200,000
Number of commercial milestone | Milestone					4	4
Eisai First Amended Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement initiation date					2012-05	2012-05
Eisai Second Amended Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement initiation date					2013-11	2013-11
Manufacturing and supply commitment period					2 years	2 years
Eisai Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement initial term					2 years	2 years
Collaboration agreement extended initial term					6 months	6 months
Collaboration agreement extension fee | SFr						SFr 2,000,000
Proceeds from sale of inventory	$ 10,000,000
Eisai Agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional manufacturing support payments | SFr						13,000,000
Manufacturing support payments to be received | SFr						SFr 6,000,000
License Deliverable
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated selling price of the deliverables for revenue recognition								64,000,000
Inventory Deliverable
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated selling price of the deliverables for revenue recognition					$ 30,800,000
Consideration allocated to deliverable revenue								0
Carrying value of inventory recognized in cost of product sales								$ 0
Recognized revenues on net product sales					2,200,000
Cost of product sales					300,000
Deferred revenues					29,400,000
Inventory					4,100,000
Manufacturing and Supply Commitment Deliverable
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated selling price of the deliverables for revenue recognition					20,800,000
Consideration allocated to deliverable revenue					2,100,000
Net product sales					500,000
Manufacturing and Supply Commitment Deliverable | Other Eisai Collaboration Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Management support payment recognized					1,600,000
Development Milestones | Axovant
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement					4,000,000
Development Milestones | Boehringer Ingelheim
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement					7,000,000
Development Milestones | Boehringer Ingelheim | Non-substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement					30,000,000
Regulatory Milestones | Axovant
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement					37,500,000
Regulatory Milestones | Boehringer Ingelheim | Non-substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement					84,000,000
Commercial Milestones | Boehringer Ingelheim | Non-substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement					$ 130,000,000

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	May 09, 2017	Apr. 21, 2017	Apr. 17, 2017	Jan. 04, 2017	Mar. 31, 2017	Mar. 31, 2016	Apr. 30, 2017
Stockholders Equity [Line Items]
Aggregate gross proceeds from sale of common stock					$ 5,267,000	$ 127,000
Equity Distribution Agreement | Citigroup Global Markets Inc. | ATM Offering
Stockholders Equity [Line Items]
Aggregate gross proceeds from sale of common stock					$ 6,400,000
Common Stock | Equity Distribution Agreement | Citigroup Global Markets Inc. | ATM Offering
Stockholders Equity [Line Items]
Sale of common stock shares					4,264,970
Maximum | Equity Distribution Agreement | Citigroup Global Markets Inc. | ATM Offering
Stockholders Equity [Line Items]
Aggregate value of common stock available				$ 50,000,000
Percentage of sales agent commission on gross proceeds				3.00%
Reimbursement of sales agent expenses				$ 50,000
Weighted Average | Common Stock | Equity Distribution Agreement | Citigroup Global Markets Inc. | ATM Offering
Stockholders Equity [Line Items]
Average market price per share					$ 1.51
Subsequent Event
Stockholders Equity [Line Items]
Aggregate gross proceeds from sale of common stock	$ 75,500,000
Subsequent Event | Citigroup Global Markets Inc. and Leerink Partners LLC | Underwritten Public Offering
Stockholders Equity [Line Items]
Percentage of underwriter discounts and commissions on per share offering price			6.00%
Subsequent Event | Equity Distribution Agreement | Citigroup Global Markets Inc. | ATM Offering
Stockholders Equity [Line Items]
Aggregate gross proceeds from sale of common stock	$ 900,000
Proceeds from sale of common stock not yet received							$ 900,000
Subsequent Event | Common Stock | Citigroup Global Markets Inc. and Leerink Partners LLC | Underwritten Public Offering
Stockholders Equity [Line Items]
Sale of common stock shares		69,000,000	60,000,000
Aggregate gross proceeds from sale of common stock		$ 74,600,000
Public offering price per share			$ 1.15
Subsequent Event | Common Stock | Equity Distribution Agreement | Citigroup Global Markets Inc. | ATM Offering
Stockholders Equity [Line Items]
Sale of common stock shares	625,262
Subsequent Event | Maximum | Common Stock | Citigroup Global Markets Inc. and Leerink Partners LLC | Underwritten Public Offering
Stockholders Equity [Line Items]
Additional sale of common stock shares			9,000,000
Subsequent Event | Weighted Average | Common Stock | Equity Distribution Agreement | Citigroup Global Markets Inc. | ATM Offering
Stockholders Equity [Line Items]
Average market price per share	$ 1.46

Share-based Compensation - Shar

Share-based Compensation - Share Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 1,838	$ 2,809
Total share-based compensation expense capitalized into inventory		37
Cost of product sales
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	51	20
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	379	1,763
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 1,408	$ 1,026

Share-based Compensation - Summ

Share-based Compensation - Summary of Stock Option Activity (Detail) - Stock options shares in Thousands	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Number of stock options
Outstanding at January 1, 2017 | shares	25,200
Granted | shares	15,325
Exercised | shares	(15)
Forfeited/cancelled/expired | shares	(1,076)
Outstanding at March 31, 2017 | shares	39,434
Weighted- Average Exercise Price
Outstanding at January 1, 2017 | $ / shares	$ 3.03
Granted | $ / shares	1.49
Exercised | $ / shares	1.28
Forfeited/cancelled/expired | $ / shares	9
Outstanding at March 31, 2017 | $ / shares	$ 2.27

Share-based Compensation - Su37

Share-based Compensation - Summary of Restricted Stock Unit Awards Activity (Detail) - Restricted Stock Units (RSU) shares in Thousands	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Number of Restricted Stock Units
Unvested at January 1, 2017 | shares	25
Unvested at March 31, 2017 | shares	25
Weighted- Average Grant-Date Fair Value
Unvested at January 1, 2017 | $ / shares	$ 4.26
Unvested at March 31, 2017 | $ / shares	$ 4.26

Share-based Compensation - Addi

Share-based Compensation - Additional Information (Detail) - Performance restricted stock units - shares	1 Months Ended		3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period		3 years
Units granted (shares)	745,000	695,000
Shares cancelled due to management changes			355,556

Concentrations of Credit Risk39

Concentrations of Credit Risk and Major Customers - Percentages of Total Revenues (Detail) - Sales Revenue, Net	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Concentration Risk [Line Items]
Percentage of revenues	100.00%	100.00%
Eisai Agreement
Concentration Risk [Line Items]
Percentage of revenues	64.10%	57.70%
Boehringer Ingelheim Agreement
Concentration Risk [Line Items]
Percentage of revenues	18.20%	13.60%
Toll manufacturing agreements
Concentration Risk [Line Items]
Percentage of revenues	10.80%	10.40%
Axovant Agreement
Concentration Risk [Line Items]
Percentage of revenues	6.60%	6.00%
Other collaboration agreements
Concentration Risk [Line Items]
Percentage of revenues	0.30%	12.30%

Net Loss Per Share - Potentiall

Net Loss Per Share - Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss Attributable to Stockholders of Arena Per Share (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss attributable to stockholders of Arena per share (shares)	31,731	19,405
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss attributable to stockholders of Arena per share (shares)	31,643	19,091
RSUs and unvested restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss attributable to stockholders of Arena per share (shares)	88	314

Appointment of Chief Medical 41

Appointment of Chief Medical Officer (Details) - 2013 Long-Term Incentive Plan as Amended - Chief Medical Officer	1 Months Ended
	Mar. 31, 2017shares
Management Changes [Line Items]
Options granted (shares)	1,293,500
Term	7 years
Vesting period	4 years
Vesting percentage	25.00%
Cliff Vesting After One Year
Management Changes [Line Items]
Vesting period	1 year
Vesting Years Two Through Four
Management Changes [Line Items]
Vesting period	3 years

Beacon Discovery, Inc. - Reconc

Beacon Discovery, Inc. - Reconciliation of Equity Attributable to Stockholders of Arena and Equity Attributable to Beacon (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Reconciliation Of Stockholders Equity [Line Items]
Balance at January 1, 2017	$ 40,395
Net loss	(22,497)	$ (21,548)
Translation gain	804	$ 2,591
Other	7,954
Balance at March 31, 2017	26,656
Equity Attributable to Stockholders of Arena
Reconciliation Of Stockholders Equity [Line Items]
Balance at January 1, 2017	39,904
Net loss	(22,053)
Translation gain	804
Other	7,954
Balance at March 31, 2017	26,609
Equity Attributable to Noncontrolling Interest in Consolidated Variable Interest Entity | Variable Interest Entity, Primary Beneficiary
Reconciliation Of Stockholders Equity [Line Items]
Balance at January 1, 2017	491
Net loss	(444)
Balance at March 31, 2017	$ 47

Beacon Discovery, Inc. - Summar

Beacon Discovery, Inc. - Summary of Assets and Liabilities Included in Condensed Consolidated Balance Sheet (Detail) - Variable Interest Entity, Primary Beneficiary - Beacon Discovery, Inc. $ in Thousands	Mar. 31, 2017USD ($)
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	$ 947
Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena	135
Cash and Cash Equivalents
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	306
Accounts Receivable
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	10
Prepaid Expense and Other Current Assets
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	14
Land, Property and Equipment, Net
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	617
Accounts Payable and Other Accrued Liabilities
Variable Interest Entity [Line Items]
Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena	$ 135