Arrowhead Pharmaceuticals (ARWR) 10-Q 6 Feb 24 2024 Q1 Quarterly report Financial data

Cover

Cover - shares	3 Months Ended
	Dec. 31, 2023	Jan. 31, 2024
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Dec. 31, 2023
Document Transition Report	false
Entity File Number	001-38042
Entity Registrant Name	ARROWHEAD PHARMACEUTICALS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-0408024
Entity Address, Address Line One	177 E. Colorado Blvd
Entity Address, Address Line Two	Suite 700
Entity Address, City or Town	Pasadena
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	91105
City Area Code	626
Local Phone Number	304-3400
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	ARWR
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		123,896,914
Entity Central Index Key	0000879407
Current Fiscal Year End Date	--09-30
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
Amendment Flag	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Current assets:
Cash, cash equivalents and restricted cash	$ 58,215	$ 110,891
Available-for-sale securities, at fair value	162,064	292,735
Prepaid expenses	10,516	8,813
Other current assets	7,104	7,082
Total current assets	237,899	419,521
Property, plant and equipment, net	333,411	290,262
Intangible assets, net	9,837	10,262
Right-of-use assets	44,907	45,297
Other assets	232	210
Total Assets	626,286	765,552
Current liabilities:
Accounts payable	4,460	35,866
Accrued expenses	46,604	39,763
Accrued payroll and benefits	8,777	17,963
Lease liabilities	3,421	10,563
Deferred revenue	0	866
Other liabilities	461	435
Total current liabilities	63,723	105,456
Long-term liabilities:
Lease liabilities, net of current portion	115,157	104,608
Liability related to the sale of future royalties	273,692	268,326
Total long-term liabilities	388,849	372,934
Commitments and contingencies (Note 7)
Noncontrolling interest and stockholders’ equity:
Common stock, $0.001 par value: Authorized 290,000 shares; issued and outstanding 107,500 and 107,312 shares	200	200
Additional paid-in capital	1,320,356	1,300,395
Accumulated other comprehensive loss	(1,255)	(3,222)
Accumulated deficit	(1,158,894)	(1,026,030)
Total Arrowhead Pharmaceuticals, Inc. stockholders’ equity	160,407	271,343
Noncontrolling interest	13,307	15,819
Total noncontrolling interest and stockholders’ equity	173,714	287,162
Total Liabilities, Noncontrolling Interest and Stockholders’ Equity	$ 626,286	$ 765,552

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2023	Sep. 30, 2023
Statement of Financial Position [Abstract]
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	290,000,000	290,000,000
Common stock, shares issued (in shares)	107,500,000	107,312,000
Common stock, shares outstanding (in shares)	107,500,000	107,312,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Income Statement [Abstract]
Revenue	$ 3,551	$ 62,546
Operating expenses:
Research and development	116,491	83,695
General and administrative	23,605	20,985
Total operating expenses	140,096	104,680
Operating loss	(136,545)	(42,134)
Other income (expense):
Interest income	2,802	2,682
Interest expense	(5,367)	(2,849)
Other, net	421	507
Total other (expense) income	(2,144)	340
Loss before income tax (benefit) expense and noncontrolling interest	(138,689)	(41,794)
Income tax (benefit) expense	(3,313)	17
Net loss including noncontrolling interest	(135,376)	(41,811)
Net loss attributable to noncontrolling interest, net of tax	(2,512)	(486)
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (132,864)	$ (41,325)
Net loss per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ (1.24)	$ (0.39)
Diluted (in dollars per share)	$ (1.24)	$ (0.39)
Weighted-average shares used in calculating
Basic (in shares)	107,415	106,039
Diluted (in shares)	107,415	106,039
Other comprehensive loss, net of tax:
Change in unrealized losses on available-for-sale securities	$ 1,909	$ 0
Foreign currency translation adjustments	58	(122)
Comprehensive loss	$ (133,409)	$ (41,933)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders’ Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Non- controlling Interest
Beginning balance at Sep. 30, 2022	$ 418,339	$ 198	$ 1,219,213	$ (136)	$ (820,755)	$ 19,819
Beginning balance (in shares) at Sep. 30, 2022		105,960,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	19,390		19,390
Exercise of stock options	576		576
Exercise of stock options (in shares)		82,000
Common stock - restricted stock units vesting	0	$ 1	(1)
Common stock - restricted stock units vesting (in shares)		98,000
Foreign currency translation adjustments	(122)			(122)
Change in unrealized losses on available-for-sale securities	0
Net loss	(41,811)				(41,325)	(486)
Ending balance at Dec. 31, 2022	396,372	$ 199	1,239,178	(258)	(862,080)	19,333
Ending balance (in shares) at Dec. 31, 2022		106,140,000
Beginning balance at Sep. 30, 2023	287,162	$ 200	1,300,395	(3,222)	(1,026,030)	15,819
Beginning balance (in shares) at Sep. 30, 2023		107,312,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	19,694		19,694
Exercise of stock options	$ 267		267
Exercise of stock options (in shares)	33,825	34,000
Common stock - restricted stock units vesting (in shares)		154,000
Foreign currency translation adjustments	$ 58			58
Change in unrealized losses on available-for-sale securities	1,909			1,909
Net loss	(135,376)				(132,864)	(2,512)
Ending balance at Dec. 31, 2023	$ 173,714	$ 200	$ 1,320,356	$ (1,255)	$ (1,158,894)	$ 13,307
Ending balance (in shares) at Dec. 31, 2023		107,500,000

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (135,376)	$ (41,811)
Adjustments to reconcile net loss to net cash flow from operating activities
Stock-based compensation	19,694	19,390
Depreciation and amortization	4,263	2,689
(Accretion) amortization of note premiums/discounts	(835)	690
Realized gain on investments	(80)	0
Non-cash interest expense on liability related to the sale of future royalties	5,367	2,849
Changes in operating assets and liabilities:
Accounts receivable	0	(38,157)
Prepaid expenses and other current assets	(1,748)	10,529
Accounts payable	(12,463)	(2,072)
Accrued expenses	408	(7,203)
Deferred revenue	(866)	(22,979)
Operating lease, net	3,796	559
Net cash used in operating activities	(117,840)	(75,516)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(68,656)	(38,911)
Purchases of investments	0	(111,199)
Proceeds from sales and maturities of investments	133,495	69,416
Net cash provided by (used in) investing activities	64,839	(80,694)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from the exercises of stock options	267	576
Proceeds from the sale of future royalties	0	250,000
Net cash provided by financing activities	267	250,576
Net (decrease) increase in cash, cash equivalents and restricted cash	(52,734)	94,366
Effect of exchange rate on cash, cash equivalents and restricted cash	58	(122)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH:
BEGINNING OF PERIOD	110,891	108,005
END OF PERIOD	58,215	202,249
Supplementary disclosure of cash flows:
Interest paid	0	0
Income taxes (paid) refunded	(999)	0
Supplemental disclosure of noncash investing activities:
Capital expenditures included in accrued expenses	$ 11,290	$ 14,044

Organization and Significant Ac

Organization and Significant Accounting Policies	3 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Organization and Significant Accounting Policies	ORGANIZATION AND SIGNIFICANT ACCOUNTING POLICIES General and Recent Developments Arrowhead Pharmaceuticals, Inc. and its subsidiaries (referred to herein collectively as the “Company”) are primarily engaged in developing medicines that treat intractable diseases by silencing the genes that cause them. Using a broad portfolio of RNA chemistries and efficient modes of delivery, the Company’s therapies trigger the RNA interference mechanism to induce rapid, deep and durable knockdown of target genes. RNA interference (“RNAi”) is a mechanism present in living cells that inhibits the expression of a specific gene, thereby affecting the production of a specific protein. The Company’s RNAi-based therapeutics may leverage this natural pathway of gene silencing to target and shut down specific disease-causing genes. The following table presents the Company’s current pipeline: Therapeutic Area Name Stage Product Rights Cardiometabolic Plozasiran (ARO-APOC3) Two Phase 2b and one Phase 3 Arrowhead Zodasiran (ARO-ANG3) Two Phase 2b Arrowhead Olpasiran Phase 3 Amgen Pulmonary ARO-RAGE Phase 1/2a Arrowhead ARO-MUC5AC Phase 1/2a Arrowhead ARO-MMP7 Phase 1/2a Arrowhead Liver GSK-4532990 Phase 2b GSK Fazirsiran Phase 3 Takeda and Arrowhead JNJ-3989 Phase 2 GSK ARO-C3 Phase 1/2a Arrowhead ARO-PNPLA3 Phase 1 Arrowhead ARO-CFB Phase 1/2a Arrowhead Muscle ARO-DUX4 Phase 1/2a Arrowhead ARO-DM1 Phase 1/2a Arrowhead Central Nervous System (CNS) Various Pre-Clinical Arrowhead The Company operates lab facilities in California and Wisconsin, where its research and development activities, including the development of RNAi therapeutics, take place. The Company’s principal executive offices are located in Pasadena, California. During the first quarter of fiscal 2024, the Company continued to develop and advance its pipeline and partnered candidates. Several key recent developments include: • Filed an application for clearance to initiate a Phase 1/2a clinical trial of ARO-CFB, being developed as a potential treatment for complement mediated renal disease; • Filed an application for clearance to initiate a Phase 1/2a clinical trial of ARO-DM1, being developed as a potential treatment for type 1 myotonic dystrophy (DM1), the most common adult-onset muscular dystrophy; and • Entered into an Amended and Restated License Agreement with GSK, pursuant to which GSK received a worldwide, exclusive license to develop and commercialize JNJ-3989 (formerly ARO-HBV). JNJ-3989 had previously been licensed to Janssen Pharmaceuticals, Inc. See Note 2. Consolidation and Basis of Presentation The interim Consolidated Financial Statements include the accounts of Arrowhead Pharmaceuticals, Inc. and its subsidiaries (wholly-owned subsidiaries and a variable interest entity for which the Company is the primary beneficiary). Subsidiaries refer to Arrowhead Madison, Inc., Visirna Therapeutics, Inc. (“Visirna”), and Arrowhead Australia Pty Ltd. For subsidiaries in which the Company owns or is exposed to less than 100% of the economics, the Company records net loss attributable to noncontrolling interests in its consolidated statements of operations equal to the percentage of the economic or ownership interests retained in such entity by the respective noncontrolling party. The interim Consolidated Financial Statements have been prepared in conformity with U.S. generally accepted accounting principles (“GAAP”). The financial data of the Company included herein are unaudited. In the opinion of management, all material adjustments of a normal recurring nature have been made to present fairly the Company’s financial position at December 31, 2023 and the results of operations and cash flows for the periods presented. All intercompany transactions and balances have been eliminated. Certain financial information that is normally included in annual financial statements prepared in accordance with GAAP, but that is not required for interim reporting purposes, has been omitted from the accompanying interim consolidated financial statements and related notes. Readers are urged to review the Company’s Annual Report on Form 10-K for the year ended September 30, 2023 for more complete descriptions and discussions. Operating results and cash flows for the three months ended December 31, 2023 are not necessarily indicative of the results that may be expected for the fiscal year ending September 30, 2024. Liquidity The Company’s primary sources of financing have been through the sale of its equity securities, revenue from its licensing and collaboration agreements and the sale of certain future royalties. Research and development activities have required significant capital investment since the Company’s inception and are expected to continue to require significant cash expenditure in the future, particularly as the Company’s pipeline of drug candidates and its headcount have both expanded. Additionally, significant capital investment will be required as the Company’s pipeline matures into later stage clinical trials. As of December 31, 2023, the Company had $58.2 million in cash, cash equivalents and restricted cash ($6.9 million in restricted cash) and $162.1 million in available-for-sale debt securities to fund operations. During the three months ended December 31, 2023, the Company’s cash, cash equivalents and restricted cash and investments balance decreased by $183.3 million which was primarily due to the ongoing expenses related to the Company’s research and development programs and general and administrative expenses and capital expenditures. During the three months ended December 31, 2022, the Company received the $250.0 million upfront payment from Royalty Pharma (Note 11). On January 2, 2024, the Company entered into an underwriting agreement with Jefferies LLC, BofA Securities, Inc., and Cowen and Company, LLC, as representatives of the several underwriters. The Company issued 15,790,000 shares of common stock at a price of $28.50 per share. The aggregate purchase price paid by investors was $450.0 million and the Company received net proceeds of $429.0 million after deducting advisory fees and offering expenses. In total, the Company is eligible to receive up to $2.8 billion in developmental, regulatory and sales milestones, and may receive various royalties on net sales from its licensing and collaboration agreements, subject to the terms and conditions of those agreements. The revenue recognition for these collaboration agreements is discussed further in Note 2. Summary of Significant Accounting Policies There have been no changes to the significant accounting policies disclosed in the Company’s most recent Annual Report on Form 10-K for the fiscal year ended September 30, 2023. Uncertainty in Income Taxes The Company recorded an income tax benefit of $3.3 million and $0 for the three months ended December 31, 2023 and 2022, respectively. The income tax benefit is primarily due to the discrete change in the Company’s uncertain tax positions related to the statute of limitation expiration. Recent Accounting Pronouncements In December 2023, the Financial Accounting Standards Board (“FASB”) issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures , to improve its income tax disclosure requirements. Under the ASU, entities must annually (1) disclose specific categories in the rate reconciliation and (2) provide additional information for reconciling items that meet a quantitative threshold. This ASU will become effective for the Company beginning on October 1, 2025. The Company does not expect any material impact on its consolidated financial statements and related disclosures resulting from applying this ASU.

Collaboration and License Agree

Collaboration and License Agreements	3 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration and License Agreements	COLLABORATION AND LICENSE AGREEMENTS The following table provides a summary of revenue recognized: Three Months Ended December 31, 2023 2022 (in thousands) GSK $ 2,685 $ (677) Horizon — 21,667 Takeda 866 16,312 Janssen — 244 Amgen — 25,000 Total $ 3,551 $ 62,546 The following table summarizes the balance of receivables and contract liabilities related to the Company’s collaboration and license agreements: December 31, 2023 September 30, 2023 (in thousands) Receivables included in accounts receivable $ — $ — Contract liabilities included in deferred revenue $ — $ 866 Glaxosmithkline Intellectual Property (No. 3) Limited (“GSK”) GSK License Agreement On November 22, 2021, GSK and the Company entered into an Exclusive License Agreement (the “GSK License Agreement”). Under the GSK License Agreement, GSK has received an exclusive license for GSK-4532990 (formerly ARO-HSD). The exclusive license is worldwide with the exception of greater China. GSK is wholly responsible for all clinical development and commercialization of GSK-4532990 in its territory. GSK dosed the first patient in a Phase 2b trial in March 2023 and paid a $30.0 million milestone payment to the Company in the third quarter of fiscal 2023. The Company is eligible for an additional payment of $100.0 million upon achieving the first patient dosed in a Phase 3 trial. Furthermore, should the Phase 3 trial read out positively, and the potential new medicine receives regulatory approval in major markets, the deal provides for commercial milestone payments to the Company of up to $190.0 million at first commercial sale, and up to $590.0 million in sales-related milestone payments. The Company is further eligible to receive tiered royalties on net product sales in a range of mid-teens to twenty percent. GSK-HBV Agreement On December 11, 2023, the Company entered into an Amended and Restated License Agreement with GSK (the “GSK-HBV Agreement”) pursuant to which GSK received a worldwide, exclusive license to develop and commercialize JNJ-3989 (formerly ARO-HBV), the Company’s third-generation subcutaneously administered RNAi therapeutic candidate being developed as a potential therapy for patients with chronic hepatitis B virus infection. JNJ-3989 had previously been licensed to Janssen in October 2018. Under the terms of the GSK-HBV Agreement, the Company received $2.7 million in December 2023, upon signing the amended GSK-HBV Agreement. The Company is eligible to receive up to $832.5 million in development and sales milestone payments under the GSK-HBV Agreement. There were no contract assets and liabilities recorded as of December 31, 2023. Horizon Therapeutics Ireland DAC (“Horizon”) In June 2021, Horizon and the Company entered into a collaboration and license agreement (the “Horizon License Agreement”). Under the terms of the Horizon License Agreement, Horizon received a worldwide exclusive license for HZN-457, a clinical-stage medicine being developed by Horizon as a potential treatment for people with uncontrolled gout. At the inception of the Horizon License Agreement, the Company identified one distinct performance obligation. The Company determined that the key deliverables included the license and certain R&D services, including the Company’s responsibilities to conduct all activities through the preclinical stages of development of HZN-457 (the “Horizon R&D Services”). The Company received a $40.0 million upfront payment in July 2021. Revenue was recognized on a straight-line basis over the timeframe for completing the Horizon R&D Services, concluding in the first quarter of 2023. Further, the Company received an additional $15.0 million upon Horizon’s initiation of a Phase 1 clinical trial in January 2023. On October 6, 2023, Amgen completed its acquisition of Horizon and subsequently notified the Company of Amgen’s intent to terminate the HZN-457 license. Horizon exercised its right to terminate the Horizon License Agreement for convenience, which took effect on December 21, 2023. Takeda Pharmaceutical Company Limited (“Takeda”) In October 2020, Takeda and the Company entered into an Exclusive License and Co-Funding Agreement (the “Takeda License Agreement”). Under the Takeda License Agreement, Takeda and the Company will co-develop the Company’s Fazirsiran program (formerly TAK-999 and ARO-AAT), the Company’s second-generation subcutaneously administered RNAi therapeutic candidate being developed as a treatment for liver disease associated with alpha-1 antitrypsin deficiency. Within the United States, fazirsiran, if approved, will be co-commercialized under a 50/50 profit sharing structure. Outside the United States, Takeda received an exclusive license to commercialize fazirsiran and will lead the global commercialization strategy, while the Company will be eligible to receive tiered royalties of 20% to 25% on net sales. At the inception of the Takeda License Agreement, the Company identified one distinct performance obligation. The Company determined that the key deliverables included the license and certain R&D services including the Company’s responsibilities to complete the initial portion of the SEQUOIA study, to complete the ongoing Phase 2 AROAAT2002 study and to ensure certain manufacturing of fazirsiran drug product is completed and delivered to Takeda (the “Takeda R&D Services”). Due to the specialized and unique nature of these Takeda R&D Services and their direct relationship with the license, the Company determined that these deliverables represent one distinct bundle and, thus, one performance obligation. Beyond the Takeda R&D Services, which are the responsibility of the Company, Takeda will be responsible for managing future clinical development and commercialization outside the United States. Within the United States, the Company will also participate in co-development and co-commercialization efforts and will co-fund these efforts with Takeda as part of the 50/50 profit sharing structure within the United States. The Company considers the collaborative activities, including the co-development and co-commercialization, to be a separate unit of account within Topic 808, and as such, these co-funding amounts are recorded as research and development expenses or general and administrative expenses, as appropriate. Under the terms of the Takeda License Agreement, the Company received $300.0 million as an upfront payment in January 2021 and an additional $40.0 million upon Takeda’s initiation of a Phase 3 REDWOOD clinical study of fazirsiran in March 2023, and is eligible to receive up to $527.5 million in additional potential development, regulatory and commercial milestones. The Company has allocated the total $300.0 million initial transaction price to its one distinct performance obligation for the fazirsiran license and the associated Takeda R&D Services. Revenue is recognized using the input method (based on actual patient visits completed versus total estimated visits completed for the ongoing SEQUOIA and AROAAT2002 clinical studies). The Company previously expected these clinical trials to extend to September 2025 in order to demonstrate long term safety and efficacy in the open label extension (OLE) part of the studies; however, in August 2023, Takeda initiated a Phase 3 OLE study, concluding the Phase 2 study visits for patients in the SEQUOIA and AROAAT2002 studies by December 31, 2023. Consequently, the Company adjusted its revenue recognition estimates in the fiscal year 2023 to align with the revised performance period, resulting in accelerated revenue of $70.5 million, or $0.66 per diluted share, for the year ended September 30, 2023. The remaining $0.9 million of deferred revenue was recognized for the three months ended December 31, 2023. There were no further contract liabilities as of December 31, 2023 . The Company also recorded $11.1 million as accrued expenses as of December 31, 2023 that was primarily driven by co-development and co-commercialization activities. Janssen Pharmaceuticals, Inc. (“Janssen”) On April 7, 2023, Janssen voluntarily terminated its collaboration agreement with the Company and the Company regained full rights to ARO-PNPLA3, formerly called JNJ-75220795. ARO-PNPLA3 is in Phase 1 clinical trials, which are now being developed by the Company. Further, on December 11, 2023, the Company entered into the GSK-HBV Agreement, as discussed above, pursuant to which GSK received an exclusive license for JNJ-3989 (formerly ARO-HBV). JNJ-3989 had previously been licensed to Janssen in October 2018. Amgen Inc. (“Amgen”) In September 2016, Amgen and the Company entered into two collaboration and license agreements and a common stock purchase agreement. Under the Second Collaboration and License Agreement (the “Olpasiran Agreement”), Amgen received a worldwide, exclusive license to the Company’s novel RNAi olpasiran (previously referred to as AMG- 890 or ARO-LPA) program. These RNAi molecules are designed to reduce elevated lipoprotein(a), which is a genetically validated, independent risk factor for atherosclerotic cardiovascular disease. Under the Olpasiran Agreement, Amgen is wholly responsible for clinical development and commercialization. Under the Olpasiran Agreement, the Company has received $35.0 million in upfront payments and $21.5 million in the form of an equity investment by Amgen in the Company’s common stock. Further, the Company received additional an $55.0 million in milestone payments; $10.0 million upon Amgen’s initiation of a Phase 1 study in September 2018, $20.0 million upon its initiation of a Phase 2 clinical study in July 2020, and $25.0 million upon its first subject enrollment in a Phase 3 trial in December 2022. The Company has substantially completed its performance obligations under the Olpasiran Agreement. There were no contract assets and liabilities recorded as of December 31, 2023. In November 2022, Royalty Pharma and the Company entered into the Royalty Pharma Agreement. In consideration for the payments under the Royalty Pharma Agreement, Royalty Pharma is entitled to receive all royalties otherwise payable by Amgen to the Company under the Olpasiran Agreement. The Company remains eligible to receive up to an additional $535.0 million in remaining development, regulatory and sales milestone payments payable from Amgen and Royalty Pharma. See Note 11. Visirna Therapeutics, Inc. (“Visirna”) In April 2022, the Company and Visirna, its subsidiary, entered into a License Agreement (the “Visirna License Agreement”), pursuant to which Visirna received an exclusive license to develop, manufacture and commercialize four of the Company’s RNAi-based investigational cardiometabolic medicines in Greater China (including the People’s Republic of China, Hong Kong, Macau and Taiwan).

Balance Sheet Accounts

Balance Sheet Accounts	3 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Accounts	BALANCE SHEET ACCOUNTS Property, Plant and Equipment The following table summarizes the Company’s major classes of property, plant and equipment: December 31, 2023 September 30, 2023 (in thousands) Land $ 2,996 $ 2,996 Building 71,797 — Research equipment 59,956 56,509 Furniture 2,761 1,540 Computers and software 883 700 Leasehold improvements 103,893 103,813 Construction in progress 136,914 166,655 379,200 332,213 Less: Accumulated depreciation and amortization (45,789) (41,951) Property, plant and equipment, net $ 333,411 $ 290,262 Depreciation and amortization expense for property and equipment for the three months ended December 31, 2023 and 2022 was $3.8 million and $2.3 million, respectively. As of December 31, 2023, the Company completed the build out of one of its laboratory and office facilities in Verona, Wisconsin, which resulted in the reclassification of related construction in progress to building. Further, the Company commenced depreciation on the newly completed facility over a 39-year period. Accrued Expenses Accrued expenses consist of the following: December 31, 2023 September 30, 2023 (in thousands) Accrued R&D expenses $ 18,106 $ 16,125 Accrued R&D expenses; co-development 11,104 5,895 Accrued capital expenditure 11,290 14,044 Other 6,104 3,699 Total accrued expense $ 46,604 $ 39,763

Investments

Investments	3 Months Ended
	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Investments	INVESTMENTS The Company’s investments consisted of the following: As of December 31, 2023 (in thousands) Adjusted Basis Gross Gross Fair Value Available-for-sale debt securities $ 163,119 $ — $ (1,055) $ 162,064 Total current investments $ 163,119 $ — $ (1,055) $ 162,064 As of September 30, 2023 (in thousands) Adjusted Basis Gross Gross Fair Value Available-for-sale debt securities $ 295,699 $ — $ (2,964) $ 292,735 Total current investments $ 295,699 $ — $ (2,964) $ 292,735

Intangible Assets

Intangible Assets	3 Months Ended
	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets	INTANGIBLE ASSETS Intangible assets subject to amortization include patents and a license agreement capitalized as part of the Novartis RNAi asset acquisition in March 2015. The following table presents the components of intangible assets: Gross Carrying Amount Accumulated Amortization Impairment Net Carrying Amount Useful Lives (in thousands) (in years) As of December 31, 2023 Patents $ 21,728 $ 13,709 $ — $ 8,019 14 License 3,129 1,311 — 1,818 21 Total intangible assets, net $ 24,857 $ 15,020 $ — $ 9,837 As of September 30, 2023 Patents $ 21,728 $ 13,321 $ — $ 8,407 14 License 3,129 1,274 — 1,855 21 Total intangible assets, net $ 24,857 $ 14,595 $ — $ 10,262 Intangible assets are reviewed annually for impairment and more frequently if potential impairment indicators exist. No impairment indicators were identified during the three months ended December 31, 2023 and 2022. Intangible assets with definite useful lives are amortized on a straight-line basis over their useful lives. Intangible assets amortization expense was $0.4 million for each of the three months ended December 31, 2023 and 2022. None of the intangible assets with definite useful lives are anticipated to have a residual value. The following table presents the estimated future amortization expense related to intangible assets as of December 31, 2023: Amortization Expense Year Ending September 30, (in thousands) 2024 (remainder) $ 1,275 2025 1,700 2026 1,700 2027 1,700 2028 1,700 Thereafter 1,762 Total $ 9,837

Stockholders' Equity

Stockholders' Equity	3 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Stockholders' Equity	STOCKHOLDERS’ EQUITY The following table summarizes the Company’s shares of common stock and preferred stock: Shares Par Value Authorized Issued Outstanding (in thousands) As of December 31, 2023 Common stock $ 0.001 290,000 107,500 107,500 Preferred stock $ 0.001 5,000 — — As of September 30, 2023 Common stock $ 0.001 290,000 107,312 107,312 Preferred stock $ 0.001 5,000 — — As of December 31, 2023 and September 30, 2023, respectively, 12,559,380 and 12,709,837 shares of common stock were reserved for issuance upon exercise of options and vesting of restricted stock units granted or available for grant under the Company’s 2004 Equity Incentive Plan, 2013 Incentive Plan, and 2021 Incentive Plan, as well as for inducement grants made to new employees under Rule 5635(c)(4) of the Nasdaq Listing Rules. On January 2, 2024, the Company entered into an underwriting agreement with Jefferies LLC, BofA Securities, Inc., and Cowen and Company, LLC, as representatives of the several underwriters. The Company issued 15,790,000 shares of common stock at an offering price of $28.50 per share. The aggregate purchase price paid by investors was $450.0 million and the Company received net proceeds of $429.0 million after deducting advisory fees and offering expenses. On December 2, 2022, the Company entered into an open market sale agreement (the “Open Market Sale Agreement”), pursuant to which the Company may, from time to time, sell up to $250,000,000 in shares of the Company’s common stock through Jefferies LLC, acting as the sales agent and/or principal, in an at-the-market offering (“ATM Offering”). The Company is not required to sell shares under the Open Market Sale Agreement. The Company will pay Jefferies LLC a commission of up to 3.0% of the aggregate gross proceeds received from all sales of the common stock under the Open Market Sale Agreement. Unless otherwise terminated, the ATM Offering shall terminate upon the earlier of (i) the sale of all shares of common stock subject to the Sales Agreement and (ii) the termination of the Sales Agreement as permitted therein. The Company and Jefferies may each terminate the Open Market Sale Agreement at any time upon prior notice. As of December 31, 2023, no shares have been issued under the Open Market Sale Agreement.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES Litigation From time to time, the Company may be subject to various claims and legal proceedings in the ordinary course of business. If the potential loss from any claim, asserted or unasserted, or legal proceeding is considered probable and the amount is reasonably estimable, the Company will accrue a liability for the estimated loss. There were no contingent liabilities recorded as of December 31, 2023 and September 30, 2023. Commitments

Leases

Leases	3 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Leases	LEASES Pasadena, California : The Company leases 49,000 square feet of office space located at 177 East Colorado Blvd. for its corporate headquarters from 177 Colorado Owner, LLC, which lease expires on April 30, 2027. The lease contains an option to renew for one additional five-year term. San Diego, California : The Company leases 144,000 square feet of office and research and development laboratory space located at 10102 Hoyt Park, San Diego, California, which lease expires on April 30, 2038. Pursuant to the lease, within twelve months of the expiration of the initial 15-year term, the Company has the option to extend the lease for up to one additional ten-year term, with certain annual increases in base rent. The lease agreement grants the Company the right to receive an Additional Tenant Improvement Allowance (“ATIA”) funded by the lessor, with a maximum amount of $7.2 million, subject to a 7% interest per annum over the base term. Further, on September 25, 2023, the Company executed the first amendment to the lease, which grants a second ATIA with a maximum amount of $23.6 million, bearing interest at a rate of 9% per annum over the base term. The Company has received $30.8 million ATIA from the lessor as of December 31, 2023. As a result, the Company remeasured its lease liability and right-of-use assets to reflect these additional allowances and the related increased lease payments. The Company has further concluded that these ATIAs have no effects on the classification of the lease. The Company previously subleased additional research and development space in San Diego, California, which subleases ended during the fiscal year of 2023. Madison, Wisconsin : The Company leases space for office and laboratory facilities, which expires on September 30, 2031. The lease contains options to renew for two terms of five years. After accounting for additional rental square feet added pursuant to amendments to the lease agreement in 2019 and 2020, the Company currently leases a total of 115,000 square feet. The components of lease assets and liabilities along with their classification on the Company’s consolidated balance sheets were as follows: Lease Assets and Liabilities Classification December 31, 2023 September 30, 2023 (in thousands) Operating lease assets Right-of-use assets $ 44,907 $ 45,297 Current operating lease liabilities Lease liabilities 3,421 10,563 Non-current operating lease liabilities Lease liabilities, net of current portion 115,157 104,608 Three Months Ended December 31, Lease Cost Classification 2023 2022 Operating lease cost Research and development $ 2,994 $ 2,069 General and administrative expense 476 533 Variable lease cost (1) Research and development 779 210 General and administrative expense — — Total $ 4,249 $ 2,812 (1) Variable lease cost is primarily related to operating expenses associated with the Company’s operating leases. There was $0 and $0.1 million short-term lease cost during the three months ended December 31, 2023, and 2022, respectively. The following table presents maturities of operating lease liabilities on an undiscounted basis as of December 31, 2023: Year Amounts (in thousands) 2024 (remainder of fiscal year) $ 9,044 2025 15,356 2026 15,696 2027 14,869 2028 13,511 2029 and thereafter 128,356 Total $ 196,832 Less imputed interest $ (78,254) Total operating lease liabilities (includes current portion) $ 118,578 Supplemental cash flow and other information related to leases was as follows: Three Months Ended December 31, 2023 2022 (in thousands) Cash received for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 3,099 $ — Right-of-use assets obtained in exchange for amended operating lease liabilities $ 64 $ — Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 2,080 $ 1,331 December 31, 2023 2022 Weighted-average remaining lease term (in years) 13.3 6.9 Weighted-average discount rate 8.0 % 8.5 %

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Stock-Based Compensation	STOCK-BASED COMPENSATION The Company has three plans that provide for equity-based compensation. Under the 2004 Equity Incentive Plan (the “2004 Plan”) and the 2013 Incentive Plan (the “2013 Plan”), 0 and 3,363,299 shares, respectively, of the Company’s common stock are reserved for grants of stock options and restricted stock awards to employees and directors as of December 31, 2023. On March 18, 2021, the Company’s Board of Directors approved the Arrowhead Pharmaceuticals, Inc. 2021 Incentive Plan (the “2021 Plan”), which authorized 8,000,000 shares (subject to certain adjustments) available for grants of stock options, stock appreciation rights, restricted and unrestricted stock, performance awards, cash awards and other awards convertible into or otherwise based on shares of the Company’s common stock. The maximum number of shares authorized under the 2021 Plan will be (i) reduced by any shares subject to awards made under the 2013 Plan after January 1, 2021, and (ii) increased by any shares subject to outstanding awards under the 2013 Plan as of January 1, 2021 that, after January 1, 2021, are canceled, expired, forfeited or otherwise not issued under such awards (other than as a result of being tendered or withheld to pay the exercise price or withholding taxes in connection with any such awards) or settled in cash. As of December 31, 2023, the total number of shares available for issuance was 5,774,487 shares, which includes 158,678 and 77,014 shares that were forfeited under the 2013 and 2021 Plans, respectively, and 2,411,054 shares have been granted under the 2021 Plan. In addition, there were 691,245 shares reserved for options and 684,900 shares reserved for restricted stock units issued as inducement grants to new employees granted outside of the Company’s equity-based compensation plans under Rule 5635(c)(4) of the Nasdaq Listing Rules. The following table presents a summary of awards outstanding: As of December 31, 2023 2004 Plan 2013 Plan 2021 Plan Inducement Awards Total Granted and outstanding awards: Options — 1,476,799 32,151 691,245 2,200,195 Restricted stock units — 1,886,500 2,013,298 684,900 4,584,698 Total — 3,363,299 2,045,449 1,376,145 6,784,893 The following table summarizes stock-based compensation expenses included in operating expenses: Three Months Ended December 31, 2023 2022 Research and development 7,823 8,402 General and administrative 9,862 10,987 Total $ 17,685 $ 19,389 Stock Option Awards The following table presents a summary of the stock option activity for the three months ended December 31, 2023: Shares Weighted- Weighted- Aggregate Outstanding at September 30, 2023 2,263,477 $ 22.68 Granted — — Cancelled or expired (29,457) 61.11 Exercised (33,825) 8.30 Outstanding at December 31, 2023 2,200,195 $ 22.46 4.1 years $ 31,862,157 Exercisable at December 31, 2023 2,118,298 $ 21.72 3.9 years $ 31,840,866 The aggregate intrinsic values represent the amount by which the market price of the underlying stock exceeds the exercise price of the option. The total intrinsic value of the options exercised during the three months ended December 31, 2023 and 2022 was $0.6 million and $2.3 million, respectively. Stock-based compensation expense related to stock options outstanding for the three months ended December 31, 2023 and 2022, was $1.5 million and $2.4 million, respectively. As of December 31, 2023, the pre-tax compensation expense for all outstanding unvested stock options in the amount of $1.4 million will be recognized in the Company’s results of operations over a weighted average period of 5 months. The fair value of each stock option award is estimated on the date of grant using the Black-Scholes option pricing model. The Black-Scholes option pricing model was developed for use in estimating the fair value of traded options, which do not have vesting restrictions and are fully transferable. The determination of the fair value of each stock option is affected by the Company’s stock price on the date of grant, as well as assumptions regarding a number of highly complex and subjective variables. Because the Company’s employee stock options have characteristics significantly different from those of traded options, and because changes in the subjective input assumptions can materially affect the fair value estimate, the existing models do not necessarily provide a reliable single measure of the fair value of its employee stock options. No options were granted during the three months ended December 31, 2023 and 2022. Visirna ESOP : On October 1, 2023, Visirna, a subsidiary of the Company, granted 7,500,000 stock options to its employees from the Employee Stock Option Plan (the “Visirna ESOP”), which authorizes 20,000,000 shares for issuance. The Visirna ESOP is independently managed by Visirna, including the valuation process. For the three months ended December 31, 2023, stock-based compensation expense related to the Visirna ESOP was $2.0 million. Restricted Stock Units Restricted Stock Units (“RSUs”), including market-based, time-based and performance-based awards, have been granted under the Company’s 2013 and 2021 Plans and as inducements grants granted outside of the Company’s equity-based compensation plans. At vesting, each outstanding RSU will be exchanged for one share of the Company’s common stock. RSU awards generally vest subject to the satisfaction of service requirements or the satisfaction of both service requirements and achievement of certain performance targets. The following table summarizes the activity of the Company’s RSUs: Number of Weighted- Outstanding at September 30, 2023 4,241,640 $ 58.43 Granted 511,290 29.48 Vested (153,457) 47.76 Forfeited (14,775) 51.21 Outstanding at December 31, 2023 4,584,698 $ 55.58 The fair value of RSUs was determined based on the closing price of the Company’s common stock on the grant date, with consideration given to the probability of achieving service and/or performance conditions for awards. For the three months ended December 31, 2023 and 2022, the Company recorded $16.2 million and $17.0 million of expense related to RSUs, respectively. As of December 31, 2023, there was $91.9 million of total unrecognized compensation cost related to RSUs that is expected to be recognized over a weighted-average period of 1.5 years.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair Value Measurements	FAIR VALUE MEASUREMENTS The Company employs a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. The Company’s valuation techniques and inputs used to measure fair value and the definition of the three levels (Level 1, Level 2, and Level 3) of the fair value hierarchy are disclosed in Note 10 - Fair Value Measurements of Notes to Consolidated Financial Statements of Part IV, “Item 15. Exhibits and Financial Statement Schedules” of its Annual Report on Form 10-K for the year ended September 30, 2023. The Company uses prices and inputs that are current as of the measurement date, including during periods of market disruption. In periods of market disruption, the ability to observe prices and inputs may be reduced for many instruments. This condition could cause an instrument to be reclassified from Level 1 to Level 2, or from Level 2 to Level 3. The Company recognizes transfers between levels at either the actual date of the event or a change in circumstances that caused the transfer. At December 31, 2023 and September 30, 2023, the Company did not have any financial assets or financial liabilities based on Level 3 measurements. The following table presents information about the Company’s assets and liabilities measured at fair value on a recurring basis, and indicates the fair value hierarchy of the valuation techniques utilized by the Company: December 31, 2023 Level 1 Level 2 Level 3 Total (in thousands) Available-for-sale debt securities U.S. government bonds $ 10,859 $ — $ — $ 10,859 Municipal securities — 7,153 — 7,153 Commercial notes — — — — Corporate debt securities — 144,052 — 144,052 Total available-for sale debt securities 10,859 151,205 — 162,064 Money market instruments 6,489 — — 6,489 Total financial assets $ 17,348 $ 151,205 $ — $ 168,553 September 30, 2023 Level 1 Level 2 Level 3 Total (in thousands) Available-for-sale debt securities U.S. government bonds $ 31,553 $ — $ — $ 31,553 Municipal securities — 7,093 — 7,093 Commercial notes — 22,205 — 22,205 Corporate debt securities — 231,884 — 231,884 Total available-for-sale debt securities 31,553 261,182 — 292,735 Money market instruments 347 — — 347 Total financial assets $ 31,900 $ 261,182 $ — $ 293,082

Liability Related to the Sale o

Liability Related to the Sale of Future Royalties	3 Months Ended
	Dec. 31, 2023
Liability Related To The Sale Of Future Royalties [Abstract]
Liability Related to the Sale of Future Royalties	LIABILITY RELATED TO THE SALE OF FUTURE ROYALTIES In November 2022, the Company and Royalty Pharma entered into the Royalty Pharma Agreement, pursuant to which Royalty Pharma agreed to pay up to $410.0 million in cash to the Company in consideration for the Company’s future royalty interest in olpasiran, a small interfering RNA (siRNA) originally developed by the Company and licensed to Amgen in September 2016 under the Olpasiran Agreement. Pursuant to the Royalty Pharma Agreement, Royalty Pharma paid $250.0 million upfront and agreed to pay up to an additional $160.0 million in aggregate one-time milestone payments due if and when the following milestone events occur: (i) $50.0 million on completion of enrollment in the OCEAN Phase 3 clinical trial for olpasiran, (ii) $50.0 million upon receipt of FDA approval of olpasiran for an approved indication (reduction in the risk of myocardial infarction, urgent coronary revascularization, or coronary heart disease death in adults with established cardiovascular disease and elevated Lp(a)), and (iii) $60.0 million upon Royalty Pharma’s receipt of at least $70.0 million of royalty payments under the Royalty Pharma Agreement in any single calendar year. In consideration for the payment of the foregoing amounts under the Royalty Pharma Agreement, Royalty Pharma is entitled to receive all royalties otherwise payable by Amgen to the Company under the Olpasiran Agreement. The Company remains eligible to receive any milestone payments potentially payable by Amgen under the Olpasiran Agreement. The Company has evaluated the terms of the Royalty Pharma Agreement and concluded in accordance with the relevant accounting guidance that the Company accounted for the transaction as debt and the funding of $250.0 million from Royalty Pharma was recorded as a liability related to the sale of future royalties on its consolidated balance sheets. The Company is not obligated to repay this upfront funding received under the Royalty Pharma Agreement. This liability is amortized over the expected repayment term using an effective interest rate method. The effective interest rate is calculated based on the rate that would enable the debt to be repaid in full over the anticipated life of the arrangement. The interest rate may vary during the term of the agreement depending on a number of factors, including the amount and timing of forecasted net revenues which affects the repayment timing and ultimate amount of repayment. The Company will evaluate the effective interest rate periodically based on its current revenue forecasts utilizing the prospective method. For the three months ended December 31, 2023 and 2022, the Company recognized non-cash interest expense of $5.4 million and $2.8 million, respectively, on the consolidated statements of operations and comprehensive loss.

Earnings Per Share

Earnings Per Share	3 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Earnings Per Share	EARNINGS PER SHARE The following table presents the computation of basic and diluted earnings per share for the three months ended December 31, 2023 and 2022. Three Months Ended December 31, 2023 2022 (in thousands, except per share amounts) Numerator: Net loss attributable to Arrowhead Pharmaceuticals, Inc. $ (132,864) $ (41,325) Denominator: Weighted-average basic shares outstanding 107,415 106,039 Effect of dilutive securities — — Weighted-average diluted shares outstanding 107,415 106,039 Basic earnings per share $ (1.24) $ (0.39) Diluted earnings per share $ (1.24) $ (0.39)

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Pay vs Performance Disclosure
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (132,864)	$ (41,325)

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Dec. 31, 2023
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Organization and Significant _2

Organization and Significant Accounting Policies (Policies)	3 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements In December 2023, the Financial Accounting Standards Board (“FASB”) issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures , to improve its income tax disclosure requirements. Under the ASU, entities must annually (1) disclose specific categories in the rate reconciliation and (2) provide additional information for reconciling items that meet a quantitative threshold. This ASU will become effective for the Company beginning on October 1, 2025. The Company does not expect any material impact on its consolidated financial statements and related disclosures resulting from applying this ASU.

Organization and Significant _3

Organization and Significant Accounting Policies (Tables)	3 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Schedule of Company's Current Pipeline	The following table presents the Company’s current pipeline: Therapeutic Area Name Stage Product Rights Cardiometabolic Plozasiran (ARO-APOC3) Two Phase 2b and one Phase 3 Arrowhead Zodasiran (ARO-ANG3) Two Phase 2b Arrowhead Olpasiran Phase 3 Amgen Pulmonary ARO-RAGE Phase 1/2a Arrowhead ARO-MUC5AC Phase 1/2a Arrowhead ARO-MMP7 Phase 1/2a Arrowhead Liver GSK-4532990 Phase 2b GSK Fazirsiran Phase 3 Takeda and Arrowhead JNJ-3989 Phase 2 GSK ARO-C3 Phase 1/2a Arrowhead ARO-PNPLA3 Phase 1 Arrowhead ARO-CFB Phase 1/2a Arrowhead Muscle ARO-DUX4 Phase 1/2a Arrowhead ARO-DM1 Phase 1/2a Arrowhead Central Nervous System (CNS) Various Pre-Clinical Arrowhead

Collaboration and License Agr_2

Collaboration and License Agreements (Tables)	3 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Revenue	The following table provides a summary of revenue recognized: Three Months Ended December 31, 2023 2022 (in thousands) GSK $ 2,685 $ (677) Horizon — 21,667 Takeda 866 16,312 Janssen — 244 Amgen — 25,000 Total $ 3,551 $ 62,546
Schedule of Receivables and Contract Liabilities	The following table summarizes the balance of receivables and contract liabilities related to the Company’s collaboration and license agreements: December 31, 2023 September 30, 2023 (in thousands) Receivables included in accounts receivable $ — $ — Contract liabilities included in deferred revenue $ — $ 866

Balance Sheet Accounts (Tables)

Balance Sheet Accounts (Tables)	3 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Property and Equipment	The following table summarizes the Company’s major classes of property, plant and equipment: December 31, 2023 September 30, 2023 (in thousands) Land $ 2,996 $ 2,996 Building 71,797 — Research equipment 59,956 56,509 Furniture 2,761 1,540 Computers and software 883 700 Leasehold improvements 103,893 103,813 Construction in progress 136,914 166,655 379,200 332,213 Less: Accumulated depreciation and amortization (45,789) (41,951) Property, plant and equipment, net $ 333,411 $ 290,262
Schedule of Accrued Expenses	Accrued expenses consist of the following: December 31, 2023 September 30, 2023 (in thousands) Accrued R&D expenses $ 18,106 $ 16,125 Accrued R&D expenses; co-development 11,104 5,895 Accrued capital expenditure 11,290 14,044 Other 6,104 3,699 Total accrued expense $ 46,604 $ 39,763

Investments (Tables)

Investments (Tables)	3 Months Ended
	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Summary of Short-term and Long-term Investments and Marketable Securities	The Company’s investments consisted of the following: As of December 31, 2023 (in thousands) Adjusted Basis Gross Gross Fair Value Available-for-sale debt securities $ 163,119 $ — $ (1,055) $ 162,064 Total current investments $ 163,119 $ — $ (1,055) $ 162,064 As of September 30, 2023 (in thousands) Adjusted Basis Gross Gross Fair Value Available-for-sale debt securities $ 295,699 $ — $ (2,964) $ 292,735 Total current investments $ 295,699 $ — $ (2,964) $ 292,735

Intangible Assets (Tables)

Intangible Assets (Tables)	3 Months Ended
	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Finite-Lived Intangible Assets	The following table presents the components of intangible assets: Gross Carrying Amount Accumulated Amortization Impairment Net Carrying Amount Useful Lives (in thousands) (in years) As of December 31, 2023 Patents $ 21,728 $ 13,709 $ — $ 8,019 14 License 3,129 1,311 — 1,818 21 Total intangible assets, net $ 24,857 $ 15,020 $ — $ 9,837 As of September 30, 2023 Patents $ 21,728 $ 13,321 $ — $ 8,407 14 License 3,129 1,274 — 1,855 21 Total intangible assets, net $ 24,857 $ 14,595 $ — $ 10,262
Schedule of Intangible Assets Future Amortization Expense	The following table presents the estimated future amortization expense related to intangible assets as of December 31, 2023: Amortization Expense Year Ending September 30, (in thousands) 2024 (remainder) $ 1,275 2025 1,700 2026 1,700 2027 1,700 2028 1,700 Thereafter 1,762 Total $ 9,837

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	3 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Schedule of Shares of Common Stock and Preferred Stock	The following table summarizes the Company’s shares of common stock and preferred stock: Shares Par Value Authorized Issued Outstanding (in thousands) As of December 31, 2023 Common stock $ 0.001 290,000 107,500 107,500 Preferred stock $ 0.001 5,000 — — As of September 30, 2023 Common stock $ 0.001 290,000 107,312 107,312 Preferred stock $ 0.001 5,000 — —

Leases (Tables)

Leases (Tables)	3 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Schedule of Lease Assets and Liabilities and Lease Cost	The components of lease assets and liabilities along with their classification on the Company’s consolidated balance sheets were as follows: Lease Assets and Liabilities Classification December 31, 2023 September 30, 2023 (in thousands) Operating lease assets Right-of-use assets $ 44,907 $ 45,297 Current operating lease liabilities Lease liabilities 3,421 10,563 Non-current operating lease liabilities Lease liabilities, net of current portion 115,157 104,608 Three Months Ended December 31, Lease Cost Classification 2023 2022 Operating lease cost Research and development $ 2,994 $ 2,069 General and administrative expense 476 533 Variable lease cost (1) Research and development 779 210 General and administrative expense — — Total $ 4,249 $ 2,812 (1) Variable lease cost is primarily related to operating expenses associated with the Company’s operating leases.
Summary of Maturities of Operating Lease Liabilities on an Undiscounted Basis	The following table presents maturities of operating lease liabilities on an undiscounted basis as of December 31, 2023: Year Amounts (in thousands) 2024 (remainder of fiscal year) $ 9,044 2025 15,356 2026 15,696 2027 14,869 2028 13,511 2029 and thereafter 128,356 Total $ 196,832 Less imputed interest $ (78,254) Total operating lease liabilities (includes current portion) $ 118,578 Supplemental cash flow and other information related to leases was as follows: Three Months Ended December 31, 2023 2022 (in thousands) Cash received for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 3,099 $ — Right-of-use assets obtained in exchange for amended operating lease liabilities $ 64 $ — Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 2,080 $ 1,331 December 31, 2023 2022 Weighted-average remaining lease term (in years) 13.3 6.9 Weighted-average discount rate 8.0 % 8.5 %

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Summary of Awards Outstanding	The following table presents a summary of awards outstanding: As of December 31, 2023 2004 Plan 2013 Plan 2021 Plan Inducement Awards Total Granted and outstanding awards: Options — 1,476,799 32,151 691,245 2,200,195 Restricted stock units — 1,886,500 2,013,298 684,900 4,584,698 Total — 3,363,299 2,045,449 1,376,145 6,784,893
Share-Based Payment Arrangement, Expensed	The following table summarizes stock-based compensation expenses included in operating expenses: Three Months Ended December 31, 2023 2022 Research and development 7,823 8,402 General and administrative 9,862 10,987 Total $ 17,685 $ 19,389
Summarized Information about Stock Options	The following table presents a summary of the stock option activity for the three months ended December 31, 2023: Shares Weighted- Weighted- Aggregate Outstanding at September 30, 2023 2,263,477 $ 22.68 Granted — — Cancelled or expired (29,457) 61.11 Exercised (33,825) 8.30 Outstanding at December 31, 2023 2,200,195 $ 22.46 4.1 years $ 31,862,157 Exercisable at December 31, 2023 2,118,298 $ 21.72 3.9 years $ 31,840,866
Summary of Share Activity Related to RSUs	The following table summarizes the activity of the Company’s RSUs: Number of Weighted- Outstanding at September 30, 2023 4,241,640 $ 58.43 Granted 511,290 29.48 Vested (153,457) 47.76 Forfeited (14,775) 51.21 Outstanding at December 31, 2023 4,584,698 $ 55.58

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair Value Measurements for Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table presents information about the Company’s assets and liabilities measured at fair value on a recurring basis, and indicates the fair value hierarchy of the valuation techniques utilized by the Company: December 31, 2023 Level 1 Level 2 Level 3 Total (in thousands) Available-for-sale debt securities U.S. government bonds $ 10,859 $ — $ — $ 10,859 Municipal securities — 7,153 — 7,153 Commercial notes — — — — Corporate debt securities — 144,052 — 144,052 Total available-for sale debt securities 10,859 151,205 — 162,064 Money market instruments 6,489 — — 6,489 Total financial assets $ 17,348 $ 151,205 $ — $ 168,553 September 30, 2023 Level 1 Level 2 Level 3 Total (in thousands) Available-for-sale debt securities U.S. government bonds $ 31,553 $ — $ — $ 31,553 Municipal securities — 7,093 — 7,093 Commercial notes — 22,205 — 22,205 Corporate debt securities — 231,884 — 231,884 Total available-for-sale debt securities 31,553 261,182 — 292,735 Money market instruments 347 — — 347 Total financial assets $ 31,900 $ 261,182 $ — $ 293,082

Earnings Per Share (Tables)

Earnings Per Share (Tables)	3 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Schedule of Earnings Per Share, Basic and Diluted	The following table presents the computation of basic and diluted earnings per share for the three months ended December 31, 2023 and 2022. Three Months Ended December 31, 2023 2022 (in thousands, except per share amounts) Numerator: Net loss attributable to Arrowhead Pharmaceuticals, Inc. $ (132,864) $ (41,325) Denominator: Weighted-average basic shares outstanding 107,415 106,039 Effect of dilutive securities — — Weighted-average diluted shares outstanding 107,415 106,039 Basic earnings per share $ (1.24) $ (0.39) Diluted earnings per share $ (1.24) $ (0.39)

Organization and Significant _4

Organization and Significant Accounting Policies - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Jan. 02, 2024	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Organization And Significant Accounting Policies [Line Items]
Cash, cash equivalents and restricted cash		$ 58,200
Restricted cash		6,900
Available-for-sale securities, at fair value		162,064		$ 292,735
Increase (decrease) in cash and investments		(183,300)
Development regulatory and sales milestones payments		2,800,000
Income tax (benefit) expense		(3,313)	$ 17
Subsequent Event | Underwriting Agreement
Organization And Significant Accounting Policies [Line Items]
Shares of common stock issued (in shares)	15,790,000
Common stock offering price (in dollars per share)	$ 28.50
Aggregate purchase price paid by investors	$ 450,000
Net proceeds	$ 429,000
Royalty Pharma
Organization And Significant Accounting Policies [Line Items]
Upfront milestone payment received		$ 250,000

Collaboration and License Agr_3

Collaboration and License Agreements - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Revenue	$ 3,551	$ 62,546
GSK
Disaggregation of Revenue [Line Items]
Revenue	2,685	(677)
Horizon
Disaggregation of Revenue [Line Items]
Revenue	0	21,667
Takeda
Disaggregation of Revenue [Line Items]
Revenue	866	16,312
Janssen
Disaggregation of Revenue [Line Items]
Revenue	0	244
Amgen
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 25,000

Collaboration and License Agr_4

Collaboration and License Agreements - Receivables and Contract Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Receivables included in accounts receivable	$ 0	$ 0
Contract liabilities included in deferred revenue	$ 0	$ 866

Collaboration and License Agr_5

Collaboration and License Agreements - Glaxosmithkline Intellectual Property (No. 3) Limited (Details) - USD ($)	1 Months Ended
	Dec. 31, 2023	Oct. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Nov. 22, 2021
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract liabilities	$ 0		$ 866,000
GSK | GSK License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment receivable				$ 30,000,000
Milestone payment receivable upon achievement of phase two and first patient dosed in phase three					$ 100,000,000
GSK | GSK License Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Commercial milestone payments at first commercial sale					190,000,000
Sales-related milestone payments					$ 590,000,000
GSK | GSK-HBV Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Initial transaction price	2,700,000
Contract assets	0
Contract liabilities	$ 0
GSK | GSK-HBV Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development and sales milestone payments		$ 832,500,000

Collaboration and License Agr_6

Collaboration and License Agreements - Horizon Therapeutics Ireland DAC (Details) - Horizon - Horizon License Agreement $ in Millions	1 Months Ended
	Jul. 31, 2021 USD ($)	Jun. 30, 2021 obligation	Jan. 31, 2023 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of distinct performance obligations | obligation		1
Initial transaction price	$ 40
Milestone payment			$ 15

Collaboration and License Agr_7

Collaboration and License Agreements - Takeda Pharmaceutical Company Limited (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Oct. 31, 2020 USD ($) obligation bundle	Dec. 31, 2023 USD ($) $ / shares	Dec. 31, 2022 USD ($) $ / shares	Sep. 30, 2023 USD ($) $ / shares	Mar. 31, 2023 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development regulatory and sales milestones payments		$ 2,800,000
Revenues		$ 3,551	$ 62,546
Diluted (in dollars per share) | $ / shares		$ (1.24)	$ (0.39)
Accrued expenses		$ 46,604		$ 39,763
Takeda | License and Co-Funding Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of distinct performance obligations | obligation	1
Number of distinct bundle | bundle	1
Initial transaction price	$ 300,000
Milestone payment					$ 40,000
Revenues				$ 70,500
Diluted (in dollars per share) | $ / shares				$ 0.66
Deferred revenue recognized		900
Accrued expenses		11,100
Takeda | License and Co-Funding Agreement | Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of eligible to receive tiered royalties on net sales	20%
Takeda | License and Co-Funding Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of eligible to receive tiered royalties on net sales	25%
Development regulatory and sales milestones payments		$ 527,500

Collaboration and License Agr_8

Collaboration and License Agreements - Amgen, Inc (Details)	1 Months Ended
	Sep. 30, 2016 USD ($) agreement	Dec. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jul. 31, 2020 USD ($)	Sep. 30, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract liabilities		$ 0	$ 866,000
Amgen | Olpasiran Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of agreements | agreement	2
Upfront payment	$ 35,000,000
Proceeds from the issuance of common stock	$ 21,500,000
Milestone payments		55,000,000		$ 25,000,000	$ 20,000,000	$ 10,000,000
Contract assets		0
Contract liabilities		0
Amgen | Olpasiran Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional remaining development regulatory and sales milestones payments		$ 535,000,000

Collaboration and License Agr_9

Collaboration and License Agreements - Visirna Therapeutics, Inc. (Details) - USD ($)	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 3,551,000	$ 62,546,000
Deferred revenue	0		$ 866,000
Visirna Therapeutics, Inc. | Visirna License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	85,989	$ 749,262
Contract assets	0
Deferred revenue	$ 0

Balance Sheet Accounts - Summar

Balance Sheet Accounts - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 379,200		$ 332,213
Less: Accumulated depreciation and amortization	(45,789)		(41,951)
Property, plant and equipment, net	333,411		290,262
Depreciation and amortization expense for property and equipment	3,800	$ 2,300
Land
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,996		2,996
Building
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 71,797		0
Useful life	39 years
Research equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 59,956		56,509
Furniture
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,761		1,540
Computers and software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	883		700
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	103,893		103,813
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 136,914		$ 166,655

Balance Sheet Accounts - Accrue

Balance Sheet Accounts - Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued R&D expenses	$ 18,106	$ 16,125
Accrued R&D expenses; co-development	11,104	5,895
Accrued capital expenditure	11,290	14,044
Other	6,104	3,699
Total accrued expense	$ 46,604	$ 39,763

Investments - Summary of Short-

Investments - Summary of Short-term, Long-term Investments and Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Current investments, Available-for-sale, Adjusted Basis	$ 163,119	$ 295,699
Current investments, Available-for-sale, Gross Unrealized Gains	0	0
Current investments, Available-for-sale, Gross Unrealized Losses	(1,055)	(2,964)
Current investments, Available-for-sale, Fair Value	162,064	292,735
Debt securities
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Current investments, Available-for-sale, Adjusted Basis	163,119	295,699
Current investments, Available-for-sale, Gross Unrealized Gains	0	0
Current investments, Available-for-sale, Gross Unrealized Losses	(1,055)	(2,964)
Current investments, Available-for-sale, Fair Value	$ 162,064	$ 292,735

Intangible Assets - Schedule of

Intangible Assets - Schedule of Intangible Assets (Details) - USD ($)	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Finite Lived Intangible Assets [Line Items]
Net Carrying Amount	$ 9,837,000		$ 10,262,000
Amortization expense	400,000	$ 400,000
Novartis
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	24,857,000		24,857,000
Accumulated Amortization	15,020,000		14,595,000
Impairment	0		0
Net Carrying Amount	9,837,000		10,262,000
Novartis | Patents
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	21,728,000		21,728,000
Accumulated Amortization	13,709,000		13,321,000
Impairment	0		0
Net Carrying Amount	$ 8,019,000		$ 8,407,000
Useful Lives	14 years		14 years
Novartis | License
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 3,129,000		$ 3,129,000
Accumulated Amortization	1,311,000		1,274,000
Impairment	0		0
Net Carrying Amount	$ 1,818,000		$ 1,855,000
Useful Lives	21 years		21 years

Intangible Assets - Expected Fu

Intangible Assets - Expected Future Amortization (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
2024 (remainder)	$ 1,275
2025	1,700
2026	1,700
2027	1,700
2028	1,700
Thereafter	1,762
Net Carrying Amount	$ 9,837	$ 10,262

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($)	Jan. 02, 2024	Dec. 02, 2022	Dec. 31, 2023	Sep. 30, 2023
Class Of Stock [Line Items]
Common stock, par value (in dollars per share)			$ 0.001	$ 0.001
Common stock, shares authorized (in shares)			290,000,000	290,000,000
Common stock, shares issued (in shares)			107,500,000	107,312,000
Common stock, shares outstanding (in shares)			107,500,000	107,312,000
Preferred stock, par value (in dollars per share)			$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)			5,000,000	5,000,000
Preferred stock, shares issued (in shares)			0	0
Preferred stock, shares outstanding (in shares)			0	0
Common stock, share reserve for issuance (in shares)			5,774,487
Underwriting Agreement | Subsequent Event
Class Of Stock [Line Items]
Shares of common stock issued (in shares)	15,790,000
Common stock offering price (in dollars per share)	$ 28.50
Aggregate purchase price paid by investors	$ 450,000,000
Net proceeds	$ 429,000,000
At The Market Agreement
Class Of Stock [Line Items]
Shares issued			0
At The Market Agreement | Maximum
Class Of Stock [Line Items]
Common stock shares value reserved for future issuance		$ 250,000,000
Percentage of commission to sales agent		3%
2004 Equity Incentive Plan, 2013 Equity Incentive Plan, 2021 Equity Incentive Plan and Inducement Grants
Class Of Stock [Line Items]
Common stock, share reserve for issuance (in shares)			12,559,380	12,709,837

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) ft² in Thousands	Dec. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)	Dec. 20, 2021 ft²
Other Commitments [Line Items]
Contingent liabilities	$ 0	$ 0
Drug Manufacturing Facility | Wisconsin
Other Commitments [Line Items]
Planned area of the site (in sq ft) | ft²			160
Laboratory and Office Facility | Wisconsin
Other Commitments [Line Items]
Planned area of the site (in sq ft) | ft²			140
Facilities
Other Commitments [Line Items]
Capital expenditures incurred	224,700,000
Facilities | Minimum
Other Commitments [Line Items]
Amount intends to invest for buildout of the facilities	60,000,000
Facilities | Maximum
Other Commitments [Line Items]
Amount intends to invest for buildout of the facilities	$ 73,000,000

Leases - Narrative (Details)

Leases - Narrative (Details) ft² in Thousands, $ in Millions	3 Months Ended
	Dec. 31, 2023 USD ($) ft² option	Dec. 31, 2022 USD ($)	Sep. 25, 2023 USD ($)
Lessee Lease Description [Line Items]
Short-term lease cost | $	$ 0	$ 0.1
Corporate Headquarters In Pasadena | California
Lessee Lease Description [Line Items]
Office space leases (in sq ft) | ft²	49
Corporate Headquarters In Pasadena | California | Colorado Owner, LLC
Lessee Lease Description [Line Items]
Number of options to renew | option	1
Operating lease renewal term	5 years
Research Facility in San Diego | California
Lessee Lease Description [Line Items]
Office space leases (in sq ft) | ft²	144
Operating lease renewal term	10 years
Lease term	15 years
Maximum additional tenant improvement allowance | $	$ 7.2		$ 23.6
Additional tenant improvement allowance interest per annum	7%		9%
Additional tenant improvement allowance liability | $	$ 30.8
Research Facility in San Diego | California | Maximum
Lessee Lease Description [Line Items]
Number of options to renew | option	1
Research Facility in Madison | Wisconsin
Lessee Lease Description [Line Items]
Office space leases (in sq ft) | ft²	115
Number of options to renew | option	2
Operating lease renewal term	5 years

Leases - Components of Lease As

Leases - Components of Lease Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Lease Assets and Liabilities
Right-of-use assets	$ 44,907		$ 45,297
Lease liabilities	3,421		10,563
Lease liabilities, net of current portion	115,157		$ 104,608
Lease Cost
Total	4,249	$ 2,812
Research and development
Lease Cost
Operating lease cost	2,994	2,069
Variable lease cost	779	210
General and administrative expense
Lease Cost
Operating lease cost	476	533
Variable lease cost	$ 0	$ 0

Leases - Summary of Maturities

Leases - Summary of Maturities of Operating Lease Liabilities on an Undiscounted Basis (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Leases [Abstract]
2024 (remainder of fiscal year)	$ 9,044
2025	15,356
2026	15,696
2027	14,869
2028	13,511
2029 and thereafter	128,356
Total	196,832
Less imputed interest	(78,254)
Total operating lease liabilities (includes current portion)	$ 118,578

Leases - Supplemental Cash Flow

Leases - Supplemental Cash Flow and Other Information Related to Leases (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Cash received for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 3,099	$ 0
Right-of-use assets obtained in exchange for amended operating lease liabilities	64	0
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 2,080	$ 1,331
Weighted-average remaining lease term (in years)	13 years 3 months 18 days	6 years 10 months 24 days
Weighted-average discount rate	8%	8.50%

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended
	Oct. 01, 2023	Dec. 31, 2023	Dec. 31, 2022	Mar. 18, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, share reserve for issuance (in shares)		5,774,487
Granted (in shares)		0
Stock-based compensation expense		$ 17,685	$ 19,389
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Intrinsic value of options exercised		600	2,300
Stock-based compensation expense		1,500	2,400
Unrecognized pre-tax compensation expense		$ 1,400
Weighted average period to recognize pre-tax compensation expense		5 months
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 16,200	$ 17,000
Weighted average period to recognize pre-tax compensation expense		1 year 6 months
Unrecognized pre-tax compensation expense		$ 91,900
2004 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance (in shares)		0
2013 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance (in shares)		3,363,299
Number of shares forfeited (in shares)		158,678
2021 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized (in shares)				8,000,000
Number of shares forfeited (in shares)		77,014
Granted (in shares)		2,411,054
Inducement Awards | Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance (in shares)		691,245
Inducement Awards | Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance (in shares)		684,900
Visirna ESOP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized (in shares)	20,000,000
Granted (in shares)	7,500,000
Stock-based compensation expense		$ 2,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Granted and Outstanding Shares (Details) - shares	Dec. 31, 2023	Sep. 30, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options outstanding (in shares)	2,200,195	2,263,477
Total (in shares)	6,784,893
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment award (in shares)	4,584,698	4,241,640
2004 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options outstanding (in shares)	0
Total (in shares)	0
2004 Equity Incentive Plan | Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment award (in shares)	0
2013 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options outstanding (in shares)	1,476,799
Total (in shares)	3,363,299
2013 Incentive Plan | Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment award (in shares)	1,886,500
2021 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options outstanding (in shares)	32,151
Total (in shares)	2,045,449
2021 Incentive Plan | Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment award (in shares)	2,013,298
Inducement Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options outstanding (in shares)	691,245
Total (in shares)	1,376,145
Inducement Awards | Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment award (in shares)	684,900

Stock-Based Compensation - Expe

Stock-Based Compensation - Expenses Included in Operating Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 17,685	$ 19,389
Research and development
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	7,823	8,402
General and administrative expense
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 9,862	$ 10,987

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summarize Information about Stock Options (Details)	3 Months Ended
	Dec. 31, 2023 USD ($) $ / shares shares
Shares
Beginning balance (in shares) | shares	2,263,477
Granted (in shares) | shares	0
Cancelled (in shares) | shares	(29,457)
Exercised (in shares) | shares	(33,825)
Ending balance (in shares) | shares	2,200,195
Number of Options Outstanding, Exercisable (in shares) | shares	2,118,298
Weighted- Average Exercise Price Per Share
Beginning balance (in dollars per share) | $ / shares	$ 22.68
Granted (in dollars per share) | $ / shares	0
Cancelled (in dollars per share) | $ / shares	61.11
Exercised (in dollars per share) | $ / shares	8.30
Ending balance (in dollars per share) | $ / shares	22.46
Weighted-Average Exercise Price Per Share, Exercisable (in dollars per share) | $ / shares	$ 21.72
Weighted- Average Remaining Contractual Term (Years)	4 years 1 month 6 days
Weighted-Average Remaining Contractual Term, Exercisable	3 years 10 months 24 days
Aggregate Intrinsic Value | $	$ 31,862,157
Aggregate Intrinsic Value, Exercisable | $	$ 31,840,866

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of RSUs Activity (Details) - Restricted Stock Units (RSUs)	3 Months Ended
	Dec. 31, 2023 $ / shares shares
Number of RSUs
Beginning of period (in shares) | shares	4,241,640
Granted (in shares) | shares	511,290
Vested (in shares) | shares	(153,457)
Forfeited (in shares) | shares	(14,775)
End of period (in shares) | shares	4,584,698
Weighted- Average Grant Date Fair Value Per Share
Beginning balance (in dollars per share) | $ / shares	$ 58.43
Granted (in dollars per share) | $ / shares	29.48
Vested (in dollars per share) | $ / shares	47.76
Forfeited (in dollars per share) | $ / shares	51.21
Ending balance (in dollars per share) | $ / shares	$ 55.58

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Measurements for Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	$ 162,064	$ 292,735
Total financial assets	168,553	293,082
Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market instruments	6,489	347
U.S. government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	10,859	31,553
Municipal securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	7,153	7,093
Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	0	22,205
Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	144,052	231,884
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	10,859	31,553
Total financial assets	17,348	31,900
Level 1 | Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market instruments	6,489	347
Level 1 | U.S. government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	10,859	31,553
Level 1 | Municipal securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	0	0
Level 1 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	0	0
Level 1 | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	0	0
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	151,205	261,182
Total financial assets	151,205	261,182
Level 2 | Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market instruments	0	0
Level 2 | U.S. government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	0	0
Level 2 | Municipal securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	7,153	7,093
Level 2 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	0	22,205
Level 2 | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	144,052	231,884
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	0	0
Total financial assets	0	0
Level 3 | Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market instruments	0	0
Level 3 | U.S. government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	0	0
Level 3 | Municipal securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	0	0
Level 3 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	0	0
Level 3 | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities	$ 0	$ 0

Liability Related to the Sale_2

Liability Related to the Sale of Future Royalties (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Nov. 30, 2022	Dec. 31, 2023	Dec. 31, 2022
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-cash interest expense on liability related to the sale of future royalties		$ 5,367	$ 2,849
Royalty Pharma Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment	$ 250,000
Milestone payment receivable	160,000
Milestone payment receivable upon achievement of enrollment in phase 3 clinical trial	50,000
Milestone payment receivable upon FDA approval	50,000
Milestone payment receivable upon receipt of royalty payments	60,000
Royalty payment threshold	70,000
Liability related to the sale of future royalties		250,000
Non-cash interest expense on liability related to the sale of future royalties		$ 5,400	$ 2,800
Royalty Pharma Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Initial transaction price	$ 410,000

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Numerator:
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (132,864)	$ (41,325)
Denominator:
Weighted-average basic shares outstanding (in shares)	107,415	106,039
Effect of dilutive securities (in shares)	0	0
Weighted-average diluted shares outstanding (in shares)	107,415	106,039
Basic (in dollars per share)	$ (1.24)	$ (0.39)
Diluted (in dollars per share)	$ (1.24)	$ (0.39)
Potentially dilutive securities excluded from computation of diluted earnings per share (in shares)	3,544	3,327