Arrowhead Pharmaceuticals (ARWR) 10-Q 9 May 24 2024 Q2 Quarterly report Financial data

Cover

Cover - shares	6 Months Ended
	Mar. 31, 2024	May 01, 2024
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2024
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		124,200,230
Entity Central Index Key	0000879407
Current Fiscal Year End Date	--09-30
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q2
Amendment Flag	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2024	Sep. 30, 2023
Current assets:
Cash, cash equivalents and restricted cash	$ 127,704	$ 110,891
Available-for-sale securities, at fair value	395,410	292,735
Prepaid expenses	11,022	8,813
Other current assets	7,514	7,082
Total current assets	541,650	419,521
Property, plant and equipment, net	359,252	290,262
Intangible assets, net	9,412	10,262
Right-of-use assets	44,626	45,297
Other assets	210	210
Total Assets	955,150	765,552
Current liabilities:
Accounts payable	8,521	35,866
Accrued expenses	37,055	39,763
Accrued payroll and benefits	13,741	17,963
Lease liabilities	5,285	10,563
Deferred revenue	0	866
Other liabilities	573	435
Total current liabilities	65,175	105,456
Long-term liabilities:
Lease liabilities, net of current portion	113,632	104,608
Liability related to the sale of future royalties	280,938	268,326
Total long-term liabilities	394,570	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 124,133 and 107,312 shares	217	200
Additional paid-in capital	1,768,866	1,300,395
Accumulated other comprehensive loss	(1,095)	(3,222)
Accumulated deficit	(1,284,194)	(1,026,030)
Total Arrowhead Pharmaceuticals, Inc. stockholders’ equity	483,794	271,343
Noncontrolling interest	11,611	15,819
Total noncontrolling interest and stockholders’ equity	495,405	287,162
Total Liabilities, Noncontrolling Interest and Stockholders’ Equity	$ 955,150	$ 765,552

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2024	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)	124,133,000	107,312,000
Common stock, shares outstanding (in shares)	124,133,000	107,312,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive (Loss) Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Income Statement [Abstract]
Revenue	$ 0	$ 146,267	$ 3,551	$ 208,813
Operating expenses:
Research and development	101,122	74,881	217,613	158,576
General and administrative	25,069	23,221	48,674	44,206
Total operating expenses	126,191	98,102	266,287	202,782
Operating (loss) income	(126,191)	48,165	(262,736)	6,031
Other income (expense):
Interest income	6,250	4,560	9,052	7,242
Interest expense	(7,244)	(5,057)	(12,611)	(7,906)
Other, net	189	8	610	515
Total other expense	(805)	(489)	(2,949)	(149)
(Loss) income before income tax (benefit) expense and noncontrolling interest	(126,996)	47,676	(265,685)	5,882
Income tax (benefit) expense	0	0	(3,313)	17
Net (loss) income including noncontrolling interest	(126,996)	47,676	(262,372)	5,865
Net loss attributable to noncontrolling interest, net of tax	(1,696)	(999)	(4,208)	(1,485)
Net (loss) income attributable to Arrowhead Pharmaceuticals, Inc.	$ (125,300)	$ 48,675	$ (258,164)	$ 7,350
Net (loss) income per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ (1.02)	$ 0.46	$ (2.24)	$ 0.07
Diluted (in dollars per share)	$ (1.02)	$ 0.45	$ (2.24)	$ 0.07
Weighted-average shares used in calculating
Basic (in shares)	123,285	106,757	115,307	106,394
Diluted (in shares)	123,285	108,143	115,307	107,893
Other comprehensive (loss) income, net of tax:
Change in unrealized losses on available-for-sale securities	$ 216	$ 0	$ 2,125	$ 0
Foreign currency translation adjustments	(56)	(74)	2	(196)
Comprehensive (loss) income	$ (126,836)	$ 47,602	$ (260,245)	$ 5,669

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)
Net income (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, 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]
Foreign currency translation adjustments	(196)
Change in unrealized losses on available-for-sale securities	0
Net income (loss)	5,865
Ending balance at Mar. 31, 2023	465,106	$ 199	1,260,310	(332)	(813,405)	18,334
Ending balance (in shares) at Mar. 31, 2023		106,869,000
Beginning balance at Dec. 31, 2022	396,372	$ 199	1,239,178	(258)	(862,080)	19,333
Beginning balance (in shares) at Dec. 31, 2022		106,140,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	20,612		20,612
Exercise of stock options	520		520
Exercise of stock options (in shares)		64,000
Common stock - restricted stock units vesting (in shares)		665,000
Foreign currency translation adjustments	(74)			(74)
Change in unrealized losses on available-for-sale securities	0
Net income (loss)	47,676				48,675	(999)
Ending balance at Mar. 31, 2023	465,106	$ 199	1,260,310	(332)	(813,405)	18,334
Ending balance (in shares) at Mar. 31, 2023		106,869,000
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)		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 income (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
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]
Exercise of stock options (in shares)	154,347
Foreign currency translation adjustments	$ 2
Change in unrealized losses on available-for-sale securities	2,125
Net income (loss)	(262,372)
Ending balance at Mar. 31, 2024	495,405	$ 217	1,768,866	(1,095)	(1,284,194)	11,611
Ending balance (in shares) at Mar. 31, 2024		124,133,000
Beginning balance at Dec. 31, 2023	173,714	$ 200	1,320,356	(1,255)	(1,158,894)	13,307
Beginning balance (in shares) at Dec. 31, 2023		107,500,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	17,750		17,750
Exercise of stock options	1,512		1,512
Exercise of stock options (in shares)		120,000
Common stock - restricted stock units vesting	0	$ 1	(1)
Common stock - restricted stock units vesting (in shares)		723,000
Common stock issued, net of offering costs	429,265	$ 16	429,249
Common stock issued, net of offering costs (in shares)		15,790,000
Foreign currency translation adjustments	(56)			(56)
Change in unrealized losses on available-for-sale securities	216			216
Net income (loss)	(126,996)				(125,300)	(1,696)
Ending balance at Mar. 31, 2024	$ 495,405	$ 217	$ 1,768,866	$ (1,095)	$ (1,284,194)	$ 11,611
Ending balance (in shares) at Mar. 31, 2024		124,133,000

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
CASH FLOWS FROM OPERATING ACTIVITIES:
Net (loss) income	$ (262,372)	$ 5,865
Adjustments to reconcile net loss to net cash flow from operating activities
Stock-based compensation	37,444	40,002
Depreciation and amortization	8,788	5,358
Amortization (accretion) of note premiums/discounts	896	(82)
Realized gain on investments	(80)	0
Non-cash interest expense on liability related to the sale of future royalties	12,612	7,906
Changes in operating assets and liabilities:
Accounts receivable	0	(68,024)
Prepaid expenses and other current assets	(2,643)	20,309
Accounts payable	(8,402)	6,688
Accrued expenses	(11)	(27,279)
Deferred revenue	(866)	(99,135)
Operating lease, net	4,417	1,205
Net cash used in operating activities	(210,217)	(107,187)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(102,731)	(66,225)
Purchases of investments	(309,982)	(192,528)
Proceeds from sales and maturities of investments	208,615	141,994
Net cash used in investing activities	(204,098)	(116,759)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from the exercises of stock options	1,779	1,096
Proceeds from the issuance of common stock, net of offering costs	429,265	0
Proceeds from the sale of future royalties	0	250,000
Net cash provided by financing activities	431,044	251,096
Net increase in cash, cash equivalents and restricted cash	16,729	27,150
Effect of exchange rate on cash, cash equivalents and restricted cash	84	(196)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH:
BEGINNING OF PERIOD	110,891	108,005
END OF PERIOD	127,704	134,959
Supplementary disclosure of cash flows:
Income taxes paid	(3,014)	0
Supplemental disclosure of noncash investing activities:
Capital expenditures included in accrued expenses	$ 7,265	$ 12,831

Organization and Significant Ac

Organization and Significant Accounting Policies	6 Months Ended
	Mar. 31, 2024
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) Phase 3 Arrowhead zodasiran (ARO-ANG3) 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. Thus far in fiscal 2024, the Company has continued to develop and advance its pipeline and partnered candidates. Several key recent developments include: • Completed enrollment in Amgen’s Phase 3 OCEAN(a) - outcome trial of olpasiran, triggering a $50.0 million milestone payment to the Company, which was paid in the third quarter of fiscal 2024; • Presented final data from the double-blind treatment period of the Company’s Phase 2 SHASTA-2 study of investigational plozasiran in patients with severe Hypertriglyceridemia. Results from the SHASTA-2 study showed dramatic, consistent, and sustained reductions in Apolipoprotein C-III (APOC3) and triglycerides and improvement in multiple atherogenic lipoprotein levels; • Announced an Expanded Access Program (EAP) to make investigational plozasiran available outside of a clinical trial for qualifying patients with familial chylomicronemia syndrome (FCS); • Initiated 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; • 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; • 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 March 31, 2024 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 six months ended March 31, 2024 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 March 31, 2024, the Company had $127.7 million in cash, cash equivalents and restricted cash ($2.2 million in restricted cash) and $395.4 million in available-for-sale securities to fund operations. During the six months ended March 31, 2024, the Company’s cash, cash equivalents and restricted cash and investments balance increased by $119.5 million which was primarily due to the net proceeds of $429.3 million from the underwritten offering in January 2024 discussed below, offset by ongoing expenses related to the Company’s research and development programs, general and administrative expenses and capital expenditures. 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 $429.3 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 six months ended March 31, 2024 and 2023, 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	6 Months Ended
	Mar. 31, 2024
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 March 31, Six Months Ended March 31, 2024 2023 2024 2023 (in thousands) GSK $ — $ 30,000 $ 2,685 $ 29,323 Horizon — — — 21,667 Takeda — 116,156 866 132,468 Janssen — 111 — 355 Amgen — — — 25,000 Total $ — $ 146,267 $ 3,551 $ 208,813 The following table summarizes the balance of receivables and contract liabilities related to the Company’s collaboration and license agreements: March 31, 2024 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 March 31, 2024. 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 was 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 Phase 2 study visits for patients in the SEQUOIA and AROAAT2002 studies concluded by December 31, 2023, and the Company has substantially completed its performance obligation under the Takeda license agreement. As such, all revenue has been fully recognized as of December 31, 2023. There were no further deferred revenue and contract liabilities as of March 31, 2024 . The Company has recorded $13.8 million as accrued expenses as of March 31, 2024 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 March 31, 2024. In November 2022, Royalty Pharma Investments 2019 ICAV (“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	6 Months Ended
	Mar. 31, 2024
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: March 31, 2024 September 30, 2023 (in thousands) Land $ 2,996 $ 2,996 Building 75,262 — Research equipment 62,926 56,509 Furniture 4,150 1,540 Computers and software 923 700 Leasehold improvements 104,723 103,813 Construction in progress 158,243 166,655 409,223 332,213 Less: Accumulated depreciation and amortization (49,971) (41,951) Property, plant and equipment, net $ 359,252 $ 290,262 Depreciation and amortization expense for property, plant and equipment for the three months ended March 31, 2024 and 2023 was $4.1 million and $2.2 million, respectively. Depreciation and amortization expense for property and equipment for the six months ended March 31, 2024 and 2023 was $7.9 million and $4.5 million, respectively. During the first quarter of fiscal 2024, the Company completed the build out of one of its laboratory and office facilities in Verona, Wisconsin, which resulted in the reclassification of $75.3 million from construction in progress to building as of March 31, 2024. Further, the Company commenced depreciation on the newly completed facility over a 39-year period. Accrued Expenses Accrued expenses consist of the following: March 31, 2024 September 30, 2023 (in thousands) Accrued R&D expenses $ 14,443 $ 16,125 Accrued R&D expenses; co-development 13,751 5,895 Accrued capital expenditures 7,265 14,044 Other 1,596 3,699 Total accrued expense $ 37,055 $ 39,763

Investments

Investments	6 Months Ended
	Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract]
Investments	INVESTMENTS The Company’s investments consisted of the following: As of March 31, 2024 (in thousands) Adjusted Basis Gross Gross Fair Value Available-for-sale securities $ 396,249 $ 11 $ (850) $ 395,410 Total current investments $ 396,249 $ 11 $ (850) $ 395,410 As of September 30, 2023 (in thousands) Adjusted Basis Gross Gross Fair Value Available-for-sale securities $ 295,699 $ 3 $ (2,967) $ 292,735 Total current investments $ 295,699 $ 3 $ (2,967) $ 292,735

Intangible Assets

Intangible Assets	6 Months Ended
	Mar. 31, 2024
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 March 31, 2024 Patents $ 21,728 $ 14,097 $ — $ 7,631 14 License 3,129 1,348 — 1,781 21 Total intangible assets, net $ 24,857 $ 15,445 $ — $ 9,412 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 six months ended March 31, 2024 and 2023. 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 March 31, 2024 and 2023, and $0.9 million for each of six months ended March 31, 2024 and 2023. 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 March 31, 2024: Amortization Expense Year Ending September 30, (in thousands) 2024 (remainder) $ 850 2025 1,700 2026 1,700 2027 1,700 2028 1,700 Thereafter 1,762 Total $ 9,412

Stockholders' Equity

Stockholders' Equity	6 Months Ended
	Mar. 31, 2024
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 March 31, 2024 Common stock $ 0.001 290,000 124,133 124,133 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 March 31, 2024 and September 30, 2023, respectively, 11,723,683 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, 2021 Incentive Plan, as well as for other 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 $429.3 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 March 31, 2024, no shares have been issued under the Open Market Sale Agreement.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Mar. 31, 2024
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 March 31, 2024. Commitments The Company owns land in the Verona Technology Park in Verona, Wisconsin, which is being developed into an approximately 160,000 square foot drug manufacturing facility and an approximately 140,000 square foot laboratory and office facility which will support the Company’s manufacturing process development and analytical activities. During the first quarter of fiscal 2024, the Company completed the build out of one of its laboratory and office facilities.

Leases

Leases	6 Months Ended
	Mar. 31, 2024
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 received $30.8 million ATIA from the lessor during the first quarter of fiscal 2024. 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 March 31, 2024 September 30, 2023 (in thousands) Operating lease assets Right-of-use assets $ 44,626 $ 45,297 Current operating lease liabilities Lease liabilities 5,285 10,563 Non-current operating lease liabilities Lease liabilities, net of current portion 113,632 104,608 Three Months Ended March 31, Six Months Ended March 31, Lease Cost Classification 2024 2023 2024 2023 (in thousands) Operating lease cost Research and development $ 2,572 $ 2,343 $ 5,566 $ 4,412 General and administrative expense 491 500 967 1,033 Variable lease cost (1) Research and development 836 160 1,615 370 General and administrative expense — — — — Total $ 3,899 $ 3,003 $ 8,148 $ 5,815 (1) Variable lease cost is primarily related to operating expenses associated with the Company’s operating leases. There was no short-term lease cost during the first half of fiscal 2024. There was $0.4 million and $0.7 million short-term lease cost during the three and six months ended March 31, 2023, respectively. The following table presents maturities of operating lease liabilities on an undiscounted basis as of March 31, 2024: Year Amounts (in thousands) 2024 (remainder of fiscal year) $ 6,997 2025 15,356 2026 15,696 2027 14,869 2028 13,511 2029 and thereafter 128,356 Total $ 194,785 Less imputed interest $ (75,868) Total operating lease liabilities (includes current portion) $ 118,917 Supplemental cash flow and other information related to leases was as follows: Three Months Ended March 31, Six Months Ended March 31, 2024 2023 2024 2023 (in thousands) Cash received for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ — $ 8,918 $ 3,099 $ 17,929 Right-of-use assets obtained in exchanged for amended operating lease liabilities $ — $ — $ 64 $ 22,582 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 2,046 $ 1,098 $ 4,016 $ 2,196 March 31, 2024 2023 Weighted-average remaining lease term (in years) 13.0 6.6 Weighted-average discount rate 8.0 % 8.5 %

Stock-Based Compensation

Stock-Based Compensation	6 Months Ended
	Mar. 31, 2024
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 2,967,887 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 March 31, 2024. 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 March 31, 2024, the total number of shares available for issuance was 4,553,827 shares, which includes 158,678 and 134,389 shares that were forfeited under the 2013 and 2021 Plans, respectively, and 3,689,089 shares have been granted under the 2021 Plan. In addition, there were 688,165 shares reserved for options and 637,563 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 March 31, 2024 2004 Plan 2013 Plan 2021 Plan Inducement Awards Total Granted and outstanding awards: Options — 1,358,377 32,151 688,165 2,078,693 Restricted stock units — 1,609,510 2,844,090 637,563 5,091,163 Total — 2,967,887 2,876,241 1,325,728 7,169,856 The following table summarizes stock-based compensation expenses included in operating expenses: Three Months Ended March 31, Six Months Ended March 31, 2024 2023 2024 2023 Research and development $ 7,097 $ 8,745 $ 15,413 $ 17,147 General and administrative 9,491 11,868 18,860 22,855 Total $ 16,588 $ 20,613 $ 34,273 $ 40,002 Stock Option Awards The following table presents a summary of the stock option activity for the six months ended March 31, 2024: Shares Weighted- Weighted- Aggregate Outstanding at September 30, 2023 2,263,477 $ 22.68 Granted — — Cancelled or expired (30,437) 61.11 Exercised (154,347) 11.53 Outstanding at March 31, 2024 2,078,693 $ 23.04 4.0 years $ 26,788,979 Exercisable at March 31, 2024 2,054,866 $ 22.72 3.9 years $ 26,788,746 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 March 31, 2024 and 2023 was $2.5 million and $1.4 million, respectively. The total intrinsic value of the options exercised during the six months ended March 31, 2024 and 2023 was $3.1 million and $3.6 million, respectively Stock-based compensation expense related to stock options outstanding for the three months ended March 31, 2024 and 2023, was $0.6 million and $2.2 million, respectively. Stock-based compensation expense related to stock options outstanding for the six months ended March 31, 2024 and 2023, was $2.1 million and $4.6 million, respectively. As of March 31, 2024, the pre-tax compensation expense for all outstanding unvested stock options in the amount of $0.8 million will be recognized in the Company’s results of operations over a weighted average period of 4 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 six months ended March 31, 2024 and 2023. 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 and six months ended March 31, 2024, stock-based compensation expense related to the Visirna ESOP was $1.2 million and $3.2 million, respectively. 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 1,838,025 30.75 Vested (876,352) 53.60 Forfeited (112,150) 44.16 Outstanding at March 31, 2024 5,091,163 $ 49.58 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 March 31, 2024 and 2023, the Company recorded $16.0 million and $18.4 million of expense related to RSUs, respectively. For the six months ended March 31, 2024 and 2023, the Company recorded $32.2 million and $35.4 million of expense related to RSUs, respectively. As of March 31, 2024, there was $112.0 million of total unrecognized compensation cost related to RSUs that is expected to be recognized over a weighted-average period of 1.7 years.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Mar. 31, 2024
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 March 31, 2024 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: March 31, 2024 Level 1 Level 2 Level 3 Total (in thousands) Available-for-sale securities U.S. Treasuries $ 4,963 $ — $ — $ 4,963 U.S. government bonds — 48,637 — 48,637 Municipal securities — 3,949 — 3,949 Commercial notes — 120,710 — 120,710 Corporate debt securities — 217,151 — 217,151 Total available-for-sale securities 4,963 390,447 — 395,410 Cash equivalents Money market instruments 77,080 — — 77,080 U.S. Treasuries 4,993 — — 4,993 Commercial notes — 4,997 — 4,997 Total cash equivalents 82,073 4,997 — 87,070 Total financial assets $ 87,036 $ 395,444 $ — $ 482,480 September 30, 2023 Level 1 Level 2 Level 3 Total (in thousands) Available-for-sale 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 securities 31,553 261,182 — 292,735 Cash equivalents Money market instruments 347 — — 347 Total cash equivalents 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	6 Months Ended
	Mar. 31, 2024
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. The Company records the obligations at their carrying value using the effective interest method. In order to amortize the sale of future royalties, the Company utilizes the prospective method to estimate the future royalties to be paid by the Company to the counterparty over the life of the arrangement. Under the prospective method, a new effective interest rate is determined based on the revised estimate of remaining cash flows. The new rate is the discount rate that equates the present value of the revised estimate of remaining cash flows with the carrying amount of the debt, and it will be used to recognize non-cash interest expense for the remaining periods. The Company periodically assesses the amount and the timing of expected royalty payments using a combination of internal projections and forecasts from external sources. The estimates of future net product sales (and resulting royalty payments) are based on key assumptions including population, penetration, probability of success and sales price, among others. To the extent such payments are greater or less than the Company’s initial estimates or the timing of such payments is materially different than its original estimates, the Company will prospectively adjust the amortization of the royalty financing obligations and the effective interest rate. As of March 31, 2024, the estimated effective interest rate was 9.3%. The following table presents the activity with respect to the liability related to the sale of future royalties. Carrying Amount (in thousands) Carrying value as of September 30, 2023 $ 268,326 Non-cash interest expense recognized 12,612 Carrying value as of March 31, 2024 $ 280,938

Net Loss Per Share

Net Loss Per Share	6 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Net Loss Per Share	NET LOSS PER SHARE The following table presents the computation of basic and diluted net loss per share for the three and six months ended March 31, 2024 and 2023. Three Months Ended March 31, Six Months Ended March 31, 2024 2023 2024 2023 (in thousands, except per share amounts) Numerator: Net (loss) income attributable to Arrowhead Pharmaceuticals, Inc. $ (125,300) $ 48,675 $ (258,164) $ 7,350 Denominator: Weighted-average basic shares outstanding 123,285 106,757 115,307 106,394 Effect of dilutive securities — 1,386 — 1,499 Weighted-average diluted shares outstanding 123,285 108,143 115,307 107,893 Basic net (loss) gain per share $ (1.02) $ 0.46 $ (2.24) $ 0.07 Diluted net (loss) gain per share $ (1.02) $ 0.45 $ (2.24) $ 0.07 The following table sets forth the potentially dilutive securities that have been excluded from the calculation of diluted net loss per share because to include them would be anti-dilutive. Three Months Ended March 31, Six Months Ended March 31, 2024 2023 2024 2023 (in thousands) Options 595 767 657 779 Restricted stock units 3,411 3,583 3,931 3,290 Total 4,006 4,350 4,588 4,069

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Pay vs Performance Disclosure
Net (loss) income attributable to Arrowhead Pharmaceuticals, Inc.	$ (125,300)	$ 48,675	$ (258,164)	$ 7,350

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended	6 Months Ended
	Mar. 31, 2024 shares	Mar. 31, 2024 shares
Trading Arrangements, by Individual
Material Terms of Trading Arrangement		During the quarter ended March 31, 2024, the following directors and officers (as defined in Exchange Act Rule 16a-1(f)) adopted certain trading plans intended to satisfy Rule 10b5-1(c): Name Title Adoption Date Plan Start Date Plan End Date Shares Vesting and Subject to Sell-To- Cover (1) Other James Hamilton Chief Discovery and Translational Medicine 3/26/2024 1/6/2025 1/17/2025 60,000 n/a Ken Myszkowski Chief Financial Officer 3/12/2024 6/11/2024 2/28/2025 n/a 115,000 Ken Myszkowski Chief Financial Officer 3/28/2024 1/6/2025 1/17/2025 15,000 n/a Ken Myszkowski Chief Financial Officer 3/28/2024 1/6/2025 1/17/2025 15,000 n/a Ken Myszkowski Chief Financial Officer 3/28/2024 1/6/2025 1/17/2025 15,000 n/a Ken Myszkowski Chief Financial Officer 3/28/2024 1/6/2025 1/17/2025 18,750 n/a Patrick O'Brien Chief Operating Officer and General Counsel 3/21/2024 1/6/2025 1/17/2025 67,500 n/a William Waddill Board Member 3/27/2024 12/16/2024 12/31/2024 n/a 7,495 (1)
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false
James Hamilton [Member]
Trading Arrangements, by Individual
Name	James Hamilton
Title	Chief Discovery and Translational Medicine
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	3/26/2024
Arrangement Duration	11 days
Aggregate Available	60,000	60,000
Patrick O'Brien [Member]
Trading Arrangements, by Individual
Name	Patrick O'Brien
Title	Chief Operating Officer and General Counsel
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	3/21/2024
Arrangement Duration	11 days
Aggregate Available	67,500	67,500
William Waddill [Member]
Trading Arrangements, by Individual
Name	William Waddill
Title	Board Member
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	3/27/2024
Arrangement Duration	15 days
Aggregate Available	7,495	7,495
Ken Myszkowski March 12 Plan [Member] | Ken Myszkowski [Member]
Trading Arrangements, by Individual
Name	Ken Myszkowski
Title	Chief Financial Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	3/12/2024
Arrangement Duration	262 days
Aggregate Available	115,000	115,000
Ken Myszkowski March 28 Plan 1 [Member] | Ken Myszkowski [Member]
Trading Arrangements, by Individual
Name	Ken Myszkowski
Title	Chief Financial Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	3/28/2024
Arrangement Duration	11 days
Aggregate Available	15,000	15,000
Ken Myszkowski March 28 Plan 2 [Member] | Ken Myszkowski [Member]
Trading Arrangements, by Individual
Name	Ken Myszkowski
Title	Chief Financial Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	3/28/2024
Arrangement Duration	11 days
Aggregate Available	15,000	15,000
Ken Myszkowski March 28 Plan 3 [Member] | Ken Myszkowski [Member]
Trading Arrangements, by Individual
Name	Ken Myszkowski
Title	Chief Financial Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	3/28/2024
Arrangement Duration	11 days
Aggregate Available	15,000	15,000
Ken Myszkowski March 28 Plan 4 [Member] | Ken Myszkowski [Member]
Trading Arrangements, by Individual
Name	Ken Myszkowski
Title	Chief Financial Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	3/28/2024
Arrangement Duration	11 days
Aggregate Available	18,750	18,750

Organization and Significant _2

Organization and Significant Accounting Policies (Policies)	6 Months Ended
	Mar. 31, 2024
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)	6 Months Ended
	Mar. 31, 2024
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) Phase 3 Arrowhead zodasiran (ARO-ANG3) 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)	6 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Revenue	The following table provides a summary of revenue recognized: Three Months Ended March 31, Six Months Ended March 31, 2024 2023 2024 2023 (in thousands) GSK $ — $ 30,000 $ 2,685 $ 29,323 Horizon — — — 21,667 Takeda — 116,156 866 132,468 Janssen — 111 — 355 Amgen — — — 25,000 Total $ — $ 146,267 $ 3,551 $ 208,813
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: March 31, 2024 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)	6 Months Ended
	Mar. 31, 2024
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: March 31, 2024 September 30, 2023 (in thousands) Land $ 2,996 $ 2,996 Building 75,262 — Research equipment 62,926 56,509 Furniture 4,150 1,540 Computers and software 923 700 Leasehold improvements 104,723 103,813 Construction in progress 158,243 166,655 409,223 332,213 Less: Accumulated depreciation and amortization (49,971) (41,951) Property, plant and equipment, net $ 359,252 $ 290,262
Schedule of Accrued Expenses	Accrued expenses consist of the following: March 31, 2024 September 30, 2023 (in thousands) Accrued R&D expenses $ 14,443 $ 16,125 Accrued R&D expenses; co-development 13,751 5,895 Accrued capital expenditures 7,265 14,044 Other 1,596 3,699 Total accrued expense $ 37,055 $ 39,763

Investments (Tables)

Investments (Tables)	6 Months Ended
	Mar. 31, 2024
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 March 31, 2024 (in thousands) Adjusted Basis Gross Gross Fair Value Available-for-sale securities $ 396,249 $ 11 $ (850) $ 395,410 Total current investments $ 396,249 $ 11 $ (850) $ 395,410 As of September 30, 2023 (in thousands) Adjusted Basis Gross Gross Fair Value Available-for-sale securities $ 295,699 $ 3 $ (2,967) $ 292,735 Total current investments $ 295,699 $ 3 $ (2,967) $ 292,735

Intangible Assets (Tables)

Intangible Assets (Tables)	6 Months Ended
	Mar. 31, 2024
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 March 31, 2024 Patents $ 21,728 $ 14,097 $ — $ 7,631 14 License 3,129 1,348 — 1,781 21 Total intangible assets, net $ 24,857 $ 15,445 $ — $ 9,412 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 March 31, 2024: Amortization Expense Year Ending September 30, (in thousands) 2024 (remainder) $ 850 2025 1,700 2026 1,700 2027 1,700 2028 1,700 Thereafter 1,762 Total $ 9,412

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	6 Months Ended
	Mar. 31, 2024
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 March 31, 2024 Common stock $ 0.001 290,000 124,133 124,133 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)	6 Months Ended
	Mar. 31, 2024
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 March 31, 2024 September 30, 2023 (in thousands) Operating lease assets Right-of-use assets $ 44,626 $ 45,297 Current operating lease liabilities Lease liabilities 5,285 10,563 Non-current operating lease liabilities Lease liabilities, net of current portion 113,632 104,608 Three Months Ended March 31, Six Months Ended March 31, Lease Cost Classification 2024 2023 2024 2023 (in thousands) Operating lease cost Research and development $ 2,572 $ 2,343 $ 5,566 $ 4,412 General and administrative expense 491 500 967 1,033 Variable lease cost (1) Research and development 836 160 1,615 370 General and administrative expense — — — — Total $ 3,899 $ 3,003 $ 8,148 $ 5,815 (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 March 31, 2024: Year Amounts (in thousands) 2024 (remainder of fiscal year) $ 6,997 2025 15,356 2026 15,696 2027 14,869 2028 13,511 2029 and thereafter 128,356 Total $ 194,785 Less imputed interest $ (75,868) Total operating lease liabilities (includes current portion) $ 118,917 Supplemental cash flow and other information related to leases was as follows: Three Months Ended March 31, Six Months Ended March 31, 2024 2023 2024 2023 (in thousands) Cash received for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ — $ 8,918 $ 3,099 $ 17,929 Right-of-use assets obtained in exchanged for amended operating lease liabilities $ — $ — $ 64 $ 22,582 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 2,046 $ 1,098 $ 4,016 $ 2,196 March 31, 2024 2023 Weighted-average remaining lease term (in years) 13.0 6.6 Weighted-average discount rate 8.0 % 8.5 %

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Summary of Awards Outstanding	The following table presents a summary of awards outstanding: As of March 31, 2024 2004 Plan 2013 Plan 2021 Plan Inducement Awards Total Granted and outstanding awards: Options — 1,358,377 32,151 688,165 2,078,693 Restricted stock units — 1,609,510 2,844,090 637,563 5,091,163 Total — 2,967,887 2,876,241 1,325,728 7,169,856
Share-Based Payment Arrangement, Expensed	The following table summarizes stock-based compensation expenses included in operating expenses: Three Months Ended March 31, Six Months Ended March 31, 2024 2023 2024 2023 Research and development $ 7,097 $ 8,745 $ 15,413 $ 17,147 General and administrative 9,491 11,868 18,860 22,855 Total $ 16,588 $ 20,613 $ 34,273 $ 40,002
Summarized Information about Stock Options	The following table presents a summary of the stock option activity for the six months ended March 31, 2024: Shares Weighted- Weighted- Aggregate Outstanding at September 30, 2023 2,263,477 $ 22.68 Granted — — Cancelled or expired (30,437) 61.11 Exercised (154,347) 11.53 Outstanding at March 31, 2024 2,078,693 $ 23.04 4.0 years $ 26,788,979 Exercisable at March 31, 2024 2,054,866 $ 22.72 3.9 years $ 26,788,746
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 1,838,025 30.75 Vested (876,352) 53.60 Forfeited (112,150) 44.16 Outstanding at March 31, 2024 5,091,163 $ 49.58

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Mar. 31, 2024
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: March 31, 2024 Level 1 Level 2 Level 3 Total (in thousands) Available-for-sale securities U.S. Treasuries $ 4,963 $ — $ — $ 4,963 U.S. government bonds — 48,637 — 48,637 Municipal securities — 3,949 — 3,949 Commercial notes — 120,710 — 120,710 Corporate debt securities — 217,151 — 217,151 Total available-for-sale securities 4,963 390,447 — 395,410 Cash equivalents Money market instruments 77,080 — — 77,080 U.S. Treasuries 4,993 — — 4,993 Commercial notes — 4,997 — 4,997 Total cash equivalents 82,073 4,997 — 87,070 Total financial assets $ 87,036 $ 395,444 $ — $ 482,480 September 30, 2023 Level 1 Level 2 Level 3 Total (in thousands) Available-for-sale 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 securities 31,553 261,182 — 292,735 Cash equivalents Money market instruments 347 — — 347 Total cash equivalents 347 — — 347 Total financial assets $ 31,900 $ 261,182 $ — $ 293,082

Liability Related to the Sale_2

Liability Related to the Sale of Future Royalties (Tables)	6 Months Ended
	Mar. 31, 2024
Liability Related To The Sale Of Future Royalties [Abstract]
Schedule of Liability Related to the Sale of Future Royalties	The following table presents the activity with respect to the liability related to the sale of future royalties. Carrying Amount (in thousands) Carrying value as of September 30, 2023 $ 268,326 Non-cash interest expense recognized 12,612 Carrying value as of March 31, 2024 $ 280,938

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	6 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Schedule of Earnings Per Share, Basic and Diluted	The following table presents the computation of basic and diluted net loss per share for the three and six months ended March 31, 2024 and 2023. Three Months Ended March 31, Six Months Ended March 31, 2024 2023 2024 2023 (in thousands, except per share amounts) Numerator: Net (loss) income attributable to Arrowhead Pharmaceuticals, Inc. $ (125,300) $ 48,675 $ (258,164) $ 7,350 Denominator: Weighted-average basic shares outstanding 123,285 106,757 115,307 106,394 Effect of dilutive securities — 1,386 — 1,499 Weighted-average diluted shares outstanding 123,285 108,143 115,307 107,893 Basic net (loss) gain per share $ (1.02) $ 0.46 $ (2.24) $ 0.07 Diluted net (loss) gain per share $ (1.02) $ 0.45 $ (2.24) $ 0.07
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The following table sets forth the potentially dilutive securities that have been excluded from the calculation of diluted net loss per share because to include them would be anti-dilutive. Three Months Ended March 31, Six Months Ended March 31, 2024 2023 2024 2023 (in thousands) Options 595 767 657 779 Restricted stock units 3,411 3,583 3,931 3,290 Total 4,006 4,350 4,588 4,069

Organization and Significant _4

Organization and Significant Accounting Policies - Narrative (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
	Jan. 02, 2024	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023
Organization And Significant Accounting Policies [Line Items]
Cash, cash equivalents and restricted cash		$ 127,700		$ 127,700
Restricted cash		2,200		2,200
Available-for-sale securities, at fair value		395,410		395,410		$ 292,735
Increase (decrease) in cash and investments				119,500
Development regulatory and sales milestones payments		2,800,000		2,800,000
Income tax (benefit) expense		0	$ 0	(3,313)	$ 17
Underwriting Agreement
Organization And Significant Accounting Policies [Line Items]
Net proceeds	$ 429,300
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
Amgen | Royalty Pharma Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment receivable upon achievement of enrollment in phase 3 clinical trial		$ 50,000		$ 50,000

Collaboration and License Agr_3

Collaboration and License Agreements - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 146,267	$ 3,551	$ 208,813
GSK
Disaggregation of Revenue [Line Items]
Revenue	0	30,000	2,685	29,323
Horizon
Disaggregation of Revenue [Line Items]
Revenue	0	0	0	21,667
Takeda
Disaggregation of Revenue [Line Items]
Revenue	0	116,156	866	132,468
Janssen
Disaggregation of Revenue [Line Items]
Revenue	0	111	0	355
Amgen
Disaggregation of Revenue [Line Items]
Revenue	$ 0	$ 0	$ 0	$ 25,000

Collaboration and License Agr_4

Collaboration and License Agreements - Receivables and Contract Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	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	Mar. 31, 2024	Dec. 11, 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) $ in Thousands	1 Months Ended
	Oct. 31, 2020 USD ($) obligation bundle	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development regulatory and sales milestones payments		$ 2,800,000
Accrued expenses		37,055	$ 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
Accrued expenses		13,800
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	6 Months Ended
	Sep. 30, 2016 USD ($) agreement	Mar. 31, 2024 USD ($)	Mar. 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]
Proceeds from the issuance of common stock, net of offering costs		$ 429,265,000	$ 0
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, net of offering costs	$ 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 ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 0	$ 146,267	$ 3,551	$ 208,813
Deferred revenue	0		0		$ 866
Visirna Therapeutics, Inc. | Visirna License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	100	$ 900
Contract assets	0		0
Deferred revenue	$ 0		$ 0

Balance Sheet Accounts - Summar

Balance Sheet Accounts - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Sep. 30, 2023
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 409,223		$ 409,223			$ 332,213
Less: Accumulated depreciation and amortization	(49,971)		(49,971)			(41,951)
Property, plant and equipment, net	359,252		359,252			290,262
Depreciation and amortization expense for property and equipment	4,100	$ 2,200	7,900	$ 4,500
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,996		2,996			2,996
Building
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	75,262		75,262			0
Property, plant and equipment, reclassification			75,300
Useful life					39 years
Research equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	62,926		62,926			56,509
Furniture
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,150		4,150			1,540
Computers and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	923		923			700
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	104,723		104,723			103,813
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 158,243		158,243			$ 166,655
Property, plant and equipment, reclassification			$ (75,300)

Balance Sheet Accounts - Accrue

Balance Sheet Accounts - Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued R&D expenses	$ 14,443	$ 16,125
Accrued R&D expenses; co-development	13,751	5,895
Accrued capital expenditures	7,265	14,044
Other	1,596	3,699
Total accrued expense	$ 37,055	$ 39,763

Investments - Summary of Short-

Investments - Summary of Short-term, Long-term Investments and Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Sep. 30, 2023
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Current investments, Available-for-sale, Adjusted Basis	$ 396,249	$ 295,699
Current investments, Available-for-sale, Gross Unrealized Gains	11	3
Current investments, Available-for-sale, Gross Unrealized Losses	(850)	(2,967)
Current investments, Available-for-sale, Fair Value	395,410	292,735
Debt securities
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Current investments, Available-for-sale, Adjusted Basis	396,249	295,699
Current investments, Available-for-sale, Gross Unrealized Gains	11	3
Current investments, Available-for-sale, Gross Unrealized Losses	(850)	(2,967)
Current investments, Available-for-sale, Fair Value	$ 395,410	$ 292,735

Intangible Assets - Schedule of

Intangible Assets - Schedule of Intangible Assets (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023
Finite Lived Intangible Assets [Line Items]
Net Carrying Amount	$ 9,412,000		$ 9,412,000		$ 10,262,000
Amortization expense	400,000	$ 400,000	900,000	$ 900,000
Novartis
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	24,857,000		24,857,000		24,857,000
Accumulated Amortization	15,445,000		15,445,000		14,595,000
Impairment	0		0		0
Net Carrying Amount	9,412,000		9,412,000		10,262,000
Novartis | Patents
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	21,728,000		21,728,000		21,728,000
Accumulated Amortization	14,097,000		14,097,000		13,321,000
Impairment	0		0		0
Net Carrying Amount	$ 7,631,000		$ 7,631,000		$ 8,407,000
Useful Lives	14 years		14 years		14 years
Novartis | License
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 3,129,000		$ 3,129,000		$ 3,129,000
Accumulated Amortization	1,348,000		1,348,000		1,274,000
Impairment	0		0		0
Net Carrying Amount	$ 1,781,000		$ 1,781,000		$ 1,855,000
Useful Lives	21 years		21 years		21 years

Intangible Assets - Expected Fu

Intangible Assets - Expected Future Amortization (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Sep. 30, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
2024 (remainder)	$ 850
2025	1,700
2026	1,700
2027	1,700
2028	1,700
Thereafter	1,762
Net Carrying Amount	$ 9,412	$ 10,262

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($)	Jan. 02, 2024	Dec. 02, 2022	Mar. 31, 2024	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)			124,133,000	107,312,000
Common stock, shares outstanding (in shares)			124,133,000	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)			4,553,827
Underwriting Agreement
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	$ 429,300,000
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)			11,723,683	12,709,837

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Detail) ft² in Thousands	Mar. 31, 2024 USD ($)	Dec. 20, 2021 ft²
Other Commitments [Line Items]
Contingent liabilities	$ 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	247,000,000
Facilities | Minimum
Other Commitments [Line Items]
Amount intends to invest for buildout of the facilities	37,000,000
Facilities | Maximum
Other Commitments [Line Items]
Amount intends to invest for buildout of the facilities	$ 51,000,000

Leases - Narrative (Details)

Leases - Narrative (Details) ft² in Thousands, $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2023 USD ($)	Mar. 31, 2024 USD ($) ft² option	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Sep. 25, 2023 USD ($)
Lessee Lease Description [Line Items]
Short-term lease cost | $	$ 0.4	$ 0	$ 0.7
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		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023
Lease Assets and Liabilities
Right-of-use assets	$ 44,626		$ 44,626		$ 45,297
Lease liabilities	5,285		5,285		10,563
Lease liabilities, net of current portion	113,632		113,632		$ 104,608
Lease Cost
Total	3,899	$ 3,003	8,148	$ 5,815
Research and development
Lease Cost
Operating lease cost	2,572	2,343	5,566	4,412
Variable lease cost	836	160	1,615	370
General and administrative expense
Lease Cost
Operating lease cost	491	500	967	1,033
Variable lease cost	$ 0	$ 0	$ 0	$ 0

Leases - Summary of Maturities

Leases - Summary of Maturities of Operating Lease Liabilities on an Undiscounted Basis (Details) $ in Thousands	Mar. 31, 2024 USD ($)
Leases [Abstract]
2024 (remainder of fiscal year)	$ 6,997
2025	15,356
2026	15,696
2027	14,869
2028	13,511
2029 and thereafter	128,356
Total	194,785
Less imputed interest	(75,868)
Total operating lease liabilities (includes current portion)	$ 118,917

Leases - Supplemental Cash Flow

Leases - Supplemental Cash Flow and Other Information Related to Leases (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Cash received for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 0	$ 8,918	$ 3,099	$ 17,929
Right-of-use assets obtained in exchange for amended operating lease liabilities	0	0	64	22,582
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 2,046	$ 1,098	$ 4,016	$ 2,196
Weighted-average remaining lease term (in years)	13 years	6 years 7 months 6 days	13 years	6 years 7 months 6 days
Weighted-average discount rate	8%	8.50%	8%	8.50%

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Oct. 01, 2023	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Mar. 18, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, share reserve for issuance (in shares)		4,553,827		4,553,827
Granted (in shares)				0	0
Stock-based compensation expense		$ 16,588	$ 20,613	$ 34,273	$ 40,002
Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Intrinsic value of options exercised		2,500	1,400	3,100	3,600
Stock-based compensation expense		600	2,200	2,100	4,600
Unrecognized pre-tax compensation expense		800		$ 800
Weighted average period to recognize pre-tax compensation expense				4 months
Restricted stock units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		16,000	$ 18,400	$ 32,200	$ 35,400
Weighted average period to recognize pre-tax compensation expense				1 year 8 months 12 days
Unrecognized pre-tax compensation expense		$ 112,000		$ 112,000
2004 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance (in shares)		0		0
2013 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance (in shares)		2,967,887		2,967,887
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)				134,389
Granted (in shares)				3,689,089
Inducement Awards | Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance (in shares)		688,165		688,165
Inducement Awards | Restricted stock units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance (in shares)		637,563		637,563
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		$ 1,200		$ 3,200

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Granted and Outstanding Shares (Details) - shares	Mar. 31, 2024	Sep. 30, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options outstanding (in shares)	2,078,693	2,263,477
Total (in shares)	7,169,856
Restricted stock units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment award (in shares)	5,091,163	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
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)	1,358,377
Total (in shares)	2,967,887
2013 Incentive Plan | Restricted stock units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment award (in shares)	1,609,510
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)	2,876,241
2021 Incentive Plan | Restricted stock units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment award (in shares)	2,844,090
Inducement Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options outstanding (in shares)	688,165
Total (in shares)	1,325,728
Inducement Awards | Restricted stock units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment award (in shares)	637,563

Stock-Based Compensation - Expe

Stock-Based Compensation - Expenses Included in Operating Expenses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 16,588	$ 20,613	$ 34,273	$ 40,002
Research and development
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	7,097	8,745	15,413	17,147
General and administrative expense
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 9,491	$ 11,868	$ 18,860	$ 22,855

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summarize Information about Stock Options (Details)	6 Months Ended
	Mar. 31, 2024 USD ($) $ / shares shares	Mar. 31, 2023 shares
Shares
Beginning balance (in shares) | shares	2,263,477
Granted (in shares) | shares	0	0
Cancelled (in shares) | shares	(30,437)
Exercised (in shares) | shares	(154,347)
Ending balance (in shares) | shares	2,078,693
Number of Options Outstanding, Exercisable (in shares) | shares	2,054,866
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	11.53
Ending balance (in dollars per share) | $ / shares	23.04
Weighted-Average Exercise Price Per Share, Exercisable (in dollars per share) | $ / shares	$ 22.72
Weighted- Average Remaining Contractual Term (Years)	4 years
Weighted-Average Remaining Contractual Term, Exercisable	3 years 10 months 24 days
Aggregate Intrinsic Value | $	$ 26,788,979
Aggregate Intrinsic Value, Exercisable | $	$ 26,788,746

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of RSUs Activity (Details) - Restricted stock units	6 Months Ended
	Mar. 31, 2024 $ / shares shares
Number of RSUs
Beginning of period (in shares) | shares	4,241,640
Granted (in shares) | shares	1,838,025
Vested (in shares) | shares	(876,352)
Forfeited (in shares) | shares	(112,150)
End of period (in shares) | shares	5,091,163
Weighted- Average Grant Date Fair Value Per Share
Beginning balance (in dollars per share) | $ / shares	$ 58.43
Granted (in dollars per share) | $ / shares	30.75
Vested (in dollars per share) | $ / shares	53.60
Forfeited (in dollars per share) | $ / shares	44.16
Ending balance (in dollars per share) | $ / shares	$ 49.58

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	Mar. 31, 2024	Sep. 30, 2023
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 395,410	$ 292,735
Cash equivalents	87,070	347
Total financial assets	482,480	293,082
Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	77,080	347
U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	4,993
Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	4,997
U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	4,963
U.S. government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	48,637	31,553
Municipal securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	3,949	7,093
Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	120,710	22,205
Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	217,151	231,884
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	4,963	31,553
Cash equivalents	82,073	347
Total financial assets	87,036	31,900
Level 1 | Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	77,080	347
Level 1 | U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	4,993
Level 1 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Level 1 | U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	4,963
Level 1 | U.S. government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	31,553
Level 1 | Municipal securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Level 1 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Level 1 | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	390,447	261,182
Cash equivalents	4,997	0
Total financial assets	395,444	261,182
Level 2 | Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 2 | U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Level 2 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	4,997
Level 2 | U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0
Level 2 | U.S. government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	48,637	0
Level 2 | Municipal securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	3,949	7,093
Level 2 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	120,710	22,205
Level 2 | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	217,151	231,884
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Cash equivalents	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]
Cash equivalents	0	0
Level 3 | U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Level 3 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Level 3 | U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0
Level 3 | U.S. government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Level 3 | Municipal securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Level 3 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Level 3 | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 0	$ 0

Liability Related to the Sale_3

Liability Related to the Sale of Future Royalties - Narrative (Details) - Royalty Pharma Agreement - USD ($) $ in Millions	1 Months Ended
	Nov. 30, 2022	Mar. 31, 2024
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment	$ 250
Milestone payment receivable	160
Milestone payment receivable upon FDA approval	50
Milestone payment receivable upon receipt of royalty payments	60
Royalty payment threshold	70
Liability related to the sale of future royalties		$ 250
Liability related to the sale of future royalties, interest rate		9.30%
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment receivable upon achievement of enrollment in phase 3 clinical trial		$ 50
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Initial transaction price	$ 410

Liability Related to the Sale_4

Liability Related to the Sale of Future Royalties - Activity (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2024	Mar. 31, 2024	Mar. 31, 2023
Liability, Sale Of Future Royalties [Roll Forward]
Carrying value as of September 30, 2023		$ 268,326
Non-cash interest expense recognized	$ 12,612	12,612	$ 7,906
Carrying value as of March 31, 2024	$ 280,938	$ 280,938

Net Loss Per Share - Basic and

Net Loss Per Share - Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Numerator:
Net (loss) income attributable to Arrowhead Pharmaceuticals, Inc.	$ (125,300)	$ 48,675	$ (258,164)	$ 7,350
Denominator:
Weighted-average basic shares outstanding (in shares)	123,285	106,757	115,307	106,394
Effect of dilutive securities (in shares)	0	1,386	0	1,499
Weighted-average diluted shares outstanding (in shares)	123,285	108,143	115,307	107,893
Basic net (loss) gain per share (in dollars per share)	$ (1.02)	$ 0.46	$ (2.24)	$ 0.07
Diluted net (loss) gain per share (in dollars per share)	$ (1.02)	$ 0.45	$ (2.24)	$ 0.07

Net Loss Per Share - Antiduliti

Net Loss Per Share - Antidulitive (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted earnings per share (in shares)	4,006	4,350	4,588	4,069
Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted earnings per share (in shares)	595	767	657	779
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted earnings per share (in shares)	3,411	3,583	3,931	3,290