S-8

As filed with the Securities and Exchange Commission on February 28, 2024

Registration No. 333-268665               

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549

FORM S-8
REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

ARROWHEAD PHARMACEUTICALS, INC.

(Exact Name of Registrant as Specified in its Charter)

Delaware	46-0408024
(State or Other Jurisdiction of Incorporation or Organization)	(I.R.S. Employer Identification No.)

177 E. Colorado Blvd, Suite 700
Pasadena, CA 91105
(Address of Principal Executive Offices)

ARROWHEAD PHARMACEUTICALS, INC. INDUCEMENT PLAN
(Full Title of the Plan)

Christopher Anzalone
President and Chief Executive Officer
177 E. Colorado Blvd, Suite 700
Pasadena, CA 91105
Telephone: (626) 304-3400
(Name, address, and telephone number, including area code, of agent for service)

Copy to:

Ryan A. Murr
Gibson, Dunn & Crutcher LLP
555 Mission Street, Suite 3000

San Francisco, California 94105
Telephone: (415) 393-8373
Facsimile: (415) 374-8430

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer”, “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

Large accelerated filer	☒	Accelerated filer	☐
Non-accelerated filer	☐	Smaller reporting company	☐
Emerging growth company	☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☐

Introduction

This registration statement on Form S-8 is being filed by Arrowhead Pharmaceuticals, Inc. (the “Registrant” or the “Company”) to register a total of 832,950 shares of Common Stock, par value $0.001 per share (“Common Stock”), issuable under the Arrowhead Pharmaceuticals, Inc. Inducement Plan (the “Plan”), including previously announced restricted stock units granted at various dates from March 29, 2023 to December 28, 2023 to new employees as inducement awards in connection with the commencement of employment pursuant to NASDAQ Listing Rule 5635(c)(4).

Part I

INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

Information required by Part I to be contained in the Section 10(a) prospectus is omitted from this registration statement in accordance with Rule 428 under the Securities Act of 1933, as amended (the “Securities Act”), and the Note to Part I of Form S-8.

Part II

INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3. Incorporation of Documents by Reference.

The Securities and Exchange Commission (the “Commission”) allows us to “incorporate by reference” the information we file with them, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be part of this registration statement, and later information filed with the Commission will update and supersede this information. We hereby incorporate by reference into this registration statement the following documents previously filed with the Commission:

•The Company’s annual report on Form 10-K for the fiscal year ended September 30, 2023, filed on November 29, 2023;

•The Company’s current reports on Form 8-K filed on December 5, 2023, January 4, 2024, and January 22, 2024;

•The Company’s quarterly report on Form 10-Q for the period ended December 31, 2023, filed on February 6, 2023; and

•The description of the Company’s Common Stock contained in its registration statement on Form 8-A/A, filed on April 7, 2016, including any amendments or reports filed for the purpose of updating such description.

All documents that the Registrant subsequently files pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act prior to the filing of a post-effective amendment to the registration statement which indicates that all of the shares of Common Stock offered have been sold or which deregisters all of such shares then remaining unsold, shall be deemed to be incorporated by reference in this registration statement and to be a part hereof from the date of the filing of such documents, except as to any document or report (or any portion thereof) that is not deemed filed under such provisions. For the purposes of this registration statement, any statement contained in a document incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or superseded to the extent that a statement contained herein or in any other subsequently filed document which also is or is deemed to be incorporated by reference herein modifies or supersedes such statement. Any such statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this registration statement.

Under no circumstances will any information furnished under Items 2.02 or 7.01 of Form 8-K be deemed incorporated herein by reference unless such Form 8-K expressly provides to the contrary.

Item 4. Description of Securities.

Not applicable.

Item 5. Interests of Named Experts and Counsel.

None.

Item 6. Indemnification of Directors and Officers.

The Company’s Certificate of Incorporation, as amended, provides for the elimination of personal monetary liability of directors to the fullest extent permissible under Delaware law. Delaware law does not permit the elimination or limitation of director monetary liability for: (i) breaches of the director’s duty of loyalty to the corporation or its stockholders; (ii) acts or omissions not in good faith or involving intentional misconduct or knowing violations of law; (iii) the unlawful payment of dividends or unlawful stock repurchases or redemptions or (iv) transactions in which the director received an improper personal benefit.

Section 145 of the Delaware General Corporation Law permits a Delaware corporation to indemnify, on certain terms and conditions, any person who was or is a party or is threatened to be made a party to any threatened pending or completed action, suit or proceeding, whether civil, criminal, administrative or investigative, by reason of the fact that he is or was a director, officer, employee or agent of the corporation or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise against expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by him in connection with such action. The Certificate of Incorporation and Bylaws of the Company require the Company to indemnify the Company’s directors and officers to the fullest extent permitted under Delaware law.

We have entered into indemnification agreements with each of our directors and our executive officers. These agreements provide that we will indemnify each of our directors and executive officers to the fullest extent permitted by Delaware law. We will advance expenses, including attorneys’ fees, judgments, fines and settlement amounts, to each indemnified director and executive officer in connection with any proceeding in which indemnification is available and we will indemnify our directors and officers for any action or proceeding arising out of that person’s services as an officer or director brought on behalf of the Company or in furtherance of our rights.

We also maintain general liability insurance which covers certain liabilities of our directors and officers arising out of claims based on acts or omissions in their capacities as directors or officers, including liabilities under the Securities Act.

Item 7. Exemption from Registration Claimed.

Not applicable.

Item 8. Exhibits

Exhibit No.	Description
4.1	Form of Common Stock Certificate of Arrowhead Pharmaceuticals, Inc. (1)
4.2	Amended and Restated Certificate of Incorporation of Arrowhead Research Corporation (effecting, among other things a change in the corporation’s name from “Arrowhead Research Corporation” to “Arrowhead Pharmaceuticals, Inc.”), filed with the Secretary of the State of Delaware on April 5, 2016 (1)
4.3	Certificate of Amendment to the Amended and Restated Certificate of Incorporation of Arrowhead Pharmaceuticals, Inc. (effecting an increase in the total number of shares the corporation may issue), filed with the Secretary of the State of Delaware on April 27, 2023) (2)
4.4	Second Amended and Restated Bylaws of Arrowhead Pharmaceuticals, Inc., as amended January 24, 2023 (3)
4.5	Arrowhead Pharmaceuticals, Inc. 2021 Incentive Plan (4)
4.6	Arrowhead Pharmaceuticals, Inc. Inducement Plan*
5.1	Opinion of Gibson, Dunn & Crutcher LLP*
23.1	Consent of Rose, Snyder & Jacobs LLP*
23.2	Consent of Gibson, Dunn & Crutcher LLP (filed as a part of Exhibit 5.1)
24.1	Power of Attorney (set forth on signature page of this registration statement)
99.1	Form of RSU Agreement for Officers and Certain Other Employees (Arrowhead Pharmaceuticals, Inc. 2021 Incentive Plan- Inducement Award) (5)
99.2	Form of RSU Agreement for Employees (Arrowhead Pharmaceuticals, Inc. 2021 Incentive Plan- Inducement Award) (5)
107	Calculation of Filing Fee Table*

*    Filed herewith.

(1)    Filed as an exhibit to the Registrant’s Current Report on Form 8-K, filed on April 6, 2016 and incorporated herein by reference.

(2)    Filed as an exhibit to the Registrant’s Current Report on Form 10-Q, filed on May 2, 2023 and incorporated herein by reference.

(3)    Filed as an exhibit to the Registrant’s Current Report on Form 8-K, filed on January 30, 2023 and incorporated herein by reference.

(4)    Filed as an exhibit to the Registrant’s Registration Statement on Form S-8, filed on May 18, 2021 and incorporated herein by reference.

(5)    Filed as an exhibit to the Registrant’s Registration Statement on Form S-8, filed on December 22, 2021 and incorporated herein by reference.

Item 9. Undertakings.

(a)The undersigned Registrant hereby undertakes:

(1)To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement:

(i)To include any prospectus required by Section 10(a)(3) of the Securities Act;

(ii)To reflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information in the registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than 20 percent change in the maximum aggregate offering price set forth in the “Calculation of Registration Fee” table in the effective registration statement;

(iii)To include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement;

Provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) of this section do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the Commission by the Registrant pursuant to Section 13 or Section 15(d) of the Securities Exchange Act (15 U.S.C. 78m or 78o(d)) that are incorporated by reference in the registration statement.

(2)That, for the purpose of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(3)To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

(b)The undersigned Registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, each filing of the Registrant’s annual report pursuant to Section 13(a) or Section 15(d) of the Exchange Act (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Exchange Act) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(c)Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the Registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the Registrant of expenses incurred or paid by a director, officer or controlling person of the Registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the Registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

SIGNATURES

Pursuant to the requirements of the Securities Act, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Pasadena, California, on February 28, 2024.

Arrowhead Pharmaceuticals, Inc.

By:    /s/ Christopher Anzalone
Christopher Anzalone
President and Chief Executive Officer

POWER OF ATTORNEY

Each of the undersigned hereby constitutes and appoints each of Christopher Anzalone and Kenneth A. Myszkowski, his or her attorney-in-fact, with power of substitution, in his or her name and in the capacity indicated below, to sign any and all further amendments (including post-effective amendments) to this registration statement on Form S-8 and to file the same, with exhibits thereto and other documents in connection therewith, with the Commission, hereby ratifying and confirming all that said attorney-in-fact, or his substitute or substitutes, may do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act, this registration statement has been signed by the following persons in the capacities and on the date indicated.

Signature	Title	Date
/s/ Christopher Anzalone	President, Chief Executive Officer and Director (Principal Executive Officer)	February 28, 2024
/s/ Kenneth A. Myszkowski	Chief Financial Officer (Principal Financial and Accounting Officer)	February 28, 2024
/s/ Mauro Ferrari	Director	February 28, 2024
/s/ Douglass Given	Director	February 28, 2024
/s/ Michael S. Perry	Director	February 28, 2024
/s/ William Waddill	Director	February 28, 2024
/s/ Adeoye Olukotun	Director	February 28, 2024
/s/ Victoria Vakiener	Director	February 28, 2024

EX-FILING FEES

Exhibit 107 CALCULATION OF FILING FEE TABLE Form S-8 Arrowhead Pharmaceuticals, Inc. Table 1 – Newly Registered Securities Security Type Security Class Title Fee Calculatio n Rule Amount Registered (1) Proposed Maximum Offering Price Per Unit Maximum Aggregate Offering Price Fee Rate Amount of Registration Fee Equity Common stock, par value $0.001 per share 457(a) 832,950(2) $29.67 (3) $24,713,627 $147.60 per $1,000,000 $3,647.73 Total Offering Amounts $24,713,627 $3,647.73 Total Fee Offsets $0 Net Fee Due $3,647.73 (1) Pursuant to Rule 416 of the Securities Act of 1933, as amended (the “Securities Act”), this registration statement also covers any additional shares of common stock, par value $0.001 per share (“Common Stock”), that become issuable by reason of any stock split, stock dividend or similar transaction effected without the receipt of consideration which results in an increase in the number of the registrant’s outstanding shares of Common Stock. (2) Represents shares of Common Stock issuable upon vesting granted under the Plan, including restricted stock unit awards granted to 166 employees from March 29, 2023 through December 28, 2023 as inducement awards made pursuant to NASDAQ Listing Rule 5635(c)(4). (3) Estimated solely for the purpose of calculating the registration fee pursuant to Rule 457 of the Securities Act, and based on the average of the high and low sale prices of the registrant’s Common Stock, as reported on the Nasdaq Global Select Market, on February 22, 2024.

EX-4.6

1 ARROWHEAD PHARMACEUTICALS, INC. INDUCEMENT PLAN 1. DEFINED TERMS Exhibit A, which is incorporated by reference, defines the terms used in the Plan and sets forth certain operational rules related to those terms. 2. PURPOSE The Plan has been established to advance the interests of the Company by providing for the grant to Participants of Stock-based and other incentive Awards. Each Award granted under the Plan is intended to qualify as an employment inducement award pursuant to Nasdaq Listing Rule 5635(c)(4), and shall be interpreted and administered accordingly. 3. ADMINISTRATION The Administrator has discretionary authority, subject only to the express provisions of the Plan, to interpret the Plan; determine eligibility for and grant Awards; determine, modify or waive the terms and conditions of any Award; prescribe forms, rules and procedures relating to the Plan; determine whether Awards should be settled in cash and/or shares of Stock; and otherwise do all things necessary or appropriate to carry out the purposes of the Plan. Determinations of the Administrator made under the Plan will be conclusive and will bind all parties. 4. LIMITS ON AWARDS UNDER THE PLAN (a) Number of Shares. The maximum number of shares of Stock that may be delivered in satisfaction of Awards under the Plan is 555,300. For purposes of this Section 4(a), the aggregate number of shares of Stock available for issuance under this Plan at any time shall not be reduced by (i) shares subject to Awards that have been retained or withheld by the Company in payment or satisfaction of the exercise price or tax withholding obligation of an Award, (ii) shares of Stock underlying the portion of any Award that expires, terminates or is forfeited or is settled in cash prior to the issuance of Stock thereunder, or (iii) shares subject to Awards that otherwise do not result in the issuance of shares in connection with payment or settlement thereof. To the extent consistent with applicable legal requirements (including applicable stock exchange requirements), Stock issued under awards of an acquired company that are converted, replaced or adjusted in connection with the acquisition shall not reduce the number of shares of Stock available for Awards under the Plan. In addition, shares that have been delivered (either actually or by attestation) to the Company in payment or satisfaction of the exercise price or tax withholding obligation of an Award shall be available for issuance under this Plan. (b) Type of Shares. Stock delivered by the Company under the Plan may be authorized but unissued Stock or previously issued Stock acquired by the Company. No fractional shares of Stock will be delivered under the Plan. 5. ELIGIBILITY AND PARTICIPATION The Administrator will select Participants from among only individuals who, at the time of grant (i) has not previously been an Employee or director, or (ii) is commencing employment following a bona fide period of non-employment by the Company or any subsidiary; provided that in each case, the grant of the Award is made as a material inducement to his or her commencement as an Employee of the Company or a subsidiary.

2 6. RULES APPLICABLE TO AWARDS (a) All Awards. (1) Award Provisions. The Administrator will determine the terms of all Awards, subject to the limitations provided herein. By accepting (or, under such rules as the Administrator may prescribe, being deemed to have accepted) an Award, the Participant will be deemed to have agreed to the terms of the Award and the Plan. Notwithstanding any provision of this Plan to the contrary, awards of an acquired company that are converted, replaced or adjusted in connection with the acquisition may contain terms and conditions that are inconsistent with the terms and conditions specified herein, as determined by the Administrator. (2) Term of Plan. No Awards may be made after ten years from the Date of Adoption, but previously granted Awards may continue beyond that date in accordance with their terms. (3) Transferability. Except as the Administrator otherwise expressly provides in accordance with the third sentence of this Section 6(a)(3), no Award may be transferred other than by will or by the laws of descent and distribution. During a Participant’s lifetime, except as the Administrator otherwise expressly provides in accordance with the third sentence of this Section 6(a)(3), SARs and Stock Options may be exercised only by the Participant. The Administrator may permit the gratuitous transfer (i.e., transfer not for value) of Awards to any transferee eligible to be covered by the provisions of Form S-8 (under the Securities Act of 1933, as amended), subject to such limitations as the Administrator may impose. (4) Vesting, etc. Subject to Section 6(a)(5), the Administrator will determine the time or times at which an Award will vest or become exercisable and the terms on which a Stock Option or SAR will remain exercisable. Without limiting the foregoing, the Administrator may at any time accelerate the vesting or exercisability of an Award, regardless of any adverse or potentially adverse tax or other consequences resulting from such acceleration. Unless the Administrator expressly provides otherwise, however, the following rules will apply if a Participant’s Employment ceases: (A) Immediately upon the cessation of the Participant’s Employment and except as provided in (B), (C), and (D) below, each Stock Option and SAR that is then held by the Participant or by the Participant’s permitted transferees, if any, will cease to be exercisable and will terminate and all other Awards that are then held by the Participant or by the Participant’s permitted transferees, if any, to the extent not already vested will be forfeited. (B) Subject to (C), (D) and (E) below, all Stock Options and SARs held by the Participant or the Participant’s permitted transferees, if any, immediately prior to the cessation of the Participant’s Employment, to the extent then exercisable, will remain exercisable for the lesser of (i) a period of three months and (ii) the period ending on the latest date on which such Stock Option or SAR could have been exercised without regard to this Section 6(a)(4), and will thereupon immediately terminate. (C) All Stock Options and SARs held by a Participant or the Participant’s permitted transferees, if any, immediately prior to the cessation of the Participant’s Employment due to his or her death, to the extent then exercisable, will remain exercisable for the lesser of (i) a period of twelve months and (ii) the period ending on the latest date on which such Stock Option or SAR could have been exercised without regard to this Section 6(a)(4), and will thereupon immediately terminate. (D) All Stock Options and SARs held by a Participant or the Participant’s permitted transferees, if any, immediately prior to the cessation of the Participant’s Employment due to his or her “permanent and total disability” (within the meaning of Section 22(e)(3) of the Code), to the extent then exercisable, will remain exercisable for the lesser of (i) a period of six months and (ii) the period ending on the latest date on which such Stock Option or SAR could have been exercised without regard to this Section 6(a)(4), and will thereupon immediately terminate. (E) All Awards (whether or not exercisable) held by a Participant or the Participant’s permitted transferees, if any, immediately prior to the cessation of the Participant’s Employment will immediately terminate upon such cessation of Employment if the termination is for Cause or occurs in circumstances that in the sole determination of the Administrator would have constituted grounds for the Participant’s Employment to be terminated for Cause. (5) Additional Restrictions. The Administrator may cancel, rescind, withhold or otherwise limit or restrict any Award at any time if the Participant is not in compliance with all applicable provisions of the Award agreement and the Plan, or if the Participant breaches any agreement with the Company or its Affiliates with respect to non-competition, non-

3 solicitation or confidentiality. Without limiting the generality of the foregoing, the Administrator may recover Awards made under the Plan and payments under or gain in respect of any Award in accordance with all applicable Company clawback or recoupment policy, as such policy may be amended and in effect from time to time, or as otherwise required by law or applicable stock exchange listing standards, including, without limitation, the Arrowhead Pharmaceuticals, Inc. Compensation Recoupment (Clawback) Policy. (6) Taxes. The grant of an Award and the delivery, vesting and retention of Stock, cash or other property under an Award are conditioned upon full satisfaction by the Participant of all tax withholding requirements with respect to the Award. The Administrator will prescribe such rules for the withholding of taxes as it deems necessary or appropriate. The Administrator may, but need not, hold back shares of Stock from an Award or permit a Participant to tender previously owned shares of Stock in satisfaction of tax withholding requirements (only up to the amount permitted that will not cause an adverse accounting consequence or cost). (7) Dividend Equivalents, Etc. The Administrator may provide for the payment of amounts (on terms and subject to conditions established by the Administrator) in lieu of cash dividends or other cash distributions with respect to Stock subject to an Award whether or not the holder of such Award is otherwise entitled to share in the actual dividend or distribution in respect of such Award. Any entitlement to dividend equivalents or similar entitlements will be established and administered either consistent with an exemption from, or in compliance with, the requirements of Section 409A. Dividends or dividend equivalent amounts payable in respect of Awards that are subject to restrictions will be subject to the same vesting and forfeiture restrictions as apply to the Awards to which they relate. (8) Rights Limited. Nothing in the Plan or in any Award will be construed as giving any person the right to continued employment or service with the Company or its Affiliates, or any rights as a stockholder except as to shares of Stock actually issued under the Plan; nor will anything in the Plan or in any Award affect the right of the Company or its Affiliates to discharge or discipline a Participant at any time. The loss of existing or potential profit in Awards will not constitute an element of damages in the event of termination of Employment for any reason, even if the termination is in violation of an obligation of the Company or any Affiliate to the Participant. (9) Coordination with Other Plans. Awards under the Plan may be granted in tandem with, or in satisfaction of or substitution for, other Awards under the Plan or awards made under other compensatory plans or programs of the Company or its Affiliates. (10) Section 409A. Each Award will contain such terms as the Administrator determines, and will be construed and administered, such that the Award either qualifies for an exemption from the requirements of Section 409A or satisfies such requirements. (b) Stock Options and SARs. Only Stock Options that are not intended to qualify as an “incentive stock option” within the meaning of Section 422 of the Code may be granted under the Plan. (1) Time And Manner Of Exercise. Unless the Administrator expressly provides otherwise, no Stock Option or SAR will be deemed to have been exercised until the Administrator receives a notice of exercise (in form acceptable to the Administrator), which may be an electronic notice, signed (including electronic signature in form acceptable to the Administrator) by the appropriate person and accompanied by any payment required under the Award. A Stock Option or SAR exercised by any person other than the Participant will not be deemed to have been exercised until the Administrator has received such evidence as it may require that the person exercising the Award has the right to do so. (2) Exercise Price. The exercise price (or the base value from which appreciation is to be measured) of each Award requiring exercise will be no less than 100% of the Fair Market Value of the Stock subject to the Award, determined as of the date of grant, or such higher amount as the Administrator may determine in connection with the grant. Except in connection with a corporate transaction involving the Company (which term shall include, without limitation, any stock dividend, stock split, extraordinary cash dividend, recapitalization, reorganization, merger, consolidation, split-up, spin-off, combination, or exchange of shares) or as otherwise contemplated by Section 7 of the Plan, the Company may not, without obtaining stockholder approval, (A) amend the terms of outstanding Stock Options or SARs to reduce the exercise price or base value of such Stock Options or SARs, (B) cancel outstanding Stock Options or SARs in exchange for Stock Options or SARs with an exercise price or base value that is less than the exercise price or base value of the original Stock Options or SARs, or (C) cancel outstanding Stock Options or SARs that have an exercise price or base value greater than the Fair Market Value of a share of Stock on the date of such cancellation in exchange for cash or other consideration.

4 (3) Payment Of Exercise Price. Where the exercise of an Award is to be accompanied by payment, payment of the exercise price will be by cash or check acceptable to the Administrator or by such other legally permissible means, if any, as may be acceptable to the Administrator. (4) Maximum Term. Stock Options and SARs will have a maximum term not to exceed ten (10) years from the date of grant; provided, however, that, if a Participant still holding an outstanding but unexercised Stock Option or SAR ten (10) years from the date of grant (or, in the case of a Stock Option or SAR with a maximum term of less than ten (10) years, such maximum term) is prohibited by applicable law or a written policy of the Company applicable to similarly situated employees from engaging in any open-market sales of Stock, and if at such time the Stock is publicly traded (as determined by the Administrator), the maximum term of such Award will instead be deemed to expire on the thirtieth (30th) day following the date the Participant is no longer prohibited from engaging in such open market sales. 7. EFFECT OF CERTAIN TRANSACTIONS (a) Mergers, etc. Except as otherwise provided in an Award agreement, the following provisions will apply in the event of a Covered Transaction: (1) Assumption or Substitution. If the Covered Transaction is one in which there is an acquiring or surviving entity, the Administrator may (but, for the avoidance of doubt, need not) provide (i) for the assumption or continuation of some or all outstanding Awards or any portion thereof or (ii) for the grant of new awards in substitution therefor by the acquiror or survivor or an affiliate of the acquiror or survivor. (2) Cash-Out of Awards. Subject to Section 7(a)(6) below, the Administrator may (but, for the avoidance of doubt, need not) provide for payment (a “cash-out”), with respect to some or all Awards or any portion thereof, equal in the case of each affected Award or portion thereof to the excess, if any, of (A) the Fair Market Value of one share of Stock (as determined by the Administrator in its reasonable discretion) times the number of shares of Stock subject to the Award or such portion, over (B) the aggregate exercise or purchase price, if any, under the Award or such portion (in the case of an SAR, the aggregate base value above which appreciation is measured), in each case on such payment terms (which need not be the same as the terms of payment to holders of Stock) and other terms, and subject to such conditions, as the Administrator determines, it being understood that if the exercise or purchase price (or base value) of an Award is equal to or greater than the Fair Market Value of one share of Stock (as determined in accordance with this Section 7(a)(2)), the Award may be cancelled with no payment due hereunder. (3) Acceleration of Certain Awards. Subject to Section 7(a)(6) below, the Administrator may (but, for the avoidance of doubt, need not) provide that any Award requiring exercise will become exercisable, in full or in part and/or that the delivery of any shares of Stock remaining deliverable under any outstanding Award of Stock Units (including Restricted Stock Units and Performance Awards to the extent consisting of Stock Units) will be accelerated in full or in part, in each case on a basis that gives the holder of the Award a reasonable opportunity, as determined by the Administrator, following exercise of the Award or the delivery of the shares, as the case may be, to participate as a stockholder in the Covered Transaction. (4) Change in Control. Notwithstanding anything herein to the contrary, unless otherwise expressly provided for in the Award agreement or another contract, including an employment or severance agreement or severance plan, or under the terms of a transaction constituting a Change in Control, in the event of a Change in Control, each outstanding Award will fully vest (which, in the case of Performance Awards shall be to their maximal value) and become exercisable immediately prior to the consummation of such Change in Control, and the Administrator shall notify the Participant in writing or electronically that the Award will be fully vested and exercisable for a period of at least fifteen (15) days from the date of such notice, (which notice may be delivered prior to the consummation of the Change in Control) and the Award will terminate upon the expiration of such period if not otherwise assumed or substituted pursuant to Section 7(a)(1) above or cashed-out pursuant to Section 7(a)(2) above. (5) Termination of Awards Upon Consummation of Covered Transaction. Except as the Administrator may otherwise determine in any case and except for the 15-day exercise period set forth above in Section 7(a)(4), each Award will automatically terminate (and in the case of outstanding shares of Restricted Stock, will automatically be forfeited) upon consummation of the Covered Transaction, other than Awards assumed pursuant to Section 7(a)(1) above. (6) Additional Limitations. Any share of Stock and any cash or other property delivered pursuant to Section 7(a)(2) or Section 7(a)(3) above with respect to an Award may, in the discretion of the Administrator, contain such restrictions, if any, as the Administrator deems appropriate to reflect any performance or other vesting conditions to which the

5 Award was subject and that did not lapse (and were not satisfied) in connection with the Covered Transaction. For purposes of the immediately preceding sentence, a cash-out under Section 7(a)(2) above or acceleration under Section 7(a)(3) above will not, in and of itself, be treated as the lapsing (or satisfaction) of a performance or other vesting condition. In the case of Restricted Stock that does not vest and is not forfeited in connection with the Covered Transaction, the Administrator may require that any amounts delivered, exchanged or otherwise paid in respect of such Stock in connection with the Covered Transaction be placed in escrow or otherwise made subject to such restrictions as the Administrator deems appropriate to carry out the intent of the Plan. (b) Changes in and Distributions With Respect to Stock. (1) Basic Adjustment Provisions. In the event of a stock dividend, stock split or combination of shares (including a reverse stock split), recapitalization, reclassification or other distribution of the Company’s equity securities without the receipt of consideration by the Company, or other change in the Company’s capital structure that constitutes an equity restructuring within the meaning of FASB ASC 718, the Administrator will make appropriate adjustments to the maximum number of shares specified in Section 4(a) that may be delivered under the Plan and will also make appropriate adjustments to the number and kind of shares of stock or securities subject to Awards then outstanding or subsequently granted, any exercise or purchase prices (or base values) relating to Awards and any other provision of Awards affected by such change. (2) Certain Other Adjustments. The Administrator may also make adjustments of the type described in Section 7(b)(1) above to take into account distributions to stockholders other than those provided for in Section 7(a) and 7(b)(1), or any other event, if the Administrator determines that adjustments are appropriate to avoid distortion in the operation of the Plan, having due regard for the requirements of Section 409A, where applicable. (3) Continuing Application of Plan Terms. References in the Plan to shares of Stock will be construed to include any stock or securities resulting from an adjustment pursuant to this Section 7. 8. LEGAL CONDITIONS ON DELIVERY OF STOCK The Company will not be obligated to deliver any shares of Stock pursuant to the Plan or to remove any restriction from shares of Stock previously delivered under the Plan until: (i) the Company is satisfied that all legal matters in connection with the issuance and delivery of such shares have been addressed and resolved; (ii) if the outstanding Stock is at the time of delivery listed on any stock exchange or national market system, the shares to be delivered have been listed or authorized to be listed on such exchange or system upon official notice of issuance; and (iii) all conditions of the Award have been satisfied or waived. The Company may require, as a condition to exercise of the Award, such representations or agreements as counsel for the Company may consider appropriate to avoid violation of the Securities Act of 1933, as amended, or any applicable state or non-U.S. securities law. Any Stock required to be issued to Participants under the Plan will be evidenced in such manner as the Administrator may deem appropriate, including book-entry registration or delivery of stock certificates. In the event that the Administrator determines that Stock certificates will be issued to Participants under the Plan, the Administrator may require that certificates evidencing Stock issued under the Plan bear an appropriate legend reflecting any restriction on transfer applicable to such Stock, and the Company may hold the certificates pending lapse of the applicable restrictions. 9. AMENDMENT AND TERMINATION 10. OTHER COMPENSATION ARRANGEMENTS The Administrator may at any time or times amend the Plan or any outstanding Award for any purpose which may at the time be permitted by law, and may at any time terminate the Plan as to any future grants of Awards; provided, that except as otherwise expressly provided in the Plan the Administrator may not, without the Participant’s consent, alter the terms of an Award so as to affect materially and adversely the Participant’s rights under the Award, unless the Administrator expressly reserved the right to do so at the time the Award was granted or unless the Administrator determines in its sole discretion and prior to the date of any Covered Transaction that such amendment or alteration either (i) is required or advisable in order for the Company, the Plan or the Award to satisfy any law or regulation or to meet the requirements of or avoid adverse financial accounting consequences under any accounting standard, or (ii) is not reasonably likely to significantly diminish the benefits provided under such Award, or that any such diminishment has been adequately compensated.

6 The existence of the Plan or the grant of any Award will not in any way affect the Company’s right to Award a person bonuses or other compensation in addition to Awards under the Plan. 11. MISCELLANEOUS (a) Waiver of Jury Trial. By accepting an Award under the Plan, each Participant waives any right to a trial by jury in any action, proceeding or counterclaim concerning any rights under the Plan and any Award, or under any amendment, waiver, consent, instrument, document or other agreement delivered or which in the future may be delivered in connection therewith, and agrees that any such action, proceedings or counterclaim will be tried before a court and not before a jury. By accepting an Award under the Plan, each Participant certifies that no officer, representative, or attorney of the Company has represented, expressly or otherwise, that the Company would not, in the event of any action, proceeding or counterclaim, seek to enforce the foregoing waivers. Notwithstanding anything to the contrary in the Plan, nothing herein is to be construed as limiting the ability of the Company and a Participant to agree to submit disputes arising under the terms of the Plan or any Award made hereunder to binding arbitration or as limiting the ability of the Company to require any eligible individual to agree to submit such disputes to binding arbitration as a condition of receiving an Award hereunder. (b) Limitation of Liability. Notwithstanding anything to the contrary in the Plan, neither the Company, nor any Affiliate, nor the Administrator, nor any person acting on behalf of the Company, any Affiliate, or the Administrator, will be liable to any Participant or to the estate or beneficiary of any Participant or to any other holder of an Award by reason of any acceleration of income, or any additional tax (including any interest and penalties), asserted by reason of the failure of an Award to satisfy the requirements of Section 409A or by reason of Section 4999 of the Code, or otherwise asserted with respect to the Award. 12. ESTABLISHMENT OF SUB-PLANS The Administrator may from time to time establish one or more sub-plans under the Plan for purposes of satisfying applicable blue sky, securities or tax laws of various jurisdictions. The Administrator will establish such sub-plans by adopting supplements to the Plan setting forth (i) such limitations on the Administrator’s discretion under the Plan as it deems necessary or desirable and (ii) such additional terms and conditions not otherwise inconsistent with the Plan as it deems necessary or desirable. All supplements so established will be deemed to be part of the Plan, but each supplement will apply only to Participants within the affected jurisdiction (as determined by the Administrator). 13. GOVERNING LAW The laws of the State of Delaware will govern all questions concerning the construction, validity and interpretation of this Plan, without regard to that state’s conflict of laws rules.

7 EXHIBIT A Definition of Terms The following terms, when used in the Plan, will have the meanings and be subject to the provisions set forth below: “Administrator”: The Compensation Committee, except that the Compensation Committee may delegate (i) to one or more of its members (or one or more other members of the Board (including the full Board)) such of its duties, powers and responsibilities as it may determine; (ii) to one or more officers of the Company the power to grant Awards to the extent permitted by Section 157(c) of the Delaware General Corporation Law; and (iii) to such Employees or other persons as it determines such ministerial tasks as it deems appropriate; provided, however, that all Awards granted under the Plan must be approved by the Compensation Committee. In the event of any delegation described in the preceding sentence, the term “Administrator” will include the person or persons so delegated to the extent of such delegation. “Affiliate”: Any corporation or other entity that stands in a relationship to the Company that would result in the Company and such corporation or other entity being treated as one employer under Section 414(b) and Section 414(c) of the Code. “Award”: Any or a combination of the following: (i) Stock Options. (ii) SARs. (iii) Restricted Stock. (iv) Unrestricted Stock. (v) Stock Units, including Restricted Stock Units. (vi) Performance Awards. (vii) Cash Awards. (viii) Awards (other than Awards described in (i) through (vii) above) that are convertible into or otherwise based on Stock. “Board”: The Board of Directors of the Company. “Cash Award”: An Award denominated in cash. “Cause”: In the case of any Participant who is party to an employment or severance-benefit agreement that contains a definition of “Cause,” the definition set forth in such agreement will apply with respect to such Participant under the Plan for so long as such agreement is in effect. In the case of any other Participant, “Cause” will mean, as determined by the Administrator in its reasonable judgment, (i) a substantial failure of the Participant to perform the Participant’s duties and responsibilities to the Company or subsidiaries or substantial negligence in the performance of such duties and responsibilities; (ii) the commission by the Participant of a felony or a crime involving moral turpitude; (iii) the commission by the Participant of theft, fraud, embezzlement, material breach of trust or any material act of dishonesty involving the Company or any of its subsidiaries; (iv) a significant violation by the Participant of the code of conduct of the Company or its subsidiaries of any material policy of the Company or its subsidiaries, or of any statutory or common law duty of loyalty to the Company or its subsidiaries; (v) material breach of any of the terms of the Plan or any Award made under the Plan, or of the terms of any other agreement between the Company or subsidiaries and the Participant; or (vi) other conduct by the Participant that could be expected to be harmful to the business, interests or reputation of the Company. “Change in Control”: The first to occur of (i) a “person,” as such term is used in Sections 13(d) and 14(d) of the Exchange Act (other than the Company or one of its subsidiaries or an employee benefit plan of the Company or any

8 of its subsidiaries, including any trustee of such plan acting as trustee) becoming the “beneficial owner” (as defined in Rule 13d-3 under the Exchange Act), directly or indirectly, of securities of the Company representing fifty percent (50%) or more of the combined voting power of the Company’s then outstanding securities entitled to vote generally in the election of directors; (ii) a consummation of (x) a merger or consolidation of the Company with any other corporation, other than a merger or consolidation that would result in the voting securities of the Company outstanding immediately prior thereto continuing to represent (either by remaining outstanding or by being converted into voting securities of the surviving entity) more than fifty percent (50%) of the total voting power represented by the voting securities of the Company or such surviving entity outstanding immediately after such merger or consolidation, or (y) the sale or disposition by the Company of all or substantially all the Company’s assets; or (iii) a change in the composition of the Board, as a result of which fewer than a majority of the directors are Incumbent Directors. “Incumbent Directors” shall mean directors who either (A) are directors of the Company as of the Date of Adoption, or (B) are elected, or nominated for election, to the Board with the affirmative votes of at least a majority of the Incumbent Directors at the time of such election or nomination (but shall not include an individual whose election or nomination is in connection with an actual or threatened proxy contest relating to the election of directors to the Company). For the avoidance of doubt, a transaction will not constitute a Change in Control if: (i) its sole purpose is to change the jurisdiction of the Company’s incorporation, or (ii) its sole purpose is to create a holding company that will be owned in substantially the same proportions by the persons who held the Company’s securities immediately before such transaction. “Code”: The U.S. Internal Revenue Code of 1986 as from time to time amended and in effect, or any successor statute as from time to time in effect. “Compensation Committee”: The Compensation Committee of the Board. “Company”: Arrowhead Pharmaceuticals, Inc. “Covered Transaction”: Any of (i) a consolidation, merger, or similar transaction or series of related transactions, including a sale or other disposition of stock, in which the Company is not the surviving corporation or which results in the acquisition of all or substantially all of the Company’s then outstanding common stock by a single person or entity or by a group of persons and/or entities acting in concert; (ii) a sale or transfer of all or substantially all the Company’s assets; or (iii) a dissolution or liquidation of the Company. Where a Covered Transaction involves a tender offer that is reasonably expected to be followed by a merger described in clause (i) (as determined by the Administrator), the Covered Transaction will be deemed to have occurred upon consummation of the tender offer. “Date of Adoption”: The date the Plan was adopted by the Board. “Employee”: Any person who is employed by the Company or an Affiliate. “Employment”: A Participant’s employment or other service relationship with the Company and its Affiliates. Employment will be deemed to continue, unless the Administrator expressly provides otherwise, so long as the Participant is employed by, or otherwise is providing services as a director, consultant, or advisor to the Company or an Affiliate. If a Participant’s employment or other service relationship is with an Affiliate and that entity ceases to be an Affiliate, the Participant’s Employment will be deemed to have terminated when the entity ceases to be an Affiliate unless the Participant transfers Employment to the Company or its remaining Affiliates. Notwithstanding the foregoing and the definition of “Affiliate” above, in construing the provisions of any Award relating to the payment of “nonqualified deferred compensation” (subject to Section 409A) upon a termination or cessation of Employment, references to termination or cessation of employment, separation from service, retirement or similar or correlative terms will be construed to require a “separation from service” (as that term is defined in Section 1.409A-1(h) of the Treasury Regulations, after giving effect to the presumptions contained therein) from the Company and from all other corporations and trades or businesses, if any, that would be treated as a single “service recipient” with the Company under Section 1.409A-1(h)(3) of the Treasury Regulations. The Company may, but need not, elect in writing, subject to the applicable limitations under Section 409A, any of the special elective rules prescribed in Section 1.409A-1(h) of the Treasury Regulations for purposes of determining whether a “separation from service” has occurred. Any such written election will be deemed a part of the Plan. “Exchange Act”: The Securities Exchange Act of 1934, as amended. “Fair Market Value”: As of any date, the value of the Stock determined as follows:

9 (i) If the Stock is listed on any established stock exchange or traded on any established market, the Fair Market Value of a share of Stock as of any date of determination will be, unless otherwise determined by the Board, the closing sales price for such stock as quoted on such exchange or market (or the exchange or market with the greatest volume of trading in the Stock) on the date of determination, as reported in a source the Board deems reliable. (ii) Unless otherwise provided by the Board, if there is no closing sales price for the Stock on the date of determination, then the Fair Market Value will be the closing selling price on the last preceding date for which such quotation exists. (iii) In the absence of such markets for the Stock, the Fair Market Value will be determined by the Board in good faith and in a manner that complies with Sections 409A of the Code. “Participant”: A person who is granted an Award under the Plan. “Performance Award”: An Award subject to Performance Criteria. “Performance Criteria”: Specified criteria, other than the mere continuation of Employment or the mere passage of time, the satisfaction of which is a condition for the grant, exercisability, vesting or full enjoyment of an Award. Such criteria shall be determined by the Administrator and may include any measure of performance (measured either absolutely or by reference to an index or indices and determined either on a consolidated basis or, as the context permits, on a divisional, subsidiary, line of business, project or geographical basis or in combinations thereof) as deemed appropriate by the Administrator. A Performance Criterion and any targets with respect thereto determined by the Administrator need not be based upon an increase, a positive or improved result or avoidance of loss. “Plan”: The Arrowhead Pharmaceuticals, Inc. Inducement Plan, as from time to time amended and in effect. “Restricted Stock”: Stock subject to restrictions requiring that it be redelivered or offered for sale to the Company if specified conditions are not satisfied. “Restricted Stock Unit”: A Stock Unit that is, or as to which the delivery of Stock or cash in lieu of Stock is, subject to the satisfaction of specified performance or other vesting conditions. “SAR”: A right entitling the holder upon exercise to receive an amount (payable in cash and/or in shares of Stock of equivalent value) equal to the excess of the Fair Market Value of the shares of Stock subject to the right over the base value from which appreciation under the SAR is to be measured. “Section 409A”: Section 409A of the Code. “Stock”: Common stock of the Company, par value $0.001 per share. “Stock Option”: An option entitling the holder to acquire shares of Stock upon payment of the exercise price. “Stock Unit”: An unfunded and unsecured promise, denominated in shares of Stock, to deliver Stock or cash measured by the value of Stock in the future. “Unrestricted Stock”: Stock not subject to any restrictions under the terms of the Award.

EX-5.1

Abu Dhabi  Beijing  Brussels  Century City  Dallas  Denver  Dubai  Frankfurt  Hong Kong  Houston  London  Los Angeles Munich  New York  Orange County  Palo Alto  Paris  San Francisco  Singapore  Washington, D.C. Exhibit 5.1 February 28, 2024 Arrowhead Pharmaceuticals, Inc. 177 E. Colorado Blvd, Suite 700 Pasadena, CA 91105 Re: Arrowhead Pharmaceuticals, Inc. Registration Statement on Form S-8 Ladies and Gentlemen: We have examined the Registration Statement on Form S-8 (the “Registration Statement”) of Arrowhead Pharmaceuticals, Inc., a Delaware corporation (the “Company”), to be filed with the Securities and Exchange Commission (the “Commission”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), in connection with the offering by the Company of up to 555,300 shares of the Company’s Common Stock, par value $0.001 per share (the “Shares”). The Shares subject to the Registration Statement are to be issued under the Arrowhead Pharmaceuticals, Inc. Inducement Plan (the “Plan”) and include certain previously issued restricted stock unit awards granted at various dates from March 29, 2023 through December 28, 2023 to new employees as inducement awards in connection with the commencement of employment. We have examined the originals, or photostatic or certified copies, of such records of the Company and certificates of officers of the Company and of public officials and such other documents as we have deemed relevant and necessary as the basis for the opinions set forth below. In our examination, we have assumed the genuineness of all signatures, the legal capacity and competency of all natural persons, the authenticity of all documents submitted to us as originals and the conformity to original documents of all documents submitted to us as copies. We have also assumed that there are no agreements or understandings between or among the Company and any participants in the Plan that would expand, modify or otherwise affect the terms of the Plan or the respective rights or obligations of the participants thereunder. Finally, we have assumed the accuracy of all other information provided to us by the Company during the course of our investigations, on which we have relied in issuing the opinion expressed below. Based upon the foregoing examination and in reliance thereon, and subject to the qualifications, assumptions and limitations stated herein and in reliance on the statements of fact contained in the documents that we have examined, we are of the opinion that the

Arrowhead Pharmaceuticals, Inc. February 28, 2024 Page 2 Shares, when issued and sold in accordance with the terms set forth in the Plan and against payment therefor, and when the Registration Statement has become effective under the Securities Act, will be validly issued, fully paid and non-assessable. We render no opinion herein as to matters involving the laws of any jurisdiction other than the Delaware General Corporation Law (the “DGCL”). We are not admitted to practice in the State of Delaware; however, we are generally familiar with the DGCL as currently in effect and have made such inquiries as we consider necessary to render the opinions above. This opinion is limited to the effect of the current state of the DGCL and the facts as they currently exist. We assume no obligation to revise or supplement this opinion in the event of future changes in such law or the interpretations thereof or such facts. We consent to the filing of this opinion as an exhibit to the Registration Statement. In giving these consents, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the Rules and Regulations of the Commission. Very truly yours, /s/ Gibson, Dunn & Crutcher LLP

EX-23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM We hereby consent to the incorporation by reference, in this Registration Statement on Form S-8 of our report dated November 29, 2023, with respect to the consolidated financial statements of Arrowhead Pharmaceuticals, Inc. and Subsidiaries appearing in the Company’s Annual Report on Form 10-K for the year ended September 30, 2023. Rose, Snyder & Jacobs LLP Encino, California February 28, 2024