SEC Form DEFA14A filed by Prothena Corporation plc

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

Filed by the Registrant ☒

Filed by a Party other than the Registrant ☐

Check the appropriate box:

PROTHENA CORPORATION PUBLIC LIMITED COMPANY

(Name of registrant as specified in its charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): November 6, 2025

PROTHENA CORPORATION PUBLIC LIMITED COMPANY

(Exact name of registrant as specified in its charter)

77 Sir John Rogerson’s Quay, Block C

Grand Canal Docklands

Dublin 2, D02 VK60, Ireland

(Address of principal executive offices, including Zip Code)

Registrant’s telephone number, including area code: 011-353-1-236-2500

(Former Name or Former Address, if Changed Since Last Report.)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

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 13(a) of the Exchange Act. ☐

Item 2.02. Results of Operations and Financial Condition.

The information in Item 2.02 of this Current Report on Form 8-K and Exhibit 99.1 attached hereto shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section. It may only be incorporated by reference in another filing under the Exchange Act or the Securities Act of 1933, as amended, if such subsequent filing specifically incorporate by reference the information furnished pursuant to Item 2.02 (including Exhibit 99.1) of this Current Report.

On November 6, 2025, Prothena Corporation plc issued a press release announcing its financial results for the third quarter ended September 30, 2025. A copy of that press release is furnished as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference.

Item 9.01. Financial Statements and Exhibits

(d) Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Exhibit 99.1

PRESS RELEASE

Prothena Reports Third Quarter 2025 Financial Results and Business Highlights

DUBLIN, Ireland, November 6, 2025 — Prothena Corporation plc (NASDAQ:PRTA), a late-stage clinical biotechnology company with a robust pipeline of investigational therapeutics built on protein dysregulation expertise, today reported financial results for the third quarter and first nine months of 2025 and provided business highlights.

“We are pleased with the advancement of our late-stage partnered clinical programs. Novo Nordisk recently initiated the Phase 3 CLEOPATTRA clinical trial evaluating coramitug in ATTR-CM and Roche plans to initiate the Phase 3 PARAISO clinical trial evaluating prasinezumab in early-stage Parkinson’s disease by the end of 2025. Recently, Bristol Myers Squibb obtained Fast Track designation from the U.S. FDA for BMS-986446, an anti-MTBR-tau antibody, for the treatment of Alzheimer’s disease. BMS-986446 is currently in an ongoing Phase 2 trial with primary completion expected in the first half of 2027,” said Gene Kinney, Ph.D., President and Chief Executive Officer, Prothena. “We look forward to Novo Nordisk presenting Phase 2 coramitug results in a late-breaking presentation at the American Heart Association Scientific Sessions 2025. In addition, our Prothena scientists will be presenting a poster on our TDP-43 CYTOPE^®, a therapeutic modality enabling cytosolic delivery of macromolecules, which demonstrated a reduction in intracellular TDP-43 pathology in a preclinical ALS mouse model at Neuroscience 2025 hosted by the Society of Neuroscience. We look forward to sharing more about CYTOPE and its potential applications in the future.”

Third Quarter, Recent Business Highlights and Upcoming Milestones

Updates on Active Clinical Development Portfolio

Prasinezumab, a potential first-in-class antibody for the treatment of Parkinson’s disease that is designed to target a key epitope within the C-terminus of alpha-synuclein and is the focus of a worldwide collaboration with Roche.

Coramitug (formerly PRX004), a potential first-in-class amyloid depleter antibody for the treatment of ATTR amyloidosis with cardiomyopathy (ATTR-CM) designed to deplete the pathogenic, non-native forms of the transthyretin (TTR) protein, is being developed by Novo Nordisk as part of its up to $1.2 billion acquisition of Prothena’s ATTR amyloidosis business and pipeline.

BMS-986446 (formerly PRX005), a potential best-in-class antibody for the treatment of Alzheimer’s disease that specifically targets a key epitope within the microtubule binding region (MTBR) of tau, a protein implicated in the causal pathophysiology of Alzheimer’s disease.

PRX019, a potential treatment of neurodegenerative diseases in development in collaboration with Bristol Myers Squibb.

Upcoming Scientific and Investor Conferences

Novo Nordisk to present Phase 2 results for coramitug in a late-breaking session at the American Heart Association Scientific Sessions on November 10, 2025 in New Orleans, LA

Prothena poster presentation on a TDP-43 CYTOPE^®, a proprietary modality enabling cytosolic delivery of macromolecules for precision targeting of intracellular disease targets, in an ALS mouse model at Neuroscience 2025 annual meeting organized by Society for Neuroscience (SfN) on November 19, 2025 in San Diego, CA

Members of the senior management team will present and participate in investor meetings at the following upcoming investor conferences:

Third Quarter and First Nine Months of 2025 Financial Results

For the third quarter and first nine months of 2025, Prothena reported net loss of $36.5 million and $222.5 million, respectively, as compared to a net loss of $59.0 million and $64.4 million for the third quarter and first nine months of 2024, respectively. The first nine months of 2025 includes $33.1 million of restructuring charges associated with the discontinuation of the birtamimab program and the reduction in workforce announced in June 2025, and a $43.2 million net non-cash income tax expense to book a full valuation allowance against its federal deferred tax assets. Net loss per share was $0.68 and $4.13 for the third quarter and first nine months of 2025, respectively, as compared to a net loss per share of $1.10 and $1.20 for the third quarter and first nine months of 2024, respectively.

Prothena reported total revenue of $2.4 million and $9.7 million for the third quarter and first nine months of 2025, respectively, as compared to total revenue of $1.0 million and $133.0 million for the third quarter and first nine months of 2024, respectively. Total revenue for the third quarter and first nine months of 2025 was primarily from collaboration revenue from Bristol Myers Squibb related to the partial performance of our PRX019 Phase 1 clinical trial obligation. Total revenue for the first nine months of 2024, was primarily from collaboration revenue from Bristol Myers Squibb.

Research and development (R&D) expenses totaled $28.9 million and $120.3 million for the third quarter and first nine months of 2025, respectively, as compared to $50.7 million and $172.3 million for the third quarter and first nine months of 2024, respectively. The decrease in R&D expenses for the third quarter and first nine months of 2025 compared to the same periods in the prior year was primarily due to lower clinical trial expenses, lower personnel expenses, lower manufacturing and lower consulting expenses. R&D expenses included non-cash share-based compensation expense of $2.5 million and $12.0 million for the third quarter and first nine months of 2025, respectively, as compared to $5.1 million and $16.2 million for the third quarter and first nine months of 2024, respectively.

General and administrative (G&A) expenses totaled $13.2 million and $46.7 million for the third quarter and first nine months of 2025, respectively, as compared to $16.8 million and $50.4 million for the third quarter and first nine months of 2024, respectively. The decrease in G&A expenses for the third quarter and first nine months of 2025 compared to the same periods in the prior year was primarily due to lower personnel expenses. G&A expenses included non-cash share-based compensation expense of $4.7 million and $16.5 million for the third quarter and first nine months of 2025, respectively, as compared to $5.9 million and $19.2 million for the third quarter and first nine months of 2024, respectively.

Total non-cash share-based compensation expense was $7.2 million for the third quarter of 2025 and $30.6 million for the first nine months of 2025 which included $2.1 million in non-cash share-based compensation expense related to restructuring charges, as compared to $11.0 million and $35.4 million for the third quarter and first nine months of 2024, respectively.

As of September 30, 2025, Prothena had $331.7 million in cash, cash equivalents and restricted cash, and no debt.

As of October 31, 2025, Prothena had approximately 53.8 million ordinary shares outstanding.

2025 Financial Guidance

The Company continues to expect its full year net cash used in operating and investing activities to be $170 to $178 million and to end the year with approximately $298 million (midpoint) in cash, cash equivalents, and restricted cash. The estimated full year 2025 net cash used from operating and investing activities is primarily driven by an estimated net loss of $240 to $248 million, which includes an estimated $36 million of non-cash share-based compensation expense and a $44.9 million non-cash income tax expense to book a full valuation allowance against its federal deferred tax assets.

Share Redemption Program

Prothena will convene an Extraordinary General Meeting (“EGM”) of shareholders on November 19, 2025 to vote on a proposal to approve a reduction in Prothena’s share capital to create distributable reserves, subject to confirmation by the Irish High Court, to support a potential share redemption program. The Board of Directors is seeking shareholder approval of such a proposal to create flexibility to potentially return capital to shareholders via a share redemption program through open market purchases or other permissible means in 2026. Any such program would be subject to the discretion of the Board of Directors and Prothena’s then-current financial condition.

About Prothena

Prothena Corporation plc is a late-stage clinical biotechnology company with expertise in protein dysregulation and a pipeline of investigational therapeutics with the potential to change the course of devastating neurodegenerative and rare peripheral amyloid diseases. Fueled by its deep scientific expertise built over decades of research, Prothena is advancing a pipeline of therapeutic candidates for a number of indications and novel targets for which its ability to integrate scientific insights around neurological dysfunction and the biology of misfolded proteins can be leveraged. Prothena’s pipeline includes both wholly-owned and partnered programs being developed for the potential treatment of diseases including ATTR amyloidosis with cardiomyopathy, Alzheimer’s disease, Parkinson’s disease and a number of other neurodegenerative diseases. For more information, please visit the Company’s website at www.prothena.com and follow the Company on X (formerly Twitter) @ProthenaCorp.

Forward-Looking Statements

This press release contains forward-looking statements. These statements relate to, among other things, the sufficiency of our cash position to fund advancement of a broad pipeline and completion of our ongoing clinical trials; the continued advancement of our discovery, preclinical, and clinical pipeline, and expected milestones in 2025, 2026, 2027, and beyond; the treatment potential, designs, proposed mechanisms of action, and potential administration of prasinezumab, coramitug, BMS-986446, and PRX019; plans for ongoing and future clinical trials of prasinezumab, coramitug, BMS-986446, and PRX019; the expected timing of

reporting data from pre-clinical studies and clinical trials, including data from Novo Nordisk’s Phase 2 clinical trial evaluating coramitug on November 10, 2025 and our TDP-43 CYTOPE^® program on November 19, 2025; projections regarding peak sales and patient population for prasinezumab; timing of and amounts we may receive under our collaborations with Novo Nordisk and Bristol Myers Squibb; our anticipated net cash burn from operating and investing activities for 2025 and expected cash balance at the end of 2025; our estimated net loss and non-cash share-based compensation expense for 2025; and the potential and ability to return capital to shareholders via a share redemption program or other permissible means if shareholders approve a reduction in share capital to create distributable reserves. These statements are based on estimates, projections and assumptions that may prove not to be accurate, and actual results could differ materially from those anticipated due to known and unknown risks, uncertainties and other factors, including but not limited to uncertainties related to the completion of operational and financial closing procedures, audit adjustments and other developments that may arise that would require adjustments to the preliminary financial results included in this press release, as well as those described in the “Risk Factors” sections of our Quarterly Report on Form 10-Q to be filed with the Securities and Exchange Commission (SEC) on November 6, 2025, and discussions of potential risks, uncertainties, and other important factors in our subsequent filings with the SEC. We undertake no obligation to update publicly any forward-looking statements contained in this press release as a result of new information, future events, or changes in our expectations.

PROTHENA CORPORATION PLC

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS

(unaudited - amounts in thousands except per share data)

PROTHENA CORPORATION PLC

CONDENSED CONSOLIDATED BALANCE SHEETS

(unaudited—amounts in thousands)

Contacts:

Mark Johnson, CFA, Vice President, Investor Relations

650-837-8550

[email protected]

[email protected]