Prothena : Director's Report and Financial Statements for Fiscal Year 2025

About this update from Prothena Corporation Plc

Prothena Corporation plc Directors' Report and Consolidated Financial Statements For the Year Ended 31 December 2025 Registered number: 518146 Directors' Report and Financial Statements Table of Contents Page Directors' Report ....................................................................................................................................................... 1 Statement of Directors' Responsibilities ................................................................................................................... 51 Independent Auditor's Report ................................................................................................................................... 52 Group Financial Statements .................................................................................................................................. Consolidated Balance Sheets ......................................................................................................................... 60 Consolidated Profit and Loss Account ............................................................................................................... 61 Consolidated Statements of Cash Flows ............................................................................................................ 62 Consolidated Statements of Shareholders' Equity ............................................................................................. 64 Notes Forming Part of the Consolidated Financial Statements .......................................................................... 65 Parent Company Financial Statements ................................................................................................................. Parent Company Balance Sheet .......................................................................................................................... 92 Parent Company Statement of Cash Flows ........................................................................................................ 93 Parent Company Statement of Shareholders' Equity .......................................................................................... 94 Parent Company Notes Forming Part of the Financial Statements ..................................................................... 95 Directors' Report For the year ended 31 December 2025 ‌The directors present their annual report and audited financial statements for Prothena Corporation plc ("Prothena" or "the Company") and its subsidiary undertakings (collectively "the group") for the year ended December 31, 2025. The consolidated financial statements can be found from pages 60 to 91. The directors have elected to prepare the Consolidated Financial Statements in accordance with Section 279 of the Companies Act 2014, which provides that a true and fair view of the assets and liabilities, financial position and profit or loss of a company and its subsidiary undertakings may be given by preparing its group financial statements in accordance with U.S. accounting standards ("US GAAP"), as defined by Section 279(1) of the Companies Act 2014, to the extent that the use of those standards in the preparation of the financial statements does not contravene any provision of Part 6 of the Companies Act 2014. PRINCIPAL ACTIVITIES Prothena is a late-stage clinical biotechnology company with expertise in protein dysregulation with the potential to change the course of devastating neurodegenerative and rare peripheral amyloid diseases. Fueled by our deep scientific expertise built over decades of research, we are advancing a pipeline of therapeutic candidates for a number of indications and novel targets for which our ability to integrate scientific insights around neurological dysfunction and the biology of misfolded proteins can be leveraged. The Company's pipeline includes both wholly-owned and partnered programs being developed for the potential treatment of diseases including Parkinson's disease, ATTR amyloidosis with cardiomyopathy, Alzheimer's disease, Amyotrophic lateral sclerosis (ALS) and a number of other neurodegenerative diseases. Prothena is developing and applying its proprietary CYTOPE® technology to target a broad spectrum of intracellular disease pathways in the brain and periphery. The Company was formed on September 26, 2012, under the laws of Ireland and re-registered as an Irish public limited company on October 25, 2012. The Company's ordinary shares began trading on The Nasdaq Global Market under the symbol "PRTA" on December 21, 2012, and currently trade on The Nasdaq Global Select Market. BASIS OF PREPARATION The accompanying consolidated financial statements include the accounts of Prothena Corporation plc and our subsidiary undertakings. PRINCIPAL RISKS AND UNCERTAINTIES You should carefully consider the risks described below, together with all of the other information included in this Directors' Report in considering our business and prospects. Set forth below and elsewhere in this report and in other documents we file with the U.S. Securities and Exchange Commission (the "SEC") are descriptions of certain risks, uncertainties, and other factors that could cause our actual results to differ materially from those anticipated. If any of the following risks, other unknown risks, or risks that we think are immaterial occur, our business, financial condition, results of operations, cash flows, or growth prospects could be adversely impacted, in which case, the market price of our ordinary shares could decline, and you may lose all or part of your investment in our ordinary shares. Additional risks and uncertainties not presently known to us or that we currently deem immaterial also may impair our business operations. Risks Relating to Our Financial Position, Our Need for Additional Capital, and Our Business We anticipate that we will incur losses for the foreseeable future and we may never sustain profitability. We may not generate the cash that is necessary to finance our operations in the foreseeable future. We incurred net losses of $244.1 million $122.3 million and $147.0 million for the years ended December 31, 2025, 2024, and 2023, respectively. As 1 Directors' Report (continued) of December 31, 2025, we had an accumulated profit and loss deficit of $491.3 million. We expect to continue to incur substantial losses for the foreseeable future as we: wind down the Phase 1 clinical trials for PRX012, and support the Phase 1 clinical trial for PRX019 and potential additional clinical trials for these and other programs; develop and possibly commercialize our drug candidates; undertake nonclinical development of other drug candidates and initiate clinical trials, if supported by nonclinical data; pursue our early stage research and seek to identify additional drug candidates; and potentially acquire rights from third parties to drug candidates or technologies through licenses, acquisitions, or other means. We must generate significant revenue to achieve and maintain profitability. Even if we succeed in discovering, developing, and commercializing one or more drug candidates, we may not be able to generate sufficient revenue and we may never be able to achieve or sustain profitability. We will require additional capital to fund our operations, and if we are unable to obtain such capital, we will be unable to successfully develop and commercialize drug candidates. As of December 31, 2025, we had cash and cash equivalents of $307.5 million. The majority of such cash is held in accounts at U.S. banking institutions that we believe are of high quality. Cash held in depository accounts may exceed the $250,000 Federal Deposit Insurance Corporation insurance limits. If such banking institutions were to fail, we could lose all or a portion of those amounts held in excess of such insurance limitations. Although we believe, based on our current business plans, that our existing cash and cash equivalents will be sufficient to meet our obligations for at least the next twelve months, we anticipate that we will require additional capital in order to continue the research and development, and eventual commercialization, of our drug candidates. Our future capital requirements will depend on many factors that are currently unknown to us, including, without limitation: the timing of progress, results, and costs of our clinical trials, including the Phase 3 clinical trial for prasinezumab being conducted by Roche, the Phase 3 clinical trial for coramitug (formerly PRX004) being conducted by Novo Nordisk, the Phase 2 clinical trial for BMS-986446 being conducted by BMS, and the Phase 1 clinical trial for PRX019; the timing, initiation, progress, results, and costs of these and our other research, development, and possible commercialization activities; the results of our research, nonclinical studies, and clinical trials; the costs of manufacturing our drug candidates for clinical development as well as for future commercialization needs; if and when appropriate, the costs of preparing for commercialization of our drug candidates; the costs of preparing, filing, and prosecuting patent applications, and maintaining, enforcing, and defending intellectual property-related claims; our ability to establish strategic collaborations, licensing, or other arrangements; the timing, receipt, and amount of any capital investments, cost-sharing contributions or reimbursements, milestone payments, or royalties that we might receive under current or potential future collaborations; the costs to satisfy our obligations under current and potential future collaborations; and 2 Directors' Report (continued) the timing, receipt, and amount of revenues or royalties, if any, from any approved drug candidates. We have based our expectations relating to liquidity and capital resources on assumptions that may prove to be wrong, and we could use our available capital resources sooner than we currently expect. Because of the numerous risks and uncertainties associated with the development and commercialization of our drug candidates, we are unable to estimate the amounts of increased capital outlays and operating expenses associated with completing the development and commercialization of our current drug candidates. In the pharmaceutical industry, the research and development process is lengthy and involves a high degree of risk and uncertainty. This process is conducted in various stages and, during each stage, there is substantial risk that drug candidates in our research and development pipeline will experience difficulties, delays or failures. This makes it difficult to estimate the total costs to complete our clinical trials and to estimate anticipated completion dates with any degree of accuracy, which raises concerns that attempts to quantify costs and provide estimates of timing may be misleading by implying a greater degree of certainty than actually exists. In order to develop and obtain regulatory approval for our drug candidates we will need to raise substantial additional funds. We expect to raise any such additional funds through public or private equity or debt financings, collaborative agreements with corporate partners, or other arrangements. Our ability to raise additional capital, including our ability to secure new collaborations, may also be adversely impacted by global economic conditions, including any disruptions to, and volatility in, the credit and financial markets in the United States and worldwide, geopolitical turmoil, and the ongoing conflict in Israel and any potential escalation or geographic expansion of such conflict, which could heighten other risks identified in this report. We cannot assure that additional funds will be available when we need them on terms that are acceptable to us or at all. If we raise additional funds by issuing equity securities, including pursuant to our Amended Distribution Agreement (as may be further amended from time to time, and as discussed below), substantial dilution to existing shareholders would result. If we raise additional funds by incurring debt financing, the terms of the debt may involve significant cash payment obligations as well as covenants and specific financial ratios that may restrict our ability to operate our business. We may be required to relinquish rights to our technologies or drug candidates or grant licenses on terms that are not favorable to us in order to raise additional funds through strategic alliances, joint ventures, or licensing arrangements. If adequate funds are not available on a timely basis, we may be required to: terminate or delay clinical trials or other development activities for one or more of our drug candidates; delay arrangements for activities that may be necessary to commercialize our drug candidates; curtail or eliminate our drug research and development programs that are designed to identify new drug candidates; or cease operations. In addition, if we do not meet our payment obligations to third parties as they come due, we may be subject to litigation claims. Even if we are successful in defending against these claims, litigation could result in substantial costs and distract management and may have unfavorable results that could further adversely impact our financial condition. Our future success depends on our ability to retain key personnel and to attract, retain, and motivate qualified personnel. We are highly dependent on key personnel, including Dr. Gene G. Kinney, our President and Chief Executive Officer. There can be no assurance that we will be able to retain Dr. Kinney or any of our key personnel. The loss of the services of Dr. Kinney or any other person on whom we are highly dependent might impede the achievement of our research, development, and commercial objectives. We do not carry "key person" insurance covering any members of our senior management. Attracting and retaining qualified scientific and other personnel are critical to our growth and future success. Competition for qualified personnel in our industry is intense. We may not be able to attract and retain these personnel on acceptable terms given that competition. Additionally, we may not be able to integrate and motivate qualified personnel to enable them to 3

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