Company: PRTA
Filing Date: 2025-11-06
Form Type: DEFA14A
Source: 0001193125-25-269620
Chunk: 3

Company: PROTHENA CORP PUBLIC LTD CO
Filing Date: 2025-11-06
Form: DEFA14A
Chunk 3
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 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-CMand 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-tauantibody, for the treatment of Alzheimer’s disease. BMS-986446is 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-43CYTOPE ®, a therapeutic modality enabling cytosolic delivery of macromolecules, which demonstrated a reduction in intracellular TDP-43pathology 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-classantibody for the treatment of Parkinson’s disease that is designed to target a key epitope within the C-terminusof alpha-synuclein and is the focus of a worldwide collaboration with Roche.

| • |     | Partner Roche to initiate the Phase 3 PARAISO clinical trial evaluating prasinezumab for early-stage 
 Parkinson’s disease by the end of 2025 (NCT07174310)                                                 |

| • |     | Roche has stated that prasinezumab has peak sales potential greater than$3.5 billion (unadjusted) and could 
 be the first disease-modifying treatment for a condition that affects