Company: PRTA
Filing Date: 2025-11-06
Form Type: DEFA14A
Source: 0001193125-25-269620
Chunk: 4

Company: PROTHENA CORP PUBLIC LTD CO
Filing Date: 2025-11-06
Form: DEFA14A
Chunk 4
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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 Sessionson November 10, 2025 in New Orleans, LA

| • |     | Title: Primary results from the phase 2 randomized, placebo controlled, blinded trial of the monoclonal 
 antibody coramitug in transthyretin amyloid cardiomyopathy (ATTR-CM)                                    |

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

| • |     | Title: Treatment with a cell-internalizing antibody targeting pTDP43 reduces intraneuronal pathology in 
 a mouse model of ALS                                                                                    |

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