Company: PRTA
Filing Date: 2025-11-06
Form Type: DEFA14A
Source: 0001193125-25-269620
Chunk: 2

Company: PROTHENA CORP PUBLIC LTD CO
Filing Date: 2025-11-06
Form: DEFA14A
Chunk 2
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.01. Financial Statements and Exhibits (d) Exhibits.

| Exhibit No. 
 99.1        |     | Description                                                                 
 Press Release dated November 6, 2025                                        |
|:------------|:----|:----------------------------------------------------------------------------|
| 104         |     | Cover Page Interactive Data File (embedded within the Inline XBRL document) |

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.

| Date: November 6, 2025 |     | PROTHENA CORPORATION PLC |     |                                                    |
|:-----------------------|:----|:-------------------------|:----|:---------------------------------------------------|
|                        |     | By:                      |     | /s/ Tran B. Nguyen                                 |
|                        |     | Name:                    |     | Tran B. Nguyen                                     |
|                        |     | Title:                   |     | Chief Financial Officer and Chief Strategy Officer |

Exhibit 99.1 PRESS RELEASE Prothena Reports Third Quarter 2025 Financial Results and Business Highlights

| • |     | Net cash used in operating and investing activities was $40.6 million and $140.4 million for the third                
 quarter and first nine months of 2025, respectively; quarter-end cash and restricted cash position was $331.7 million |

| • |     | Roche to initiate the Phase 3 PARAISO clinical trial evaluating prasinezumab, a potential first-in-class anti-alpha-synuclein antibody, for early-stage Parkinson’s disease by end of 2025 |

| • |     | Novo Nordisk initiated the Phase 3 CLEOPATTRA clinical trial evaluating coramitug, a potential first-in-class amyloid depleter, for ATTR amyloidosis with cardiomyopathy |

| • |     | Bristol Myers Squibb obtained Fast Track designation from the U.S. FDA for                         
 BMS-986446 (PRX005), an anti-MTBR-tau-targeting antibody, for the treatment of Alzheimer’s disease |

| • |     | Potential to earn up to $105 million in aggregate clinical milestone payments by end of 2026 related to the                                         
 advancement of coramitug for ATTR amyloidosis with cardiomyopathy by Novo Nordisk and PRX019 for neurodegenerative diseases by Bristol Myers Squibb |

DUBLIN, Ireland,