Company: PRTA
Filing Date: 2025-09-25
Form Type: PRE 14A
Source: 0001140361-25-036135
Chunk: 0

Company: PROTHENA CORP PUBLIC LTD CO
Filing Date: 2025-09-25
Form: PRE 14A
Chunk 0
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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:

| ☒ | Preliminary Proxy Statement |

| ☐ | Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) |

| ☐ | Definitive Proxy Statement |

| ☐ | Definitive Additional Materials |

| ☐ | Soliciting Material Pursuant to § 240.14a-12 |

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) Payment of Filing Fee (Check all boxes that apply):

| ☒ |     | No fee required.                                                                                    |
| ☐ |     | Fee paid previously with preliminary materials.                                                     |
| ☐ |     | Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11 |

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[ ], 2025 Dear Shareholder: You are cordially invited to attend the Extraordinary General Meeting of Shareholders (the “EGM”) of Prothena Corporation plc (the “Company”) to be held on Wednesday, November 19, 2025, at 4:00 p.m. local time, at the offices of A&L Goodbody LLP, 25 North Wall Quay, Dublin 1, D01 H104, Ireland. Details of the business to be presented at the EGM may be found in the Notice of Extraordinary General Meeting of Shareholders and the Proxy Statement accompanying this letter. At the EGM, we are asking shareholders to approve a reduction of our Company capital to facilitate the creation of distributable reserves. The reduction of capital will give our Board of Directors greater flexibility with respect to its ability to make future distributions to shareholders, including by way of a potential share redemption program as referenced in our press release dated August 4, 2025. We hope that you will participate in the meeting by voting through acceptable means as described in this Proxy Statement as promptly as possible. Your vote is important – so please exercise your