Company: PRTA
Filing Date: 2025-11-06
Form Type: DEFA14A
Source: 0001193125-25-269620
Chunk: 1

Company: PROTHENA CORP PUBLIC LTD CO
Filing Date: 2025-11-06
Form: DEFA14A
Chunk 1
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 Act (17 CFR 230.425) |

| ☐ | Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 
 CFR 240.14a-12)                                                        |

| ☐ | Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b)) |

| ☐ | Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c)) |

Securities registered pursuant to Section 12(b) of the Act:

| Title of Each Class                        |     | Trading 
 Symbol  |     | Name of Each Exchange           
 on Which Registered             |
| Ordinary Shares, par value $0.01 per share |     | PRTA    |     | The Nasdaq Global Select Market |

Indicate by check mark whether the registrant is an emerging growth company as defined in as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2of the Securities Exchange Act of 1934 (§240.12b-2of this chapter). Emerging growth company ☐ If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Item 2.02. Results of Operations and Financial Condition. The information in Item 2.02 of this Current Report on Form 8-Kand Exhibit 99.1 attached hereto shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section. It may only be incorporated by reference in another filing under the Exchange Act or the Securities Act of 1933, as amended, if such subsequent filing specifically incorporate by reference the information furnished pursuant to Item 2.02 (including Exhibit 99.1) of this Current Report. On November 6, 2025, Prothena Corporation plc issued a press release announcing its financial results for the third quarter ended September 30, 2025. A copy of that press release is furnished as Exhibit 99.1 to this Current Report on Form 8-Kand is incorporated herein by reference. Item 9