Prothena cuts workforce by about 63% after ending birtamimab development

Prothena Corporation plc (Nasdaq: PRTA) announced on June 18, 2025 an approximate 63% reduction in its workforce. The company said the cut is meant to bring operating costs down to what it needs for its remaining wholly-owned programs, its obligations to partnered programs and its business development work. It reported the reduction in a Form 8-K under Item 2.05.

What the filing says

The reduction follows the company's May 23, 2025 decision to discontinue further development of birtamimab, a treatment for AL amyloidosis. Prothena expects the reduction to be substantially complete by the end of the fourth quarter of 2025.

The company expects to recognize approximately $16 to $20 million in total. That covers severance and related benefits for the employees laid off, plus non-cash share-based compensation from accelerating stock options. The filing says these charges are primarily one-time severance payments and are primarily cash charges. It adds that the company may also incur other charges or cash spending it does not yet expect, because of events that may occur as a result of the restructuring.

Affected U.S. employees are eligible for severance under the Prothena Biosciences Inc. Amended and Restated Severance Plan. Affected non-U.S. employees are eligible for statutory or contractual payments under their employment agreements and local law. For certain senior executives, some stock options will vest early and their exercise periods will be extended.

Two executives leave

Under Item 5.02, the company said that, in connection with the workforce reduction, announced the same day, the employment of Carol D. Karp, Chief Regulatory Officer, and David A. Ford, Chief People Officer, will end effective August 1, 2025. Their departures trigger payments and benefits under the same severance plan, along with the early vesting and extended exercise periods described above. The filing says those terms are set out in the Definitive Proxy Statement on Schedule 14A filed on March 28, 2025.

What was being given up

The 10-K filed February 27, 2025 named birtamimab first among the company's wholly-owned programs, ahead of the Alzheimer's portfolio. It described the company as conducting the registration-enabling confirmatory Phase 3 AFFIRM-AL trial in Mayo Stage IV patients with AL amyloidosis, under a Special Protocol Assessment agreement with the FDA. The 10-K also said there is no assurance that the birtamimab Phase 3 results would be positive, and that the trial had been disrupted by the COVID-19 pandemic. In May 2025 the company ended development.

With birtamimab gone, the company's remaining wholly-owned work is in Alzheimer's disease. PRX012, an anti-Aβ antibody that the 10-K places in the Phase 1 ASCENT trials, and PRX123, a dual Aβ-tau vaccine, have both been granted FDA Fast Track Designation for that disease. Its partnered programs include prasinezumab for Parkinson's disease, developed with Roche, and BMS-986446 with Bristol Myers Squibb. In December 2024 Roche announced topline results from the Phase 2b PADOVA trial of prasinezumab, and the 10-K says Roche is further evaluating the data and will work with health authorities on next steps. The 10-K shows a net loss of $122,310 thousand for 2024, against $147,028 thousand for 2023, and cash, cash equivalents and restricted cash of $472,248 thousand at the end of 2024, down from $621,042 thousand. The 10-K also says Prothena expects losses for the foreseeable future, may never reach profitability and will need more capital to fund its operations.

Who is involved

Tran B. Nguyen, Chief Strategy Officer and Chief Financial Officer, signed the 8-K, dated June 18, 2025.

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