Why Is Prothena Stock Trading Lower On Tuesday?

Prothena Corporation's Phase 3 AFFIRM-AL trial of birtamimab for AL amyloidosis failed to meet its primary endpoint of all-cause mortality (HR=0.915, p=0.7680), as well as secondary endpoints, leading the company to discontinue birtamimab development and initiate a substantial workforce reduction. The company will now focus on other pipeline programs, including PRX012 for Alzheimer's, with data expected in August, causing PRTA stock to fall 26.80% in premarket trading.

Analysis

Prothena Corporation plc (PRTA) announced a significant setback with its Phase 3 AFFIRM-AL clinical trial for birtamimab in newly diagnosed AL amyloidosis patients. The trial, which enrolled 207 patients, failed to meet its primary endpoint of time to all-cause mortality, with a hazard ratio of 0.915 and a p-value of 0.7680. Additionally, key secondary endpoints, including the 6-minute walk test distance (p=0.5288) and Short Form-36 version 2 Physical Component Score (p=0.9597), were also not met. Consequently, Prothena will discontinue all development of birtamimab, including its open-label extension. This decision was described as surprising and disappointing by CEO Gene Kinney. Despite birtamimab being generally safe and well-tolerated, the lack of efficacy has prompted the company to initiate measures to reduce operating expenses, including an expected substantial workforce reduction, and to evaluate business options with financial advisors. The market reacted sharply to this news, with PRTA stock declining 26.80% to $4.82 in premarket trading. The company is now shifting focus to its remaining pipeline, with initial Phase 1 data for PRX012 in Alzheimer's disease anticipated in August, and program updates expected from partners Roche (mid-year), Novo Nordisk (2H25), and Bristol Myers Squibb (2026). Details regarding the planned reduction in operating expenses are expected in June.

Key Decisions for Investors

• Investors should brace for near-term volatility in PRTA stock due to the birtamimab trial failure and program discontinuation, which represents a significant loss of a late-stage asset.
• Closely monitor the upcoming Phase 1 data for PRX012 in Alzheimer's disease expected in August, as this will be a critical catalyst for the company's future valuation given the current pipeline reassessment.
• Evaluate Prothena's forthcoming plans in June regarding operating expense reductions and workforce restructuring, as these will indicate the company's financial runway and strategic refocus.
• Reassess exposure to PRTA, considering the heightened risk profile following this major clinical setback, and await further clarity on the company's strategic business options and pipeline prioritization.