Birtamimab in Patients with Mayo Stage IV AL Amyloidosis: Rationale for Confirmatory Affirm-AL Phase 3 Study Design

Background: Light chain (AL) amyloidosis is a rare, progressive, and typically fatal hematologic disorder caused by plasma cells that produce misfolded AL protein, resulting in deposits of amyloid in tissues and organs that cause organ dysfunction and failure. Birtamimab is an investigational monoclonal antibody designed to neutralize circulating soluble amyloid and deposited insoluble amyloid, thus promoting the phagocytic clearance of amyloid deposits. In 2018, the global phase 3 VITAL study in newly diagnosed, treatment-naïve patients was terminated based on a futility analysis of the composite primary endpoint (time to all-cause mortality [ACM] or time to cardiac hospitalization >90 days after first study drug infusion); the final hazard ratio (HR) numerically favored birtamimab + standard of care (SOC) over placebo + SOC (0.835, 95% CI 0.5799, 1.2011; p=0.330). Post hoc analysis of ACM over 9 months revealed a pronounced survival benefit (HR=0.413, 95% CI 0.191, 0.895; p=0.025; Figure) in a subgroup of patients at high risk for early mortality (Mayo stage IV). At 9 months, the proportions of surviving patients were 74% and 49% in the birtamimab + SOC and placebo + SOC groups, respectively. Post hoc analyses of secondary endpoints in this subgroup also supported clinical and functional benefits of birtamimab + SOC, with clinically meaningful improvements observed in health-related quality of life (assessed with 36-Item Short Form Health Survey version 2; SF-36v2) and 6-minute walk test (6MWT) distance (both nominal p<0.05) at 9 months. Across all birtamimab clinical trials, no drug-related deaths, dose-limiting toxicities, or major risks were identified.