Birtamimab is a humanized monoclonal antibody designed to target and clear misfolded forms of serum amyloid A (SAA) protein, specifically amyloid fibrils and oligomers. It is developed for the treatment of light-chain (AL) amyloidosis, a rare and often fatal disease where misfolded immunoglobulin light chains form amyloid deposits that damage organs (particularly the heart). Birtamimab binds to a cryptic epitope exposed on misfolded SAA-related amyloid, marking it for clearance by phagocytic immune cells (e.g., macrophages). Despite promising Phase I/II data, a pivotal Phase III trial in patients with newly diagnosed AL amyloidosis and cardiac involvement did not meet its primary endpoint. However, a pre-specified subgroup analysis suggested potential benefit in a very high-risk patient population, leading to further investigation.