Preclinical characterization of AT-03, a novel Serum Amyloid P fusion protein that demonstrates pan-amyloid binding and removal

The systemic amyloidoses are progressive disorders caused by extracellular deposition of insoluble amyloid fibrils leading to organ dysfunction that often proves fatal. New therapeutics aiming at removing deposited amyloid are urgently needed to improve patient outcomes. Methods We developped AT-03 (originally called SAP-scFc), a fusion protein consisting of serum amyloid P-component, which binds all types of amyloid, linked to a single chain human IgG1 Fc domain. AT-03 binding to diverse types of amyloid and phagocytic activity were assessed both in vitro and in vivo . Therapeutic efficacy was evaluated in an AA mouse model. Results AT-03 bound with high potency to AL and ATTR human amyloid extracts. In murine models, intravenously administered AT-03 bound to AA, AL and AApoA2 amyloid, including in the heart. Ex vivo AT-03 opsonization induced phagocytosis of human AL extract by activated human THP-1 macrophages and enhanced in vivo phagocytosis in mice. A single intravenous injection of SAP-scFc induced a significant reduction of splenic amyloid in a murine model of AA amyloidosis. Conclusions AT-03 binds many amyloid types and can promote macrophage-mediated phagocytosis of the deposits. Thus, AT-03 is a promising novel therapeutic agent for the removal of systemic amyloid.