Pre-Clinical Characterization of a Novel Fusion Protein (AT-03), with Pan-Amyloid Binding and Removal

Background: Amyloidosis is characterized by extracellular deposition of protein fibrils that can involve any organ or tissue in the body. It is a severe, progressive and often lethal disorder. There are approximately seventeen different systemic amyloid diseases, each associated with the deposition of a specific precursor protein. Toxic amyloid deposits result in significant organ dysfunction and increased morbidity and mortality. Reduction and removal of tissue amyloid deposits could restore organ function, improve quality of life and increase life expectancy for patients. We have developed and characterized a fusion protein consisting of serum amyloid protein (SAP) linked to a single chain human IgG1 Fc domain, designated AT-03. SAP is a pentameric serum protein that is ubiquitously associated with amyloid deposits. The high affinity and specificity for amyloid resulted in the use of radio-iodinated SAP as an imaging agent for the non-invasive detection of systemic amyloid. The SAP domain of the fusion protein targets the amyloid deposits delivering Fc domains to stimulate amyloid removal via macrophage mediated phagocytosis.

Methods: Binding of AT-03 to ATTR and AL fibrils and extracts was assessed by ELISA. Ex vivo phagocytosis of amyloid substrates was studied using activated human THP-1 cells. The biodistribution of AT-03 in murine models of AA, AApoA2 and AL amyloidosis was evaluated immunohistochemically or by SPECT imaging and microautoradiography. Clearance of AA amyloidosis in response to AT-03 therapy was accessed in a mouse model using histological scoring of amyloid burden.

Results: AT-03 bound amyloid AL and ATTR substrates with sub nanomolar EC50 values. Systemic administration of AT-03 resulted in specific binding to diverse forms of amyloid in the heart, kidney, liver and/or spleen in multiple animal models. Opsonization of amyloid deposits with AT-03 in vitro resulted in a dose dependent phagocytosis by activated THP-1 cells. A single IV dose of AT-03 resulted in significant reduction of splenic AA amyloid within 14 days post-injection.

Conclusions: We have developed a novel fusion protein with sub nanomolar pan amyloid reactivity that specifically binds diverse forms of amyloid in multiple organs and tissues in vivo and can effectuate amyloid removal. The data indicates that AT-03 may serve as a novel therapeutic for the removal of systemic amyloid deposits of diverse types. AT-03 is currently being developed for clinical evaluation.