Discovery of small-molecule reversible Factor D inhibitors targeting the alternative complement pathway

Complement is a key component of the innate immune system, recognizing pathogens and promoting their elimination through cell lysis and/or inflammatory responses1,2. Activation of complement can be initiated by three distinct routes, the classical, lectin and alternative pathways. These converge at the proteolytic cleavage of the third component of complement, C3, generating the C3a and C3b activation fragments. C3b also participates in amplification of its own production via the positive feedback loop of the alternative pathway (AP)2,3. The two proteases Factor D and Factor B are essential for this tightly regulated amplification process. Dysregulation of AP activity predisposes individuals to diverse disorders including age-related macular degeneration (AMD), paroxysmal nocturnal hemoglobinuria (PNH), atypical hemolytic uremic syndrome (aHUS) and C3 nephropathy4-7. Here, using a structure-based design approach, we describe the identification of potent and selective small-molecule reversible inhibitors of Factor D. These inhibitors efficiently blocked AP activation in human whole blood, and prevented both C3 deposition onto, and lysis of, human erythrocytes in an assay that mimics erythrocyte lytic sensitivity. These findings were confirmed with erythrocytes from PNH patients. Oral administration inhibited lipopolysaccharide (LPS)-induced AP activation both systemically and in ocular tissues in Factor D-humanized C57Bl/6 mice. These data demonstrate the feasibility of inhibiting the AP with specific small molecule antagonists and support the development of oral Factor D inhibitors that would enable the systemic treatment of many complement-mediated diseases for which there are currently either no or sub-optimal therapies