Efficient Complement Inhibition by Small Molecules in PNH and aHUS: A Novel Therapeutic Approach

The text discusses the role of Factor D, a serine protease crucial in the alternative pathway of complement (APC), and its potential as a therapeutic target for conditions such as paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS). The current standard treatment, eculizumab, requires intravenous administration and has limitations, including hemolysis in some PNH patients. A new in vitro model using a modified Ham test has been presented for testing complement inhibitors in aHUS, as opposed to PNH, which can be tested with GPI anchor protein deficient erythrocytes.

Three small molecule inhibitors of Factor D (ACH-3856, ACH-4100, ACH-4471) were studied for their binding affinity and inhibitory effects on Factor D activity and APC-mediated hemolysis. Blood samples were collected from PNH and aHUS patients with consent, and serum and erythrocytes from PNH patients on eculizumab were gathered immediately post-administration. The compounds were tested in various dilutions, and their binding was analyzed using surface plasmon resonance.

The results showed that all three molecules had high binding affinity to human Factor D and inhibited its proteolytic activity in a dose-dependent manner with low IC50 values. They also effectively reduced hemolysis in rabbit erythrocytes and C3 fragment deposition at higher concentrations. The inhibitors were further tested on erythrocytes from PNH patients and showed significant reduction in hemolysis at low concentrations. In the modified Ham test for aHUS, the addition of these inhibitors led to a substantial decrease in cell killing, indicating their potential effectiveness.

The study concludes that these Factor D inhibitors could be the first oral complement inhibitors capable of efficiently blocking hemolysis in PNH and reducing C3 fragment accumulation. Moreover, it validates the modified Ham test as a preclinical model for evaluating complement inhibitors in aHUS.