Evaluating Clinical Endpoints for C5 Inhibitors

Using free C5 levels as surrogate endpoints in complement therapies, especially for marketed Eculizumab and Ravulizumab, is grounded in the drug's specific mechanism of action and the correlation between free C5 levels and clinical outcomes. In studies targeting PNH, trials have shown that Ravulizumab maintains free C5 levels below the established threshold (<0.5 μg/mL).

RLYB116 is an affibody with a high affinity for blocking C5. To determine the free C5 levels from post-dose time points, a free C5 assay was developed on the Gyrolab platform. In the RLYB116 first-in-human (FIH) study, the measured free C5 levels seemingly deviated from the predicted post-dose free C5 levels. This discrepancy was further characterized by spiking varying concentrations of RLYB116 in human serum with endogenous C5. The results indicated that free C5 levels were overestimated when compared to free C5 concentration quantified using the same technology for Ravulizumab studies. Compared to Ravulizumab, the faster Koff for RLYB116-C5 complex likely resulted in the assay capture reagent outcompeting for C5 from the RLYB116, resulting in an apparently higher concentration of unbound C5 (i.e., free C5).

Additional experiments were conducted to determine the correlation between hemolysis levels and free C5 levels in samples treated with RLYB116. Experiments included spiking serial concentrations of RLYB116 or Ravulizumab into 100% C5 human serum and then measuring the free C5 and hemolysis levels and, adding serial concentrations of C5 and RLYB116 or Ravulizumab into C5-depleted human serum and measuring the hemolysis levels. Despite a higher measured free C5 value in RLYB116-treated samples, a similar level of hemolysis reduction (>90%) from baseline was observed, indicating complete complement inhibition is achieved in samples treated with RLYB116 or Ravulizumab.

The experimental evidence suggests that the free C5 levels obtained from RLYB116 study were significantly overestimated and it did not reflect the true complement inhibition by RLYB116. Furthermore, based on the totality of data from ex-vivo and clinical results, we conclude that RLYB116 demonstrated comparable or better potency compared to Ravulizumab, with a once weekly/self-administered dose. Additional pharmacodynamic markers, instead of free C5, should be used to support RLYB116 clinical development.

No relevant conflicts of interest to declare.