Preclinical Characterization of BCX9930, a Potent Oral Complement Factor D Inhibitor, Targeting Alternative Pathway-Mediated Diseases Including Paroxysmal Nocturnal Hemoglobinuria (PNH)

INTRODUCTION: Complement factor D is a serine protease that specifically catalyzes the cleavage of factor B [bound to C3(H₂O) or C3b], its only natural substrate, to Ba and Bb. Factor D has the lowest concentration in plasma among all the complement proteins and is the rate-limiting enzyme of the alternative pathway (AP) of complement. Factor D plays a key role in both AP-initiated C3 convertase formation and the amplification loop of the complement cascade. Therefore, targeting factor D is a promising therapeutic strategy to inhibit AP activation for treatment of AP-mediated diseases such as PNH, atypical hemolytic uremic syndrome, and C3 glomerulopathy.

Inhibition of terminal complement C5 by eculizumab or ravulizumab results in sustained inhibition of intravascular hemolysis in patients with PNH by blocking formation of the membrane attack complex (C5b-9). However, many PNH patients treated with eculizumab continue to have symptomatic anemia, with up to half of the patients requiring blood transfusions due to extravascular hemolysis as a result of ongoing AP activation and C3 opsonization on erythrocytes.

BioCryst Pharmaceuticals has developed a small-molecule, orally bioavailable human factor D inhibitor that is currently under evaluation as oral monotherapy for the treatment of patients with PNH in a Phase 1 study (BCX9930-101; NCT04330534). Here we demonstrate that BCX9930 is a potent and highly selective factor D inhibitor, and completely blocked AP activation on PNH erythrocytes in vitro.

METHODS: Factor D enzymatic activity was analyzed in vitro in an esterolytic assay with a synthetic substrate and in a proteolytic assay using its natural substrate factor B bound to C3b. The selectivity was assessed in substrate-specific assays using other human serine proteases. The inhibition of AP-mediated hemolysis was assessed in vitro with rabbit erythrocytes incubated with 10% normal human serum (NHS). The Ham test was used to evaluate the inhibition by BCX9930 of AP-mediated hemolysis of erythrocytes from PNH patients with 20% acidified NHS. Inhibition of AP-mediated C3 fragment deposition was evaluated by flow cytometry using PNH erythrocytes incubated with acidified C5-depleted NHS. Activity of the classical pathway (CP) of complement l was evaluated by measuring 50% Hemolytic Complement (CH₅₀) activity of serum and by assay of CP-mediated hemolysis of antibody-sensitized sheep erythrocytes. Pharmacodynamic effects in rhesus monkeys dosed orally with BCX9930 (100 and 200 mg twice-a-day) were assessed ex vivo with the AP Wieslab assay for the determination of C5b-9 generation after AP activation.

RESULTS: BCX9930 potently inhibited esterolytic activity of purified human factor D with a mean 50% inhibitory concentration (IC₅₀) of 14.3 nM. BCX9930 also inhibited its proteolytic activity against factor B bound to C3b with a mean IC₅₀ of 28.1 nM. BCX9930 inhibited AP-mediated hemolysis of rabbit erythrocytes with a mean IC₅₀ of 29.5 nM. Using the Ham test with erythrocytes from three PNH patients, BCX9930 completely suppressed AP-mediated hemolysis with a mean IC₅₀ of 35.4 nM. Furthermore, BCX9930 suppressed C3 fragment deposition on PNH erythrocytes with a mean IC₅₀ value of 39.3nM. BCX9930 did not inhibit enzyme activity of the human serine proteases thrombin, activated protein C, tissue plasminogen activator and trypsin up to 28 μM, and activated factor X and activated factor XII up to 50 μM. The mean IC₅₀ values of BCX9930 inhibition of C1s and plasmin were 936 and 2850 nM, respectively. BCX9930 at a concentration up to 3 μM had no effect on CP activity in in vitro assays. Oral dosing of BCX9930 in monkeys completely suppressed AP activity of serum measured by AP-dependent generation of C5b-9.

CONCLUSIONS: BCX9930 inhibitory activity on Factor D in vitro was potent and highly specific. BCX9930 completely blocked hemolysis of PNH cells in vitro and suppressed the accumulation of C3 fragments on PNH erythrocytes, indicating that BCX9930 has the potential to inhibit both intravascular and extravascular hemolysis. After oral dosing of BCX9930 in rhesus monkeys, AP activity was completely suppressed in ex vivo assays. These data demonstrate that BCX9930, an orally administered investigational drug, is a potent and selective factor D inhibitor, and that targeting factor D with BCX9930 has potential for the treatment of patients with PNH and other alternative pathway mediated diseases.