Prediction of Response to Fostamatinib Based on the Presence of Plasma Platelet Autoantibodies in Adult Patients with Immune Thrombocytopenia (ITP) — Exploratory Analyses from Phase 3 Studies

Background: Initial results of an analysis of anti-platelet autoantibodies in patients with ITP from two phase 3 studies showed that patients were more likely to respond to treatment with the SYK inhibitor fostamatinib if they were positive for anti-platelet autoantibodies. Stable platelet responses to fostamatinib were reported in 36% of 28 patients who tested positive for anti-platelet autoantibodies and in 9% of 32 patients who tested negative (p<0.02). This analysis has been expanded to include 85 patients and to evaluate various platelet antigens.

Methods: Plasma samples from 85 patients in the phase 3 studies were collected with informed consent at 2 weeks after the first dose of fostamatinib. Samples were analyzed for anti-platelet autoantibodies via Luminex beads-based PakLx assayand FACS-based indirect platelet immunofluorescence test (iPIFT) using platelets from healthy volunteers.

Results: Of 85 patient samples, 38 (45%) were positive for one or more anti-platelet autoantibodies and 19 (22%) were from patients with a stable platelet response to fostamatinib. Of 38 patients positive for autoantibodies, 13 (34%) had stable platelet responses. Of 47 patients negative for autoantibodies, 6 (13%) had stable platelet responses.

Conclusions: The presence of anti-platelet autoantibodies appeared to be associated with a stable platelet response to fostamatinib. This exploratory analysis provides support for the hypothesis that fostamatinib prevents platelet loss through inhibition of autoantibody-directed platelet destruction in ITP. Further studies are ongoing.