Spleen tyrosine kinase (Syk) signaling is central to phagocytosis‐based, antibody‐mediated platelet destruction in adults with immune thrombocytopenia (ITP). Fostamatinib, an oral Syk inhibitor, produced sustained on‐treatment responses in a phase 2 ITP study. In two parallel, phase 3, multicenter, randomized, double‐blind, placebo‐controlled trials (FIT1 and FIT2), patients with persistent/chronic ITP were randomized 2:1 to fostamatinib (n = 101) or placebo (n = 49) at 100 mg BID for 24 weeks with a dose increase in nonresponders to 150 mg BID after 4 weeks. The primary endpoint was stable response (platelets ≥50 000/μL at ≥4 of 6 biweekly visits, weeks 14‐24, without rescue therapy). Baseline median platelet count was 16 000/μL; median duration of ITP was 8.5 years. Stable responses occurred in 18% of patients on fostamatinib vs. 2% on placebo (P = .0003). Overall responses (defined retrospectively as ≥1 platelet count ≥50 000/μL within the first 12 weeks on treatment) occurred in 43% of patients on fostamatinib vs. 14% on placebo (P = .0006). Median time to response was 15 days (on 100 mg bid), and 83% responded within 8 weeks. The most common adverse events were diarrhea (31% on fostamatinib vs. 15% on placebo), hypertension (28% vs. 13%), nausea (19% vs. 8%), dizziness (11% vs. 8%), and ALT increase (11% vs. 0%). Most events were mild or moderate and resolved spontaneously or with medical management (antihypertensive, anti‐motility agents). Fostamatinib produced clinically‐meaningful responses in ITP patients including those who failed splenectomy, thrombopoietic agents, and/or rituximab. Fostamatinib is a novel ITP treatment option that targets an important mechanism of ITP pathogenesis.

To determine whether inhibition of Syk would be useful in Fc␥R-dependent cytopenias such as immune thrombocytopenic purpura (ITP) or autoimmune hemolytic anemia, mouse models were used to evaluate efficacy of R406, an inhibitor of Syk function, in treating cytopenia. Both disease models responded favorably to treatment, with amelioration of ITP being more dramatic. Thus, phase 2 clinical trial was initiated to study the effects of Syk inhibition in humans with ITP. Sixteen adults with chronic ITP were entered into an open-label, single-arm cohort doseescalation trial beginning with 75 mg and escalating as high as 175 mg twice daily. Doses were increased until a persistent response was seen, toxicity occurred, or 175 mg twice daily was reached. Eight patients achieved persistent responses with platelet counts greater than 50 ؋ 10 9 /L (50 000 mm 3 ) on more than 67% (actually 95%) of their study visits, including 3 who had not persistently responded to thrombopoietic agents. Four others had nonsustained responses. Mean peak platelet count exceeded 100 ؋ 10 9 /L (100 000 mm 3 ) in these 12 patients. Toxicity was primarily GI-related with diarrhea (urgency) and vomiting; 2 patients had transaminitis.

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