Two parallel, multicenter, randomized, placebo-controlled phase 3 studies evaluated the oral spleen tyrosine kinase inhibitor fostamatinib in adult patients with persistent/chronic immune thrombocytopenia (ITP; Study 047 in North America, Australia, and Europe, and Study 048 in Europe). Patients from both studies were enrolled in an ongoing open-label, long-term extension study (Study 049). In these studies, patients initially received fostamatinib 100 mg twice daily, and increased to 150 mg twice daily if they showed no platelet response. The current study evaluated the safety and durability of response among patients in these 3 studies who achieved stable response (SR).
In studies 047/048, 18/101 (18%) fostamatinib-treated patients achieved an SR compared with 1/49 (2%) patients who received placebo (P = .0003); SR was defined as a platelet count ≥50/nL at 4 of 6 evaluations every 2 weeks during weeks 14 to 24. In Study 049, SR was defined as a platelet count ≥50/nL within 12 weeks of starting treatment without the need for rescue therapy and ≥50/nL at 2 of the 3 subsequent monthly evaluations.
Of the 150 (82%) patients who participated in studies 047/048, 123 enrolled in Study 049. Of the fostamatinib-treated patients on Study 047 (n = 36), Study 048 (n = 43), and Study 049 (n = 44), a total of 27 (22%) patients achieved an SR; 10 of whom received placebo on studies 047/048 and received fostamatinib in Study 049. Of the 25 fostamatinib-treated patients who achieved an SR, 17 (68%) patients maintained their response as of April 14, 2017 (≥12 months following fostamatinib initiation), with a median platelet count of 95/nL. The median time to response was approximately 15.0 days, with an 80% response before 6 weeks. The median duration of response for the patients who maintained an SR (n = 17) has not yet been reached, and is estimated at >28 months (12, >28).
In studies 047/048, the most common fostamatinib-related adverse events in patients included diarrhea (29%), hypertension (20%), nausea (19%), epistaxis (16%), and ALT/AST increase (14%). In the expansion cohort (n = 123), the most common fostamatinib-related adverse events included diarrhea (28%), hypertension (15%), petechiae (15%), and epistaxis (14%). Serious adverse events occurred in 27 of 123 (22%) patients; of these, bleeding-related serious adverse events were experienced by 11 patients.
Treatment discontinuations due to adverse events were reported in 15 of 123 (12%) patients, including diarrhea (n = 5), liver enzyme elevations (n = 3), and neutropenia (n = 2).
These study results indicated that the majority of heavily pretreated patients with persistent/chronic ITP who achieved an SR to fostamatinib therapy maintained an SR for ≥12 months.
Bussel JB, et al. ASH 2017. Abstract 16.