Post-marketing surveillance of fostamatinib for primary immune thrombocytopenia in Japan: interim analysis

Fostamatinib was approved in Japan in December 2022 for primary immune thrombocytopenia (ITP). Given the limited clinical trial data, post-marketing surveillance (PMS) was conducted in Japan to collect safety and effectiveness data for fostamatinib. This interim analysis of PMS collected data between April 2023 and April 2025. Most of the 489 patients in the safety analysis set had received ≥ 3 treatment lines. Approximately 70% used fostamatinib with glucocorticoids and/or a thrombopoietin receptor agonist. Adverse drug reactions (ADRs) were reported in 49.5% (serious ADRs in 8.6%). The most common ADRs were diarrhoea (20.0%), hypertension (15.7%), hepatic function abnormal (9.6%), liver disorders (4.9%), and neutrophil count decreased (4.1%). Safety analysis revealed no major differences in the frequency of ADRs, including those of special interest, by combination treatment(s). Median platelet count increased at 2 weeks of treatment and remained above 50,000/μL thereafter, regardless of the ITP treatment combination. Further, complete response (platelet count ≥ 100,000/μL) was observed in 135 of 309 patients at 3 months and 128 of 241 patients at 6 months under continuous fostamatinib use. The results of this real-world PMS revealed no new safety concerns and support the effectiveness of fostamatinib in patients with primary ITP.