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Time Course of Response to Fostamatinib, an Oral Spleen Tyrosine Kinase (SYK) Inhibitor for the Treatment of Immune Thrombocytopenia (ITP)

1Dept. of Hematology and Oncology, St Michael's Hospital, University of Toronto, Toronto, Canada, 2Michael G. DeGroote School of Medicine, McMaster University, Hamilton, Canada, 3Divison of Hematology-Oncology, East Carolina University, Greenville, United States, 4Dept. of Medical Affairs, Rigel Pharmaceuticals, Inc, South San Francisco, United States, 5Arizona Oncology Associates, PC, Tucson, United States

Abstract Number: PB0815

Meeting: ISTH 2021 Congress

Theme: Platelet Disorders, von Willebrand Disease and Thrombotic Microangiopathies » Acquired Thrombocytopenias

Background: Immune thrombocytopenia (ITP) is a heterogeneous disease characterized by low platelet counts and increased bleeding risk. Platelet loss is mediated by a spleen tyrosine kinase (SYK) dependent pathway in macrophages. Fostamatinib is an oral SYK inhibitor approved for the treatment of ITP. In phase 3 clinical studies, fostamatinib was shown to both decrease bleeding risk and improve platelet counts, with 54% of patients achieving platelet counts ≥50,000/µL. 

Aims: To evaluate the time course of response and adverse effects associated with fostamatinib over one year.

Methods: In a post-hoc analysis, clinical endpoints were assessed in 3-month increments over the first year of the phase 3 studies. Patients with data for the full year were included. Patients received fostamatinib at 100-150 mg BID.

Results: Characteristics of Patients who Received Fostamatinib Treatment for >1 Year and <1 YearClinical endpoints assessed in 3-month increments in the 58 patients with data for all 4 quarters of the first year of treatmentCharacteristics of the 58 patients who continued treatment for ≥1 year differed from those with <1 year of treatment with regard to the number of prior treatments and baseline platelet count (see table). The patients treated for ≥1 year had a response rate of 91% (53/58) (achieved a platelet count ≥50,000/µL), 71% (41/58) within 12 weeks.
Among those treated for ≥1 year (n=58), over the first year of treatment, median platelet counts continued to increase per each 3-month period (see figure), and the incidence of bleeding events and use of rescue therapy decreased with continued treatment (see figure). Also, among these patients, the incidence of common adverse events (diarrhea [38%] and hypertension [28%]) decreased over each 3-month period of treatment.

Conclusions: In these post-hoc analyses of ITP patients with ≥1 year of treatment, fostamatinib provided durable and progressive clinical benefit with diminishing adverse events over time.

To cite this abstract in AMA style:

Sholzberg M, Arnold D, Liles D, Numerof R, Boxer M. Time Course of Response to Fostamatinib, an Oral Spleen Tyrosine Kinase (SYK) Inhibitor for the Treatment of Immune Thrombocytopenia (ITP) [abstract]. Res Pract Thromb Haemost. 2021; 5 (Suppl 1). https://abstracts.isth.org/abstract/time-course-of-response-to-fostamatinib-an-oral-spleen-tyrosine-kinase-syk-inhibitor-for-the-treatment-of-immune-thrombocytopenia-itp/. Accessed September 25, 2021.

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