Results of Phase 3 Clinical Study of Fostamatinib Conducted in Japan for Treatment of Immune Thrombocytopenia (R788-1301) Published in British Journal of Haematology

Kissei Pharmaceutical Co., Ltd. (Head Office: Matsumoto, Nagano, Japan; Chairman and CEO: Mutsuo Kanzawa; "Kissei") announced that results of the Phase 3 clinical study (R788-1301) of fostamatinib (generic name) in Japanese patients with immune thrombocytopenia were published in the British Journal of Haematology*.

The Phase 3, placebo-controlled, double-blind, parallel-group study evaluated the efficacy and safety of fostamatinib in 34 Japanese patients with chronic immune thrombocytopenic purpura (ITP). In the double-blind study period, patients were randomized to receive fostamatinib or placebo for 24 weeks.

The stable response rate**, which was the primary endpoint of the study, was 36% (8 of 22 patients) in the fostamatinib group and 0% (none of 12 patients) in the placebo group (p = 0.030). The overall response rate***, which was the secondary endpoint, was 45% (10 of 22 patients) in the fostamatinib group and 0% (none of 12 patients) in the placebo group (p = 0.006). Most adverse events were mild or moderate and were manageable. No new safety signals were identified in Japanese patients with ITP.

Fostamatinib is an oral spleen tyrosine kinase (SYK) inhibitor discovered by Rigel Pharmaceuticals, Inc. (Head Office: USA, President and CEO: Raul Rodriguez; "Rigel"). In October 2018, Kissei acquired the development and commercialization rights for this agent in Japan, China, South Korea and Taiwan from Rigel. In April 2022, Kissei announced that it submitted a new drug application for approval of manufacturing and marketing in Japan for fostamatinib as indicated for chronic ITP.

Kissei has engaged in the research and development of new drugs focusing on rare diseases and diseases for which there are no sufficient treatments. We strive to contribute to the treatment of patients suffering from serious illnesses.

*: Kuwana M, Ito T, Kowata S, Hatta Y, Fujimaki K, Naito K, et al. Fostamatinib for the treatment of Japanese patients with primary immune thrombocytopenia: A phase 3, placebo-controlled, double-blind, parallel-group study. Br J Haematol. 2022; 00: 1-10. URL https://doi.org/10.1111/bjh.18582
**: The percentage of patients who achieved a platelet count of 50,000/µL or more at four or more of the six visits from week 14 to 24
***: The percentage of patients who achieved a platelet count 50,000/µL or more at one or more of the six visits from week 2 to 12

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About Immune Thrombocytopenia (ITP)
ITP is a disease which causes serious bleeding events and bruising due to a decrease in platelet counts below 100,000/μL, despite the absence of other obvious illnesses and medications that cause thrombocytopenia. In Japan, idiopathic thrombocytopenic purpura is listed as a designated intractable disease name and used widely, while immune thrombocytopenia is the internationally accepted name for the disease. Fostamatinib is targeting chronic ITP.

The clinical symptoms of ITP include subcutaneous bleeding (petechiae or purpura) as well as bleeding from the gums or nose, and gastrointestinal, reproductive or urinary tracts, as well as intracranial bleeding.

ITP is designated as an "intractable disease" by the Minister of Health, Labour and Welfare. The number of patients with ITP is estimated to be approximately 19,000**** and 2.16 per 100,000***** people are newly diagnosed with ITP every year in Japan. The cause of ITP has still not been definitively elucidated. It is believed that one of the possible causes is the decreased platelet count as a result of the production of autoantibodies against platelets, leading to the destruction of opsonized platelets by macrophages in the spleen. ITP is currently treated with corticosteroids or thrombopoietin (TPO) receptor agonists as well as surgical removal of the spleen.

****: Estimated based on the number of patients having certificates issued for specific disease treatment (designated intractable disease)
*****: Int J Hematol, 2011, 93: 329-35

About Fostamatinib (R788)
Fostamatinib is an oral SYK inhibitor that suppresses platelet destruction by macrophages thereby preventing platelet depletion; this allows recovery of the platelet count, potentially improving the bleeding symptoms caused by chronic ITP. Fostamatinib has a different mechanism of action than existing treatments. In addition, it can be selected as a new treatment option for patients in whom conventional treatment methods such as steroids have not been sufficiently effective or are poorly tolerated.

Fostamatinib has been granted orphan drug status in the United States, Japan and Korea. It was approved by the US Food and Drug Administration in April 2018 for the treatment of adult patients with chronic ITP and launched in May 2018 in the United States. Subsequently, fostamatinib has been launched in Canada and some European countries. Kissei entered into sublicense agreements in June 2021 and August 2021 for the rights to develop and commercialize fostamatinib in South Korea and China respectively.

About Rigel Pharmaceuticals (Nasdaq: RIGL)
Rigel Pharmaceuticals, Inc., is a biotechnology company dedicated to discovering, developing and providing novel small molecule drugs that significantly improve the lives of patients with hematologic disorders, cancer and rare immune diseases. Founded in 1996, Rigel is based in South San Francisco, California, U.S.A. For more details, please visit www.rigel.com.