Definition: Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.
Input: Rigel Pharmaceuticals Inc is developing fostamatinib, a prodrug of the spleen tyrosine kinase (Syk) inhibitor R-406, for the potential treatment of autoimmune diseases such as rheumatoid arthritis (RA), idiopathic thrombocytopenic purpura (ITP) and B-cell lymphomas. Syk is a key mediator of Fc and B-cell receptor signaling in inflammatory cells, such as B-cells, mast cells, macrophages and neutrophils. Preclinical studies of R-406 or fostamatinib demonstrated a significant reduction in major inflammatory mediators such as TNFalpha, IL-1, IL-6 and IL-18, leading to reduced inflammation and bone degradation in models of RA. In a phase II clinical trial, fostamatinib treatment effectively improved American College of Rheumatology response rates in patients with RA. Preclinical studies and phase II trials also suggested the potential of using fostamatinib for the treatment of ITP and B-cell lymphomas, by increasing platelet counts and inducing response rates, respectively. Fostamatinib is orally bioavailable and was well tolerated in phase I and II trials, with the most common side effect being gastrointestinal symptoms. At the time of publication, phase II trials for fostamatinib were ongoing in patients with RA, ITP and B-cell lymphomas. The Syk inhibitor appears to be a promising therapeutic for immunological diseases, but further data are required to establish the efficacy and long-term safety of the drug in humans.
Output:
Which enzyme is inhibited by a drug fostamatinib?