Fostamatinib is primarily used in the treatment of chronic immune thrombocytopenia (ITP), a condition where the immune system mistakenly attacks and destroys platelets, leading to increased bleeding and bruising. It works by inhibiting spleen tyrosine kinase (SYK), which plays a key role in the immune response that causes platelet destruction. This helps to increase platelet counts and reduce the risk of bleeding in patients who have not responded adequately to other treatments. Additionally, fostamatinib is being investigated for its potential use in other autoimmune and inflammatory diseases, such as rheumatoid arthritis and certain types of cancer, due to its ability to modulate immune system activity.