Using Fostamatinib to Treat Post-Hematopoietic Stem Cell Transplant Immune-mediated Cytopenias

Participants will receive fostamatinib 100 mg BID for 4 weeks. On the 4-week evaluation, a) if cytopenia improves (hemoglobin =10 g/dL, platelets = 50 x 109/L), patients will continue the same dose for a total of 12 weeks, b) if refractory cytopenias persist (hemoglobin \< 10 g/dL, platelets \< 50 x 109/L), the dose will be increased to 150 mg BID. Subjects with persistent (=2 readings, 2 weeks apart) loss of hematologic response after week 5 can increase their dose to 150 mg BID until at end of the study on week 12.