Subjects who at any time during the C-935788-057 study achieved a hemoglobin response will continue at their dose and regimen from the Week 22 visit in the C-935788-057 study.

All other subjects who enter the extension study will initially receive fostamatinib 100 mg PO bid. Starting at Week 4, the initial fostamatinib dose of 100 mg PO bid will be increased to fostamatinib 150 mg PO bid if subjects have adequately tolerated the study drug, based on the Investigator's judgment.

Fostamatinib is supplied in two (2) dosage strengths: 100 mg and 150 mg.

Incidence, frequency, seriousness, and severity of adverse events that occurred during the current study

Change from baseline in blood pressure over time

Change from baseline in absolute neutrophil count (ANC) over time