This is a Phase IIa single arm open-label study to investigate the safety, tolerability, and PK of F-652 in combination with systemic corticosteroids in subjects who have undergone HSCT and have newly diagnosed grade II-IV lower GI aGVHD. The HSCT may be derived from bone marrow, peripheral blood stem cells, or cord blood. The PK of F-652 in this subject population will be investigated. Subjects may be replaced if subject withdrawal is not related to safety or treatment response.