Additional Study Information: This study is testing a drug called GDC-8264, which is being developed to treat acute graft-versus-host-disease (acute GVHD). GDC-8264 is an experimental drug, which means health authorities have not approved GDC-8264 in combination with standard medications for the treatment of acute GVHD. The purpose of this study is to evaluate the effects, good or bad, of GDC-8264 plus standard medications on patients who have been diagnosed with acute GVHD and have increased risk of poor clinical response to standard treatment. GDC-8264 will be given to you as pills every day throughout the study treatment period in addition to the standard medications, prednisone or methylprednisolone. You will receive GDC-8264 to treat acute GVHD after HSCT (hematopoietic stem cell transplantation). The goal is for GDC-8264 to reduce the amount of tissue damage caused by acute GVHD and improve outcomes for patients with more severe or high-risk disease.