A Study to Assess the Safety and Pharmacokinetics of GDC-8264 in Combination With Standard of Care in Participants With Acute Graft-Versus-Host Disease (aGVHD)

Inclusion Criteria: Diagnosis of post-allogeneic hematopoietic stem cell transplantation (HSCT) aGVHD at screening Evidence of engraftment post-transplant Diagnosis of high-risk aGVHD, per refined Minnesota high-risk aGVHD criteria during screening Initiation of treatment with systemic corticosteroids for aGVHD at a dose of prednisone ≥2 milligrams per kilograms per day (mg/kg/day) by orally (PO) or methylprednisolone ≥2 mg/kg/day intravenously (or equivalent) in divided doses at diagnosis and up to 3 days prior to or on the same day as initiation of GDC-8264 (Day 1), with no taper planned prior to Day 3 Exclusion Criteria: Evidence of relapsed, progressing, or persistent malignancy, or treatment for relapse after transplant, or requirement for rapid immune suppression withdrawal as pre-emergent treatment of early malignancy relapse Prior receipt of more than one allogeneic HSCT Prior receipt of solid organ transplantation that are target organs for aGVHD (e.g., liver transplant) Prior systemic treatment for aGVHD, except for the standard of care corticosteroid treatment initiated as part of this trial Diagnosis of chronic GVHD or overlap syndrome Uncontrolled active infection (i.e., progressive symptoms related to infection despite treatment, or persistently positive blood cultures despite treatment, or any other evidence of severe sepsis) Severe organ dysfunction (e.g., acute liver failure, renal failure requiring dialysis, ventilator support, or vasopressor therapy) Initiation or planned use of a marketed small molecule (excluding corticosteroids) or biologic therapy as treatment for aGVHD from the start of screening through the treatment period