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