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 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.

Eligibility last updated 2/1/23. Questions regarding updates should be directed to the study team contact.