Inclusion Criteria:

1. Have Karnofsky Performance Status (KPS) of ≥ 70%.
2. Have expected survival of ≥ 3 months.
3. Have histologically confirmed Grade 2 or Grade 3 astrocytoma (per the 2016 or 2021 World Health Organization [WHO] Classification of Tumors of the central nervous system)
4. Have documented IDH1-R132H gene mutation and absence of 1p19q co-deletion (i.e., non-co-deleted, or intact) and/or documented loss of nuclear ATRX expression or ATRX mutation by local testing.
5. Have measurable, magnetic resonance imaging (MRI)-evaluable, unequivocal contrast enhancing disease as determined by institution radiologist/Investigator at Screening on either 2D T1 post-contrast weighted images or 3D T1 post-contrast weighted images. Per mRANO criteria, measurable lesion is defined as at least 1 enhancing lesion measuring ≥ 1 cm x ≥ 1 cm.
6. Have recurrent or progressive disease and received prior treatment with chemotherapy, radiation, or both.
7. Surgical resection is indicated for treatment, but surgery is not urgently indicated (e.g., for whom surgery within the next 6-9 weeks is appropriate). (NOTE: This criterion only applies to participants enrolled in the perioperative phase of the study. Participants in the Safety Lead-In should not require surgery).

Exclusion Criteria:

1. Have received prior systemic anti-cancer therapy within 1 month of the first dose of IMP, radiation within 12 months of the first dose of IMP, or an investigational agent < 14 days prior to the first dose of IMP. In addition, the first dose of IMP should not occur before a period of ≥ 5 half-lives of the investigational agent has elapsed.
2. Have received 2 or more courses of radiation.
3. Have received any prior treatment with an isocitrate dehydrogenase (IDH) inhibitor; anti-programmed cell death 1 (PD1), anti-programmed cell death ligand 1 (PD-L1), or anti-PD-ligand 2 (L2) agent, or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (e.g., CTLA-4, OX 40, CD137); any other checkpoint inhibitor; bevacizumab; or any prior vaccine therapy.

Note: Other inclusion and exclusion criteria may apply.