• Be ≥12 years of age with a weight at screening ≥40 kg.

• Have documented IDH1 or IDH2 mutation based on local testing of tumor tissue by an accredited laboratory

• Have adequate renal function, defined as a creatinine clearance ≥40 mL/min based on the Cockcroft-Gault glomerular filtration rate estimation: (140 - Age) × (Weight in kg) × (0.85 if female) / 72 × serum creatinine (mg/dL).

• Have adequate bone marrow function as evidenced by:

‣ Absolute neutrophil count ≥1,500/mm3 or 1.5×109/L

⁃ Hemoglobin ≥9 g/dL or 90 g/L

⁃ Platelets ≥100,000/mm3 or 100×109/L

• Have expected survival of ≥3 months.

• KPS or LPPS ≥70 at the start of study treatment.

• Participants on corticosteroids for reasons related to glioma must be on a stable or decreasing dose (≤4mg/day dexamethasone or equivalent) for ≥5 days before the start of study treatment.

• Female participants of reproductive potential must have a negative serum pregnancy test before starting study treatment.

∙ Phase 1b ONLY:

• Have histologically confirmed Grade 2, 3 or 4 IDHm (as per WHO 2021) glioma (astrocytoma or oligodendroglioma).

‣ For oligodendroglioma: Have local testing at an accredited laboratory demonstrating presence of 1p19q co deletion

⁃ For astrocytoma: Have local testing by an accredited laboratory demonstrating lack of 1p19q co-deletion and/or documented loss of nuclear ATRX expression or ATRX mutation

• Are appropriate to receive TMZ as post-radiotherapy (RT) adjuvant therapy or as treatment for first disease recurrence after prior RT and/or chemotherapy, per Investigator judgement. For those receiving TMZ in the post-RT adjuvant setting, study treatment must begin no more than 6 weeks after completion of RT.

• Have adequate hepatic function as evidenced by:

‣ Serum total bilirubin ≤1.5×upper limit of normal (ULN); if ≥1.5×ULN and due to Gilbert syndrome, total bilirubin ≤3×ULN with direct bilirubin ≤ULN,

⁃ AST and ALT ≤ULN, and

⁃ Alkaline phosphatase ≤2.5×ULN.

∙ Phase 2 ONLY:

• Have histologically confirmed Grade 4 astrocytoma, IDHm (per 2021 WHO criteria). Those who meet the Grade 4 designation via homozygous deletion of CDKN2A/B are eligible.

• Have 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.

• Have received SOC RT with concurrent TMZ (RT-TMZ) before enrollment. Study treatment must begin no more than 6 weeks after completion of RT-TMZ.

• Have adequate hepatic function as evidenced by:

‣ Serum total bilirubin ≤1.5×ULN; if ≥1.5×ULN and due to Gilbert syndrome, total bilirubin ≤3×ULN with direct bilirubin ≤ULN,

⁃ AST and ALT at or below the upper limit of normal. An elevation ≤1.5×ULN and considered not clinically significant by the Investigator may be allowed after Medical Monitor (Sponsor) approval, and

⁃ Alkaline phosphatase ≤2.5×ULN.