The purpose of this study is to establish the recommended phase 2 dose of eflornithine in combination with temozolomide in patients whose glioblastoma or astrocytoma is newly diagnosed, and to evaluate safety and tolerability of this combination at that dose.

This open label dose escalation and expansion study will be conducted using a standard dose-escalation design with escalating doses of eflornithine plus temozolomide at the approved dose level, followed by an expansion cohort that will further evaluate safety and preliminary efficacy of the combination at the recommended phase 2 dose.

Duration of participation will be up to approximately 104 weeks in total per patient.

Screening Period - A maximum screening duration of 4 weeks.

Treatment Period - Up to approximately 104 weeks.

Follow-Up Visit - 4 weeks from last treatment.

Long-term Survival Follow-Up - up to 2 years from last treatment.

A total of up to 66 patients will be enrolled in a non-randomized fashion (patients may be added to any of the dose levels below the RP2D to a maximum of approximately 20 per dose level with the intent of further characterizing safety and pharmacokinetics).

Inclusion Criteria:

• Diagnosis of World Health Organization (WHO) G4 classified GBM, IDH-wildtype (patients with GBM) or G3 astrocytoma (IDH1 or 2 mutant; CDKN2A/B intact) per WHO 2021 tumor classification.
• Completed external beam radiation therapy per standard of care.
• Patients with GBM: Must have received at least 80% of planned daily doses of TMZ during chemoradiation. Patients with astrocytoma: Must have tolerated adjuvant TMZ treatment through at least 2 and not more than 4 cycles.
• Adequate hematologic, renal, hepatic, and other organ function as indicated by hematology and serum chemistry testing.
• Willing to abstain from intercourse or use acceptable contraceptive methods.
• If taking corticosteroids, must be on a stable or decreasing dose.

Exclusion Criteria:

• Recent history of recurrent or metastatic cancer that could confound response assessments
• Prior systemic chemotherapy other than temozolomide during external beam radiation therapy (for patients with GBM) or adjuvant temozolomide through up to 4 pre-study cycles (for patients with astrocytoma).
• Prior Optune treatment.
• Active infection or serious intercurrent medical illness.
• Poorly controlled seizures.
• Significant cardiac disease within 6 months of enrollment.
• Poorly controlled diabetes.
• Use of another investigational agent within 30 days of enrollment.