What is Temodar (Temozolomide)?
Temodar (temozolomide) is an oral alkylating chemotherapy agent used primarily to treat glioblastoma multiforme and anaplastic astrocytoma, working by methylating DNA to induce tumor cell death. 1
Mechanism of Action
Temozolomide is a second-generation alkylating prodrug that delivers methyl groups to purine bases of DNA (specifically O6-guanine, N7-guanine, and N3-adenine), causing DNA damage and tumor cell death 2. Its key pharmacological advantages include:
- Nearly 100% oral bioavailability
- Lipophilic properties and small molecular size allowing it to cross the blood-brain barrier effectively
- Achieves approximately 20% of plasma concentration in cerebrospinal fluid 2
Primary Indications
Newly Diagnosed Glioblastoma Multiforme
The standard of care is temozolomide combined with radiotherapy (the Stupp protocol), followed by maintenance temozolomide 3, 1:
- Concomitant phase: 75 mg/m² daily for 42 days during radiotherapy (60 Gy in 30 fractions)
- Maintenance phase: 150-200 mg/m² daily for 5 consecutive days every 28 days for 6 cycles 3
This regimen improves median survival to 14.6 months versus 12.1 months with radiotherapy alone 4.
Refractory Anaplastic Astrocytoma
- Initial dose: 150 mg/m² daily for 5 consecutive days per 28-day cycle
- Dose escalation: May increase to 200 mg/m² if blood counts remain adequate 1
IDH-Mutant Gliomas
For IDH-mutant, 1p19q non-codeleted anaplastic astrocytoma (grade 3):
- Adjuvant temozolomide: 150-200 mg/m² daily for days 1-5 every 4 weeks for maximum 12 months following radiotherapy 3
Dosing Schedules by Clinical Scenario
Elderly or Poor Performance Status Patients
For patients where standard 6-week radiotherapy may not be appropriate 3:
- Hypofractionated radiotherapy: 40 Gy in 15 fractions over 3 weeks
- Concurrent temozolomide: 75 mg/m²/day for 21 days
- Adjuvant temozolomide: 150-200 mg/m²/day for 5 of 28 days for up to 12 months
Alternative monotherapy option:
- 100 mg/m² daily on days 1-7 every 2 weeks until progression, OR
- 200 mg/m² daily on days 1-5 every 28 days for up to 6 cycles 3
Critical Contraindications and Precautions
MGMT Promoter Methylation Status
The benefit from temozolomide is largely limited to patients with MGMT promoter-methylated glioblastoma 5. For elderly patients:
- MGMT methylated tumors: Consider temozolomide alone
- MGMT unmethylated tumors: Hypofractionated radiotherapy alone may be preferred, though this remains controversial 5
Required Prophylaxis
Pneumocystis carinii pneumonia (PCP) prophylaxis is mandatory for all patients receiving concomitant temozolomide and radiotherapy for the 42-day regimen 1. Continue prophylaxis until lymphocyte recovery.
Common Adverse Effects
Most Frequent (≥10% incidence) 1:
- Alopecia
- Fatigue
- Nausea and vomiting
- Headache
- Constipation
- Anorexia
- Convulsions
- Rash
Hematologic Toxicities (Grade 3-4, ≥10% incidence) 1:
- Lymphopenia (most common)
- Thrombocytopenia
- Neutropenia
- Leukopenia
Serious Adverse Events
Myelosuppression is the most serious toxicity 1:
- Non-cumulative and reversible
- Thrombocytopenia occurs in <5% of patients
- Continuous administration causes profound CD4-selective lymphocytopenia 6
Myelodysplastic syndrome and secondary malignancies (including myeloid leukemia) have been observed 1.
Dose Modifications
Blood Count Requirements 1:
- Do not start next cycle unless: ANC ≥1.5 × 10⁹/L AND platelets ≥100 × 10⁹/L
- Reduce dose by 50 mg/m² (minimum 100 mg/m²) if: ANC <1.0 × 10⁹/L OR platelets <50 × 10⁹/L during any cycle
Monitoring Schedule 1:
- Concomitant phase: CBC weekly during treatment
- Maintenance phase: CBC on Day 1 and Day 22 of each cycle
- If counts drop: Weekly CBC until recovery
Important Clinical Pitfalls
Infusion timing matters: For IV formulation, bioequivalence established only with 90-minute infusion; shorter or longer infusions may result in suboptimal dosing 1
Pregnancy: Temozolomide causes fetal harm at 0.38-0.75 times the maximum recommended human dose in animal studies 1
Resistance mechanisms: Treatment failure often relates to O6-methylguanine-DNA methyltransferase (MGMT) expression, mismatch repair system defects, and cancer stem-like cells 2
Geriatric and female patients: Higher risk of developing myelosuppression 1