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Mechanism of Action
Temozolomide is not directly active but undergoes rapid nonenzymatic conversion at physiologic pH to the reactive compound 5-(3-methyltriazen- 1-yl)-imidazole-4-carboxamide (MTIC). The cytotoxicity of MTIC is thought to be primarily due to alkylation of DNA. Alkylation (methylation) occurs mainly at the O6 and N7 positions of guanine.
Temozolomide is rapidly and completely absorbed after oral administration with a peak plasma concentration (Cmax) achieved in a median Tmax of 1 hour. Food reduces the rate and extent of temozolomide absorption. Mean peak plasma concentration and AUC decreased by 32% and 9%, respectively, and median Tmax increased by 2-fold (from 1-2.25 hours) when temozolomide was administered after a modified high-fat breakfast.
A pharmacokinetic study comparing oral and intravenous temozolomide in 19 patients with primary CNS malignancies showed that 150 mg/m² TEMODAR for injection administered over 90 minutes is bioequivalent to 150 mg/m² TEMODAR oral capsules with respect to both Cmax and AUC of temozolomide and MTIC. Following a single 90-minute intravenous infusion of 150 mg/m², the geometric mean Cmax values for temozolomide and MTIC were 7.3 mcg/mL and 276 ng/mL, respectively. Following a single oral dose of 150 mg/m², the geometric mean Cmax values for temozolomide and MTIC were 7.5 mcg/mL and 282 ng/mL, respectively. Following a single 90-minute intravenous infusion of 150 mg/m², the geometric mean AUC values for temozolomide and MTIC were 24.6 mcg·hr/mL and 891 ng·hr/mL, respectively. Following a single oral dose of 150 mg/m², the geometric mean AUC values for temozolomide and MTIC were 23.4 mcg·hr/mL and 864 ng·hr/mL, respectively.
Temozolomide has a mean apparent volume of distribution of 0.4 L/kg (%CV=13%). It is weakly bound to human plasma proteins; the mean percent bound of drug-related total radioactivity is 15%.
Metabolism and Elimination
Temozolomide is spontaneously hydrolyzed at physiologic pH to the active species, MTIC and to temozolomide acid metabolite. MTIC is further hydrolyzed to 5-amino-imidazole-4-carboxamide (AIC), which is known to be an intermediate in purine and nucleic acid biosynthesis, and to methylhydrazine, which is believed to be the active alkylating species. Cytochrome P450 enzymes play only a minor role in the metabolism of temozolomide and MTIC. Relative to the AUC of temozolomide, the exposure to MTIC and AIC is 2.4% and 23%, respectively.
About 38% of the administered temozolomide total radioactive dose is recovered over 7 days: 37.7% in urine and 0.8% in feces. The majority of the recovery of radioactivity in urine is unchanged temozolomide (5.6%), AIC (12%), temozolomide acid metabolite (2.3%), and unidentified polar metabolite(s) (17%). Overall clearance of temozolomide is about 5.5 L/hr/m². Temozolomide is rapidly eliminated, with a mean elimination half-life of 1.8 hours, and exhibits linear kinetics over the therapeutic dosing range of 75 to 250 mg/m²/day.
Effect of Age
A population pharmacokinetic analysis indicated that age (range: 19-78 years) has no influence on the pharmacokinetics of temozolomide.
Effect of Gender
A population pharmacokinetic analysis indicated that women have an approximately 5% lower clearance (adjusted for body surface area) for temozolomide than men.
Effect of Race
The effect of race on the pharmacokinetics of temozolomide has not been studied.
A population pharmacokinetic analysis indicated that the oral clearance of temozolomide is similar in smokers and nonsmokers.
Effect of Renal Impairment
A population pharmacokinetic analysis indicated that creatinine clearance over the range of 36 to 130 mL/min/m² has no effect on the clearance of temozolomide after oral administration. The pharmacokinetics of temozolomide have not been studied in patients with severely impaired renal function (CLcr < 36 mL/min/m²). Caution should be exercised when TEMODAR is administered to patients with severe renal impairment [see Use in Special Populations]. TEMODAR has not been studied in patients on dialysis.
Effect of Hepatic Impairment
A study showed that the pharmacokinetics of temozolomide in patients with mild-to-moderate hepatic impairment (Child-Pugh Class I - II) were similar to those observed in patients with normal hepatic function. Caution should be exercised when temozolomide is administered to patients with severe hepatic impairment [see Use In Specific Populations].
Effect of Other Drugs on Temozolomide Pharmacokinetics
In a multiple-dose study, administration of TEMODAR Capsules with ranitidine did not change the Cmax or AUC values for temozolomide or MTIC.
A population analysis indicated that administration of valproic acid decreases the clearance of temozolomide by about 5% [see DRUG INTERACTIONS].
A population analysis did not demonstrate any influence of coadministered dexamethasone, prochlorperazine, phenytoin, carbamazepine, ondansetron, H2-receptor antagonists, or phenobarbital on the clearance of orally administered temozolomide.
Animal Toxicology and/or Pharmacology
Toxicology studies in rats and dogs identified a low incidence of hemorrhage, degeneration, and necrosis of the retina at temozolomide doses equal to or greater than 0.63 times the maximum recommended human dose (125 mg/m²). These changes were most commonly seen at doses where mortality was observed.
Newly Diagnosed Glioblastoma Multiforme
Five hundred and seventy-three patients were randomized to receive either TEMODAR (TMZ)+Radiotherapy (RT) (n=287) or RT alone (n=286). Patients in the TEMODAR+RT arm received concomitant TEMODAR (75 mg/m²) once daily, starting the first day of RT until the last day of RT, for 42 days (with a maximum of 49 days). This was followed by 6 cycles of TEMODAR alone (150 or 200 mg/m²) on Days 1 to 5 of every 28-day cycle, starting 4 weeks after the end of RT. Patients in the control arm received RT only. In both arms, focal radiation therapy was delivered as 60 Gy/30 fractions. Focal RT includes the tumor bed or resection site with a 2- to 3-cm margin. Pneumocystis carinii pneumonia (PCP) prophylaxis was required during the TMZ+RT, regardless of lymphocyte count, and was to continue until recovery of lymphocyte count to less than or equal to Grade 1.
At the time of disease progression, TEMODAR was administered as salvage therapy in 161 patients of the 282 (57%) in the RT alone arm, and 62 patients of the 277 (22%) in the TEMODAR+RT arm.
The addition of concomitant and maintenance TEMODAR to radiotherapy in the treatment of patients with newly diagnosed GBM showed a statistically significant improvement in overall survival compared to radiotherapy alone (Figure 1). The hazard ratio (HR) for overall survival was 0.63 (95% CI for HR=0.52-0.75) with a log-rank P < 0.0001 in favor of the TEMODAR arm. The median survival was increased by 2.5 months in the TEMODAR arm.
FIGURE 1: Kaplan-Meier Curves for Overall Survival
Refractory Anaplastic Astrocytoma
A single-arm, multicenter study was conducted in 162 patients who had anaplastic astrocytoma at first relapse and who had a baseline Karnofsky performance status of 70 or greater. Patients had previously received radiation therapy and may also have previously received a nitrosourea with or without other chemotherapy. Fifty-four patients had disease progression on prior therapy with both a nitrosourea and procarbazine, and their malignancy was considered refractory to chemotherapy (refractory anaplastic astrocytoma population). Median age of this subgroup of 54 patients was 42 years (19- 76). Sixty-five percent were male. Seventy-two percent of patients had a KPS of > 80. Sixty-three percent of patients had surgery other than a biopsy at the time of initial diagnosis. Of those patients undergoing resection, 73% underwent a subtotal resection and 27% underwent a gross total resection. Eighteen percent of patients had surgery at the time of first relapse. The median time from initial diagnosis to first relapse was 13.8 months (4.2-75.4).
TEMODAR Capsules were given for the first 5 consecutive days of a 28-day cycle at a starting dose of 150 mg/m²/day. If the nadir and day of dosing (Day 29, Day 1 of next cycle) absolute neutrophil count was ≥ 1.5 x 109/L (1500/μL) and the nadir and Day 29, Day 1 of next cycle platelet count was ≥ 100 x 109/L (100,000/μL), the TEMODAR dose was increased to 200 mg/m²/day for the first 5 consecutive days of a 28-day cycle.
In the refractory anaplastic astrocytoma population, the overall tumor response rate (CR+PR) was 22% (12/54 patients) and the complete response rate was 9% (5/54 patients). The median duration of all responses was 50 weeks (range: 16-114 weeks) and the median duration of complete responses was 64 weeks (range: 52-114 weeks). In this population, progression-free survival at 6 months was 45% (95% CI: 31%-58%) and progression-free survival at 12 months was 29% (95% CI: 16%-42%). Median progression-free survival was 4.4 months. Overall survival at 6 months was 74% (95% CI: 62%-86%) and 12-month overall survival was 65% (95% CI: 52%-78%). Median overall survival was 15.9 months.
Last reviewed on RxList: 1/29/2013
This monograph has been modified to include the generic and brand name in many instances.
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