O6-methylguanine formation, repair protein depletion and clinical outcome with a 4 hr schedule of temozolomide in the treatment of advanced melanoma: Results of a phase II study

O6-Methylguanine-DNA methyltransferase (MGMT) is a major determinant of resistance to temozolomide. Its levels are depleted in lymphocytes after drug administration, but there is partial recovery by 24 hr, the usual time of subsequent dosing. Administering subsequent doses of temozolomide at the MGMT nadir could enhance its effectiveness, by increasing the amount of O6-methylguanine (O6-meG) in DNA. We evaluated the efficacy of such a schedule of temozolomide and determined the kinetics of MGMT depletion and O6-meG formation in DNA following treatment. Thirty patients with advanced malignant melanoma were treated with temozolomide 1,000 mg/m2 equally split into 5 doses over a 16 hr period every 28 days. O6-meG formation was determined in peripheral blood mononuclear cell (PBMC) DNA and, in a subset of patients, in tumor tissue during the first treatment cycle. MGMT levels fell rapidly with dosing, reaching a nadir in PBMCs of 18.0 +/- 2.26% of initial levels. O6-meG levels increased during the treatment period, peaking at 11.1 +/- 1.25 micromol/mol dG in PBMCs and at 4.25 +/- 0.79 micromol/mol dG in tumor biopsies. The main toxicities were grade IV thrombocytopenia in 12 patients (42.8%) and grade IV neutropenia in 11 patients (39.2%), associated with fever in 8 cases. There were 7 responses (1 complete), for an overall response rate of 23.3%; median overall survival was 6.1 months. The compressed schedule has activity against melanoma, with greater MGMT depletion and O6-meG formation than previously reported for O6-alkylating agent regimens. Myelosuppression precludes its wider application, but MGMT in PBMCs predicted the dose intensity of temozolomide that patients could sustain, suggesting a means by which individuals suitable for this approach might be identified.