Temozolomide Pharmacodynamics in Patients with Metastatic Melanoma: DNA Damage and Activity of Repair Enzymes O6-Alkylguanine Alkyltransferase and Poly(ADP-Ribose) Polymerase-1
Plummer ER., Middleton MR., Jones C., Olsen A., Hickson I., McHugh P., Margison GP., McGown G., Thorncroft M., Watson AJ., Boddy AV., Calvert AH., Harris AL., Newell DR., Curtin NJ.
PURPOSE: Temozolomide, a DNA methylating agent used to treat melanoma, induces DNA damage, which is repaired by O(6)-alkylguanine alkyltransferase (ATase) and poly(ADP-ribose) polymerase-1 (PARP-1)-dependent base excision repair. The current study was done to define the effect of temozolomide on DNA integrity and relevant repair enzymes as a prelude to a phase I trial of the combination of temozolomide with a PARP inhibitor.EXPERIMENTAL DESIGN: Temozolomide (200 mg/m(2) oral administration) was given to 12 patients with metastatic malignant melanoma. Peripheral blood lymphocytes (PBL) were analyzed for PARP activity, DNA single-strand breakage, ATase levels, and DNA methylation. PARP activity was also measured in tumor biopsies from 9 of 12 patients and in PBLs from healthy volunteers.RESULTS: Temozolomide pharmacokinetics were consistent with previous reports. Temozolomide therapy caused a substantial and sustained elevation of N(7)-methylguanine levels, a modest and sustained reduction in ATase activity, and a modest and transient increase in DNA strand breaks and PARP activity in PBLs. PARP-1 activity in tumor homogenates was variable (828 +/- 599 pmol PAR monomer/mg protein) and was not consistently affected by temozolomide treatment.CONCLUSIONS: The effect of temozolomide reported here are consistent with those documented in previous studies with temozolomide and similar drug, dacarbazine, demonstrating that a representative patient population was investigated. Furthermore, PARP activity was not inhibited by temozolomide treatment and this newly validated pharmacodynamic assay is therefore suitable for use in a proof-of-principle phase I trial a PARP-1 inhibitor in combination with temozolomide.