Pharmacological strategies to increase the antitumor activity of methylating agents

Among methylating agents of clinical interest, temozolomide is a novel antitumor compound that has raised particular interest due to its acceptable safety profile and activity against tumors poorly responsive to conventional chemotherapy, such as malignant glioma and metastatic melanoma. Moreover, the drug has recently shown promising antitumor activity in a patient affected by primary brain lymphoma and is currently under phase II clinical trials for leptomeningeal metastases from leukemia and lymphoma or for brain metastases from lung and breast cancers. The antitumor activity of TMZ, that generates different types of methyl adducts (70% N7-methylguanine, 10% N3-methyladenine and 9% O6-methylguanine), has been mainly attributed to the formation of O6-methylguanine adducts. Indeed, tumor cell susceptibility to TMZ is strongly affected by the functional status of DNA repair systems, involved either in the removal of methyl adducts from O6G or in the apoptotic signaling triggered by O6-methylG:T mispairs. This review will focus on the different pharmacological strategies aimed at overcoming tumor resistance to TMZ such as new formulations of the drug or dosing schedules, and combined treatments with other chemotherapeutic agents, modulators of DNA repair systems, or gene therapy. The potential use of N3-methyladenine selective agents in the case of tumors tolerant to O6-methylguanine will be also discussed.