Temozolomide

Temozolomide is not active until it is converted at physiologic pH to MTIC. It is suggested that MTIC then alkylates DNA at the N7 position of guanine, O3 position of adenosine, and O6 position of guanosine, with the most common site being the N7 position. This methylation of guanine residues lead to single and double-strand DNA breaks and subsequent apoptotic cell death. It is suggested that the N7-methylguanine plays a critical role in the antitumor activity of the drug, as there is a correlation between the sensitivity of tumor cell lines to temozolomide and the activity of O6-alkylguanine alkyltransferase, which is the DNA repair protein that specifically removes alkyl groups at the O6 position of guanine. Cells lines that have lower levels of AGT are more sensitive to the cytotoxicity of temozolomide. It is also suggested that cytotoxic mechanism of temozolomide is related to the failure of the DNA MMR system to find a complementary base for methylated guanine. The DNA MMR system is involved in the formation of a number of proteins that remove methylated guanine. Evidence shows that when this repair process is targeted to the DNA strand opposite the O6-methylguanine, its inability to find the correct target leads to long-lived nicks in the DNA. The accumulation of these nicks lead to the inhibition of replication in the daughter cells, thereby blocking the cell cycle at the G_-M boundary.