Temozolomide is an oral chemotherapy drug. It is an alkylating agent used for the treatment of Grade IV astrocytoma — an aggressive brain tumor, also known as glioblastoma multiforme — as well as for treating melanoma, a form of skin cancer. Temozolomide is also indicated for relapsed Grade III anaplastic astrocytoma and not indicated for, but as of 2011 used to treat oligodendroglioma brain tumors in some countries, replacing the older (and less well tolerated) PCV (Procarbazine-Lomustine-Vincristine) regimen. This methylation damages the DNA and triggers the death of tumor cells. However, some tumor cells are able to repair this type of DNA damage, and therefore diminish the therapeutic efficacy of temozolomide, by expressing a protein O6-alkylguanine DNA alkyltransferase (AGT) encoded in humans by the O-6-methylguanine-DNA methyltransferase (MGMT) gene.
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