Interactions
As temozolomide is not metabolized in the liver and has a low affinity to plasma proteins, it is expected to have a low potential for interactions. An analysis of patient data showed no interactions with a range of other drugs; the exception is valproic acid, which slightly slows down temozolomide elimination from the body. Combining the drug with other myelosuppressants may increase the risk of myelosuppression. Pharmacology
Mechanism of action
The therapeutic benefit of temozolomide depends on its ability to alkylate/methylate DNA, which most often occurs at the N-7 or O-6 positions of guanine residues. 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. In some tumors, epigenetic silencing of the MGMT gene prevents the synthesis of this enzyme, and as a consequence such tumors are more sensitive to killing by temozolomide. Conversely, the presence of AGT protein in brain tumors predicts poor response to temozolomide and these patients receive little benefit from chemotherapy with temozolomide. Pharmacokinetics
Temozolomide is quickly and almost completely absorbed from the gut, and readily penetrates the blood–brain barrier; the concentration in the cerebrospinal fluid is 30% of the concentration in the blood plasma. Intake with food decreases maximal plasma concentrations by 33% and the area under the curve by 9%. Only 15% (10–20%) of the substance are bound to blood plasma proteins.
Adverse effects
The most common side effect is bone marrow suppression. The most common non-hematological adverse effects associated with temozolomide are nausea and vomiting, which are either self-limiting or readily controlled with standard antiemetic therapy. These latter effects are usually mild to moderate (grade 1 to 2). The incidence of severe nausea and vomiting is around 4% each. Patients who have pre-existing or a history of severe vomiting may require antiemetic therapy before initiating temozolomide treatment. Temozolomide should be administered in the fasting state, at least one hour before a meal. Antiemetic therapy may be administered before, or following, administration of temozolomide. Temozolomide is genotoxic, teratogenic and fetotoxic and should not be used during pregnancy. Lactating women should discontinue nursing while receiving the drug because of the risk of secretion into breast milk. One study indicated that women that have taken temozolomide without concomitant fertility preservation measures achieve pregnancy to a lesser rate later in life, but the study was too small to show statistical significance in the hypothesis that temozolomide would confer a risk of female infertility. In male patients, temozolomide can have genotoxic effects. Men are advised not to father a child during or up to six months after treatment and to seek advice on cryoconservation of sperm prior to treatment, because of the possibility of irreversible infertility due to temozolomide therapy. Very rarely temozolomide can cause acute respiratory failure or liver damage.