Current Translational Insights into MGMT Methylation Regulating Temozolomide Sensitivity and Resistance in Glioblastoma Multiforme

Temozolomide is used as frontline chemotherapy in the management of glioblastoma multiforme (GBM); however, its clinical utility is limited by the occurrence of significant resistance, majorly caused due to direct DNA repair. O6- methylguanine-DNA-methyltransferase (MGMT), a DNA repair protein, mediates this direct repair pathway and reverses the activity of temozolomide. We characterize and underscore the functional relevance and molecular aspects of MGMT in the development of sensitivity/resistance to temozolomide treatment. We review early translational, as well as clinical, evidence for the role of MGMT in mediating temozolomide resistance in vitro in cell lines, in vivo in small animals as well as in GBM patients. Various approaches have been delineated to mitigate MGMT-induced temozolomide resistance. The most promising means in discovery biology appears to be the co-administration of MGMT inhibitors such as O6 benzyl guanine or lomeguatrib. Surprisingly, the validation of these pharmacologic inhibitors to assess the reversal of chemoresistance by appropriately designed safety and efficacy trials in combination with temozolomide is yet to be demonstrated. Taken together, given the regulation of temozolomide resistance by MGMT, intermediate and late discovery groups may focus their efforts on pharmacologic inhibition of MGMT, singly or in combination with radiotherapy or immunotherapy, to combat temozolomide resistance in GBM patients. In addition, one may speculate that the combined clinical use of temozolomide with a drug regulator-approved MGMT inhibitor as well as an immune checkpoint inhibitor such as nivolumab may prove beneficial. Future studies may also investigate any inter-ethnic variability in population pharmacogenetics of MGMT and pharmacometric approaches to optimize cancer precision medicine.