Temozolomide Resistance: A Multifarious Review on Mechanisms Beyond O-6-Methylguanine-DNA Methyltransferase

Background: Chemotherapy with the oral alkylating agent temozolomide still prevails as a linchpin in the therapeutic regimen of glioblastoma alongside radiotherapy. Because of the impoverished prognosis and sparse chemotherapeutic medicaments associated with glioblastoma, the burgeoning resistance to temozolomide has made the whole condition almost irremediable. Objective: The present review highlights the possible mechanisms of drug resistance following chemotherapy with temozolomide. Methods: The review summarizes the recent developments, as published in articles from Scopus, PubMed, and Web of Science search engines. Description: One of the prime resistance mediators, O-6-methylguanine-DNA methyltransferase, upon activation, removes temozolomide-induced methyl adducts bound to DNA and reinstates genomic integrity. In the bargain, neoteric advances in the conception of temozolomide resistance have opened the door to explore several potential mediators like indirect DNA repair systems, efflux mechanisms, epigenetic modulation, microenvironmental influences, and autophagy-apoptosis processes that constantly lead to the failure of chemotherapy. Conclusion: This review sheds light on recent discoveries, proposed theories, and clinical developments in the field of temozolomide resistance to summarize the complex and intriguing involvement of oncobiological pathways.