SS-1 Therapeutic challenges for glioblastoma: Temozolomide and its issues?

Abstract

Glioblastoma (GBM), the most malignant form (WHO grade IV) of gliomas, remains incurable despite recent advances in medical technologies and molecular knowledge, with 5-year survival rate being just beyond 10%, thus undoubtedly leaving unmet needs to develop effective therapeutics to improve outcome. Current standard of care for patients with newly diagnosed GBM is maximum safe resection of the enhancing tumor bulk, followed by involved-field radiotherapy with temozolomide (TMZ) given concomitantly and as an adjuvant therapy thereof with or without Tumor-Treating Fields (TTF). The efficacy of TMZ for GBM was proved in 2005 by a randomized phase III trial (EORTC25981) for the first time ever in history, since then, however, any other agents have failed to show benefit to improve survival until now. O6-methylguanine-DNA methyltransferase (MGMT), a specific DNA repair enzyme, has been shown to restore the cytotoxic O6-methylguanine lesion induced by TMZ to normal, responsible for a major reason of TMZ resistance. Expression of MGMT is tightly regulated by methylation of the promoter region of the MGMT gene, where promoter methylation results in suppression of its expression. Accordingly, the promoter methylation of the MGMT gene (meth-MGMT) has been consistently associated with a better response to and outcome by TMZ treatment, and furthermore a favorable prognostic factor for patients with newly diagnosed GBM in a number of prospective clinical trials. In this line, efforts have been made to overcome TMZ resistance including intensifying dose schedulings of TMZ, one of which has been currently tested in a multi-institutional prospective phase III trial in Japan (JCOG1308C). TMZ has also been investigated in lower grade gliomas. A major issue in treating lower grade glioma with TMZ is potential development of the hypermutated tumors by virtue of O6-methylguanine-induced mutagenesis. The clinical and molecular consideration related to TMZ in glioma treatment will be presented.