O6-methylguanine-DNA methyltransferase, O6-benzylguanine, and resistance to clinical alkylators in pediatric primary brain tumor cell lines

Purpose: Primary brain tumors are the leading cause of cancer death in children. Our purpose is (a) to assess the contribution of the DNA repair protein O-6-methylguanine-DNA methyltransferase (MGMT) to the resistance of pediatric brain tumor cell lines to clinical alkylating agents and (b) to evaluate variables for maximal potentiation of cell killing by the MGMT inhibitor O-6-benzylguanine, currently in clinical trials. Few such data for pediatric glioma lines, particularly those from low-grade tumors, are currently available. Experimental design: We used clonogenic assays of proliferative survival to quantitate cytoxicity of the chloroethylating agent 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and the methylating agent temozolomide in 11 glioma and five medulloblastoma lines. Twelve lines are newly established and characterized here, nine of them from low-grade gliomas including pilocytic astrocytomas. Results: (a) MGMT is a major determinant of BCNU resistance and the predominant determinant of temozolomide resistance in both our glioma and medulloblastoma lines. On average, O-6-benzylguanine reduced LD10 for BCNU and temozolomide, 2.6- and 26-fold, respectively, in 15 MGMT-expressing lines. (b) O-6-Benzylguanine reduced D-T (the threshold dose for killing) for BCNU and temozolomide, 3.3- and 138-fold, respectively. D-T was decreased from levels higher than, to levels below, clinically achievable plasma doses for both alkylators. (c) Maximal potentiation by O-6-benzylguanine required complete and prolonged suppression of MGMT. Conclusions: Our results support the use of O-6-benzylguanine to achieve full benefit of alkylating agents, particularly temozolomide, in the chemotherapy of pediatric brain tumors.