ROLE OF O-6-METHYLGUANINE-DNA METHYLTRANSFERASE IN RESISTANCE OF HUMAN BRAIN-TUMOR CELL-LINES TO THE CLINICALLY RELEVANT METHYLATING AGENTSTEMOZOLOMIDE AND STREPTOZOTOCIN

We have analyzed the sensitivity of 14 human medulloblastoma- and glio ma-derived cell lines to the clinically used methylating agents temozo lomide and streptozotocin. The cell lines responded similarly to these agents, displaying a 3-fold range in cytotoxicity, assessed as the 10 % survival dose (LD(10)). The contribution of O-6-methylguanine-DNA me thyltransferase (MGMT) to resistance, measured as reduction in the LD( 10) by O-6-benzylguanine (O-6-BG), varied among the lines by 1 order o f magnitude for both agents. However, in all MGMT-expressing lines, O- 6-BG eliminated a threshold dose that accounted for up to one-half of the LD(10). The effect of O-6-BG on the rate of killing varied 13-fold for temozolomide and 14-fold for streptozotocin. Some lines displayed two subpopulations with different rates of killing, with one subpopul ation that comprised 20-60% of cells showing essentially no dependence of the rate of killing on MGMT, O-6-BG increased the range of the LD( 10) for both agents, The persistent, heightened variability in cytotox icity in the absence of MGMT, the lack of correlation between MGMT con tent of the lines and cytoxicity (LD(10)), and the lack of correlation between MGMT content and the contribution of MGMT to resistance (O-6- BG-mediated reduction of the LD(10)) reflect the operation of resistan ce mechanisms other than MGMT. We also analyzed sensitivity to methyl methanesulfonate, observing little dependence of resistance on MGMT an d persistent variability in cytotoxicity in the presence of O-6-BG. We discuss the implications for clinical use of methylators and O-6-BG.