Purpose and experimental design: The contributions of O6-methylguanine-DNA-methyltransferase(MGMT), p53 status, mismatch repair, and apoptotic response to the resistance of glial tumors to temozolomide (TMZ) were tested using seven established human glial tumor cell lines in culture and xenografts in athymic mice.
Results: Resistance to TMZ was only marginally dependent on MGMT activity, because subtoxic doses of TMZ easily eliminated MGMT reserves for at least 18 h after treatment. Resistance to TMZ varied most notably with the p53 status of the tumor. Tumors with wild-type (wt) p53 and a functional p53 response to DNA damage (SWB40 and SWB61) were most sensitive. The p21-related cell cycle arrest was intimately linked to TMZ toxicity because tumors with wt p53 but lacking a robust increase in p21 protein level (D-54) were resistant to TMZ. In contrast, tumors with a dysfunctional p53 cycle and a weak cell cycle response to DNA damage (SWB39 and SWB77) were extremely unresponsive to treatment even with the aid of MGMT inactivators. Notable exceptions to the above were observed with the p53 mutated tumors SWB33 and SWB95, which were arrested by TMZ in G1-S and consequently underwent apoptosis despite their failure to express p21.
Conclusions: By testing a limited number of glial tumors in cell culture and also as xenografts, we have shown that mobilization of the p53 in response to TMZ damage is likely to induce a cell cycle arrest and apoptosis in glial tumors. Additional pathways linking cell cycle arrest and apoptosis contribute to the efficacy of TMZ against p53 mutated glial tumors. The unusual resistance of tumors, of which the cell cycle was not arrested in response to TMZ treatment, was associated with allelic losses during regrowth of treated tumors. Nevertheless such resistance was not related to dysfunctional mismatch repair.