In vitro evaluation of temozolomide combined with X-irradiation

The in vitro cytotoxicity of 8-carbamoyl-3-methylimidazo[5,1-d]-1,2,3,5-tetrazine-4(3H)-one (temozolomide) with concurrent X-irradiation was examined in a human glioblastoma cell line (U373MG) as a potential radio-chemotherapeutic treatment for malignant glioma. The combination was also examined in a human colorectal adenocarcinoma (Mawi) which had 100-fold greater O-6-alkylguanine-DNA alkyltransferase (AGT) activity, a DNA-repair protein which confers resistance to temozolomide. A comparison of IC50 values indicated U373MG to be over 32-fold more sensitive to temozolomide than Mawi, but slightly more resistant to X-irradiation (p < 0.035; unpaired two-tailed t-test). Temozolomide and X-irradiation proved largely additive in U373MG by isobologram analysis (50% iso-effect) and the addition of 10 mu M temozolomide to 1-2 Gy of X-irradiation increased cell kill by 2.5- to 8.0-fold. However, the combination was antagonistic in Mawi: an effect attributed to AGT induction by X-irradiation as the antagonism was removed by coincubation with the AGT inhibitor O-6-benzylguanine (O-6-BG 1 mu M; 24 h). O-6-BG did not affect the radiation dose-response curve, but significantly increased temozolomide cytotoxicity (p < 0.015). In conclusion, the combination of temozolomide with radiation is at best additive, but could nonetheless be of considerable therapeutic benefit in glioma, particularly if administered for prolonged periods. If AGT induction compromises the efficacy of this therapy, it may be circumvented with an appropriate inhibitor such as O-6-BG.