IN-VITRO EVALUATION OF TEMOZOLOMIDE COMBINED WITH X-IRRADIATION

The in vitro cytotoxicity of 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-chemothe rapeutic treatment for malignant glioma. The combination was also exam ined in a human colorectal adenocarcinoma (Mawi) which had 100-fold gr eater O-6-alkylguanine-DNA alkyltransferase (AGT) activity, a DNA-repa ir protein which confers resistance to temozolomide. A comparison of I C50 values indicated U373MG to be over 32-fold more sensitive to temoz olomide than Mawi, but slightly more resistant to X-irradiation (p < 0 .035; unpaired two-tailed t-test). Temozolomide and X-irradiation prov ed 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 i ncreased cell kill by 2.5- to 8.0-fold. However, the combination was a ntagonistic in Mawi: an effect attributed to AGT induction by X-irradi ation as the antagonism was removed by coincubation with the AGT inhib itor O-6-benzylguanine (O-6-BG 1 mu M; 24 h). O-6-BG did not affect th e radiation dose-response curve, but significantly increased temozolom ide cytotoxicity (p < 0.015). In conclusion, the combination of temozo lomide with radiation is at best additive, but could nonetheless be of considerable therapeutic benefit in glioma, particularly if administe red for prolonged periods. If AGT induction compromises the efficacy o f this therapy, it may be circumvented with an appropriate inhibitor s uch as O-6-BG.