DEPLETION OF O6-ALKYLGUANINE-DNA ALKYLTRANSFERASE CORRELATES WITH POTENTIATION OF TEMOZOLOMIDE AND CCNU TOXICITY IN HUMAN TUMOR-CELLS

Temozolomide -methylimidazo[5,1-d]-1,2,3,5-tetrazin-4-(3H)-one) has sh own promising activity in Phase I trials against some brain (glioma) a nd skin (melanoma, mycosis fungoides) cancers. Temozolomide and lomust ine (CCNU) showed parallel toxicity in seven human tumour cell lines a nd this generally correlated (correlation coefficients 0.87 and 0.92 r espectively) with the level of expression of the DNA repair protein 06 -alkylguanine-DNA alkyltransferase (ATase, EC 2.1.1.63). Pretreating c ells with the ATase inhibitor, O6-benzylguanine (BG), potentiated cyto toxicity to a similar degree with both drugs, but did not sensitise a cell line (ZR-75-1) expressing very low levels of this protein. When B G pretreatment was combined with repeat doses of temozolomide a dramat ic potentiation (300 fold) was seen in MAWI cells, which express high levels of ATase, but not in a cell line (U373) expressing lower levels of ATase. [C-14]-labelled temozolomide uptake was similar in sensitiv e and resistant lines. Human ATase-cDNA transfected xeroderma pigmento sum (XP) fibroblasts were more resistant than XP control cells to temo zolomide and the related chloroethylating agent mitozolomide and altho ugh BG completely suppressed ATase activity in these cells, resistance was still greater than in control cells.