O6-Alkylguanine-DNA alkyltransferase activity correlates with the therapeutic response of human rhabdomyosarcoma xenografts to 1-(2-chloroethyl)-3-(trans-4-methylcyclohexyl)-1-nitrosourea.

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Abstract
Immune-deprived female CBA/CaJ mice bearing xenografts of 6 different human rhabdomyosarcoma lines were treated with 1-(2-chloroethyl)-3-(trans-4-methylcyclohexyl)-1-nitrosourea (MeCCNU). Tumor responses were compared with levels of O6-methylguanine-DNA methyltransferase activity because of evidence indicating that repair of DNA interstrand cross-link precursors, mediated by the transferase, may be an important determinant of MeCCNU cytotoxicity. Levels of methyltransferase in tumor extracts were measured by determining the loss of O6-methylguanine from 3H-labeled methylated DNA. Five of the 6 tumor lines examined showed either no response to MeCCNU or regrowth after an incomplete response. In each instance, the extent of tumor regression correlated with the level of O76-methylguanine-DNA methyltransferase activity in tumor extracts. The single highly drug sensitive line was totally devoid of the activity. O6-Methylguanine-DNA methyltransferase levels in human tumor cells may be a clinically useful predictor of sensitivity to the chloroethylnitrosoureas.