Effect of O6-alkylguanine pretreatment on the sensitivity of human colon tumor cells to the cytotoxic effects of chloroethylating agents.

Two human colon carcinoma cell lines which differ greatly in their content of O6-alkylguanine-DNA alkyltransferase were analyzed for their response to 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) and 2-chloroethyl(methylsulfonyl)methanesulfonate (ClEtSoSo) before and after treatment with O6-alkylguanines. HT29 cells contained about 17 times more alkyltransferase activity than BE cells. The alkyltransferase activity of HT29 cells was reduced by 60-80% by treatment for 24 h with 0.05-0.4 mM O6-methylguanine or O6-n-butylguanine. Such pretreatment prior to exposure to CCNU or ClEtSoSo increased the sensitivity of HT29 cells to these drugs. The exposure to O6-alkylguanines gave a greater enhancement of the toxic effects of ClEtSoSo than of CCNU. There was no significant increase in the toxicity of these agents towards the BE cells which contained much lower levels of the alkyltransferase. When added alone neither O6-methylguanine nor O6-n-butylguanine showed any toxicity towards HT29 or BE cells at the doses used. These results provide strong evidence that the formation of adducts at the O6-position of guanine by these agents contributes significantly to their lethality and that this reaction is more critical for ClEtSoSo than CCNU. The enhancement of the activity of chloroethylating agents by pretreatment with nontoxic doses of O6-alkylguanines may be clinically useful in terms of increasing their therapeutic efficacy towards cells containing high levels of alkyltransferase.