The effect of O6-benzylguanine, O6-(p-chlorobenzyl)guanine, and O6-(p-methylbenzyl)guanine on the sensitivity of various human tumor cell lines to alkylating agents is evaluated. The sensitivity of human colon tumor cells, HT29, to the chloroethylating agents, 1,3-bis(2-chloroethyl)-1-nitrosourea, 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea, 2-chloroethyl(methylsulfonyl) methanesulfonate (clomesone), and chlorozotocin was increased by pretreatment for 2 h with 25 µm of each analogue. O6-Benzylguanine was slightly more effective as a sensitizer in HT29 cells than the p-chlorobenzyl and p-methylbenzyl analogues. However, all analogues sensitized SF767 glioma cells to the cytotoxic effects of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea, 1,3-bis(2-chloroethyl)-1-nitrosourea, and clomesone to the same degree. Both cell lines were sensitized to the methylating agents streptozotocin and 5-(3-methyl-1-triazeno)imidazole-4-carboxamide, the active intermediate of 5-(3,3-dimethyl-1-triazenyl)imidazole-4-carboxamide, by pretreatment with 10 µm O6-benzylguanine for 2 h. The number of Raji cells surviving 50 µm clomesone decreased 3-fold upon pretreatment for 2 h with 1 µm O6-benzylguanine. The degree of enhancement was dependent on the amount of alkyltransferase protein present in cell lines. For example, HT29 cells (alkyltransferase activity, 381 fmol/mg protein) exhibited a greater degree of enhancement when treated with O6-benzylguanine than SF767 (77 fmol/mg protein) and M19-MEL melanoma (36 fmol/mg protein) cells. There was no enhancement observed in mer- cell lines, U251 (<2 fmol/mg protein), and BE (3 fmol/mg protein), or with alkylating agents which did not produce a cytotoxic lesion at the O6 position of guanine in DNA such as cisplatin or 4-hydroperoxycyclophosphamide. Our studies suggest that O6-benzylguanine analogues may have utility in mer+ tumors as an adjuvant to a variety of alkylating agents which produce a toxic lesion at the O6 position of guanine.
This research was supported by the National Cancer Institute, Department of Health and Human Services, through Grants CA-47228 (M.E.D.), CA-18137 (A.E.P.), and Contract NO1-CO-74101 (R.C.M.) with ABL.