DEPLETION OF O-6-METHYLGUANINE DNA METHYLTRANSFERASE AND POTENTIATION OF 1,3-BIS(2-CHLOROETHYL)-1-NITROSOUREA ANTITUMOR-ACTIVITY BY O-6-BENZYLGUANINE INVITRO

The overcoming effect of O6-benzylguanine on O6-methylguanine-DNA methyltransferase (MGMT)-mediated 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) resistance in vitro was evaluated. Depletion of MGMT activity in Mer+ HeLa S3 cells by O6-benzylguanine was dose-dependent and a complete loss of MGMT activity was achieved at a concentration of 0.5 muM. The cytotoxic potential of BCNU on MGMT proficient HeLa S3 (1.10 pmol/mg of protein), SMMC-7721 (0.72 pmol/mg of protein) and Cc801 (0.39 pmol/mg of protein) was greatly enhanced when cells were exposed to 10 muM O6-benzylguanine for 1 h, but there was a lack of potentiation of BCNU sensitivity in Mer- HeLa MR cells due to its nearly undetectable level of MGMT. There existed a correlation between the extent of enhancement and the amount of MGMT activity. The intensity of enhancement expressed as dose modifying factor = IC50 (BCNU alone)/IC50 (10 muM O6-benzylguanine + BCNU) was 4.56, 3.89, 3.67 and 0.97 in HeLa S3, SMMC-7721, Cc801 and HeLa MR cells respectively. The results further demonstrated that O6-benzylguanine may have potential utility as an adjuvant in combination chemotherapy with chloroethylnitrosourea agents.