Prolonged depletion of O6-methylguanine DNA methyltransferase activity following exposure to O6-benzylguanine with or without streptozotocin enhances 1,3-bis(2-chloroethyl)-1-nitrosourea sensitivity in vitro.

This study was undertaken to ascertain the importance of prolonged depletion of O6-methylguanine DNA methyltransferase (MGMT) activity, following O6-benzylguanine (BG) and streptozotocin (STZ) exposure, in reversing 1,3 bis(2-chloroethyl)-1-nitrosourea (BCNU) resistance in vitro. We evaluated BCNU-induced cytotoxicity and measured the temporal recovery of MGMT activity in human colon carcinoma HT-29 cells following treatment with BG, STZ, or the combination of BG and STZ. The pretreatment regimens which provided the greatest potentiation of BCNU cytotoxicity were those exhibiting the greatest temporal inhibition of MGMT activity. The combination of BG (10 microM) and STZ (1.0 mM) produced sustained inhibition of MGMT activity through 24 h and potentiated BCNU cytotoxicity by at least one log greater than either agent alone. Similarly, BG (10-100 microM) produced marked reductions in MGMT activity and increased BCNU cytotoxicity in a dose-dependent fashion. A 100-microM dose of BG inhibited MGMT activity for 48 h and potentiated BCNU induced cell kill by 3 logs greater than BCNU alone. In addition, we observed that during the period of sustained inhibition of MGMT activity, no changes in the steady-state MGMT mRNA levels occurred. We conclude that prolonged inhibition of MGMT activity is an important determinant in reversing BCNU resistance and that chemotherapeutic regimens targeting the inactivation of MGMT activity should be optimized such that MGMT activity is depleted for at least 24 h following BCNU administration.