PROLONGED DEPLETION OF O-6-METHYLGUANINE DNA METHYLTRANSFERASE ACTIVITY FOLLOWING EXPOSURE TO O-6-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 dep letion of O6 methylguanine DNA methyltransferase (MGMT) activity, foll owing O6-benzylguanine (BG) and streptozotocin (STZ) exposure, in reve rsing 1,3 bis(2-chloroethyl)-1-nitrosourea (BCNU) resistance in vitro. We evaluated BCNU-induced cytotoxicity and measured the temporal reco very of MGMT activity in human colon carcinoma HT-29 cells following t reatment with BG, STZ, or the combination of BG and STZ. The pretreatm ent regimens which provided the greatest potentiation of BCNU cytotoxi city were those exhibiting the greatest temporal inhibition of MGMT ac tivity. The combination of BG (10 muM) and STZ (1.0 mM) produced susta ined inhibition of MGMT activity through 24 h and potentiated BCNU cyt otoxicity by at least one log greater than either agent alone. Similar ly, BG (10-100 muM) produced marked reductions in MGMT activity and in creased BCNU cytotoxicity in a dose-dependent fashion. A 100-muM dose of BG inhibited MGMT activity for 48 h and potentiated BCNU induced ce ll kill by 3 logs greater than BCNU alone. In addition, we observed th at during the period of sustained inhibition of MGMT activity, no chan ges in the steady-state MGMT mRNA levels occurred. We conclude that pr olonged inhibition of MGMT activity is an important determinant in rev ersing BCNU resistance and that chemotherapeutic regimens targeting th e inactivation of MGMT activity should be optimized such that MGMT act ivity is depleted for at least 24 h following BCNU administration.