Acquired resistance to O6-benzylguanine plus chloroethylnitrosoureas in human breast cancer.

PURPOSE: O(6)-benzylguanine (BG) is a pseudosubstrate inactivator of the DNA repair protein O(6)- alkylguanine-DNA alkyltransferase (AGT) that has entered clinical trials as a potentiator of the antitumor effect of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). This study was designed to evaluate potential mechanisms of BG + BCNU resistance in breast cancer cells.

METHODS

We treated MCF-7 breast cancer cells three times with cytotoxic concentrations of BG + BCNU to isolate a population of MCF-7 cells possessing BG + BCNU resistance (BBR). We measured drug resistance, AGT activity, cross-resistance to other agents and sensitivity of AGT to BG inactivation.

RESULTS

When compared with the parent line, MCF-7BBR cells were no longer sensitized to BCNU by BG, resulting in three-fold resistance to BG + BCNU and fourfold resistance to BG + 1-(2-chloroethyl)-1-nitrosourea (CNU). In contrast, MCF-7 and MCF-7BBR cells had similar sensitivity to BCNU, CNU, temozolomide (each in the absence of BG), cisplatin, and UV light. Acquired resistance to BG + chloroethylnitrosoureas (CENU) in MCF-7BBR cells was not accompanied by an increase in AGT or altered kinetics of BG inactivation of AGT. While glutathione-S transferase activity was increased modestly, its contribution to resistance in this setting remains unclear.

CONCLUSIONS

This is the first report of acquired BG + CENU resistance in human tumor cells and the results indicate that selective pressure with BG + CENUs may result in nonresponsiveness to BG by one or more complex mechanisms. These results predict the development of acquired resistance to BG + BCNU in clinical trials.