Acquired resistance to O6-benzylguanine plus chloroethylnitrosoureas in human breast cancer.
OBJECTIVE
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.
Phillips
W P
WP
Division of Hematology-Oncology and the Case Western Reserve University/University Hospitals of Cleveland Ireland Cancer Center, BRB-3, 10900 Euclid Avenue, Cleveland OH 44106-4937, USA.
Gerson
S L
SL
D018906:Antineoplastic Agents, Alkylating / Q000494:pharmacology*; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000494:pharmacology*; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000201:enzymology / Q000378:metabolism; D002330:Carmustine / Q000494:pharmacology*; D019008:Drug Resistance, Neoplasm; D004357:Drug Synergism; D005038:Ethylnitrosourea / Q000031:analogs & derivatives* / Q000494:pharmacology; D005978:Glutathione / Q000378:metabolism; D006147:Guanine / Q000031:analogs & derivatives* / Q000494:pharmacology; D006801:Humans; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000378:metabolism; D014407:Tumor Cells, Cultured
D018906:Antineoplastic Agents, Alkylating; C064976:O(6)-benzylguanine; C017139:1-(2-chloroethyl)-1-nitrosourea; D006147:Guanine; D019853:O(6)-Methylguanine-DNA Methyltransferase; D005978:Glutathione; D005038:Ethylnitrosourea; D002330:Carmustine