Synergistic efficacy of O6-benzylguanine and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in a human colon cancer xenograft completely resistant to BCNU alone.
The DNA repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase) repairs cytotoxic DNA damage formed by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). High levels of this repair protein cause tumor drug resistance to nitrosoureas. To investigate the ability of a direct alkyltransferase inhibitor, O6-benzylguanine, to reverse the nitrosourea resistance of human colon cancer cells, we studied the VACO 6 cell line which has high alkyltransferase and is completely resistant to BCNU at maximal tolerated doses in the xenograft model. O6-Benzylguanine at 0.5 microgram/mL for 1 hr inactivated VACO 6 alkyltransferase by > 98% and reduced the IC50 of BCNU by 3- to 4-fold. Further analysis indicated that these two agents act in a highly synergistic fashion. In xenograft bearing athymic mice, dose-dependent depletion of hepatic and tumor alkyltransferase was noted. To maintain alkyltransferase depletion in the xenograft for at least 24 hr, two doses of 60 mg/kg O6-benzylguanine were given 1 hr prior and 7 hr after BCNU. Under these conditions, VACO 6 xenografts became responsive to BCNU with significant reductions (P < 0.001) in the tumor growth rate. The combination increased toxicity to the host, reducing the maximum tolerated dose of BCNU by approximately 50%. This study provides definitive evidence that high alkyltransferase activity is responsible for BCNU resistance in human colon cancer xenografts and that with careful drug scheduling, O6-benzylguanine can sensitize a tumor which is completely unresponsive to BCNU alone. Further studies which optimize the therapeutic index of BCNU and O6-benzylguanine in vivo will define the schedule to be used in broader preclinical studies.
Gerson
S L
SL
Ireland Cancer Center, University Hospitals of Cleveland, OH 44106.
Zborowska
E
E
Norton
K
K
Gordon
N H
NH
Willson
J K
JK
D019883:Alkyl and Aryl Transferases; D000818:Animals; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use*; D001835:Body Weight / Q000187:drug effects; D002330:Carmustine / Q000008:administration & dosage / Q000627:therapeutic use*; D003110:Colonic Neoplasms / Q000188:drug therapy* / Q000401:mortality / Q000473:pathology; D004351:Drug Resistance; D004357:Drug Synergism; D006147:Guanine / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000494:pharmacology; D006801:Humans; D051379:Mice; D008807:Mice, Inbred BALB C; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D014166:Transferases / Q000037:antagonists & inhibitors; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000201:enzymology
C064976:O(6)-benzylguanine; D006147:Guanine; D014166:Transferases; D019883:Alkyl and Aryl Transferases; C050956:DNA alkyltransferase; D002330:Carmustine