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

Biochem Pharmacol. 1993 Jan 26;45(2):483-91. doi: 10.1016/0006-2952(93)90086-c.


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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Alkyl and Aryl Transferases*
  • Animals
  • Antineoplastic Combined Chemotherapy Protocols / therapeutic use*
  • Body Weight / drug effects
  • Carmustine / administration & dosage
  • Carmustine / therapeutic use*
  • Colonic Neoplasms / drug therapy*
  • Colonic Neoplasms / mortality
  • Colonic Neoplasms / pathology
  • Drug Resistance
  • Drug Synergism
  • Guanine / administration & dosage
  • Guanine / analogs & derivatives*
  • Guanine / pharmacology
  • Humans
  • Mice
  • Mice, Inbred BALB C
  • Mice, Nude
  • Neoplasm Transplantation
  • Transferases / antagonists & inhibitors
  • Tumor Cells, Cultured / drug effects
  • Tumor Cells, Cultured / enzymology


  • O(6)-benzylguanine
  • Guanine
  • Transferases
  • Alkyl and Aryl Transferases
  • DNA alkyltransferase
  • Carmustine