Synergistic efficacy of O6benzylguanine and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in a human colon cancer xenograft completely resistant to BCNU alone

Stanton L. Gerson, Elizabeth Zborowska, Kathleen Norton, Nahida H. Gordon, James K V Willson

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102 Scopus citations


The DNA repair protein O6alkylguanine-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 μg 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 kgO6-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.

Original languageEnglish (US)
Pages (from-to)483-491
Number of pages9
JournalBiochemical Pharmacology
Issue number2
StatePublished - Jan 26 1993

ASJC Scopus subject areas

  • Biochemistry
  • Pharmacology


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