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.