O6-Benzylguanine (BG) is a potent inhibitor of the DNA repair
protein 06-alkylguanine
DNA alkyltransferase (AGT), and sensitizes
tumors to
BCNU in vitro and in xenografts. The combination of BG and
BCNU is now undergoing phase I clinical testing. The maximally tolerated dose of
BCNU given after BG is expected to be lower then the doses tolerated as a single agent owing to BG sensitization of hematopoietic progenitors. We have previously shown that retroviral expression of G156A mutant MGMT (deltaMGMT) in mouse and human marrow cells results in significant BG and
BCNU resistance. In this study we evaluated the effect of deltaMGMT-transduced marrow infusion on the therapeutic index of multiple BG and
BCNU treatments in
tumor-bearing nude (nu/nu athymic) mice. Prior to subcutaneous implantation of
BCNU-resistant SW480 human
colon cancer cells, cohorts of mice were given
intraperitoneal injections of nonablative doses of BG (30 mg/kg) and
BCNU (10 mg/kg, one-half of the LD10) and then infused with 1-2 x 10(6) isogeneic deltaMGMT (n = 29 mice) or lacZ-transduced (n = 20 mice) marrow cells. The xenograft-bearing mice were treated with multiple cycles of BG (30 mg/kg) and
BCNU (10-25 mg/kg). After three cycles, deltaMGMT mouse bone marrow was repopulated with CFU containing the provirus, and demonstrated a 2.7-fold increase in AGT activity and a 5.5-fold increase in
BCNU IC90 compared with LacZ mice. After five cycles, the
BCNU IC90 of CFU cells increased nine-fold over control cells, indicating selective enrichment of CFU precursor cells expressing high levels of deltaMGMT. Starting with the third cycle of
therapy, tolerance to BG and
BCNU was significantly improved in deltaMGMT mice compared with LacZ mice, as evidenced by preserved peripheral blood counts, bone marrow cellularity, and CFU content 1 and 2 weeks posttreatment and a significantly higher survival rate. Xenograft growth was significantly delayed in mice tolerating multiple cycles and higher dose intensity of BG and
BCNU as compared with mice receiving less intensive
therapy. We conclude that deltaMGMT-transduced marrow cells can improve the therapeutic index of BG and
BCNU by selectively repopulating the marrow and providing significant marrow tolerance to this combination, allowing intensive
therapy of a
BCNU-resistant
tumor.