Delta MGMT-transduced bone marrow infusion increases tolerance to O-6-benzylguanine and 1,3-bis(2-chloroethy1)-1-nitrosourea and allows intensive therapy of 1,3-bis(2-chloroethyl)-1-nitrosourea-resistant human colon cancer xenografts

O-6-Benzylguanine (BG) is a potent inhibitor of the DNA repair protein O-6- alkylguanine DNA alkyltransferase (AGT), and sensitizes tumors to BCNU in v itro and in xenografts. The combination of BG and BCNU is now undergoing ph ase 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 t o BG sensitization of hematopoietic progenitors. We have previously shown t hat retroviral expression of G156A mutant MGMT (Delta MGMT) in mouse and hu man marrow cells results in significant BG and BCNU resistance. In this stu dy we evaluated the effect of Delta MGMT-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-ha lf of the LD10) and then infused with 1-2 X 10(6) isogeneic Delta MGMT (n = 29 mice) or lacZ-transduced (n = 20 mice) marrow cells. The xenograft-bear ing mice were treated with multiple cycles of BG (30 mg/kg) and BCNU (10-25 mg/kg). After three cycles, Delta MGMT mouse bone marrow was repopulated w ith CFU containing the provirus, and demonstrated a 2.7-fold increase in AG T activity and a 5.5-fold increase in BCNU IC90 compared with LacZ mice. Af ter five cycles, the BCNU IC90 of CFU cells increased ninefold over control cells, indicating selective enrichment of CFU precursor cells expressing h igh levels of Delta MGMT. Starting with the third cycle of therapy, toleran ce to BG and BCNU was significantly improved in aMGMT mice compared with La cZ mice, as evidenced by preserved peripheral blood counts, bone marrow cel lularity, and CFU content 1 and 2 weeks posttreatment and a significantly h igher survival rate. Xenograft growth was significantly delayed in mice tol erating multiple cycles and higher dose intensity of BG and BCNU as compare d with mice receiving less intensive therapy. We conclude that Delta MGMT-t ransduced marrow cells can improve the therapeutic index of BG and BCNU by selectively repopulating the marrow and providing significant marrow tolera nce to this combination, allowing intensive therapy of a BCNU-resistant tum or.