HUMAN CD34(-UNRESPONSIVE O-6-ALKYLGUANINE-DNA ALKYLTRANSFERASE AND ARE SENSITIVE TO O-6-BENZYLGUANINE PLUS BCNU() HEMATOPOIETIC PROGENITORSHAVE LOW, CYTOKINE)

Human bone marrow (BM) cells contain low levels of the DNA repair prot ein, O-6-alkylguanine-DNA alkyltransferase, which may explain their su sceptibility to nitrosourea-induced cytotoxicity and the development o f secondary leukemia after nitrosourea treatment. Isolated CD34(+) mye loid progenitors were also found to have low levels of alkyltransferas e activity. The level of alkyltransferase in CD34(+) cells or in monon uclear BM cells did not increase after incubation with granulocyte-mac rophage colony-stimulating factor, interleukin-3, stem cell factor, th e combination, or 5637 conditioned medium. BCMU sensitivity remained u nchanged as well. In addition, O-6-benzylguanine depleted alkyltransfe rase activity in BM cells at concentrations as low as 1.5 mu mol/L aft er a 1-hour exposure. O-6-benzylguanine pretreatment markedly sensitiz ed hematopoietic progenitor colony-forming cells to BCNU, resulting in a reduction in the dose of drug (termed the dose-modification factor) required to inhibit 50% of the colony formation (IC50) of threefold t o fivefold. Since, unlike many other cell types, proliferating early ( CD34(+)) hematopoietic precursors do not induce alkyltransferase, myel osuppression may be the dose-limiting toxicity of the combination of O -6-benzylguanine plus BCNU in clinical trials. (C) 1996 by The America n Society of Hematology.