The effects of α-difluoromethylornithine (DFMO), an ornithine analogue which is an ornithine decarboxylase inhibitor, on the actions of the topoisomerase II-reactive agents 4′-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) and etoposide (VP-16) were investigated in 2 murine L1210 leukemia lines and 2 human HL-60 leukemia lines. One of the human lines was resistant to the cytotoxic and DNA cleaving effects of m-AMSA (HL-60/AMSA). In all 4 lines, α-DFMO depleted cellular putrescine and spermidine to nondetectable levels. VP-16-induced DNA cleavage (quantified using alkaline elution) was decreased in all lines following α-DFMO treatment. The m-AMSA-induced DNA cleavage was decreased in one of the L1210 lines and in the HL-60 line sensitive to m-AMSA; m-AMSA-induced DNA cleavage was increased in the other L1210 line. The low frequency of m-AMSA-induced DNA cleavage produced in HL-60/AMSA was unaffected by α-DFMO treatment. Alterations in drug-mediated DNA effects induced by α-DFMO could not be uniformly explained by α-DFMO-induced alterations in m-AMSA or VP-16 cellular uptake, as indicated by direct measurements of cell-associated drug or results of DNA cleavage assays in nuclei isolated from α-DFMO-treated cells. Exogenous putrescine prevented the effects of α-DFMO on drug-induced DNA cleavage, substantiating polyamine depletion as the cause of the altered frequency of DNA cleavage. Cytotoxicity assays in 2 of the lines demonstrated that drug-induced reductions in colony-forming ability paralleled drug-induced DNA cleavage. (2R,5R)-6-heptyne-2,5-diamine, a putrescine analogue which is also an ornithine decarboxylase inhibitor, was also used to deplete polyamine levels in HL-60. (2R,5R)-6-heptyne-2,5-diamine was more potent than α-DFMO and produced effects on m-AMSA- and VP-16-induced DNA cleavage and cytotoxicity identical to those produced by α-DFMO.
Supported by PHS Grants CA40090 (L. A. Z.), CA39809 (E. J. F.), and Program Project Grant CA13525 (L. J. M.), by Grant CH-324 (L. A. Z.) from the American Cancer Society, and by a gift to the M. D. Anderson Annual Fund for the Chemotherapy Research Program by Mr. Henry C. Beck, Jr., of Dallas, Texas.