Metabolism of 1-beta-D-arabinofuranosyl-5-azacytosine and incorporation into DNA of human T-lymphoblastic cells (Molt-4).

1-beta-D-Arabinofuranosyl-5-azacytosine (ara-5-aza-Cyd) had potent cytotoxicity against human T-type lymphoblastic cells in culture. When Molt-4 cells were exposed to ara-5-aza-Cyd for 24 h, clonogenic survival was reduced by 50 and 98% at initial concentrations of 10(-7) and 10(-6) M, respectively, compared to 3 X 10(-8) and 10(-6) M, respectively, for the same effect with 1-beta-D-arabinofuranosylcytosine (ara-C). The analogue is chemically unstable, with a t1/2 of 12 h at 37 degrees C in phosphate-buffered saline. ara-5-aza-Cyd is not significantly deaminated by human Cyd-deoxycytidine (dCyd) deaminase, in contrast to ara-C. It is phosphorylated by human cytoplasmic dCyd kinase, with a Km of 55 microM and a relative Vmax of 310% compared to dCyd. The primary metabolite (70%) in Molt-4 cells was identified as ara-5-aza-Cyd triphosphate. Thymidine but not uridine or amino acid incorporation was inhibited by ara-5-aza-Cyd. ara-5-aza-Cyd was incorporated in a dose-dependent manner into DNA, but not RNA, primarily in internucleotide linkage as the original compound. Incorporation into the cellular methanol-insoluble fraction was 3- to 5-fold higher at 8 h than was ara-C incorporation. ara-5-aza-Cyd may have a unique activity against tumor cells resistant to ara-C, particularly where high Cyd-dCyd deaminase activity is a factor. The mode of action, like that of ara-C, is probably mediated through its incorporation into DNA and inhibition of DNA synthesis.