Modulation of the cellular pharmacokinetics of ara-CTP in human leukemic blasts by dipyridamole

The effect of dipyridamole (DP) on the cellular retention of 1-beta-D-arabinofuranosylcytosine (ara-C) and its metabolites was examined in leukemic blasts that had been isolated directly from bone marrow aspirates from patients afflicted with acute myeloid leukemia (AML). When AML cells were loaded for 2 h with 1 microM [3H]-ara-C and then transferred to ara-C-free medium, total intracellular concentrations of radiolabel and [3H]-ara-C 5'-triphosphate [3H]-ara-C-CTP rapidly declined. After 8 h, total intracellular levels of tritium were 4.4 times higher if 10 microM was included in the washout medium; however, the majority of this intracellular radiolabel corresponded to [3H]-uridine arabinoside ([3H]-ara-U) and [3H]-ara-C. DP significantly increased the mean t1/2 for [3H]-ara-CTP from 102 to 136 min (P less than 0.01), but this effect was much less pronounced than that obtained for total tritium and the increase was quite variable (0-70%; median, 19%). The presence of DP in the washout medium also increased the incorporation of ara-C into DNA and the formation of ara-CDP-choline. The level of ara-CDP-choline continued to increase in both DP-containing and DP-free media for the first 4 h following drug removal and the formation of ara-CDP-choline continued during the first few hours in ara-C-free medium. At the end of the 8-h wash in DP-containing medium, the cellular concentration of ara-CDP-choline was equivalent to that found at the beginning of the washout period. Although statistically significant, the effect of DP on ara-CTP retention in AML blasts was much less pronounced than that previously observed in L5178Y leukemia. The former cells exhibited only 10% as many nucleoside transport carriers as did the L5178Y cells as measured by their capacity to bind [3H]-nitrobenzylmercaptopurine riboside (NBMPR). The effect of DP in prolonging ara-CTP retention was proportional to the number of [3H]-NBMPR binding sites. This suggests that in patients cells that exhibit extremely low transport capacity, most of the net catabolism occurs via deamination, and further inhibition of transport by DP in an effort to improve cellular retention of ara-C has little effect on ara-CTP catabolism.