Influence of fludarabine on pharmacokinetics and pharmacodynamics of cytarabine: implications for a continuous infusion schedule.

Arabinosylcytosine (ara-C) is a cytotoxic agent with major activity against acute leukemias. To exert this effect, it must first be phosphorylated to its active 5'-triphosphate, ara-CTP, which is incorporated into DNA. Our previous studies demonstrated that preincubation with arabinosyl-2-fluoroadenine (F-ara-A) increased the rate of ara-CTP accumulation in leukemia cells when incubated with 10 microM ara-C. Such concentrations of ara-C are readily obtained during intermittent bolus infusions of ara-C, and clinical trials were conducted using fludarabine in combination with 2-h infusions of intermediate-dose ara-C. During continuous infusion of ara-C, however, serum ara-C levels are <10 microM. Because the effectiveness of ara-C depends on the levels of intracellular ara-CTP and its incorporation into DNA, we sought to investigate the influence of fludarabine on pharmacodynamics of ara-C at concentrations of ara-C achieved during continuous infusion. Using the K562 human leukemic cell line, we established that incubation with 30 microM F-ara-A was able to modulate intracellular dNTP pools and achieve maximum enhancement of ara-CTP levels at all concentrations of ara-C tested (0.3-10.0 microM). The relative enhancement of ara-CTP concentrations ranged from 2.2- to 2.8-fold. Combination of F-ara-A with 1.0 and 3.0 microM ara-C also increased the incorporation of ara-CTP into DNA. To model the influence of F-ara-A on continuous infusion ara-C, cells were incubated with 1 microM ara-C alone or in combination with F-ara-A. The F-ara-A-incubated cells accumulated effective intracellular concentrations of F-ara-ATP, which resulted in greatly increased intracellular ara-CTP levels. These studies demonstrate the capacity of clinically attainable concentrations of F-ara-ATP to enhance the formation of ara-CTP at concentrations of ara-C that are achieved during a continuous infusion schedule. Given the important role intracellular ara-CTP concentrations and ara-CMP incorporation into DNA have on the ultimate cytotoxic capacity of ara-C against acute myelogenous leukemia blasts, these studies suggest a promising pharmacological model for improving the efficacy of the continuous infusion ara-C regimen.