Modulation of ara-CTP levels by fludarabine and hydroxyurea in leukemic cells

The rate of ara-cytosine triphosphate (ara-CTP) accumulation and its retent ion has been correlated with 1-beta -D-arabinofuranosylcytosine (ara-C)-med iated toxicity and clinical outcome in childhood and adult leukemia. We tes ted to what extent preincubation with the ribonucleotide reductase inhibito rs fludarabine (F-ara-A) and hydroxyurea (HU) enhanced ara-CTP levels in tw o human myeloid (HL-BD, CMK) and two lymphoblastic leukemia cell lines (MOL T-4, BLIN-1) and also in blasts from 28 children with acute leukemia (AML: 14, ALL: 14), Incubation experiments carried out with cell lines showed F-a ra-A and HU to be equipotent in increasing ara-CTP levels. The highest incr ease was observed in HL-60 cells whereas preincubation had no modulatory ef fect in MOLT-4 cells. Accordingly, modulation of intracellular ara-CTP leve ls differed between the subtypes of childhood acute leukemia: whereas in T- ALL (five) preincubation with F-ara-A and HU had no effect on intracellular ara-C metabolism, increased ara-CTP levels were seen in some cases of pre- B-ALL (seven). In myelogenous blasts (12) clinically relevant enhancement o f ara-C toxification was regularly obtained with both, F-ara-A (1,g-fold) a nd HU (1.5-fold). In conclusion, our data suggest that combinations of ara- C and ribonucleotide reductase inhibitors are apt to increase ara-CTP level s depending on the individual cell type and its sensitivity towards ara-C m odulators.