Leukaemic blasts differ from normal bone marrow mononuclear cells and CD34(+) haemopoietic stem cells in their metabolism of cytosine arabinoside

Different metabolites of cytosine arabinoside (AraC) contribute to its cyto toxicity including incorporation of AraCTP into DNA, the incorporation of A raUMP into RNA, inhibition of polymerase alpha and beta (AraCMP/CTP), an im pairment of repair mechanisms (AraCTP), alterations of phospholipid metabol ism (AraCDP-choline), a direct membrane interaction (AraC), the alteration of signal transduction pathways (AraCDP-choline, AraCTP) and the induction of apoptosis. Since little is known about the potential differences in AraC metabolism between leukaemic blasts and normal haemopoietic progenitor cel ls, the formation of all known AraC metabolites was determined in bone marr ow samples from patients with acute myeloid leukaemia (AML), healthy volunt eers and specimens of cellsorted CD34(+) haemopoietic stem cells. Highly si gnificant differences were found for phosphorylated AraC metabolites (AraCM P, -CDP, -CTP, AraUMP) between AML and normal mononuclear bone marrow (ng/1 0(7) cells respectively 1.30 v 2.66; 2.65 v 7.50; 33.68 v 99.0; 1.18 v 5.70 ). The highest differences were found for formation of AraCDP-choline (3.75 v 12.86) which might be relevant for the high efficacy of high-dose AraC r egimens, In contrast, no differences were found in the deamination product AraU (2.01 v 2.91). Only minute amounts of phosphorylated AraU derivatives were detected, providing an explanation for the lacking contribution of Ara U to cytosine arabinoside cytotoxicity. Results in normal CD34(+) haemopoie tic stem cells did not differ significantly from normal bone marrow mononuc lear cells and therefore justify their use as a surrogate in determining Ar aC-induced haematotoxicity. These data suggest a metabolic basis for the re lative selectivity of AraC cytotoxicity for AML blasts and provide a means to determine the role of different metabolites and their related mechanism of action for overall AraC cytotoxicity.