Cytidine deaminase - the methodological relevance of AraC deamination for ex vivo experiments using cultured cell lines, fresh leukemic blasts, and normal bone marrow cells

The clinical effects of cytosine arabinoside (AraC) are highly dependent on schedule and dose. Many regimens administered to patients are derived from artificial model systems involving permanent leukemic cell lines. The diff erences in pharmacokinetics between the in vivo situation and such cell lin es are largely neglected. However, cytidine deaminase activity in particula r has a major impact on AraC pharmacokinetics by degrading AraC to its inac tive metabolite AraU, and it has been shown to be of prognostic relevance i n the treatment of acute myeloid leukemia. This study therefore investigate d cytidine deaminase activities and AraC deamination in a variety of the mo st commonly used leukemic cell lines and fresh blasts and their impact on t he results of an in vitro model system. It was found that cells from differ ent cell lines (BLIN, GEM, HL60, K562, RAJI, REH, U937) vary greatly in cyt idine deaminase activity (e.g., 1.89 nmol per min/mg in K562 versus 0.01 in BLIN cells) and degrade between 18.5 (BLIN) and 96.5% (REH) of AraC to Ara U in the incubation medium. This degradation results in highly different Ar aC exposures for different cells (e.g., AUC of 960 ng per h/ml in REH versu s 4048 ng per h/ml in BLIN cells) in spite of identical starting concentrat ions of the drug. Formation of AraCTP as the main cytotoxic metabolite of A raC is significantly influenced by the differences in cell type-dependent c ytidine deaminase activity (e.g., 35.6 ng/10(7) cells in REH versus 180.2 n g/10(7) cells in BLIN cells). In contrast to permanent cell lines, fresh le ukemic blasts and normal bone marrow mononuclear cells featured low AraC de gradation in the model system.