Cytarabine (araC) is converted to araC 5'-triphosphate after entering
leukemia cells as a substrate for
nucleoside transport processes. This study tested the relationship between araC cytotoxicity, measured in an in vitro tetrazolium
dye reduction assay of cell viability, and the cellular abundance of es
nucleoside transport elements, assayed by a flow cytometric method that used the es-specific
stain, 5-(SAENTA-x8)-fluorescein (5-(Sx8)-F), in cultured
leukemia cells and in myeloblasts and lymphoblasts (blasts) from
leukemia patients. Cellular es site abundance (B(max) value for 5-(Sx8)-F binding) varied sixfold among nine leukemic myeloblast samples from patients. In cultured OCI/AML-2 myeloblasts and CCRF-CEM T-lymphoblasts, and in fresh leukemic blasts, es sites were fractionally blocked by treatment with graded concentrations of nitrobenzylthioinosine (
NBMPR), an inhibitory es site
ligand, to simulate the variation in es expression found in leukemic blasts from patients with
acute myeloid leukemia. When the cytotoxicity of a single concentration of araC was determined in
NBMPR-treated
leukemia cells, cell kill correlated closely with the intensity of 5-(Sx8)-F fluorescence (r = .92 to .99), a measure of the cell surface abundance of functional es
nucleoside transporter sites. Concentrations of
NBMPR that achieved half-maximal reduction (4.3 to 12 nmol/L) of cellular 5-(Sx8)-F fluorescence (measured by flow cytometry) approximated IC50 values (1 to 10 nmol/L) previously found for inhibition by
NBMPR of es-mediated
nucleoside fluxes in several cell types, supporting the view that 5-(Sx8)-F interacted with the estransporter. The correlation of araC cytotoxicity and the B(max) for 5-(Sx8)-F binding to es sites in cultured
leukemia cells and in leukemic blasts from acute
leukemia patients (r = .95) suggests that the flow cytometry assay of es capacity may be useful in predicting clinical response to araC.