SENSITIVITY OF ACUTE-LEUKEMIA CELLS TO CYTARABINE IS A CORRELATE OF CELLULAR ES NUCLEOSIDE TRANSPORTER SITE CONTENT MEASURED BY FLOW-CYTOMETRY WITH SAENTA-FLUORESCEIN

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 ab undance (B-max value for 5-(Sx8)-F binding) varied sixfold among nine leukemic myeloblast samples from patients. In cultured OCI/AML-5 myelo blasts and CCRF-CEM T-lymphoblasts, and in fresh leukemic blasts, es s ites were fractionally blocked by treatment with graded concentrations of nitrobenzylthioinosine (NBMPR), an inhibitory es site ligand, to s imulate the variation in es expression found in leukemic blasts from p atients 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 fluoresc ence (r = .92 to .99), a measure of the cell surface abundance of func tional es nucleoside transporter sites. Concentrations of NBMPR that a chieved 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-mediate d nucleoside fluxes in several cell types, supporting the view that 5- (Sx8)-F interacted with the es transporter. The correlation of araC cy totoxicity 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 u seful in predicting clinical response to araC. (C) 1997 by The America n Society of Hematology.