A comprehensive evaluation of nucleoside transport by tumor and by normal proliferative cells is proposed in order to define the role of transport as a determinant of therapeutic responsiveness and the selective action of nucleoside analogs. L1210 murine lymphocytic leukemia cells maintained as ascites in DB2F1 mice will serve as the primary tumor model. Other murine tumors maintained as ascitic forms in this same strain of mice will also be studied. As the limiting toxicity in mice to many nucleoside analogs is expressed in the epithelial lining of the small intestine, mature adsorptive and immature proliferative epithelial cells isolated from the villi and the crypts of the small intestine will be used to study nucleoside transport by normal drug-sensitive cells. Multi-faceted investigations with metabolically competent cells, metabolically altered (ATP depleted) cells, and membrane vesicles, as well as studies with nonmetabolizable analogs as "model" substrates, will be used to determine the kinetic and physiological properties for the transport of natural nucleosides by tumor and by normal cells. The role of metabolism, especially phosphorylation, in the transport process will also be considered. With this knowledge, competition studies between unlabeled nucleoside analogs and labeled natural nucleosides for transport by the same carrier will be carried out to elucidate the structural specificity of the transport mechanism(s) in each cell type. Additional studies measuring the transport of labeled analogs will be done as the need arises and such derivatives are available. The overall aim of these studies is to further our understanding of the structural specificity of transport processes, and to identify regions of the nucleoside molecule essential to carrier recognition. These investigations should also yield information as to whether or not the carrier-mediated uptake of nucleoside analogs contributes to the therapeutic response and selective action, or if it can be exploited to such ends by further structural modifications of the analogs.