Inhibitors of nucleoside transport. Structure-activity study using human erythrocytes
TL;DR: Nucleoside derivatives were compared in respect to their ability to inhibit a transport-dependent aspect of nucleoside metabolism in erythrocytes, the synthesis of inosine from external guanosine and hypoxanthine and the hydrophobicity of the 6-position substituents appeared to contribute importantly to inhibitory activity.
Abstract: The passage of nucleosides across the plasma membrane of erythrocytes is a membrane-mediated process which is strongly inhibited by derivatives of 9-beta-D-ribofuranosylpurine (1) with S, O, or N atoms at the purine 6 position bearing variously substituted arylalkyl groups. In this structure-activity study, nucleoside derivatives were compared in respect to their ability to inhibit a transport-dependent aspect of nucleoside metabolism in erythrocytes, the synthesis of inosine from external guanosine and hypoxanthine. 6-Benzylthio, 6-benzylamino, and 6-benzyloxy derivatives of 1 were inhibitory at 10(-5)-10(-6) M and the similarity of their activities suggested that alkylation of the transporter as the mechanism of transport inhibition was unlikely. The hydrophobicity of the 6-position substituents appeared to contribute importantly to inhibitory activity. Although replacement of the ribofuranose moiety by other sugars reduced inhibitory activity, compounds with 9-butyl groups were inhibitory. 6-[(2-Hydroxy-5-nitrobenzyl)thio] derivatives of 1 were the most potent of the inhibitors tested, being active at about 10(-7) M.
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TL;DR: The relationships between 50 per cent inhibitory concentration and dissociation constants of the complex are shown for various mechanisms of inhibition, and several methods for the estimation of very low Ki, values are presented.
483 citations
TL;DR: The chapter discusses the importance of the mechanism of permeation of nucleosides, nucleobases, and nucleotides through the cell membranes of eukaryotes and discusses the carrier model for facilitated diffusion and tests for its applicability to nucleoside and base transport.
Abstract: Publisher Summary This chapter discusses the importance of the mechanism of permeation of nucleosides, nucleobases, and nucleotides through the cell membranes of eukaryotes. Some of the reasons of its importance include (1) many anticancer and immunosuppressive agents presently in use or under development are nucleoside, nucleotide, or nucleobase analogs and a clear understanding of their mode of entry into cells and metabolism is important in the assessment of their mode of action, efficacy, and optimal administration, and (2) radioactively labeled nucleosides and nucleic acid bases are widely used as precursors to label specifically the nucleic acids of various types of organisms or of the viruses or plasmids replicating therein as well as to assess the rates of nucleic acid synthesis. An interpretation of the rates of nucleoside and base incorporation into nucleic acids, be it RNA or DNA, depends on a clear understanding of the extent to which these rates may reflect the rates of the conversion of the extracellular substrate to intracellular nucleotides, which are the direct precursors in nucleic acid synthesis. Elimination of the ambiguities inherent in metabolizing cells is of clear advantage to transport studies. Nucleoside and purine transport have been studied successfully in the absence of intracellular metabolism by the use of erythrocytes or of mutant clones of cultured animal cells that are deficient in specific metabolic enzymes and by the use of cell/substrate systems in which substrate metabolism is blocked in some other manner. The chapter also discusses the carrier model for facilitated diffusion and tests for its applicability to nucleoside and base transport.
324 citations
TL;DR: Myeloblasts have both higher araC transport rates and more nucleoside transport sites than lymphoblasts and this factor may contribute to the greater sensitivity of AML to this drug.
Abstract: Although cytosine arabinoside (araC) can induce a remission in a majority of patients presenting with acute myeloblastic leukemia (AML), a minority fail to respond and moreover the drug has less effect in acute lymphoblastic leukemia (ALL). The carrier-mediated influx of araC into purified blasts from patients with AML, ALL, and acute undifferentiated leukemia (AUL) has been compared to that of normal lymphocytes and polymorphs. Blasts showed a larger mediated influx of araC than mature cells, since mean influxes for myeloblasts and lymphoblasts were 6- and 2.3-fold greater than polymorphs and lymphocytes, respectively. Also, the mean influx for myeloblasts was fourfold greater than the mean for lymphoblasts. The number of nucleoside transport sites was estimated for each cell type by measuring the equilibrium binding of [3H]nitrobenzylthioinosine (NBMPR), which inhibits nucleoside fluxes by binding with high affinity to specific sites on the transport mechanism. The mean binding site numbers for myeloblasts and lymphoblasts were 5- and 2.8-fold greater, respectively, than for the mature cells of the same maturation series. The mean number of NBMPR binding sites for myeloblasts was fourfold greater than for lymphoblasts. Patients with AUL were heterogeneous since blasts from some gave values within the myeloblastic range and others within the lymphoblastic range. The araC influx correlated closely with the number of NBMPR binding sites measured in the same cells on the same day. Transport parameters were measured on blasts from 15 patients with AML or AUL who were then treated with standard induction therapy containing araC. Eight patients entered complete remission, while seven failed therapy, among whom were the three patients with the lowest araC influx ( 10-fold between leukemic blasts and normal leukocytes, but transport capacity related directly to the number of nucleoside transport sites on the cell. Finally, low araC transport rates or few NBMPR binding sites on blasts were observed in a subset of patients with acute leukemia who failed to achieve remission with drug combinations containing araC.
207 citations
TL;DR: Data indicate that there are two sodium-dependent mechanisms for nucleoside transport in mouse intestinal epithelial cells, and that formycin B and thymidine may serve as model substrates to distinguish between these transporters.
153 citations