Inhibition of nucleoside and nucleobase transport and nitrobenzylthioinosine binding by dilazep and hexobendine.

The transport of 500 microM uridine by human erythrocytes and S49, P388 and L1210 mouse leukemia cells, Chinese hamster ovary (CHO) cells and Novikoff rat hepatoma cells was inhibited strongly by dilazep and hexobendine with similar concentration dependence, but the sensitivity of transport in the various cell types varied greatly; IC50 values ranged from 5-30 nM for human erythrocytes and S49 and P388 cells to greater than 1 microM for CHO and Novikoff cells. The binding of nitrobenzylthioinosine (NBTI) to high-affinity sites on these cells (Kd approximately equal to 0.5 nM) was inhibited by hexobendine and dilazep in a similar pattern. Furthermore, these drugs, just as dipyridamole and papaverine, inhibited the dissociation of NBTI from high-affinity binding sites but only at concentrations 10-100 times higher than those inhibiting uridine transport. In contrast, high uridine concentrations (greater than 2 mM) accelerated the dissociation of NBTI. Dilazep also inhibited the transport of hypoxanthine, but only in those cell lines whose transporter is sensitive to inhibition by uridine and dipyridamole. Adenine transport was not inhibited significantly by dilazep in any of the cell lines tested, except for a slight inhibition in Novikoff cells. [14C]Hexobendine equilibrated across the plasma membrane in human erythrocytes within 2 sec of incubation at 25 degrees, but accumulated to 6-10 times the extracellular concentration in cells of the various cultured lines. Uptake was not affected by high concentrations of uridine, NBTI or dipyridamole. Hexobendine inhibited the growth of various cell lines to a lesser extent (IC50 = greater than or equal to 100 microM) than dipyridamole (IC50 = 15-40 microM). At 40 microM, however, it completely inhibited the growth of S49 cells that had been made nucleoside dependent by treatment with methotrexate or pyrazofurin.