Interaction of Nucleoside Analogues with the Sodium–Nucleoside Transport System in Brush Border Membrane Vesicles from Human Kidney

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Abstract
The therapeutic efficacy of nucleosides and nucleoside analogues as antitumor, antiviral, antiparasitic, and antiarrhythmic agents has been well documented. Pharmacokinetic studies suggest that many of these compounds are actively transported in the kidney. The goal of this study was to determine if therapeutically relevant nucleosides or analogues interact with the recently characterized Na+-driven nucleoside transport system of the brush border membrane of the human kidney. Brush border membrane vesicles (BBMV) were prepared from human kidney by divalent cation precipitation and differential centrifugation. The initial Na+-driven 3H-uridine uptake into vesicles was determined by rapid filtration. The effect of several naturally occurring nucleosides (cytidine, thymidine, adenosine), a pyrimidine base (uracil), a nucleotide (UMP), and several synthetic nucleoside analogues [zidovudine (AZT), cytarabine (Ara-C), and dideoxycytidine (ddC)] on Na+–uridine transport was determined. At a concentration of 100 µM the naturally occurring nucleosides, uracil, and UMP significantly inhibited Na+-uridine transport, whereas the three synthetic nucleoside analogues did not. Adenosine competitively inhibited Na+-uridine uptake with a Ki of 26.4 µM (determined by constructing a Dixon plot). These data suggest that naturally occurring nucleosides are substrates of the Na+–nucleoside transport system in the renal brush border membrane, whereas synthetic nucleoside analogues with modifications on the ribose ring are not. The Ki of adenosine is higher than clinically observed concentrations and suggests that the system may play a physiologic role in the disposition of this nucleoside.

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