RIBAVIRIN UPTAKE BY HUMAN ERYTHROCYTES AND THE INVOLVEMENT OF NITROBENZYLTHIOINOSINE-SENSITIVE (ES)-NUCLEOSIDE TRANSPORTERS

1 The major toxicity associated with oral therapy with ribavirin is an aemia, which has been postulated to occur as a result of accumulation of ribavirin triphosphate interfering with erythrocyte respiration. Th e objective of this study was to determine the mechanism by which riba virin enters into erythrocytes. 2 Entry into human erythrocytes was ex amined by measuring influx rates of [H-3]-ribavirin alone and with the inhibitor nitrobenzylthioinosine (NBMPR), and by investigating the in hibitory effects of nucleoside and nucleobase permeants on ribavirin t ransport, by use of inhibitor oil-stop methods. Transport mechanisms w ere further characterized by assessment of substrates to cause counter transport of ribavirin in preloaded erythrocytes, and by measuring the effects of ribavirin on [H-3]-NBMPR binding to erythrocyte membranes. 3 Human erythrocytes had a saturable influx mechanism for ribavirin ( K-m at 22 degrees C of 440+/-100 mu M) which was inhibited by nanomola r concentrations of NBMPR (IC50 0.99+/-0.15 nM). Nucleosides also inhi bited the influx of ribavirin (adenosine more effective than uridine) but the nucleobases hypoxanthine and adenine had no effect. In additio n, uridine caused the countertransport of ribavirin in human erythrocy tes. Entry of ribavirin into horse erythrocytes, a cell type that lack s the NBMPR-sensitive (es) nucleoside transporter, proceeded slowly an d via a pathway that was resistant to NBMPR inhibition. Ribavirin was a competitive inhibitor of adenosine influx (mean K-i 0.48+/-0.14 mM) and also inhibited NBMPR binding to erythrocyte membranes (mean K-i 2. 2+/-0.39 mM). 4 These data indicate that ribavirin is a transported pe rmeant for the es nucleoside transporter of human erythrocytes. There was no evidence for ribavirin entering cells via a nucleobase transpor ter.