Blood-to-Testis Transport of Ribavirin Involves Carrier-Mediated Processes at the Blood–Testis Barrier

Ribavirin, an antiretroviral agent targeting the hepatitis C virus, causes male reproductive toxicity. This study investigated the mechanism of ribavirin transport at the blood–testis barrier (BTB). In vivo mouse integration plot analysis after intravenous administration revealed that the net influx clearance of [³H]ribavirin in the testis was 3.6-fold greater than that of [¹⁴C]D-mannitol, a paracellular transport marker, implying transcellular transport of ribavirin across the BTB. Moreover, [³H]ribavirin uptake by TM4 cells, mouse-derived Sertoli cells, was time- and concentration-dependent, with a K_m value of 2.49 mM. S-[(4-nitrophenyl)methyl]-6-thioinosine, an inhibitor of Na⁺-independent equilibrative nucleoside transporters (ENTs), strongly inhibited the [³H]ribavirin uptake by TM4 cells at 100 µM. Compared to the uptake of [³H]adenosine, a typical endogenous nucleoside, [³H]ribavirin uptake was relatively similar to ENT2 transport. [³H]Ribavirin uptake was also observed in mouse ENT2-expressing Xenopus laevis oocytes, and gene silencing via the transfection of ENT2 small interfering RNA significantly reduced the [³H]ribavirin transport into TM4 cells by 13%. Taken together, these results suggest that ENT2 partially contributes to ribavirin transport at the BTB.