Clarification of xenobiotic recognition mechanism at the blood-placentabarrier by transporter array

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The uptake mechanisms of nucleosides at the blood-placenta barrier were clarified by using the rat syncytiotrophoblast cell line TR-TBT 18d-1. The initial uptake of [3H]uridine and [3H]adenosine by TR-TBT 18d-1 were sodium-independent and were sensitive to nitrobenzylthioinosine. The Km value of adenosine uptake was approximately 17 μM. These results suggested involvement of equilibrative nucleoside transporters ENT1 and ENT2. The uptakes were significantly reduced by several nucleoside drugs, including zidovudine (AZT), cytarabine, vidarabine, mizoribine and caffeine. The effects were small within the therapeutic concentration ranges. Therefore, these drugs at the therapeutic concentrations might have little influence on maternal-to-fetal nucleoside transfer. The uptake mechanism of AZT was characterized. Initial uptake of AZT was sodium-independent and saturable (Km, approx. 16 μM). Thymidine and 2' -deoxyuridine strongly inhibited the uptake, but nitrobenzylthioinosine, probenecid a … More nd cimetidine had little effect on the uptake. Cyclosporin A, Ko143 and probenecid had little effect on the steady state AZT accumulation, suggesting that transporter-mediated efflux of AZT is not substantial. These results indicate that the saturable AZT uptake by TR-TBT 18d-1 was mediated by a so-far unidentified transporter.A human enterocyte model Caco-2 cell exhibits estrone-3-sulfate uptake, but the responsible transporters have not been clarified, yet. We have examined the contribution of OATP2B1 to the uptake in comparison with that of its homolog genes. The OATP2B1 mRNA expression was higher than that of OATP3A1 or OATP4A1 in Caco-2 and in human jejunum biopsies. The estrone-3-sulfate uptakes normalized to OATP2B1 mRNA expression were similar in Caco-2 cells and HEK293/OATP2B1 cells. The specific activity of OATP2B1 per mRNA expression was much higher than that of OATP3A1 and OATP4A1. These results suggest that OATP2B1 is predominantly responsible for the uptake in Caco-2 cells. This method can be applied to determine the contribution of each transporter in the placenta barrier. Less