Potential of various drugs to inhibit nucleoside uptake in rat syncytiotrophoblast cell line, TR-TBT 18d-1

S-(4-Nitrobenzyl)-6-thioinosine (NBMPR) is Not a Selective Inhibitor of Equilibrative Nucleoside Transporters but Also Blocks Efflux Activity of Breast Cancer Resistance Protein

PURPOSE

S-(4-Nitrobenzyl)-6-thioinosine (NBMPR) is routinely used at concentrations of 0.10 μM and 0.10 mM to specifically inhibit transport of nucleosides mediated by equilibrative nucleoside transporters 1 (ENT1) and 2 (ENT2), respectively. We recently showed that NBMPR (0.10 mM) might also inhibit placental active efflux of [³H]zidovudine and [³H]tenofovir disoproxil fumarate. Here we test the hypothesis that NBMPR abolishes the activity of P-glycoprotein (ABCB1) and/or breast cancer resistance protein (ABCG2).

METHODS

We performed accumulation assays with Hoechst 33342 (a model dual substrate of ABCB1 and ABCG2) and bi-directional transport studies with the ABCG2 substrate [³H]glyburide in transduced MDCKII cells, accumulation studies in choriocarcinoma-derived BeWo cells, and in situ dual perfusions of rat term placenta with glyburide.

RESULTS

NBMPR inhibited Hoechst 33342 accumulation in MDCKII-ABCG2 cells (IC₅₀ = 53 μM) but not in MDCKII-ABCB1 and MDCKII-parental cells. NBMPR (0.10 mM) also inhibited bi-directional [³H]glyburide transport across monolayers of MDCKII-ABCG2 cells and blocked ABCG2-mediated [³H]glyburide efflux by rat term placenta in situ.

CONCLUSION

NBMPR at a concentration of 0.10 mM abolishes ABCG2 activity. Researchers using NBMPR to evaluate the effect of ENTs on pharmacokinetics must therefore interpret their results carefully if studying compounds that are substrates of both ENTs and ABCG2.

Contributions of Drug Transporters to Blood-Placental Barrier

The placenta is the only organ linking two different individuals, mother and fetus, termed as blood-placental barrier. The functions of the blood-placental barrier are to regulate material transfer between the maternal and fetal circulation. The main functional units are the chorionic villi within which fetal blood is separated by only three or four cell layers (placental membrane) from maternal blood in the surrounding intervillous space. A series of drug transporters such as P-glycoprotein (P-GP), breast cancer resistance protein (BCRP), multidrug resistance-associated proteins (MRP1, MRP2, MRP3, MRP4, and MRP5), organic anion-transporting polypeptides (OATP4A1, OATP1A2, OATP1B3, and OATP3A1), organic anion transporter 4 (OAT4), organic cation transporter 3 (OCT3), organic cation/carnitine transporters (OCTN1 and OCTN2), multidrug and toxin extrusion 1 (MATE1), and equilibrative nucleoside transporters (ENT1 and ENT2) have been demonstrated on the apical membrane of syncytiotrophoblast, some of which also expressed on the basolateral membrane of syncytiotrophoblast or fetal capillary endothelium. These transporters are involved in transport of most drugs in the placenta, in turn, affecting drug distribution in fetus. Moreover, expressions of these transporters in the placenta often vary along with the gestational ages and are also affected by pathophysiological factor. This chapter will mainly illustrate function and expression of these transporters in placentas, their contribution to drug distribution in fetus, and their clinical significance.

Quantification of ENT1 and ENT2 Proteins at the Placental Barrier and Contribution of These Transporters to Ribavirin Uptake

The aims of this study are to quantify the protein levels of nucleoside transporters in placental microvillous membranes (MVMs) and to clarify the contributions of these transporters to ribavirin uptake at the placental barrier. Placental MVMs of human and rat expressed equilibrative nucleoside transporter (ENT) 1 protein, whereas the expression of ENT2 protein was obscure. Maternal-to-fetal transfer of [³H]ribavirin in rats was much higher than that of [¹⁴C]sucrose. The uptake of [³H]ribavirin by rat placental trophoblast TR-TBT 18 d-1 cells, which functionally express both ENT1 and ENT2 proteins, was saturable, and was significantly inhibited by 0.1 μM nitrobenzylthioinosine, which selectively abolishes ENT1-mediated uptake. Dipyridamole at 10 μM is capable of inhibiting ENT2 as well as ENT1, but a degree of inhibition by 10 μM dipyridamole on [³H]ribavirin uptake was not much different from that by 0.1 μM nitrobenzylthioinosine (ENT1-specific inhibitor). Therefore, ENT2 may contribute little to [³H]ribavirin uptake by these cells. Rat ENT1 cRNA-injected oocytes showed increased [³H]ribavirin uptake compared with water-injected oocytes, while rat ENT2 cRNA-injected oocytes did not. In conclusion, ENT1 protein expressed in placental MVMs appears to play a predominant role in the uptake of ribavirin.

Uptake and metabolism of mizoribine, an immunosuppressant, in L5178Y-R mouse lymphoma cells in vitro and peripheral blood mononuclear cells of rats and kidney transplant recipients in vivo

The cellular uptake of mizoribine (MZR), an immunosuppressant, and metabolism of MZR to MZR-5'- monophosphate (MZRP), an active metabolite, were evaluated in L5178Y-R mouse lymphoma cells and peripheral blood mononuclear cells (PBMCs) of rats and kidney transplant recipients (KTRs, n = 22). Real-time PCR analysis revealed the expression of ENT1 and ENT2 mRNAs, but not of CNTs, in L5178Y-R cells and rat's PBMCs. In L5178Y-R cells, the uptake of MZR was suppressed by adenosine, a substrate for ENT1 and ENT2, but not by 5-(4-nitrobenzyl)-6-thioinosine (0.1 μM), an ENT1 inhibitor. Saturable metabolism of MZR to MZRP was observed. In rats, peak plasma concentrations of MZR and peak concentrations of MZR and MZRP in PBMCs were observed 3 h after oral administration. MZR disappeared from PBMCs in parallel with plasma MZR, but the disappearance of MZRP from PBMCs appeared to be slow. In KTRs, the mean plasma concentration of MZR 3-4 h after ingestion was 3.14 μg/ml and the mean MZRP concentration in PBMCs was 16.8% of MZR, reflecting the involvement of ENT in the uptake of MZR. A linear relationship was observed between plasma MZR concentrations ranging from 1 to 6 μg/ml and PBMC's MZRP concentrations ranging from 90 to 200 ng/ml.

Equilibrative Nucleoside Transporter 1 (ENT1, SLC29A1) Facilitates Transfer of the Antiretroviral Drug Abacavir across the Placenta

Abacavir is a preferred antiretroviral drug for preventing mother-to-child human immunodeficiency virus transmission; however, mechanisms of its placental transfer have not been satisfactorily described to date. Because abacavir is a nucleoside-derived drug, we hypothesized that the nucleoside transporters, equilibrative nucleoside transporters (ENTs, SLC29A) and/or Na⁺-dependent concentrative nucleoside transporters (CNTs, SLC28A), may play a role in its passage across the placenta. To test this hypothesis, we performed uptake experiments using the choriocarcinoma-derived BeWo cell line, human fresh villous fragments, and microvillous plasma membrane (MVM) vesicles. Using endogenous substrates of nucleoside transporters, [³H]-adenosine (ENTs, CNT2, and CNT3) and [³H]-thymidine (ENTs, CNT1, and CNT3), we showed significant activity of ENT1 and CNT2 in BeWo cells, whereas experiments in the villous fragments and MVM vesicles, representing a model of the apical membrane of a syncytiotrophoblast, revealed only ENT1 activity. When testing [³H]-abacavir uptakes, we showed that of the nucleoside transporters, ENT1 plays the dominant role in abacavir uptake into placental tissues, whereas contribution of Na⁺-dependent transport, most likely mediated by CNTs, was observed only in BeWo cells. Subsequent experiments with dually perfused rat term placentas showed that Ent1 contributes significantly to overall [³H]-abacavir placental transport. Finally, we quantified the expression of SLC29A in first- and third-trimester placentas, revealing that SLC29A1 is the dominant isoform. Neither SLC29A1 nor SLC29A2 expression changed over the course of placental development, but there was considerable interindividual variability in their expression. Therefore, drug-drug interactions and the effect of interindividual variability in placental ENT1 expression on abacavir disposition into fetal circulation should be further investigated to guarantee safe and effective abacavir-based combination therapies in pregnancy.

Identification of p-glycoprotein and transport mechanism of Paclitaxel in syncytiotrophoblast cells

When chemotherapy is administered during pregnancy, it is important to consider the fetus chemotherapy exposure, because it may lead to fetal consequences. Paclitaxel has become widely used in the metastatic and adjuvant settings for woman with cancer including breast and ovarian cancer. Therefore, we attempted to clarify the transport mechanisms of paclitaxel through blood-placenta barrier using rat conditionally immortalized syncytiotrophoblast cell lines (TR-TBTs). The uptake of paclitaxel was time- and temperature-dependent. Paclitaxel was eliminated about 50% from the cells within 30 min. The uptake of paclitaxel was saturable with K_m of 168 μM and 371 μM in TR-TBT 18d-1 and TR-TBT 18d-2, respectively. [³H]Paclitaxel uptake was markedly inhibited by cyclosporine and verapamil, well-known substrates of P-glycoprotein (P-gp) transporter. However, several MRP substrates and organic anions had no effect on [³H]paclitaxel uptake in TR-TBT cells. These results suggest that P-gp may be involved in paclitaxel transport at the placenta. TR-TBT cells expressed mRNA of P-gp. These findings are important for therapy of breast and ovarian cancer of pregnant women, and should be useful data in elucidating teratogenicity of paclitaxel during pregnancy.

Expression and distribution of nucleoside transporter proteins in the human syncytiotrophoblast

The plasma membrane distribution and related biological activity of nucleoside transporter proteins (NTs) were investigated in human syncytiotrophoblast from term placenta using a variety of approaches, including nucleoside uptake measurements into vesicles from selected plasma membrane domains, NT immunohistochemistry, and subcellular localization (basal, heavy, and light apical membranes as well as raft-enriched membranes from the apical domain). In contrast with other epithelia, in this epithelium, we have identified the high-affinity pyrimidine-preferring human concentrative nucleoside transporter (hCNT) 1 as the only hCNT-type protein expressed at both the basal and apical membranes. hCNT1 localization in lipid rafts is also dependent on its subcellular localization in the apical plasma membrane, suggesting a complex cellular and regional expression. Overall, this result favors the view that the placenta is a pyrimidine-preferring nucleoside sink from both maternal and fetal sides, and hCNT1 plays a major role in promoting pyrimidine salvage and placental growth. This finding may be of pharmacological relevance, because hCNT1 is known to interact with anticancer nucleoside-derived drugs and other molecules, such as nicotine and caffeine, for which a great variety of harmful effects on placental and fetal development, including intrauterine growth retardation, have been reported.

6-Mercaptopurine transport by equilibrative nucleoside transporters in conditionally immortalized rat syncytiotrophoblast cell lines TR-TBTs

Recently, more women were provided with 6-mercaptopurine (6-MP) during pregnancy. Therefore, we attempted to clarify the transport mechanisms of 6-MP through blood-placenta barrier using rat conditionally immortalized syncytiotrophoblast cell lines (TR-TBTs). The uptake of 6-MP was time- and ATP dependent, but sodium independent in TR-TBTs. 6-MP was eliminated over 50% from the cells within 30 min. The uptake of 6-MP was saturable with Michaelis-Menten constant values of 198 μM and 250 μM in TR-TBT 18d-1 and TR-TBT 18d-2, respectively. 6-Thioguanine, azathioprine, and hypoxantine, structural analogues of 6-MP, strongly inhibited [(14) C]6-MP uptake. Equilibrative nucleoside transporter (ENT) inhibitors, adenosine and uridine, significantly inhibited [(14) C]6-MP uptake. However, several organic anions and cations had no effect on [(14) C]6-MP uptake in TR-TBTs. These results suggest that sodium-independent transporters, ENTs, may be involved in 6-MP uptake at the placenta. In addition, multidrug resistance protein (MRP) inhibitors, methotrexate, probenecid, cefmetazole, and sulfinpyrazone, significantly increased the accumulation of [(14) C]6-MP in the cells. It is indicated that 6-MP may be eliminated across the blood-placental barrier via MRPs. TR-TBTs expressed mRNA of ENT1, ENT2, MRP4, and MRP5. These findings are important for the therapy of acute lymphoblastic leukemia and autoimmune diseases of pregnant women, and should be useful data in elucidating teratogenicity of 6-MP during pregnancy.

Characterization of the mechanism of zidovudine uptake by rat conditionally immortalized syncytiotrophoblast cell line TR-TBT

PURPOSE

To characterize the uptake mechanism of zidovudine (AZT), a nucleoside reverse transcriptase inhibitor, in syncytiotrophoblast cells using the TR-TBT 18d-1 cell line previously established by our group.

MATERIALS AND METHODS

The effects of several transporter inhibitors on the initial and steady-state apical uptake of AZT by TR-TBT 18d-1 were characterized, in order to identify the transporter(s) involved.

RESULTS

Initial uptake of AZT was sodium-independent and saturable; the K(m) value was about 16 microM. Nitrobenzylthioinosine (NBMPR), probenecid and cimetidine each had little effect on the saturable AZT uptake, indicating that well characterized transporters, such as organic anion transporters (OATs and OATPs), organic cation transporters (OCTs) and equilibrative nucleoside transporters (ENTs), are not involved. However, thymidine and 2'-deoxyuridine strongly inhibited AZT uptake. These results suggest that an unidentified nucleoside uptake transporter is responsible for the uptake of AZT. Cyclosporin A, Ko143 and probenecid had little effect on AZT accumulation by TR-TBT 18d-1 cells, suggesting that transporter-mediated efflux of AZT is not substantial.

CONCLUSION

Our results indicate that saturable AZT uptake into TR-TBT 18d-1 is mediated by a so-far-unidentified transporter.