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

Addressing the Clinical Importance of Equilibrative Nucleoside Transporters in Drug Discovery and Development

The US Food and Drug Administration (FDA), European Medicines Agency (EMA), and Pharmaceuticals and Medical Devices Agency (PMDA) guidances on small-molecule drug-drug interactions (DDIs), with input from the International Transporter Consortium (ITC), recommend the evaluation of nine drug transporters. Although other clinically relevant drug uptake and efflux transporters have been discussed in ITC white papers, they have been excluded from further recommendation by the ITC and are not included in current regulatory guidances. These include the ubiquitously expressed equilibrative nucleoside transporters (ENT) 1 and ENT2, which have been recognized by the ITC for their potential role in clinically relevant nucleoside analog drug interactions for patients with cancer. Although there is comparatively limited clinical evidence supporting their role in DDI risk or other adverse drug reactions (ADRs) compared with the nine highlighted transporters, several in vitro and in vivo studies have identified ENT interactions with non-nucleoside/non-nucleotide drugs, in addition to nucleoside/nucleotide analogs. Some noteworthy examples of compounds that interact with ENTs include cannabidiol and selected protein kinase inhibitors, as well as the nucleoside analogs remdesivir, EIDD-1931, gemcitabine, and fialuridine. Consequently, DDIs involving the ENTs may be responsible for therapeutic inefficacy or off-target toxicity. Evidence suggests that ENT1 and ENT2 should be considered as transporters potentially involved in clinically relevant DDIs and ADRs, thereby warranting further investigation and regulatory consideration.

Assessment of the role of nucleoside transporters, P-glycoprotein, breast cancer resistance protein, and multidrug resistance-associated protein 2 in the placental transport of entecavir using in vitro, ex vivo, and in situ methods

The nucleoside analog entecavir (ETV) is a first-line pharmacotherapy for chronic hepatitis B in adult and pediatric patients. However, due to insufficient data on placental transfer and its effects on pregnancy, ETV administration is not recommended for women after conception. To expand knowledge of safety, we focused on evaluating the contribution of nucleoside transporters (NBMPR sensitive ENTs and Na⁺ dependent CNTs) and efflux transporters, P-glycoprotein (ABCB1), breast cancer resistance protein (ABCG2), and multidrug resistance-associated transporter 2 (ABCC2), to the placental kinetics of ETV. We observed that NBMPR and nucleosides (adenosine and/or uridine) inhibited [³H]ETV uptake into BeWo cells, microvillous membrane vesicles, and fresh villous fragments prepared from the human term placenta, while Na⁺ depletion had no effect. Using a dual perfusion study in an open-circuit setup, we showed that maternal-to-fetal and fetal-to-maternal clearances of [³H]ETV in the rat term placenta were decreased by NBMPR and uridine. Net efflux ratios calculated for bidirectional transport studies performed in MDCKII cells expressing human ABCB1, ABCG2, or ABCC2 were close to the value of one. Consistently, no significant decrease in fetal perfusate was observed in the closed-circuit setup of dual perfusion studies, suggesting that active efflux does not significantly reduce maternal-to-fetal transport. In conclusion, ENTs (most likely ENT1), but not CNTs, ABCB1, ABCG2, and ABCC2, contribute significantly to the placental kinetics of ETV. Future studies should investigate the placental/fetal toxicity of ETV, the impact of drug-drug interactions on ENT1, and interindividual variability in ENT1 expression on the placental uptake and fetal exposure to ETV.

Evaluation of the maternal and developmental toxicity of 6-methylmercaptopurine riboside in rats

Thiopurine prodrugs (azathioprine, AZA, and 6-mercaptopurine, 6MP) are embryotoxic to rodents and rabbits. Little is known about the developmental toxicity of 6-methylmercaptopurine riboside (6MMPr), a thiopurine drug metabolite that is thought to mediate its liver toxicity. A limb bud assay found that 6MMPr impairs the in vitro morphogenetic differentiation of mouse limb extremities, being more potent than 6MP in the assay. This study evaluated the embryotoxicity of 6MMPr (0, 7.5, 15, 30mg/kg bw sc) in rats after single-dose exposure in mid organogenesis (GD10). One group of pregnant rats was similarly treated with 6MP (15mg/kg bw sc). After C-section (GD21), fetuses were weighed, and examined for external abnormalities. One third of each litter was examined for soft-tissue abnormalities while the remaining fetuses were cleared and stained for skeleton evaluation. 6MMPr caused a dose-dependent maternal weight loss followed by recovery before term pregnancy. Except for a nonsignificant increase in embryolethality and slight reduction in fetal weight at 30mg/kg bw, no indication of embryotoxicity was noted at this dose or at lower doses of 6MMPr. In contrast, 6MP led to nearly 98% of post-implantation losses in the presence of slight-to-mild maternal toxicity. These results are consistent with the notion that maternal treatment with 6MMPr affects embryo development, causing a nonsignificant increase in embryolethality and a slight reduction in fetal weight at 30mg/kg bw. However, there was no increase in abnormalities at this dose, which was severely toxic to the dams, as reflected in the maternal weight gain data.

Limited Impact of Murine Placental MDR1 on Fetal Exposure of Certain Drugs Explained by Bypass Transfer Between Adjacent Syncytiotrophoblast Layers

Purpose

Multidrug resistance protein 1 (MDR1) is located at the interface between two syncytiotrophoblast layers in rodent placenta, and may influence fetal drug distribution. Here, we quantitatively compare the functional impact per single MDR1 molecule of MDR1 at the placental barrier and blood-brain barrier in mice.

Methods

MDR1A and MDR1B proteins were quantified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Paclitaxel or digoxin was continuously administered to pregnant Mdr1a^−/−/Mdr1b^−/− or wild-type mice, and the drug concentrations in the maternal and fetal plasma and maternal brain were quantified by LC-MS/MS.

Results

MDR1A and MDR1B proteins are expressed in the membrane of mouse placental labyrinth, and total MDR1 at the placental barrier amounts to about 30% of that at the blood-brain barrier. The fetal-to-maternal plasma concentration ratio of digoxin was only marginally affected in Mdr1a^−/−/Mdr1b^−/− mice, while that of paclitaxel showed a several-fold increase. No such difference between the two drugs was found in the maternal brain distribution. The impact per single MDR1 molecule on the fetal distribution of digoxin was calculated to be much lower than that on the brain distribution, but this was not the case for paclitaxel. Our pharmacokinetic model indicates that the impact of placental MDR1 is inversely correlated to the ratio of permeability through gap junctions connecting the two syncytiotrophoblast layers to passive diffusion permeability.

Conclusion

Our findings indicate that murine placental MDR1 has a minimal influence on the fetal concentration of certain substrates, such as digoxin, due to bypass transfer, probably via connexin26 gap junctions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11095-022-03165-6.

Overview of Drug Transporters in Human Placenta

The transport of drugs across the placenta is a point of great importance in pharmacotherapy during pregnancy. However, the knowledge of drug transport in pregnancy is mostly based on experimental clinical data, and the underlying biological mechanisms are not fully understood. In this review, we summarize the current knowledge of drug transporters in the human placenta. We only refer to human data since the placenta demonstrates great diversity among species. In addition, we describe the experimental models that have been used in human placental transport studies and discuss their availability. A better understanding of placental drug transporters will be beneficial for the health of pregnant women who need drug treatment and their fetuses.

Differential Inhibition of Equilibrative Nucleoside Transporter 1 (ENT1) Activity by Tyrosine Kinase Inhibitors

Background and Objectives

Equilibrative nucleoside transporter (ENT) 1 is a widely-expressed drug transporter, handling nucleoside analogues as well as endogenous nucleosides. ENT1 has been postulated to be inhibited by some marketed tyrosine kinase inhibitors (TKIs). To obtain insights into this point, the interactions of 24 TKIs with ENT1 activity have been analyzed.

Methods

Inhibition of ENT1 activity was investigated in vitro through quantifying the decrease of [³H]-uridine uptake caused by TKIs in HAP1 ENT2-knockout cells, exhibiting selective ENT1 expression. TKI effects towards ENT1-mediated transport were additionally characterized in terms of their in vivo relevance and of their relationship to TKI molecular descriptors. Putative transport of the TKI lorlatinib by ENT1/ENT2 was analyzed by LC-MS/MS.

Results

Of 24 TKIs, 12 of them, each used at 10 µM, were found to behave as moderate or strong inhibitors of ENT1, i.e., they decreased ENT1 activity by at least 35%. This inhibition was concentration-dependent for at least the strongest ones (IC₅₀ less than 10 µM) and was correlated with some molecular descriptors, especially with atom-type E-state indices. Lorlatinib was notably a potent in vitro inhibitor of ENT1/ENT2 (IC₅₀ values around 1.0–2.5 µM) and was predicted to inhibit these nucleoside transporters at relevant clinical concentrations, without, however, being a substrate for them.

Conclusion

Our data unambiguously add ENT1 to the list of drug transporters inhibited by TKIs, especially by lorlatinib. This point likely merits attention in terms of possible drug–drug interactions, notably for nucleoside analogues, whose ENT1-mediated uptake into their target cells may be hampered by co-administrated TKIs such as lorlatinib.

Evaluation of the Presence and Functional Importance of Nucleoside Transporters in Lacrimal Gland for Tear Disposition of Intravenously Injected Substrate in Rabbits

Purpose: Purpose of the current study was to assess the presence and functionality of the nucleoside transporters in the lacrimal gland for the tear disposition of its substrate given intravenously in rabbits.Materials and Methods: Rabbits were divided into two groups - control and blocker pretreated. The blocker pretreated group received 5 mg/kg of dipyridamole 30 min before ribavirin (substrate), which was given at a dose of 2.5 mg/kg. All the treatments were given intravenously. Blood and tear samples were collected at 5, 15, 30, 60, 90, 120, 180, 240, 300 and 360 min (n = 4; each time point) after substrate administration. Tear samples were collected on Schirmer's strips, and plasma was separated immediately after blood collection. All the samples were stored at -80°C until analysis by LC-MS/MS.Results: Plasma ribavirin concentration for blocker pretreated group showed significantly (p < .05) higher levels at 5, 15, 30, 60, 120, 180 and 300 min as compared to the control group. Similarly, tear ribavirin concentration for blocker pretreated group also showed a significant (p < .05) increase at 5, 15, 60, 90, 180, 240 and 300 min compared to the control group. Plasma and tear AUC_(0-6) for blocker pretreated group was 1.7 (p < .001) and 2.42 (p < .001) folds higher in a significant manner as compared to the control group, respectively. Percentage penetration of ribavirin from plasma to tears was also different between control and blocker pretreated group. Permeation ratio of ribavirin from plasma to tear for blocker pretreated group was found to be 1.4-folds higher in a significant (p < .05) manner.Conclusion: It is evident from the results that nucleoside transporters are present in lacrimal gland. The blocker treatment induced increase in tear transport of ribavirin indicates the possibility of the presence of nucleoside transporters on the apical side of lacrimal acinar cells in the uptake position.

Fluorouracil uptake in triple-negative breast cancer cells: Negligible contribution of equilibrative nucleoside transporters 1 and 2

Equilibrative nucleoside transporters (ENTs) 1 and 2 reportedly accept fluorouracil as a substrate. Here, we evaluated ENT1/2 expression at the messenger RNA (mRNA), protein, and functional levels in a panel of four triple-negative breast cancer (TNBC) cell lines, BT-549, Hs578T, MDA-MB-231, and MDA-MB-435, and we examined the relationship of the observed profiles to fluorouracil sensitivity. Nitrobenzylthioinosine (NBMPR) at 0.1 μM inhibits only ENT1, while dipyridamole at 10 μM or NBMPR at 100 μM inhibits both ENT1 and ENT2. We found that the uptake of [³ H]uridine, a typical substrate of ENT1 and ENT2, was decreased to approximately 40% by 0.1 μM NBMPR. At 100 μM, NBMPR almost completely blocked the saturable uptake of [³ H]uridine, but this does not imply a functional role of ENT2, because 10 μM dipyridamole showed similar inhibition to 0.1 μM NBMPR. Expression of ENT1 mRNA was almost 1 order of magnitude higher than that of ENT2 in all TNBC cell lines. Liquid chromatography-tandem mass spectrometry(LC-MS/MS) LC-MS/MS-based targeted protein quantification showed that ENT1 protein levels were in the range of 9.3-30 fmol/μg protein in plasma membrane fraction of TNBC cell lines, whereas ENT2 protein was below the detection limit. [³ H]Fluorouracil uptake was insensitive to 0.1 μM NBMPR and 10 μM dipyridamole, suggesting a negligible contribution of ENT1 and ENT2 to fluorouracil uptake. The levels of ENT1 mRNA, ENT1 protein, ENT2 mRNA, and ENT1-mediated [³ H]uridine uptake in the four TNBC cell lines showed no correlation with fluorouracil sensitivity. These results indicate that neither ENT1 nor ENT2 contributes significantly to the fluorouracil sensitivity of TNBC cell lines.

Effects of single and repetitive valproic acid administration on the gene expression of placental transporters in pregnant rats: An analysis by gestational period

The use of valproic acid (VPA), an antiepileptic drug, during pregnancy, is known to increase various fetal risks. Since VPA has been known to inhibit histone deacetylases (HDACs); its administration could alter gene transcription levels. However, in vivo effects of VPA administration on placental transporters have not been fully elucidated. The purpose of the present study was to comprehensively evaluate the effects of single and repetitive VPA administration on the expression of placental transporters and analyze them by gestational day. We investigated 18 transporters (8 ATP-binding cassette (ABC) and 10 solute carrier (SLC) transporters) in the placentas of pregnant rats that were orally administered 400 mg/kg/day VPA for one or four days, during mid- or late gestation. In the control rats, 4 ABC transporter genes (Abcb1a, 1b, Abcc2, Abcc4) were upregulated, 3 (Abcc3, Abcc5, Abcg2) downregulated through gestation, whereas 1 (Abcc1) was not changed. Regarding SLC transporters, 6 genes (Slc7a5, Slc16a3, Slc22a3, Slc22a4, Slco2b1, Slco4a1) were increased, 1 (Slc29a1) decreased through gestation, whereas 3 (Slc7a8, Slc22a5, Slco2a1) showed no significant change. Single VPA administration altered the expression of 9 transporters and repetitive administration, 13 transporters. In particular, VPA remarkably decreased Abcc4 and Slc22a4 in late gestation and increased Abcc5 during mid-gestation. Our findings indicated that VPA administration changed transporter expression levels in rat placenta, and suggested that sensitivity to VPA differs across gestational stages.

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.