Multiple drug transporters mediate the placental transport of sulpiride

Carnitine traffic and human fertility

Carnitine is a vital molecule in human metabolism, prominently involved in fatty acid β-oxidation within mitochondria. Predominantly sourced from dietary intake, carnitine also derives from endogenous synthesis. This review delves into the complex network of carnitine transport and distribution, emphasizing its pivotal role in human fertility. Together with its role in fatty acid oxidation, carnitine modulates the acety-CoA/CoA ratio, influencing carbohydrate metabolism, lipid biosynthesis, and gene expression. The intricate regulation of carnitine homeostasis involves a network of membrane transporters, notably OCTN2, which is central in its absorption, reabsorption, and distribution. OCTN2 dysfunction, results in Primary Carnitine Deficiency (PCD), characterized by systemic carnitine depletion and severe clinical manifestations, including fertility issues. In the male reproductive system, carnitine is crucial for sperm maturation and motility. In the female reproductive system, carnitine supports mitochondrial function necessary for oocyte quality, folliculogenesis, and embryonic development. Indeed, deficiencies in carnitine or its transporters have been linked to asthenozoospermia, reduced sperm quality, and suboptimal fertility outcomes in couples. Moreover, the antioxidant properties of carnitine protect spermatozoa from oxidative stress and help in managing conditions like polycystic ovary syndrome (PCOS) and endometriosis, enhancing sperm viability and fertilization potential of oocytes. This review summarizes the key role of membrane transporters in guaranteeing carnitine homeostasis with a special focus on the implications in fertility and possible treatments of infertility and other related disorders.

Evaluation of transport mechanisms of methotrexate in human choriocarcinoma cell lines by LC-MS/MS

Methotrexate (MTX) is commonly prescribed as the initial treatment for gestational trophoblastic neoplasia (GTN), but MTX monotherapy may not be effective for high-risk GTN and choriocarcinoma. The cellular uptake of MTX is essential for its pharmacological activity. Thus, our study aimed to investigate the cellular pharmacokinetics and transport mechanisms of MTX in choriocarcinoma cells. For the quantification of MTX concentrations in cellular matrix, a liquid chromatography-tandem mass spectrometry method was created and confirmed initially. MTX accumulation in BeWo, JEG-3, and JAR cells was minimal. Additionally, the mRNA levels of folate receptor α (FRα) and breast cancer resistance protein (BCRP) were relatively high in the three choriocarcinoma cell lines, whereas proton-coupled folate transporter (PCFT), reduced folate carrier (RFC), and organic anion transporter (OAT) 4 were low. Furthermore, the expression of other transporters was either very low or undetectable. Notably, the application of inhibitors and small interfering RNAs (siRNAs) targeting FRα, RFC, and PCFT led to a notable decrease in the accumulation of MTX in BeWo cells. Conversely, the co-administration of multidrug resistance protein 1 (MDR1) and BCRP inhibitors increased MTX accumulation. In addition, inhibitors of OATs and organic-anion transporting polypeptides (OATPs) reduced MTX accumulation, while peptide transporter inhibitors had no effect. Results from siRNA knockdown experiments and transporter overexpression cell models indicated that MTX was not a substrate of nucleoside transporters. In conclusion, the results indicate that FRα and multiple transporters such as PCFT, RFC, OAT4, and OATPs are likely involved in the uptake of MTX, whereas MDR1 and BCRP are implicated in the efflux of MTX from choriocarcinoma cells. These results have implications for predicting transporter-mediated drug interactions and offer potential directions for further research on enhancing MTX sensitivity.

Downregulation of OATP2B1 by proinflammatory cytokines leads to 5-ASA hyposensitivity in Ulcerative colitis

5-Aminosalicylic acid (5-ASA) is a first-line agent in both remission and maintenance therapy for ulcerative colitis (UC). However, the mucosal concentration of 5-ASA was significantly lower in patients with severe histological inflammation, which further led to a poor response to 5-ASA treatment. Our study aimed to clarify the mechanism of 5-ASA uptake into colonic epithelial cells and to further explore the reason for the decreased colonic mucosal 5-ASA concentration in UC patients. Our results demonstrated that the colonic 5-ASA concentration was notably reduced in DSS-induced colitis mice and inversely correlated with colonic inflammation. 5-ASA was not a substrate of carnitine/organic cation transporter 1/2 (OCTN1/2) or multidrug resistance protein 1 (MDR1), whereas organic anion transporting polypeptide 2B1 (OATP2B1) and sodium-coupled monocarboxylate transporter 1 (SMCT1) mediated the uptake of 5-ASA, with a greater contribution from OATP2B1 than SMCT1. Inhibitors and siRNAs targeting OATP2B1 significantly reduced 5-ASA absorption in colonic cell lines. Moreover, OATP2B1 expression was dramatically downregulated in colon tissues from UC patients and dextran sodium sulfate (DSS)-induced colitis mice, and was also negatively correlated with colonic inflammation. Mechanistically, mixed proinflammatory cytokines downregulated the expression of OATP2B1 in a time- and concentration-dependent manner through the hepatocyte nuclear factor 4 α (HNF4α) pathway. In conclusion, OATP2B1 was the pivotal transporter involved in colonic 5-ASA uptake, which indicated that inducing OATP2B1 expression may be a strategy to promote 5-ASA uptake and further improve the concentration and anti-inflammatory efficacy of 5-ASA in UC.

Impact of P-Glycoprotein-Mediated Drug-Endogenous Substrate Interactions on Androgen and Blood-Brain Barrier Permeability

This report focuses on pharmacokinetic drug-endogenous substrate interactions (DEIs). We hypothesized that P-glycoprotein (P-gp)-mediated DEI might affect androgen kinetics, especially its blood-brain barrier (BBB) permeability. The intracellular accumulation of the endogenous substrates of P-gp, testosterone (TES) and androstenedione (ADO) was increased by several tested drugs in uptake studies using P-gp overexpressing cells, indicating that these drugs inhibit P-gp-mediated efflux of TES of ADO from the cells. In a transport study using rat BBB kit, we found that the BBB limited the penetration of TES and ADO into the central nervous system. In addition, tested drugs that cause DEI were found to increase BBB permeability of TES and ADO via P-gp inhibition. In short, this study provides new findings regarding the possibility that DEI may affect the kinetics of endogenous substrates of P-gp.

Inhibition of multiple uptake transporters in cardiomyocytes/mitochondria alleviates doxorubicin-induced cardiotoxicity

Doxorubicin (DOX) has been widely used to treat various tumors; however, DOX-induced cardiotoxicity limits its utilization. Since high accumulation of DOX in cardiomyocytes/mitochondria is the key reason, we aimed to clarify the mechanisms of DOX uptake and explore whether selectively inhibiting DOX uptake transporters would attenuate DOX accumulation and cardiotoxicity. Our results demonstrated that OCTN1/OCTN2/PMAT (organic cation/carnitine transporter 1/2 or plasma membrane monoamine transporter), especially OCTN2, played crucial roles in DOX uptake in cardiomyocytes, while OCTN2 and OCTN1 contributed to DOX transmembrane transport in mitochondria. Metformin (1-100 μM) concentration-dependently reduced DOX (5 μM for accumulation, 500 nM for cytotoxicity) concentration and toxicity in cardiomyocytes/mitochondria via inhibition of OCTN1-, OCTN2- and PMAT-mediated DOX uptake but did not affect its efflux. Furthermore, metformin (iv: 250 and 500 mg/kg or ig: 50, 100 and 200 mg/kg) could dose-dependently reduce DOX (8 mg/kg) accumulation in mouse myocardium and attenuated its cardiotoxicity. In addition, metformin (1-100 μM) did not impair DOX efficacy in breast cancer or leukemia cells. In conclusion, our study clarified the role of multiple transporters, especially OCTN2, in DOX uptake in cardiomyocytes/mitochondria; metformin alleviated DOX-induced cardiotoxicity without compromising its antitumor efficacy by selective inhibition of multiple transporters mediated DOX accumulation in myocardium/mitochondria.

Transfer of brotizolam, periciazine, and sulpiride in cord blood and breast milk, and alprazolam in breast milk: a case report

Background

A high prevalence of mental disorders including depression, anxiety, somatoform, and dissociative disorder is reported during pregnancy, however, information on the transfer of antipsychotics across the placenta and into breast milk is limited. We evaluated brotizolam, periciazine and sulpiride in cord blood, maternal serum, and breast milk, and alprazolam in breast milk.

Case presentation

A 38-year-old woman with dissociative disorder was treated with brotizolam, propericiazine, and sulpiride during pregnancy and lactation, and alprazolam during lactation. The drug concentration ratios for both cord blood and maternal serum were 33.3 and 61.5% for brotizolam and sulpiride, respectively, and periciazine (metabolite of propericiazine) was not detected in the cord blood. In breast milk, alprazolam (0.9 ng/mL), sulpiride (445.8 ng/mL), and periciazine (0.3 ng/mL) concentrations were noted at 7.5 h after the last dose on postpartum, whereas brotizolam was not detected. The relative infant doses via breast milk were 1.4, 2.7 and 0.02% of the maternal daily dose, respectively. The neonate had no congenital anomalies and did not experience any severe withdrawal symptoms after birth.

Conclusion

Use of brotizolam, propericiazine, and sulpiride during pregnancy and lactation, and use of alprazolam during lactation were acceptable in this case.

L-tetrahydropalmatine reduces oxaliplatin accumulation in the dorsal root ganglion and mitochondria through selectively inhibiting the transporter-mediated uptake thereby attenuates peripheral neurotoxicity

Oxaliplatin (OXA) is a third-generation platinum drug; however, its application is greatly limited due to the severe peripheral neurotoxicity. This study aims to confirm the transport mechanism of OXA and to explore whether L-tetrahydropalmatine (L-THP) would alleviate OXA-induced peripheral neurotoxicity by selectively inhibiting these uptake transporters in vitro and in vivo. Our results revealed that organic cation transporter 2 (OCT2), organic cation/carnitine transporter 1 (OCTN1) and organic cation/carnitine transporter 2 (OCTN2) were involved in the uptake of OXA in dorsal root ganglion (DRG) neurons and mitochondria, respectively. L-THP (1-100 μM) reduced OXA (40 μM) induced cytotoxicity in MDCK-hOCT2 (Madin-Darby canine kidney, MDCK), MDCK-hOCTN1, MDCK-hOCTN2, and rat primary DRG cells, and decreased the accumulation of OXA in above cells and rat DRG mitochondria, but did not affect its efflux from MDCK-hMRP2 cells. Furthermore, Co-administration of L-THP (5-20 mg/kg for mice, 10-40 mg/kg for rats; twice a week, iv or ig) attenuated OXA (8 mg/kg for mice, 4 mg/kg for rats; twice a week, iv) induced peripheral neurotoxicity and reduced the platinum concentration in the DRG. Whereas, L-THP (1-100 μM for cells; 10-20 mg/kg for mice) did not impair the antitumour efficacy of OXA (40 μM for cells; 8 mg/kg for mice) in HT29 tumour-bearing nude mice nor in tumour cells (HT29 and SW620 cells). In conclusion, OCT2, OCTN1 and OCTN2 contribute to OXA uptake in the DRG and mitochondria. L-THP attenuates OXA-induced peripheral neurotoxicity via inhibiting OXA uptake but without impairing the antitumour efficacy of OXA. L-THP is a potential candidate drug to attenuate OXA-induced peripheral neurotoxicity.

Organic Cation Transporter Expression and Function in the CNS

The blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) represent major control checkpoints protecting the CNS, by exerting selective control over the movement of organic cations and anions into and out of the CNS compartment. In addition, multiple CNS cell types, e.g., astrocytes, ependymal cells, microglia, contribute to processes that maintain the status quo of the CNS milieu. To fulfill their roles, these barriers and cell types express a multitude of transporter proteins from dozens of different transporter families. Fundamental advances over the past few decades in our knowledge of transporter substrates, expression profiles, and consequences of loss of function are beginning to change basic theories regarding the contribution of various cell types and clearance networks to coordinated neuronal signaling, complex organismal behaviors, and overall CNS homeostasis. In particular, transporters belonging to the Solute Carrier (SLC) superfamily are emerging as major contributors, including the SLC22 organic cation/anion/zwitterion family of transporters (includes OCT1-3 and OCTN1-3), the SLC29 facilitative nucleoside family of transporters (includes PMAT), and the SLC47 multidrug and toxin extrusion family of transporters (includes MATE1-2). These transporters are known to interact with neurotransmitters, antidepressant and anxiolytic agents, and drugs of abuse. Clarifying their contributions to the underlying mechanisms regulating CNS permeation and clearance, as well as the health status of astrocyte, microglial and neuronal cell populations, will drive new levels of understanding as to maintenance of the CNS milieu and approaches to new therapeutics and therapeutic strategies in the treatment of CNS disorders. This chapter highlights organic cation transporters belonging to the SLC superfamily known to be expressed in the CNS, providing an overview of their identification, mechanism of action, CNS expression profile, interaction with neurotransmitters and antidepressant/antipsychotic drugs, and results from behavioral studies conducted in loss of function models (knockout/knockdown).

Emerging organic contaminants and odorous compounds in secondary effluent wastewater: Identification and advanced treatment

This study aims to address organic micropollutants in secondary effluents from municipal wastewater treatment plants (WWTPs) by first identification of micropollutants in different treatment units, and second by evaluating an advanced treatment process for removals of micropollutants. In secondary effluents, 28 types of pharmaceutical and personal care products (PPCPs), 5 types of endocrine disrupting chemicals (EDCs) and 3 types of odorous compounds are detected with total concentrations of 513 ± 57.8 ng/L, 991 ± 36.5 ng/L, 553 ± 48.3 ng/L, respectively. An integrated process consisting of in-situ ozonation, ceramic membrane filtration (CMF) and biological active carbon (BAC) filtration is investigated in a pilot scale (1000 m³/d) for removal of micropollutants in secondary effluents. The total removal efficiencies of PPCPs, EDCs and odorous compounds are 98.5%, 95.4%, and 91.1%, respectively. Removal mechanisms of emerging organic contaminants (EOCs) and odorous compounds are discussed based on their physicochemical properties. The remarkable removal efficiencies of micropollutants by the pilot system is attributed to synergistic effects of combining ozonation, ceramic membrane filtration and BAC filtration. This study provides a cost-effective and robust technology with the capability of treating secondary effluents for reuse applications.

General Overview of Organic Cation Transporters in Brain

Inhibitors of Na⁺/Cl^- dependent high affinity transporters for norepinephrine (NE), serotonin (5-HT), and/or dopamine (DA) represent frequently used drugs for treatment of psychological disorders such as depression, anxiety, obsessive-compulsive disorder, attention deficit hyperactivity disorder, and addiction. These transporters remove NE, 5-HT, and/or DA after neuronal excitation from the interstitial space close to the synapses. Thereby they terminate transmission and modulate neuronal behavioral circuits. Therapeutic failure and undesired central nervous system side effects of these drugs have been partially assigned to neurotransmitter removal by low affinity transport. Cloning and functional characterization of the polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2), OCT3 (SLC22A3) and the plasma membrane monoamine transporter PMAT (SLC29A4) revealed that every single transporter mediates low affinity uptake of NE, 5-HT, and DA. Whereas the organic transporters are all located in the blood brain barrier, OCT2, OCT3, and PMAT are expressed in neurons or in neurons and astrocytes within brain areas that are involved in behavioral regulation. Areas of expression include the dorsal raphe, medullary motoric nuclei, hypothalamic nuclei, and/or the nucleus accumbens. Current knowledge of the transport of monoamine neurotransmitters by the organic cation transporters, their interactions with psychotropic drugs, and their locations in the brain is reported in detail. In addition, animal experiments including behavior tests in wildtype and knockout animals are reported in which the impact of OCT2, OCT3, and/or PMAT on regulation of salt intake, depression, mood control, locomotion, and/or stress effect on addiction is suggested.

Insight into the transplacental transport mechanism of methoxylated polybrominated diphenyl ethers using a BeWo cell monolayer model

Methoxylated polybrominated diphenyl ethers (MeO-PBDEs), a type of emerging environmental contaminants, can accumulate through the food chain and eventually enter the human body. For pregnant women, these chemicals may be transplacentally transported to their fetuses, causing early intrauterine exposure. This study was designed to explore the transport process and characteristics of MeO-PBDEs using a BeWo cell monolayer model to simulate the placental barrier effect. Concentration-dependent transplacental transport indicates that the transport of MeO-PBDEs may be dominated by passive diffusion within the studied concentration range. According to the apparent permeability coefficients, MeO-BDE congeners investigated can be classified as poorly transported compounds, with the exception of MeO-BDE28. Time-dependent transplacental transport was observed (R² = 0.97-0.99), which showed that the intracellular accumulation of MeO-PBDEs followed pseudo-first-order kinetics process. The transport process of MeO-PBDEs in the BeWo cell assay was not found to be sensitive to the pH of 6.5-7.4. An efflux transporter, breast cancer resistance protein, may be involved in the transport process of some MeO-PBDE congeners, and influx transporters, including organic anion transporters and organic cation transporters, may also be involved in the transport process. Although the present results indicated the possible transplacental transport mechanism, more molecular biological studies should be conducted to advance the understanding of the transport mechanisms.

Organic Cation Transporters in Health and Disease

The organic cation transporters (OCTs) OCT1, OCT2, OCT3, novel OCT (OCTN)1, OCTN2, multidrug and toxin exclusion (MATE)1, and MATE kidney-specific 2 are polyspecific transporters exhibiting broadly overlapping substrate selectivities. They transport organic cations, zwitterions, and some uncharged compounds and operate as facilitated diffusion systems and/or antiporters. OCTs are critically involved in intestinal absorption, hepatic uptake, and renal excretion of hydrophilic drugs. They modulate the distribution of endogenous compounds such as thiamine, L-carnitine, and neurotransmitters. Sites of expression and functions of OCTs have important impact on energy metabolism, pharmacokinetics, and toxicity of drugs, and on drug-drug interactions. In this work, an overview about the human OCTs is presented. Functional properties of human OCTs, including identified substrates and inhibitors of the individual transporters, are described. Sites of expression are compiled, and data on regulation of OCTs are presented. In addition, genetic variations of OCTs are listed, and data on their impact on transport, drug treatment, and diseases are reported. Moreover, recent data are summarized that indicate complex drug-drug interaction at OCTs, such as allosteric high-affinity inhibition of transport and substrate dependence of inhibitor efficacies. A hypothesis about the molecular mechanism of polyspecific substrate recognition by OCTs is presented that is based on functional studies and mutagenesis experiments in OCT1 and OCT2. This hypothesis provides a framework to imagine how observed complex drug-drug interactions at OCTs arise. Finally, preclinical in vitro tests that are performed by pharmaceutical companies to identify interaction of novel drugs with OCTs are discussed. Optimized experimental procedures are proposed that allow a gapless detection of inhibitory and transported drugs.

Organic Cation Transporter 1 and 3 Contribute to the High Accumulation of Dehydrocorydaline in the Heart

Dehydrocorydaline (DHC), one of the main active components of Corydalis yanhusuo, is an important remedy for the treatment of coronary heart disease. Our previous study revealed a higher unbound concentration of DHC in the heart than plasma of mice after oral administration of C. yanhusuo extract or DHC, but the underlying uptake mechanism remains unelucidated. In our investigations, we studied the transport mechanism of DHC in transgenic cells, primary neonatal rat cardiomyocytes, and animal experiments. Using quantitative real-time polymerase chain reaction and Western blotting, we found that uptake transporters expressed in the mouse heart include organic cation transporter 1/3 (OCT1/3) and carnitine/organic cation transporter 1/2 (OCTN1/2). The accumulation experiments in transfected cells showed that DHC was a substrate of OCT1 and OCT3, with K _m of 11.29 ± 3.3 and 8.96 ± 3.7 μM, respectively, but not a substrate of OCTN1/2. Additionally, a higher efflux level (1.71-fold of MDCK-mock) of DHC was observed in MDCK-MDR1 cells than in MDCK-mock cells. Therefore, DHC is a weak substrate for MDR1. Studies using primary neonatal rat cardiomyocytes showed that OCT1/3 inhibitors (quinidine, decynium-22, and levo-tetrahydropalmatine) prevented the accumulation of DHC, whereas OCTN2 inhibitors (mildronate and l-carnitine) did not affect its accumulation. Moreover, the coadministration of OCT1/3 inhibitors (levo-tetrahydropalmatine, THP) decreased the concentration of DHC in the mouse heart. Based on these findings, DHC may be accumulated partly by OCT1/3 transporters and excreted by MDR1 in the heart. THP could alter the distribution of DHC in the mouse heart. SIGNIFICANCE STATEMENT: We reported the cardiac transport mechanism of dehydrocorydaline, highly distributed to the heart after oral administration of Corydalis yanhusuo extract or dehydrocorydaline only. Dehydrocorydaline (an OCT1/3 and MDR1 substrate) accumulation in primary cardiomyocytes may be related to the transport activity of OCT1/3. This ability, hampered by selective inhibitors (levo-tetrahydropalmatine, an inhibitor of OCT1/3), causes a nearly 40% reduction in exposure of the heart to dehydrocorydaline. These results suggest that OCT1/3 may contribute to the uptake of dehydrocorydaline in the heart.

Cellular uptake properties of lamotrigine in human placental cell lines: Investigation of involvement of organic cation transporters (SLC22A1-5)

Lamotrigine (LTG) is an important antiepileptic drug for the treatment of seizures in pregnant women with epilepsy. However, it is not known if the transport of LTG into placental cells occurs via a carrier-mediated pathway. The aim of this study was to investigate the uptake properties of LTG into placental cell lines (BeWo and JEG-3), and to determine the involvement of organic cation transporters (OCTs, SLC22A1-3) and organic cation/carnitine transporter (OCTNs, SLC22A4-5) in the uptake process. The uptake of LTG at 37 °C was higher than that at 4 °C. OCT1 and OCTNs were detected in both cell lines. The uptake of LTG was not greatly affected by the extracellular pH, Na⁺-free conditions, or the presence of l-carnitine, suggesting that OCTNs were not involved. Although several potent inhibitors of OCTs (chloroquine, imipramine, quinidine, and verapamil) inhibited LTG uptake, other typical inhibitors had no effect. In addition, siRNA targeted to OCT1 had no significant effect on LTG uptake. The mRNA expression in human term placenta followed the order OCTN2 > OCT3 > OCTN1 > OCT1 ≈ OCT2. These observations suggested that LTG uptake into placental cells was carrier-mediated, but that OCTs and OCTNs were not responsible for the placental transport process.

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.

Transepithelial transport mechanisms of 7,8-dihydroxyflavone, a small molecular TrkB receptor agonist, in human intestinal Caco-2 cells

7,8-Dihydroxyflavone (7,8-DHF), as a high-affinity TrkB receptor agonist, has been extensively explored in many human disorders involving brain-derived neurotrophic factor (BDNF) such as Alzheimer's disease, Parkinson's disease, depression, and obesity. However, to date, the transepithelial transport mechanisms of 7,8-DHF in the intestines remain unclear. The aim of our work was to quantify and to characterize in vitro transport of naturally occurring 7,8-DHF distinguished by its physicochemical and pharmacological properties. We discussed the transport mechanisms of 7,8-DHF using the Caco-2 cell model to determine the bi-directional permeability with different environmental factors (time, concentration, pH, metabolic inhibitors etc.). The influx and efflux characteristics of 7,8-DHF were also clarified. 7,8-DHF was poorly transported across Caco-2 cell monolayers by mainly passive diffusion via a transcellular pathway and not a paracellular pathway. The transport of 7,8-DHF was time and concentration-dependent in both the apical (AP) to basolateral (BL) side and the reverse direction. Interestingly, decreasing the pH from 7.4 to 6.0 markedly enhanced 7,8-DHF transport. It is noteworthy that 7,8-DHF transport was strongly inhibited by metabolic inhibitors and was highly dependent on temperature. The efflux ratio (ER) values at different concentrations were all above 1.5, indicating the existence of the efflux transporter. We found that breast cancer resistance protein (BCRP) was not involved in 7,8-DHF secretion and that the transport mechanism of 7,8-DHF was passive transport with an active efflux mediated by P-glycoprotein (P-gp) and multidrug resistance associated proteins (MRPs), particularly MRP 2. Moreover, the use of various influx transporter inhibitors in Caco-2 cells showed that organic cation transporters (OCTs) and organic anion-transporting polypeptides (OATPs) participated in 7,8-DHF transport. Taken together, the elucidated transport characteristics of 7,8-DHF provide useful information for designing novel and efficient delivery systems and avoiding food-food or food-drug interactions.

Multiple Drug Transporters Contribute to the Placental Transfer of Emtricitabine

Emtricitabine (FTC) is a first-line antiviral drug recommended for the treatment of AIDS during pregnancy. We hypothesized that transporters located in the placenta contribute to FTC transfer across the blood-placenta barrier. BeWo cells, cell models with stable or transient expression of transporter genes, primary human trophoblast cells (PHTCs), and small interfering RNAs (siRNAs) were applied to demonstrate which transporters were involved. FTC accumulation in BeWo cells was reduced markedly by inhibitors of equilibrative nucleoside transporters (ENTs), concentrative nucleoside transporters (CNTs), organic cation transporters (OCTs), and organic cation/carnitine transporter 1 (OCTN1) and increased by inhibitors of breast cancer resistance protein (BCRP) and multidrug resistance-associated proteins (MRPs). ENT1, CNT1, OCTN1, MRP1/2/3, and BCRP, but not ENT2, CNT3, OCTN2, or multidrug resistance protein 1 (MDR1), were found to transport FTC. FTC accumulation in PHTCs was decreased significantly by inhibitors of ENTs and OCTN1. These results suggest that ENT1, CNT1, and OCTN1 probably contribute to FTC uptake from maternal circulation to trophoblasts and that ENT1, CNT1, and MRP1 are likely involved in FTC transport between trophoblasts and fetal blood, whereas BCRP and MRP1/2/3 facilitate FTC transport from trophoblasts to maternal circulation. Coexistence of tenofovir or efavirenz with FTC in the cell medium did not influence FTC accumulation in BeWo cells or PHTCs.

Specificity of the ergothioneine transporter natively expressed in HeLa cells

Ergothioneine is a biologically important compound that has been shown to be transported by the organic cation transporter novel type 1 (OCTN1). Following this discovery, a variety of alternate functions for OCTN1 have been suggested including an integral function in the extra-neuronal cholinergic system. The present study reaffirms the primacy of ergothioneine over these alternate substrates using natively expressed OCTN1 in HeLa cells. Besides the general transport inhibitors, quinidine, verapamil and pyrilamine no other putative substrate inhibited ergothioneine transport significantly, with only a slight inhibition demonstrated by carnitine. Even compounds structurally similar to ergothioneine failed to inhibit ergothioneine uptake, suggesting high selectivity of OCTN1. Ergothioneine was found to be avidly accumulated even at low concentrations (300 nM) by HeLa cells.

Maternal Plasma l-Carnitine Reduction During Pregnancy Is Mainly Attributed to OCTN2-Mediated Placental Uptake and Does Not Result in Maternal Hepatic Fatty Acid β-Oxidation Decline

l-Carnitine (l-Car) plays a crucial role in fatty acid β-oxidation. However, the plasma l-Car concentration in women markedly declines during pregnancy, but the underlying mechanism and its consequences on maternal hepatic β-oxidation have not yet been clarified. Our results showed that the plasma l-Car level in mice at gestation day (GD) 18 was significantly lower than that in nonpregnant mice, and the mean fetal-to-maternal plasma l-Car ratio in GD 18 mice was 3.0. Carnitine/organic cation transporter 2 (OCTN2) was highly expressed in mouse and human placenta and upregulated as gestation proceeds in human placenta, whereas expressions of carnitine transporter (CT) 1, CT2, and amino acid transporter B^0,+ were extremely low. Further study revealed that renal peroxisome proliferator-activated receptor α (PPARα) and OCTN2 were downregulated and the renal l-Car level was reduced, whereas the urinary excretion of l-Car was lower in late pregnant mice than in nonpregnant mice. Meanwhile, progesterone (pregnancy-related hormone) downregulated the expression of renal OCTN2 via PPARα-mediated pathway, and inhibited the activity of OCTN2, but estradiol, corticosterone, and cortisol did not. Unexpectedly, the maternal hepatic level of l-Car and β-hydroxybutyrate (an indicator of mitochondrial β-oxidation), and mRNA levels of several enzymes involved in fatty acid β-oxidation in GD 18 mice were higher than that in nonpregnant mice. In conclusion, OCTN2 mediated l-Car transfer across the placenta played a major role in maternal plasma l-Car reduction during pregnancy, which did not subsequently result in maternal hepatic fatty acid β-oxidation decrease.

OCTN: A Small Transporter Subfamily with Great Relevance to Human Pathophysiology, Drug Discovery, and Diagnostics

OCTN is a small subfamily of membrane transport proteins that belongs to the larger SLC22 family. Two of the three members of the subfamily, namely, OCTN2 and OCTN1, are present in humans. OCTN2 plays a crucial role in the absorption of carnitine from diet and in its distribution to tissues, as demonstrated by the occurrence of severe pathologies caused by malfunctioning or altered expression of this transporter. These findings suggest avoiding a strict vegetarian diet during pregnancy and in childhood. Other roles of OCTN2 are related to the traffic of carnitine derivatives in many tissues. The role of OCTN1 is still unclear, despite the identification of some substrates such as ergothioneine, acetylcholine, and choline. Plausibly, the transporter acts on the control of inflammation and oxidative stress, even though knockout mice do not display phenotypes. A clear role of both transporters has been revealed in drug interaction and delivery. The polyspecificity of the OCTNs is at the base of the interactions with drugs. Interestingly, OCTN2 has been recently exploited in the prodrug approach and in diagnostics. A promising application derives from the localization of OCTN2 in exosomes that represent a noninvasive diagnostic tool.

Multiple organic cation transporters contribute to the renal transport of sulpiride

Sulpiride, a selective dopamine D2 receptor blocker, is used widely for the treatment of schizophrenia, depression and gastric/duodenal ulcers. Because the great majority of sulpiride is positively charged at physiological pH 7.4, and ~70% of the dose recovered in urine is in the unchanged form after human intravenous administration of sulpiride, it is believed that transporters play an important role in the renal excretion of sulpiride. The aim of the present study was to explore which transporters contribute to the renal disposition of sulpiride. The results demonstrated that sulpiride was a substrate of human carnitine/organic cation transporter 1 (hOCTN1) and 2 (hOCTN2), human organic cation transporter 2 (hOCT2), human multidrug and toxin efflux extrusion protein 1 (hMATE1) and 2-K (hMATE2-K). Sulpiride accumulation from the basolateral (BL) to the apical (AP) side in MDCK-hOCT2/pcDNA3.1 cell monolayers was much greater than that in MDCK-hOCT2/hMATE1 cells, and cimetidine dramatically reduced the intracellular accumulation of sulpiride from BL to AP. In addition, the accumulation of sulpiride in mouse primary renal tubular cells (mPRTCs) was markedly reduced by inhibitors of Oct2 and Octns. The results implied that OCTN1, OCTN2, OCT2, MATE1 and MATE2-K probably contributed to the renal transfer of sulpiride, in which OCT2 mediated the uptake of sulpiride from the bloodstream to the proximal tubular cells, while MATEs contributed to the sulpiride efflux from the proximal tubular cells to the renal lumen, and OCTNs participated in both renal secretion and reabsorption.