Contribution of CNT1 and ENT1 to ribavirin uptake in human hepatocytes

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 Km 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.

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.

Multiple Computational Approaches for Predicting Drug Interactions with Human Equilibrative Nucleoside Transporter 1

Equilibrativenucleoside transporters (ENTs) participate in the pharmacokinetics and disposition of nucleoside analog drugs. Understanding drug interactions with the ENTs may inform and facilitate the development of new drugs, including chemotherapeutics and antivirals that require access to sanctuary sites such as the male genital tract. This study created three-dimensional pharmacophores for ENT1 and ENT2 substrates and inhibitors using K_t and IC₅₀ data curated from the literature. Substrate pharmacophores for ENT1 and ENT2 are distinct, with partial overlap of hydrogen bond donors, whereas the inhibitor pharmacophores predominantly feature hydrogen bond acceptors. Mizoribine and ribavirin mapped to the ENT1 substrate pharmacophore and proved to be substrates of the ENTs. The presence of the ENT-specific inhibitor 6-S-[(4-nitrophenyl)methyl]-6-thioinosine (NBMPR) decreased mizoribine accumulation in ENT1 and ENT2 cells (ENT1, ∼70% decrease, P = 0.0046; ENT2, ∼50% decrease, P = 0.0012). NBMPR also decreased ribavirin accumulation in ENT1 and ENT2 cells (ENT1: ∼50% decrease, P = 0.0498; ENT2: ∼30% decrease, P = 0.0125). Darunavir mapped to the ENT1 inhibitor pharmacophore and NBMPR did not significantly influence darunavir accumulation in either ENT1 or ENT2 cells (ENT1: P = 0.28; ENT2: P = 0.53), indicating that darunavir's interaction with the ENTs is limited to inhibition. These computational and in vitro models can inform compound selection in the drug discovery and development process, thereby reducing time and expense of identification and optimization of ENT-interacting compounds. SIGNIFICANCE STATEMENT: This study developed computational models of human equilibrative nucleoside transporters (ENTs) to predict drug interactions and validated these models with two compounds in vitro. Identification and prediction of ENT1 and ENT2 substrates allows for the determination of drugs that can penetrate tissues expressing these transporters.

Impact of Viral Inflammation on the Expression of Renal Drug Transporters in Pregnant Rats

Inflammation impacts the expression and function of drug transporters at term-gestation; however, the impact of inflammation on the expression of drug transporters at mid-gestation is largely unknown. Since renal drug transporters play a key role in the clearance of many drugs prescribed during pregnancy, our objective was to study the impact of the viral mimetic poly I:C on the expression of renal transporters in pregnant rats at mid-gestation. Poly I:C (10 mg/kg) or saline was administered intraperitoneally to pregnant Sprague-Dawley rats on gestational day 14. Expression of renal transporters was measured at 6, 24, and 48 h by qRT-PCR and Western blot. The mRNA levels of Mdr1a, Mrp4, Oct2, Octn1, Octn2, Mate1, Oat1-3, Urat1, Oatp4c1, Ent1, and Pept2 were significantly lower in the poly I:C group at 6 h. At 24 h, only the mRNA levels of Oct2, Oatp4c1, and Ent1 were decreased compared to saline. Poly I:C significantly decreased protein expression of Urat1 at 24 h, and P-gp, Oct2, Mate1, Oat1, Oat3 at 48 h,. Poly I:C imposed significant reductions in the expression of several key renal transporters at mid-gestation in pregnant rats. Thus, viral infection may impact renal excretion of drug transporter substrates, potentially leading to drug-disease 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.

Loss of ENT1 increases cell proliferation in the annulus fibrosus of the intervertebral disc

Mice lacking equilibrative nucleoside transporter 1 (ENT1 ^-/- ) demonstrate progressive calcification of spinal tissues including the annulus fibrosus (AF) of the intervertebral disc (IVD). We previously established ENT1 as the primary nucleoside transporter in the AF and demonstrated dysregulation of biomineralization pathways. To identify cellular pathways altered by loss of ENT1, we conducted microarray analysis of AF tissue from wild-type (WT) and ENT1 ^-/- mice before calcification (2 months of age) and associated with calcification (6 months of age). Bioinformatic analyses identified cell cycle dysregulation in ENT1 ^-/- AF tissues and implicated the E2f family of transcription factors as potential effectors. Quantitative polymerase chain reaction analysis confirmed increased expression of multiple E2f transcription factors and E2f interacting proteins ( Rb1 and Cdk2) in ENT1 ^-/- AF cells compared with WT at 6 months of age. At this time point, ENT1 ^-/- AF tissues showed increased JNK MAPK pathway activation, CDK1, minichromosome maintenance complex component 5 (Mcm5), and proliferating cell nuclear antigen (PCNA) protein expression, and PCNA-positive proliferating cells compared with WT controls. The current study demonstrates that loss of ENT1-mediated adenosine transport leads to increased cell proliferation in the AF of the IVD.

mRNA Expression and Activity of Nucleoside Transporters in Human Hepatoma HepaRG Cells

The HepaRG cell line is a highly differentiated human hepatoma cell line, displaying the expression of various drug transporters. However, functional expression of nucleoside transporters remains poorly characterized in HepaRG cells, although these transporters play a key role in hepatic uptake of antiviral and anticancer drugs. The present study was, therefore, designed to characterize the expression, activity and regulation of equilibrative (ENT) and concentrative (CNT) nucleoside transporter isoforms in differentiated HepaRG cells. These cells were found to exhibit a profile of nucleoside transporter mRNAs similar to that found in human hepatocytes, i.e., notable expression of ENT1, ENT2 and CNT1, with very low or no expression of CNT2 and CNT3. ENT1 activity was, next, demonstrated to be the main uridine transport activity present in HepaRG cells, like in cultured human hepatocytes. Various physiological factors, such as protein kinase C (PKC) activation or treatment by inflammatory cytokines or hepatocyte growth factor (HGF), were additionally found to regulate expression of ENT1, ENT2 and CNT1; PKC activation and HGF notably concomitantly induced mRNA expression and activity of ENT1 in HepaRG cells. Overall, these data suggest that HepaRG cells may be useful for analyzing cellular pharmacokinetics of nucleoside-like drugs in human hepatic cells, especially of those handled by ENT1.

The role of drug transporters in the kidney: lessons from tenofovir

Tenofovir disoproxil fumarate, the prodrug of nucleotide reverse transcriptase inhibitor tenofovir, shows high efficacy and relatively low toxicity in HIV patients. However, long-term kidney toxicity is now acknowledged as a modest but significant risk for tenofovir-containing regimens, and continuous use of tenofovir in HIV therapy is currently under question by practitioners and researchers. Co-morbidities (hepatitis C, diabetes), low body weight, older age, concomitant administration of potentially nephrotoxic drugs, low CD4 count, and duration of therapy are all risk factors associated with tenofovir-associated tubular dysfunction. Tenofovir is predominantly eliminated via the proximal tubules of the kidney, therefore drug transporters expressed in renal proximal tubule cells are believed to influence tenofovir plasma concentration and toxicity in the kidney. We review here the current evidence that the actions, pharmacogenetics, and drug interactions of drug transporters are relevant factors for tenofovir-associated tubular dysfunction. The use of creatinine and novel biomarkers for kidney damage, and the role that drug transporters play in biomarker disposition, are discussed. The lessons learnt from investigating the role of transporters in tenofovir kidney elimination and toxicity can be utilized for future drug development and clinical management programs.