Predicting Drug Interactions with Human Equilibrative Nucleoside Transporters 1 and 2 Using Functional Knockout Cell Lines and Bayesian Modeling

Translational Experimental Basis of Indirect Adenosine Receptor Agonist Stimulation for Bone Regeneration: A Review

Bone regeneration remains a significant clinical challenge, often necessitating surgical approaches when healing bone defects and fracture nonunions. Within this context, the modulation of adenosine signaling pathways has emerged as a promising therapeutic option, encouraging osteoblast activation and tempering osteoclast differentiation. A literature review of the PubMed database with relevant keywords was conducted. The search criteria involved in vitro or in vivo models, with clear methodological descriptions. Only studies that included the use of indirect adenosine agonists, looking at the effects of bone regeneration, were considered relevant according to the eligibility criteria. A total of 29 articles were identified which met the inclusion and exclusion criteria, and they were reviewed to highlight the preclinical translation of adenosine agonists. While preclinical studies demonstrate the therapeutic potential of adenosine signaling in bone regeneration, its clinical application remains unrealized, underscoring the need for further clinical trials. To date, only large, preclinical animal models using indirect adenosine agonists have been successful in stimulating bone regeneration. The adenosine receptors (A1, A2A, A2B, and A3) stimulate various pathways, inducing different cellular responses. Specifically, indirect adenosine agonists act to increase the extracellular concentration of adenosine, subsequently agonizing the respective adenosine receptors. The agonism of each receptor is dependent on its expression on the cell surface, the extracellular concentration of adenosine, and its affinity for adenosine. This comprehensive review analyzed the multitude of indirect agonists currently being studied preclinically for bone regeneration, discussing the mechanisms of each agonist, their cellular responses in vitro, and their effects on bone formation in vivo.

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.

In Vitro and In Vivo Models for Drug Transport Across the Blood-Testis Barrier

The blood-testis barrier (BTB) is a selectively permeable membrane barrier formed by adjacent Sertoli cells (SCs) in the seminiferous tubules of the testes that develops intercellular junctional complexes to protect developing germ cells from external pressures. However, due to this inherent defense mechanism, the seminiferous tubule lumen can act as a pharmacological sanctuary site for latent viruses (e.g., Ebola, Zika) and cancers (e.g., leukemia). Therefore, it is critical to identify and evaluate BTB carrier-mediated drug delivery pathways to successfully treat these viruses and cancers. Many drugs are unable to effectively cross cell membranes without assistance from carrier proteins like transporters because they are large, polar, and often carry a charge at physiologic pH. SCs express transporters that selectively permit endogenous compounds, such as carnitine or nucleosides, across the BTB to support normal physiologic activity, although reproductive toxicants can also use these pathways, thereby circumventing the BTB. Certain xenobiotics, including select cancer therapeutics, antivirals, contraceptives, and environmental toxicants, are known to accumulate within the male genital tract and cause testicular toxicity; however, the transport pathways by which these compounds circumvent the BTB are largely unknown. Consequently, there is a need to identify the clinically relevant BTB transport pathways in in vitro and in vivo BTB models that recapitulate human pharmacokinetics and pharmacodynamics for these xenobiotics. This review summarizes the various in vitro and in vivo models of the BTB reported in the literature and highlights the strengths and weaknesses of certain models for drug disposition studies. SIGNIFICANCE STATEMENT: Drug disposition to the testes is influenced by the physical, physiological, and immunological components of the blood-testis barrier (BTB). But many compounds are known to cross the BTB by transporters, resulting in pharmacological and/or toxicological effects in the testes. Therefore, models that assess drug transport across the human BTB must adequately account for these confounding factors. This review identifies and discusses the benefits and limitations of various in vitro and in vivo BTB models for preclinical drug disposition studies.

Transporter Inhibition Profile for the Antivirals Tilorone, Quinacrine and Pyronaridine

Pyronaridine, tilorone and quinacrine are cationic molecules that have in vitro activity against Ebola, SARS-CoV-2 and other viruses. All three molecules have also demonstrated in vivo activity against Ebola in mice, while pyronaridine showed in vivo efficacy against SARS-CoV-2 in mice. We have recently tested these molecules and other antivirals against human organic cation transporters (OCTs) and apical multidrug and toxin extruders (MATEs). Quinacrine was found to be an inhibitor of OCT2, while tilorone and pyronaridine were less potent, and these displayed variability depending on the substrate used. To assess whether any of these three molecules have other potential interactions with additional transporters, we have now screened them at 10 μM against various human efflux and uptake transporters including P-gp, OATP1B3, OAT1, OAT3, MRP1, MRP2, MRP3, BCRP, as well as confirmational testing against OCT1, OCT2, MATE1 and MATE2K. Interestingly, in this study tilorone appears to be a more potent inhibitor of OCT1 and OCT2 than pyronaridine or quinacrine. However, both pyronaridine and quinacrine appear to be more potent inhibitors of MATE1 and MATE2K. None of the three compounds inhibited MRP1, MRP2, MRP3, OAT1, OAT3, P-gp or OATP1B3. Similarly, we previously showed that tilorone and pyronaridine do not inhibit OATP1B1 and have confirmed that quinacrine behaves similarly. In total, these observations suggest that the three compounds only appear to interact with OCTs and MATEs to differing extents, suggesting they may be involved in fewer clinically relevant drug-transporter interactions involving pharmaceutical substrates of the other major transporters tested.

Virtual screening and molecular dynamics simulations provide insight into repurposing drugs against SARS-CoV-2 variants Spike protein/ACE2 interface

After over two years of living with Covid-19 and hundreds of million cases worldwide there is still an unmet need to find proper treatments for the novel coronavirus, due also to the rapid mutation of its genome. In this context, a drug repositioning study has been performed, using in silico tools targeting Delta Spike protein/ACE2 interface. To this aim, it has been virtually screened a library composed by 4388 approved drugs through a deep learning-based QSAR model to identify protein-protein interactions modulators for molecular docking against Spike receptor binding domain (RBD). Binding energies of predicted complexes were calculated by Molecular Mechanics/Generalized Born Surface Area from docking and molecular dynamics simulations. Four out of the top twenty ranking compounds showed stable binding modes on Delta Spike RBD and were evaluated also for their effectiveness against Omicron. Among them an antihistaminic drug, fexofenadine, revealed very low binding energy, stable complex, and interesting interactions with Delta Spike RBD. Several antihistaminic drugs were found to exhibit direct antiviral activity against SARS-CoV-2 in vitro, and their mechanisms of action is still debated. This study not only highlights the potential of our computational methodology for a rapid screening of variant-specific drugs, but also represents a further tool for investigating properties and mechanisms of selected drugs.

Machine Learning Models Identify New Inhibitors for Human OATP1B1

The uptake transporter OATP1B1 (SLC01B1) is largely localized to the sinusoidal membrane of hepatocytes and is a known victim of unwanted drug-drug interactions. Computational models are useful for identifying potential substrates and/or inhibitors of clinically relevant transporters. Our goal was to generate OATP1B1 in vitro inhibition data for [3H] estrone-3-sulfate (E3S) transport in CHO cells and use it to build machine learning models to facilitate a comparison of seven different classification models (Deep learning, Adaboosted decision trees, Bernoulli naïve bayes, k-nearest neighbors (knn), random forest, support vector classifier (SVC), logistic regression (lreg), and XGBoost (xgb)] using ECFP6 fingerprints to perform 5-fold, nested cross validation. In addition, we compared models using 3D pharmacophores, simple chemical descriptors alone or plus ECFP6, as well as ECFP4 and ECFP8 fingerprints. Several machine learning algorithms (SVC, lreg, xgb, and knn) had excellent nested cross validation statistics, particularly for accuracy, AUC, and specificity. An external test set containing 207 unique compounds not in the training set demonstrated that at every threshold SVC outperformed the other algorithms based on a rank normalized score. A prospective validation test set was chosen using prediction scores from the SVC models with ECFP fingerprints and were tested in vitro with 15 of 19 compounds (84% accuracy) predicted as active (≥20% inhibition) showed inhibition. Of these compounds, six (abamectin, asiaticoside, berbamine, doramectin, mobocertinib, and umbralisib) appear to be novel inhibitors of OATP1B1 not previously reported. These validated machine learning models can now be used to make predictions for drug-drug interactions for human OATP1B1 alongside other machine learning models for important drug transporters in our MegaTrans software.

Physiological Characterization of the Transporter-Mediated Uptake of the Reversible Male Contraceptive H2-Gamendazole Across the Blood-Testis Barrier

The blood-testis barrier (BTB) is formed by a tight network of Sertoli cells (SCs) to limit the movement of reproductive toxicants from the blood into the male genital tract. Transporters expressed at the basal membranes of SCs also influence the disposition of drugs across the BTB. The reversible, nonhormonal contraceptive, H2-gamendazole (H2-GMZ), is an indazole carboxylic acid analog that accumulates over 10 times more in the testes compared with other organs. However, the mechanism(s) by which H2-GMZ circumvents the BTB are unknown. This study describes the physiologic characteristics of the carrier-mediated process(es) that permit H2-GMZ and other analogs to penetrate SCs. Uptake studies were performed using an immortalized human SC line (hT-SerC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Uptake of H2-GMZ and four analogs followed Michaelis-Menten transport kinetics (one analog exhibited poor penetration). H2-GMZ uptake was strongly inhibited by indomethacin, diclofenac, MK-571, and several analogs. Moreover, H2-GMZ uptake was stimulated by an acidic extracellular pH, reduced at basic pHs, and independent of extracellular Na+, K+, or Cl- levels, which are intrinsic characteristics of OATP-mediated transport. Therefore, the characteristics of H2-GMZ transport suggest that one or more OATPs may be involved. However, endogenous transporter expression in wild-type Chinese hamster ovary (CHO), Madin-Darby canine kidney (MDCK), and human embryonic kidney-293 (HEK-293) cells limited the utility of heterologous transporter expression to identify a specific OATP transporter. Altogether, characterization of the transporters involved in the flux of H2-GMZ provides insight into the selectivity of drug disposition across the human BTB to understand and overcome the pharmacokinetic and pharmacodynamic difficulties presented by this barrier. SIGNIFICANCE STATEMENT: Despite major advancements in female contraceptives, male alternatives, including vasectomy, condom usage, and physical withdrawal, are antiquated and the widespread availability of nonhormonal, reversible chemical contraceptives is nonexistent. Indazole carboxylic acid analogs such as H2-GMZ are promising new reversible, antispermatogenic drugs that are highly effective in rodents. This study characterizes the carrier-mediated processes that permit H2-GMZ and other drugs to enter Sertoli cells and the observations made here will guide the development of drugs that effectively circumvent the BTB.

Localization of Xenobiotic Transporters Expressed at the Human Blood-Testis Barrier

The blood-testis barrier (BTB) is formed by basal tight junctions between adjacent Sertoli cells (SCs) of the seminiferous tubules and acts as a physical barrier to protect developing germ cells in the adluminal compartment from reproductive toxicants. Xenobiotics, including antivirals, male contraceptives, and cancer chemotherapeutics, are known to cross the BTB, although the mechanisms that permit barrier circumvention are generally unknown. This study used immunohistological staining of human testicular tissue to determine the site of expression for xenobiotic transporters that facilitate transport across the BTB. Organic anion transporter (OAT) 1, OAT2, and organic cation transporter, novel (OCTN) 1 primarily localized to the basal membrane of SCs, whereas OCTN2, multidrug resistance protein (MRP) 3, MRP6, and MRP7 localized to SC basal membranes and peritubular myoid cells (PMCs) surrounding the seminiferous tubules. Concentrative nucleoside transporter (CNT) 2 localized to Leydig cells (LCs), PMCs, and SC apicolateral membranes. Organic cation transporter (OCT) 1, OCT2, and OCT3 mostly localized to PMCs and LCs, although there was minor staining in developing germ cells for OCT3. Organic anion transporting polypeptide (OATP) 1A2, OATP1B1, OATP1B3, OATP2A1, OATP2B1, and OATP3A1-v2 localized to SC basal membranes with diffuse staining for some transporters. Notably, OATP1C1 and OATP4A1 primarily localized to LCs. Positive staining for multidrug and toxin extrusion protein (MATE) 1 was only observed throughout the adluminal compartment. Definitive staining for CNT1, OAT3, MATE2, and OATP6A1 was not observed. The location of these transporters is consistent with their involvement in the movement of xenobiotics across the BTB. Altogether, the localization of these transporters provides insight into the mechanisms of drug disposition across the BTB and will be useful in developing tools to overcome the pharmacokinetic and pharmacodynamic difficulties presented by the BTB. SIGNIFICANCE STATEMENT: Although the total mRNA and protein expression of drug transporters in the testes has been explored, the localization of many transporters at the blood-testis barrier (BTB) has not been determined. This study applied immunohistological staining in human testicular tissues to identify the cellular localization of drug transporters in the testes. The observations made in this study have implications for the development of drugs that can effectively use transporters expressed at the basal membranes of Sertoli cells to bypass the BTB.

PF-07321332 (Nirmatrelvir) Does Not Interact with Human ENT1 or ENT2: Implications for COVID-19 Patients

The ongoing pandemic of severe acute respiratory syndrome‐coronavirus 2 (SARS‐CoV‐2) and subsequently, coronavirus disease 2019 (COVID‐19), has led to the deaths of over 6.1 million people and sparked a greater interest in virology to expedite the development process for antivirals. The US Food and Drug Administration (FDA) granted emergency use authorization for three antivirals: remdesivir, molnupiravir, and nirmatrelvir. Remdesivir and molnupiravir are nucleoside analogs that undergo biotransformation to form active metabolites that incorporate into new viral RNA to stall replication. Unlike remdesivir or molnupiravir, nirmatrelvir is a protease inhibitor that covalently binds to the SARS‐CoV‐2 3C‐like protease to interrupt the viral replication cycle. A recent study identified that remdesivir and the active metabolite of molnupiravir, EIDD‐1931, are substrates of equilibrative nucleoside transporters 1 and 2 (ENT1 and 2). Despite the ubiquitous expression of the ENTs, the preclinical efficacy of remdesivir and molnupiravir is not reflected in wide‐scale SARS‐CoV‐2 clinical trials. Interestingly, downregulation of ENT1 and ENT2 expression has been shown in lung epithelial and endothelial cells in response to hypoxia and acute lung injury, although it has not been directly studied in patients with COVID‐19. It is possible that the poor efficacy of remdesivir and molnupiravir in these patients may be partially attributed to the repression of ENTs in the lungs, but further studies are warranted. This study investigated the interaction between nirmatrelvir and the ENTs and found that it was a poor inhibitor of ENT‐mediated [³H]uridine uptake at 300 μM. Unlike for remdesivir or EIDD‐1931, ENT activity is unlikely to be a factor for nirmatrelvir disposition in humans; however, whether this contributes to the similar in vitro and clinical efficacy will require further mechanistic studies.

Remdesivir and EIDD-1931 Interact with Human Equilibrative Nucleoside Transporters 1 and 2: Implications for Reaching SARS-CoV-2 Viral Sanctuary Sites

Equilibrative nucleoside transporters (ENTs) are present at the blood-testis barrier (BTB), where they can facilitate antiviral drug disposition to eliminate a sanctuary site for viruses detectable in semen. The purpose of this study was to investigate ENT-drug interactions with three nucleoside analogs, remdesivir, molnupiravir, and molnupiravir's active metabolite, β-d-N⁴-hydroxycytidine (EIDD-1931), and four non-nucleoside molecules repurposed as antivirals for coronavirus disease 2019 (COVID-19). The study used three-dimensional pharmacophores for ENT1 and ENT2 substrates and inhibitors and Bayesian machine learning models to identify potential interactions with these transporters. In vitro transport experiments demonstrated that remdesivir was the most potent inhibitor of ENT-mediated [³H]uridine uptake (ENT1 IC₅₀: 39 μM; ENT2 IC₅₀: 77 μM), followed by EIDD-1931 (ENT1 IC₅₀: 259 μM; ENT2 IC₅₀: 467 μM), whereas molnupiravir was a modest inhibitor (ENT1 IC₅₀: 701 μM; ENT2 IC₅₀: 851 μM). Other proposed antivirals failed to inhibit ENT-mediated [³H]uridine uptake below 1 mM. Remdesivir accumulation decreased in the presence of 6-S-[(4-nitrophenyl)methyl]-6-thioinosine (NBMPR) by 30% in ENT1 cells (P = 0.0248) and 27% in ENT2 cells (P = 0.0054). EIDD-1931 accumulation decreased in the presence of NBMPR by 77% in ENT1 cells (P = 0.0463) and by 64% in ENT2 cells (P = 0.0132), which supported computational predictions that both are ENT substrates that may be important for efficacy against COVID-19. NBMPR failed to decrease molnupiravir uptake, suggesting that ENT interaction is likely inhibitory. Our combined computational and in vitro data can be used to identify additional ENT-drug interactions to improve our understanding of drugs that can circumvent the BTB. SIGNIFICANCE STATEMENT: This study identified remdesivir and EIDD-1931 as substrates of equilibrative nucleoside transporters 1 and 2. This provides a potential mechanism for uptake of these drugs into cells and may be important for antiviral potential in the testes and other tissues expressing these transporters.

Review of Transporter Substrate, Inhibitor, and Inducer Characteristics of Cladribine

Cladribine is a nucleoside analog that is phosphorylated in its target cells (B- and T-lymphocytes) to its active adenosine triphosphate form (2-chlorodeoxyadenosine triphosphate). Cladribine tablets 10 mg (Mavenclad^®) administered for up to 10 days per year in 2 consecutive years (3.5-mg/kg cumulative dose over 2 years) are used to treat patients with relapsing multiple sclerosis. The ATP-binding cassette, solute carrier, and nucleoside transporter substrate, inhibitor, and inducer characteristics of cladribine are reviewed in this article. Available evidence suggests that the distribution of cladribine across biological membranes is facilitated by a number of uptake and efflux transporters. Among the key ATP-binding cassette efflux transporters, only breast cancer resistance protein has been shown to be an efficient transporter of cladribine, while P-glycoprotein does not transport cladribine well. Intestinal absorption, distribution throughout the body, and intracellular uptake of cladribine appear to be exclusively mediated by equilibrative and concentrative nucleoside transporters, specifically by ENT1, ENT2, ENT4, CNT2 (low affinity), and CNT3. Renal excretion of cladribine appears to be most likely driven by breast cancer resistance protein, ENT1, and P-glycoprotein. The latter may play a role despite its poor cladribine transport efficiency in view of the renal abundance of P-glycoprotein. There is no evidence that solute carrier uptake transporters such as organic anion transporting polypeptides, organic anion transporters, and organic cation transporters are involved in the transport of cladribine. Available in vitro studies examining the inhibitor characteristics of cladribine for a total of 13 major ATP-binding cassette, solute carrier, and CNT transporters indicate that in vivo inhibition of any of these transporters by cladribine is unlikely.

Testicular disposition of clofarabine in rats is dependent on equilibrative nucleoside transporters

Acute lymphoblastic leukemia (ALL) is the most common cancer in children and adolescents. Although the 5-year survival rate is high, some patients respond poorly to chemotherapy or have recurrence in locations such as the testis. The blood-testis barrier (BTB) can prevent complete eradication by limiting chemotherapeutic access and lead to testicular relapse unless a chemotherapeutic is a substrate of drug transporters present at this barrier. Equilibrative nucleoside transporter (ENT) 1 and ENT2 facilitate the movement of substrates across the BTB. Clofarabine is a nucleoside analog used to treat relapsed or refractory ALL. This study investigated the role of ENTs in the testicular disposition of clofarabine. Pharmacological inhibition of the ENTs by 6-nitrobenzylthioinosine (NBMPR) was used to determine ENT contribution to clofarabine transport in primary rat Sertoli cells, in human Sertoli cells, and across the rat BTB. The presence of NBMPR decreased clofarabine uptake by 40% in primary rat Sertoli cells (p = .0329) and by 53% in a human Sertoli cell line (p = .0899). Rats treated with 10 mg/kg intraperitoneal (IP) injection of the NBMPR prodrug, 6-nitrobenzylthioinosine 5'-monophosphate (NBMPR-P), or vehicle, followed by an intravenous (IV) bolus 10 mg/kg dose of clofarabine, showed a trend toward a lower testis concentration of clofarabine than vehicle (1.81 ± 0.59 vs. 2.65 ± 0.92 ng/mg tissue; p = .1160). This suggests that ENTs could be important for clofarabine disposition. Clofarabine may be capable of crossing the human BTB, and its potential use as a first-line treatment to avoid testicular relapse should be considered.

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.

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.