Saturable Hepatic Extraction of Gemcitabine Involves Biphasic Uptake Mediated by Nucleoside Transporters Equilibrative Nucleoside Transporter 1 and 2

The Role of Solute Carrier Transporters in Efficient Anticancer Drug Delivery and Therapy

Transporter-mediated drug resistance is a major obstacle in anticancer drug delivery and a key reason for cancer drug therapy failure. Membrane solute carrier (SLC) transporters play a crucial role in the cellular uptake of drugs. The expression and function of the SLC transporters can be down-regulated in cancer cells, which limits the uptake of drugs into the tumor cells, resulting in the inefficiency of the drug therapy. In this review, we summarize the current understanding of low-SLC-transporter-expression-mediated drug resistance in different types of cancers. Recent advances in SLC-transporter-targeting strategies include the development of transporter-utilizing prodrugs and nanocarriers and the modulation of SLC transporter expression in cancer cells. These strategies will play an important role in the future development of anticancer drug therapies by enabling the efficient delivery of drugs into cancer cells.

Maternal and Fetal Expression of ATP-Binding Cassette and Solute Carrier Transporters Involved in the Brain Disposition of Drugs

Evidence from preclinical and clinical studies demonstrate that pregnancy is a physiological state capable of modifying drug disposition. Factors including increased hepatic metabolism and renal excretion are responsible for impacting disposition, and the role of membrane transporters expressed in biological barriers, including the placental- and blood-brain barriers, has received considerable attention. In this regard, the brain disposition of drugs in the mother and fetus has been the subject of studies attempting to characterize the mechanisms by which pregnancy could alter the expression of ATP-binding cassette (ABC) and solute carrier (SLC) transporters. This chapter will summarize findings of the influence of pregnancy on the maternal and fetal expression of ABC and SLC transporters in the brain and the consequences of such changes on the disposition of therapeutic drugs.

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.

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

Equilibrative nucleoside transporters (ENTs) 1 and 2 facilitate nucleoside transport across the blood-testis barrier (BTB). Improving drug entry into the testes with drugs that use endogenous transport pathways may lead to more effective treatments for diseases within the reproductive tract. In this study, CRISPR/CRISPR-associated protein 9 was used to generate HeLa cell lines in which ENT expression was limited to ENT1 or ENT2. We characterized uridine transport in these cell lines and generated Bayesian models to predict interactions with the ENTs. Quantification of [³H]uridine uptake in the presence of the ENT-specific inhibitor S-(4-nitrobenzyl)-6-thioinosine (NBMPR) demonstrated functional loss of each transporter. Nine nucleoside reverse-transcriptase inhibitors and 37 nucleoside/heterocycle analogs were evaluated to identify ENT interactions. Twenty-one compounds inhibited uridine uptake and abacavir, nevirapine, ticagrelor, and uridine triacetate had different IC₅₀ values for ENT1 and ENT2. Total accumulation of four identified inhibitors was measured with and without NBMPR to determine whether there was ENT-mediated transport. Clofarabine and cladribine were ENT1 and ENT2 substrates, whereas nevirapine and lexibulin were ENT1 and ENT2 nontransported inhibitors. Bayesian models generated using Assay Central machine learning software yielded reasonably high internal validation performance (receiver operator characteristic > 0.7). ENT1 IC₅₀-based models were generated from ChEMBL; subvalidations using this training data set correctly predicted 58% of inhibitors when analyzing activity by percent uptake and 63% when using estimated-IC₅₀ values. Determining drug interactions with these transporters can be useful in identifying and predicting compounds that are ENT1 and ENT2 substrates and can thereby circumvent the BTB through this transepithelial transport pathway in Sertoli cells. SIGNIFICANCE STATEMENT: This study is the first to predict drug interactions with equilibrative nucleoside transporter (ENT) 1 and ENT2 using Bayesian modeling. Novel CRISPR/CRISPR-associated protein 9 functional knockouts of ENT1 and ENT2 in HeLa S3 cells were generated and characterized. Determining drug interactions with these transporters can be useful in identifying and predicting compounds that are ENT1 and ENT2 substrates and can circumvent the blood-testis barrier through this transepithelial transport pathway in Sertoli cells.

Pilot Study to Predict Pharmacokinetics of a Therapeutic Gemcitabine Dose From a Microdose

Microdose studies are exploratory trials to determine early drug pharmacokinetics in humans. In this trial we examined whether the pharmacokinetics of gemcitabine at a therapeutic dose could be predicted from the pharmacokinetics of a microdose. In this prospective, open-label microdosing study, a gemcitabine microdose (100 µg) was given intravenously to participants on day 1, followed by a therapeutic dose (1250 mg/m² ) on day 2. Gemcitabine and its metabolite 2',2'-difluorodeoxyuracil (dFdU) were quantified in plasma and intracellularly by using liquid chromatography-mass spectrometry). Noncompartmental pharmacokinetic analysis was performed. Ten patients participated in this study. The mean area under the plasma concentration-time curve (AUC_0-8 ) of gemcitabine after microdosing was 0.00074 h·mg/L and after therapeutic dosing was 16 h·mg/L. The mean AUC_0-8 of dFdU following the microdose and therapeutic dose were 0.022 h·mg/L and 169 h·mg/L, respectively. Exposure to gemcitabine after the therapeutic dose was within 2-fold of the exposure following a microdose, when linearly extrapolated to 1250 mg/m² . However, the shape of the concentration-time curve was different, as reflected by poor scalability in volume of distribution (939 L versus 222 L). Furthermore, intracellularly phosphorylated gemcitabine and phosphorylated dFdU levels could not be predicted from the microdose. The AUC_0-8 of gemcitabine at therapeutic dose was accurately predicted by the pharmacokinetics of a microdose, when linearly extrapolated to 1250 mg/m² . Volume of distribution, elimination rate constant, and intracellular pharmacokinetics of the therapeutic dose could not be predicted from the microdose, which demonstrates limitations of the microdose approach in this case.

Numerical analysis of apparent decitabine uptake in HCT116 cells: Incorporation of a bidirectional first-order kinetic parameter for ENT1 transport and Michaelis-Menten parameters for subsequent phosphorylation

Decitabine (DAC), a DNA methylation inhibitor, is transported into cancer cells mainly via equilibrative nucleoside transporter 1 (ENT1) and subsequently phosphorylated by deoxycytidine kinase (dCK). We previously reported that apparent DAC uptake into cells may be described using a simple compartment model with clearance for facilitated diffusion (PS) and subsequent phosphorylation (CL_met). In the present study, time course of apparent intracellular [³H]-DAC uptake was analyzed numerically, and PS and CL_met values were calculated using the compartment model in human colon cancer HCT116 cells. PS at 0.1 μM [³H]-DAC was markedly decreased in the presence of 100 μM irinotecan or etoposide, while CL_met was markedly decreased in the presence of 100 μM cytarabine or gemcitabine. CL_met at 0.1-10 μM [³H]-DAC varied in a concentration-dependent manner and was described by Michaelis-Menten parameters K_m,met and V_max,met. In conclusion, DAC uptake mainly via ENT1 may be described by a bidirectional first-order kinetic parameter, while phosphorylation by dCK may be described by Michaelis-Menten parameters.

Chemoresistance in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC), generally known as pancreatic cancer (PC), ranks the fourth leading cause of cancer-related deaths in the western world. While the incidence of pancreatic cancer is displaying a rising tendency every year, the mortality rate has not decreased significantly because of late diagnosis, early metastasis, and limited reaction to chemotherapy or radiotherapy. Adjuvant chemotherapy after surgical resection is typically the preferred option to treat early pancreatic cancer. Although 5-fluorouracil/leucovorin with irinotecan and oxaliplatin (FOLFIRINOX) and gemcitabine/nab-paclitaxel can profoundly improve the prognosis of advanced pancreatic cancer, the development of chemoresistance still leads to poor clinical outcomes. Chemoresistance is multifactorial as a result of the interaction among pancreatic cancer cells, cancer stem cells, and the tumor microenvironment. Nevertheless, more pancreatic cancer patients will benefit from precision treatment and targeted drugs. Therefore, we outline new perspectives for enhancing the efficacy of gemcitabine after reviewing the related factors of gemcitabine metabolism, mechanism of action, and chemoresistance.

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.

Contribution of equilibrative nucleoside transporters 1 and 2 to gemcitabine uptake in pancreatic cancer cells

Hepatic arterial infusion (HAI) chemotherapy is expected to be a more effective and safer method to treat the hepatic metastasis of pancreatic cancer than intravenous (iv) administration because of higher tumor exposure and lower systemic exposure. To clarify the uptake mechanism of nucleoside anticancer drugs, including gemcitabine (GEM), in pancreatic cancer, we investigated the uptakes of radiolabeled uridine (a general substrate of nucleoside transporters) and GEM in pancreatic cancer cell lines MIA-PaCa2 and As-PC1. Uridine uptake was inhibited by non-labeled GEM and also by S-(4-nitrobenzyl)-6-thioinosine (NBMPR; an inhibitor of equilibrative nucleoside transporters, ENTs) in a concentration-dependent manner, suggesting that ENTs contribute to uridine uptake in pancreatic cancer cells. As for GEM, saturable uptake was mediated by high- and low-affinity components with K_m values of micromolar and millimolar orders, respectively. Uptake was inhibited in a concentration-dependent manner by NBMPR and was sodium ion-independent. Moreover, the concentration dependence of uptake in the presence of 0.1 μM NBMPR showed a single low-affinity site. These results indicated that the high- and low-affinity sites correspond to hENT1 and hENT2, respectively. The results indicated that at clinically relevant hepatic concentrations of GEM in GEM-HAI therapy, the metastatic tumor exposure of GEM is predominantly determined by hENT2 under unsaturated conditions, suggesting that hENT2 expression in metastatic tumor would be a candidate biomarker for indicating anticancer therapy with GEM-HAI.

Antitumor effect of gemcitabine-loaded albumin nanoparticle on gemcitabine-resistant pancreatic cancer induced by low hENT1 expression

Purpose

Gemcitabine is currently the standard first-line chemotherapeutic drug for treating pancreatic cancer. However, many factors can contribute to gemcitabine resistance. One of the most important reasons is the low hENT1 expression. In this study, we tested the antitumor effect of gemcitabine-loaded human serum albumin nanoparticle (GEM-HSA-NP) on gemcitabine-resistant pancreatic cancer induced by low hENT1 expression.

Materials and methods

S-(4-nitrobenzyl)-6-thioinosine was utilized to inhibit the activity of hENT1 and simulate low hENT1 expression. Growth inhibition assays and cell cycle and apoptosis analyses were performed on human pancreatic cancer cell lines such as BxPC-3 and SW1990. The in vivo antitumor effect was studied by using patient-derived xenograft (PDX) models. The in vivo toxicity assessment was performed on healthy Kunming mice.

Results

In in vitro studies, GEM-HSA-NP showed its ability to inhibit cell proliferation, arrest cell cycle and induce apoptosis when tumor cells were resistant to gemcitabine. In in vivo studies, GEM-HSA-NP was more effective than gemcitabine on inhibiting tumor growth whether the expression levels of hENT1 were high or low in PDX models. The in vivo toxicity assessment showed that the biotoxicity of GEM-HSA-NP did not increase compared with gemcitabine.

Conclusion

GEM-HSA-NP can overcome gemcitabine resistance induced by low hENT1 expression, which suggests its potential role for the clinical application.

Comparative Evaluation of Plasma Bile Acids, Dehydroepiandrosterone Sulfate, Hexadecanedioate, and Tetradecanedioate with Coproporphyrins I and III as Markers of OATP Inhibition in Healthy Subjects

Multiple endogenous compounds have been proposed as candidate biomarkers to monitor organic anion transporting polypeptide (OATP) function in preclinical species or humans. Previously, we demonstrated that coproporphyrins (CPs) I and III are appropriate clinical markers to evaluate OATP inhibition and recapitulate clinical drug-drug interactions (DDIs). In the present study, we investigated bile acids (BAs) dehydroepiandrosterone sulfate (DHEAS), hexadecanedioate (HDA), and tetradecanedioate (TDA) in plasma as endogenous probes for OATP inhibition and compared these candidate probes to CPs. All probes were determined in samples from a single study that examined their behavior and their association with rosuvastatin (RSV) pharmacokinetics after administration of an OATP inhibitor rifampin (RIF) in healthy subjects. Among endogenous probes examined, RIF significantly increased maximum plasma concentration (C_max) and area under the concentration-time curve (AUC)_(0-24h) of fatty acids HDA and TDA by 2.2- to 3.2-fold. For the 13 bile acids in plasma examined, no statistically significant changes were detected between treatments. Changes in plasma DHEAS did not correlate with OATP1B inhibition by RIF. On the basis of the magnitude of effects for the endogenous compounds that demonstrated significant changes from baseline over interindividual variations, the overall rank order for the AUC change was found to be CP I > CP III > HDA ≈ TDA ≈ RSV > > BAs. Collectively, these results reconfirmed that CPs are novel biomarkers suitable for clinical use. In addition, HDA and TDA are useful for OATP functional assessment. Since these endogenous markers can be monitored in conjunction with pharmacokinetics analysis, the CPs and fatty acid dicarboxylates, either alone or in combination, offer promise of earlier diagnosis and risk stratification for OATP-mediated DDIs.