Equilibrative nucleoside transporters (ENT) 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/Cas9 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 [ 3 H]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 thirty-seven nucleoside/heterocycle analogs were evaluated to identify ENT interactions. Twenty-one compounds inhibited uridine uptake and abacavir, nevirapine, ticagrelor, and uridine triacetate had different IC 50 values for ENT1 and ENT2. Total accumulation of four identified inhibitors was measured with and without NBMPR to determine if there was ENT-mediated transport. Clofarabine and cladribine were ENT1 and ENT2 substrates, while nevirapine and lexibulin were ENT1 and ENT2 non-transported inhibitors. Bayesian models generated using Assay Central ® machine learning software yielded reasonably high internal validation performance (ROC > 0.7). ENT1 IC 50-based models were generated from ChEMBL; subvalidations using this training dataset correctly predicted 58% of inhibitors when analyzing activity by percent uptake and 63% when using estimated-IC 50 values. Determining drug interactions with these transporters can be useful in identifying and predicting compounds that are ENT1 and ENT2 This article has not been copyedited and formatted. The final version may differ from this version.

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 its active metabolite, EIDD-1931 and four nonnucleoside molecules repurposed as antivirals for COVID-19. The study used 3D 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 [ 3 H] uridine uptake

Equilibrative nucleoside transporters (ENTs) transport nucleosides across the blood-testis barrier (BTB). ENTs are of interest to study the disposition of nucleoside reverse-transcriptase inhibitors (NRTIs) in the human male genital tract because of their similarity in structure to nucleosides. HeLa S3 cells express ENT1 and ENT2 and were used to compare relative interactions of these transporters with selected NRTIs. Inhibition of [ 3 H] uridine uptake by NBMPR was biphasic, with IC 50 values of 11.3 nM for ENT1 and 9.6 mM for ENT2. Uptake measured with 100 nM NBMPR represented ENT2-mediated transport; subtracting that from total uptake represented ENT1-mediated transport. The kinetics of ENT1-and ENT2-mediated [ 3 H]uridine uptake revealed no difference in J max (16.53 and 30.40 pmol cm 22 min 21 ) and an eightfold difference in K t (13.6 and 108.9 mM). The resulting fivefold difference in intrinsic clearance (J max /K t ) for ENT1-and ENT2 transport accounted for observed inhibition of [ 3 H]uridine uptake by 100 nM NBMPR. Millimolar concentrations of the NRTIs emtricitabine, didanosine, lamivudine, stavudine, tenofovir disoproxil, and zalcitabine had no effect on ENT transport activity, whereas abacavir, entecavir, and zidovudine inhibited both transporters with IC 50 values of ∼200 mM, 2.5 mM, and 2 mM, respectively. Using liquid chromatography-tandem mass spectrometry and [ 3 H] compounds, the data suggest that entecavir is an ENT substrate, abacavir is an ENT inhibitor, and zidovudine uptake is carrier-mediated, although not an ENT substrate. These data show that HeLa S3 cells can be used to explore complex transporter selectivity and are an adequate model for studying ENTs present at the BTB. SIGNIFICANCE STATEMENTThis study characterizes an in vitro model using S-[(4-nitrophenyl) methyl]-6-thioinosine to differentiate between equilibrative nucleoside transporter (ENT) 1-and ENT2-mediated uridine transport in HeLa cells. This provides a method to assess the influence of nucleoside reverse-transcriptase inhibitors on natively expressed transporter function. Determining substrate selectivity of the ENTs in HeLa cells can be effectively translated into the activity of these transporters in Sertoli cells that comprise the blood-testis barrier, thereby assisting targeted drug development of compounds capable of circumventing the blood-testis barrier.