Isradipine

Equilibrative nucleoside transporters (ENTs) facilitate the equilibrative movement of nucleosides and nucleobases across cell membranes in a sodium-independent manner. ENT1 (SLC29A1) and ENT2 (SLC29A2) also transport nucleoside analogs and can affect the pharmacokinetics and pharmacodynamics of drugs used in cancer, viral infections, and inflammatory disorders. ENT1 and ENT2 may be differentiated functionally by their sensitivity to inhibition by nitrobenzylthioinosine (NBMPR), and we used this difference in NBMPR sensitivity to create a HeLa-based ENT2 inhibition assay. We then screened a library of 1600 diverse compounds composed of drugs and natural products for inhibition against ENT1 and ENT2, selecting a subset of compounds for side-by-side comparison of dose-response studies. We used these screening data to build machine learning models for ENT1 and ENT2 inhibition, employing dataset balancing and conformal prediction to adjust for the asymmetrical nature of the data. A random forest model predicted a prospective test set of 44 additional molecules (from the MedChem Express Drug Repurposing Library [2700 compounds]) as potential ENT1 inhibitors with 59% accuracy. This resulted in the identification of the Food and Drug Administration-approved drugs isradipine, avanafil, and istradefylline as inhibitors of ENT1. These new experimental and computational methods and models for these clinically relevant transporters can be used to evaluate drug-transporter interactions early in drug discovery, before testing in vivo. SIGNIFICANCE STATEMENT: Recent regulatory guidance have suggest the inclusion of the equilibrative nucleoside transporters (eg, ENT1 and ENT2) as transporters with emerging clinical relevance for in vitro and in vivo assessment. We have screened over 1600 diverse molecules, allowing us to build machine learning models that in turn were further used to make predictions to validate the models. Our combined experimental and machine learning approach resulted in the identification of multiple Food and Drug Administration-approved medications as inhibitors of ENT1 or ENT2.

The threshold of toxicological concern (TTC) is an approach to risk assessment that uses acceptable low-level exposure values derived from statistical information for chemicals for which insufficient toxicological information exists. Previously, the TTC for systemic toxicity was derived from the toxicological endpoints of the external dose for each administration route in animal studies. Internal TTC (iTTC), which extends the TTC concept from external to internal dosages, expands the scope of application by standardizing exposure via different routes. Information about blood exposure to pharmaceuticals used in treatment can help determine the chemical levels tolerated in humans. Using the tolerated plasma/serum blood drug concentration of the pharmaceutical as the toxicological point of departure, a new iTTC of 0.6 nM for the no effect concentration were proposed (a conservative risk assessment considers further safety margins). It is based on the 5th percentile values of the normal distribution, which approximates the cumulative empirical distribution, and an uncertainty coefficient to convert the treatment dose to the no-effect dose. Predicting blood concentrations of chemicals to which humans are exposed and comparing them to iTTC with appropriate caution can be used as an approach to risk assessment of systemic toxicity without animal-based testing.