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Published on: May 9, 2014
Assessment of Substrate-Dependent Ligand Interactions at the Organic Cation Transporter OCT2 Using Six Model
Philip J Sandoval1, Kimberley M Zorn1, Alex M Clark1
1Department of Physiology, College of Medicine, University of Arizona, Tucson, Arizona (P.J.S., S.H.W.); Collaborations Pharmaceuticals, Inc., Raleigh, North Carolina (K.M.Z., S.E.); and Molecular Materials Informatics, Inc., Montreal, Quebec, Canada (A.M.C.).
Abstract:
Organic cation transporter (OCT) 2 mediates the entry step for organic cation secretion by renal proximal tubule cells and is a site of unwanted drug-drug interactions (DDIs). But reliance on decision tree-based predictions of DDIs at OCT2 that depend on IC50 values can be suspect because they can be influenced by choice of transported substrate; for example, IC50 values for the inhibition of metformin versus MPP transport can vary by 5- to 10-fold. However, it is not clear whether the substrate dependence of a ligand interaction is common among OCT2 substrates. To address this question, we screened the inhibitory effectiveness of 20 µM concentrations of several hundred compounds against OCT2-mediated uptake of six structurally distinct substrates: MPP, metformin, N,N,N-trimethyl-2-[methyl(7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)amino]ethanaminium (NBD-MTMA), TEA, cimetidine, and 4-4-dimethylaminostyryl-N-methylpyridinium (ASP). Of these, MPP transport was least sensitive to inhibition. IC50 values for 20 structurally diverse compounds confirmed this profile, with IC50 values for MPP averaging 6-fold larger than those for the other substrates. Bayesian machine-learning models of ligand-induced inhibition displayed generally good statistics after cross-validation and external testing. Applying our ASP model to a previously published large-scale screening study for inhibition of OCT2-mediated ASP transport resulted in comparable statistics, with approximately 75% of "active" inhibitors predicted correctly. The differential sensitivity of MPP transport to inhibition suggests that multiple ligands can interact simultaneously with OCT2 and supports the recommendation that MPP not be used as a test substrate for OCT2 screening. Instead, metformin appears to be a comparatively representative OCT2 substrate for both in vitro and in vivo (clinical) use.
Insights
Drug interactions involving organic cation transporter 2 (OCT2) vary by substrate. Metformin is a more reliable substrate for OCT2 screening than MPP, as MPP transport inhibition is less sensitive to different compounds.
Area of Science:
- Pharmacology
- Drug Metabolism and Transporter Research
Background:
- Organic cation transporter 2 (OCT2) is crucial for renal drug secretion and a common site for drug-drug interactions (DDIs).
- Current DDI predictions for OCT2 rely on IC50 values, which can be unreliable due to substrate-dependent inhibition.
- The extent of substrate dependence in OCT2 ligand interactions is not well understood.
Purpose of the Study:
- To investigate whether the substrate dependence of ligand interactions is a common characteristic among organic cation transporter 2 (OCT2) substrates.
- To identify a more representative substrate for OCT2 screening assays.
Main Methods:
- Screened inhibitory effectiveness of hundreds of compounds against OCT2-mediated uptake of six distinct substrates (MPP, metformin, NBD-MTMA, TEA, cimetidine, ASP).
- Determined IC50 values for 20 diverse compounds across these substrates.
- Developed and validated Bayesian machine-learning models for ligand-induced inhibition.
Main Results:
- MPP transport exhibited the least sensitivity to inhibition, with IC50 values averaging sixfold larger than for other substrates.
- Machine-learning models showed good predictive performance.
- An ASP model accurately predicted inhibitors in a prior screening study (75% correct).
Conclusions:
- Substrate choice significantly impacts the assessment of OCT2 inhibitors, indicating simultaneous ligand interactions.
- MPP is not recommended as a standard substrate for OCT2 screening due to its differential sensitivity.
- Metformin is proposed as a more representative substrate for both in vitro and in vivo OCT2 DDI studies.
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