Organic Cation Transporter 1 (OCT1) has historically been overlooked in regulatory guidelines, despite its potential clinical relevance. Clinical pharmacogenetic published literature identifies fenoterol, sumatriptan, morphine, O-desmethyl tramadol, tropisetron and ondansetron as clinical substrates reliant upon OCT1 for their hepatic clearance. Among these, fenoterol presents the highest risk for drug-drug interactions (DDIs) due to its narrow therapeutic window, making it a key victim/object drug for further investigation into OCT1-mediated DDIs. This is supported clinically by reported pharmacogenetic studies in which a 1.92-fold increase in exposure (AUC) of intravenous administered fenoterol is observed in individuals with a heritable OCT1 deficient phenotype that results in 70% reduced transporter function, highlighting the potential for clinical interactions with comedications that inhibit OCT1.
Cyprotex has conducted OCT1 inhibition and substrate identification assays in HEK293 transfected cells overexpressing OCT1 since 2016 in order to evaluate the precipitant (perpetrator) and object (victim) potential for investigational drugs. Due to the potential clinical importance of fenoterol DDIs, the important question Cyprotex wanted to address was the clinical translatability and surrogate capability that the established OCT1 inhibition (IC50 determination) assay, employing [14C]‑tetraethylammonium (TEA) as the substrate, had in capitulating inhibition of fenoterol transport and subsequent DDI risk prediction for investigational drugs. Initially, Cyprotex determined the OCT1-mediated uptake kinetics (Km) of fenoterol in HEK293-OCT1 cells. An appropriate fenoterol probe substrate concentration was selected for subsequent inhibition (IC50) assays to evaluate the inhibitory potencies of amitriptyline, fluoxetine, verapamil and selegiline in order to compare with their determined inhibitory parameters versus the prototypical probe substrate [14C]‑TEA.
The successful validation of this clinically relevant human OCT1 inhibition (IC50) assay using fenoterol identified verapamil, using mechanistic static quantitative DDI predictive equations, as a likely clinical precipitant drug, with potential to double fenoterol exposure via inhibition of hepatic OCT1. Importantly, when utilising either fenoterol or [14C]‑TEA as the substrate, similar OCT1 Ki values were observed leading to the same qualitative DDI risk outcome when assessed using the regulatory guideline basic static equation. This validates the use of [14C]-TEA as a suitable in vitro surrogate probe substrate for the clinically relevant fenoterol when assessing investigational drugs as inhibitors of OCT1 in order to predict the potential for OCT1-mediated drug interactions. The radiolabelled detection employed in this assay also removes a requirement to perform MS ion interference checks. This work focused on predicting fenoterol DDIs underscores the importance of assessing investigational drugs as inhibitors of OCT1 and supports the prospective regulatory requirement for OCT1 inhibition testing to guide downstream decision-making, clinical protocols, and drug labelling.
To learn more about the experimental design, detailed results and a comprehensive discussion of this data visit the research article in Expert Opinion on Drug Metabolism & Toxicology recently published by Cyprotex.1 If you would like to learn more about our transporter studies, including OCT1 assessment, please contact Cyprotex using the link below.
