Comparative Analysis of Opioids as Substrates and Inhibitors of the Human Organic Cation Transporter 1 (OCT1)

Abstract

The organic cation transporter 1 (OCT1) is primarily located in the sinusoidal membranes of hepatocytes and mediates phase 0 biotransformation of xenobiotics, including weak basic and cationic drugs. Variations in SLC22A1, the gene encoding OCT1, are common throughout the Caucasian population. Close to 9% of Caucasians are homozygotic for OCT1 alleles that encode insufficient transporters, resulting in a partial or complete loss of OCT1 transport activity in affected individuals. Clinical studies have demonstrated that diminished OCT1 transport activity leads to elevated plasma levels of opioids morphine and O-desmethyltramadol (the active metabolite of tramadol). Patients with diminished-activity OCT1 alleles require 20% less tramadol for efficient postoperative pain management and suffer more adverse effects after morphine consumption. These findings are all the more relevant when considering the high prevalence of diminished-activity OCT1 variants and the extensive use of opioids in clinical practice. The aim of this study was to perform a systematic screening of opioids in vitro in order to identify substrates and inhibitors of OCT1 beyond morphine and tramadol that might be affected by reduced OCT1 activity. The screening was performed in a four-step process that involved in silicio predictions, membrane permeability testing in an artificial membrane model (so-called parallel artificial membrane permeability assays), inhibition experiments, and transport experiments. For the inhibition experiments, we photometrically determined to what extent our test drugs inhibited the uptake of the fluorescent OCT1 substrate ASP+, while for the transport experiments we measured the uptake of test drugs into OCT1-overexpressing HEK293 cells directly via LC-MS/MS. Sixty opioids underwent the initial in silicio predictions; of those, we chose 23 opioids to run the entire course of experiments. Seven novel substrates and fourteen inhibitors of OCT1 were identified. Three of the substrates –methylnaltrexone, meptazinol, and hydromorphone– are active compounds used in clinical practice. Uptake of methylnaltrexone into OCT1-overexpressing cells was 32 to 85 times higher than that into negative control cells, making it one of the substances most dependent on OCT1-mediated transport so far reported. Inhibitors were defined as substances with IC50 values of 100 µM or below; of the fourteen inhibitors identified, nine were especially potent with IC50 values of less than 25 µM. Notably, dextromethorphan and its metabolites were among the substances with the lowest IC50 values. Three of the inhibitors –namely dextromethorphan, levorphanol, and tapentadol– could, theoretically, reach portal vein concentrations high enough to inhibit OCT1 in vivo. We also identified five opioids –codeine, hydrocodone, naltrexone, oxycodone, and oxymorphone– that can be considered pharmacokinetically “safe” with respect to variations in SLC22A1 since they do not seem to interact with OCT1 in any way. Their uptake into OCT1-overexpressing cells was less than two times the uptake into negative control cells, indicating a lack of OCT1-mediated transport. They also failed to inhibit the uptake of ASP+ into OCT1-overexpressing cells by 50 % at concentrations of 100 µM. Analysis of the in silicio data showed that opioids with low logD(7.4) and Pe values are more likely to be morphinans, and substrates or non-inhibitors of OCT1 than opioids with high logD(7.4) and Pe values. It also appears that the structures of synthetic opioids and dextromethorphan-like opioids lend themselves to more potent inhibition of OCT1 than the “classic” morphinan structure of morphine and its congeners. Overall, this study identified at least three substrates and three inhibitors that warrant further research in humans in order to gauge their pharmacological and pharmacokinetic impact on patients with diminished OCT1 transport activity. In addition, our in vitro data suggests five opioids that may be safer to use in patients with diminished OCT1 transport activity.