Activation of G-Proteins by Morphine and Codeine Congeners: Insights to the Relevance of O- and N-Demethylated Metabolites at μ- and δ-Opioid Receptors

Abstract

Phenotypic differences in analgesic sensitivity to codeine (3-methoxymorphine) results from polymorphisms in cytochrome P450 -2D6, which catalyzes O-demethylation of codeine to morphine. However, O-demethylation reportedly is not required for analgesic activity of the 7,8-saturated codeine congeners dihydrocodeine, hydrocodone, and oxycodone. This study determined the potency and efficacy of these compounds and their demethylated derivatives to stimulate -and ␦-opioid receptor-mediated G-protein activation using agonist-stimulated guanosine 5Ј-O-(3-[35 S]thio)triphosphate ([ 35 S]GTP␥S) binding. Results showed that 7,8-saturated codeine congeners were more efficacious than codeine in activating -receptors, but only dihydrocodeine was more efficacious at ␦-receptors. Hydrocodone and oxycodone were ϳ10-fold more potent than codeine and dihydrocodeine at either receptor. Morphine-like compounds with a 3-hydroxy group were ϳ30-to 100-fold more potent than their 3-methoxy analogs at the -receptor, and these compounds generally exhibited greater efficacy (e.g., morphine produced 2-fold greater maximal stimulation than codeine). Removal of the N-methyl group did not affect efficacy or potency of codeine congeners to activate -receptors, whereas this modification generally increased efficacy but decreased potency of morphine congeners. At the ␦ receptor, morphine congeners showed greater potency and structuredependent differences in efficacy compared with codeine congeners, whereas removal of the N-methyl group had effects similar to those observed at the -receptor. These results demonstrate that 7,8-saturated codeine congeners are more efficacious than codeine, which may explain their lack of requirement for 3-O-demethylation in vivo. Nonetheless, because all 7,8-saturated codeine congeners were significantly less potent than their morphine derivatives, further research is needed to understand the relationship between metabolism and in vivo activity of these compounds.The opioid analgesic, codeine (3-methoxymorphine), undergoes several metabolic routes including N-and O-demethylation, as well as glucuronidation (Dayer et al., 1988;Mikus et al., 1991;Caraco et al., 1996;Yu et al., 2002). It is well established that codeine must be O-demethylated to morphine to produce analgesia in both humans (Chen et al., 1991) and rats (Mikus et al., 1991;Cleary et al., 1994). In humans, cytochrome P450 -2D6 (CYP-2D6) catalyzes this reaction, and individuals with dysfunctional allelic variants of this gene are phenotypically described as poor metabolizers (PMs) and are less sensitive to codeine. Individuals expressing multiple copies of active CYP-2D6 are extensive metabolizers (EMs) and are more sensitive to codeine. EMs can exhibit the PM phenotype, however, when codeine is coadministered along with a CYP-2D6 inhibitor such as quinidine. The requirement for O-demethylation is not surprising given that the reported K i values for binding to -opioid receptors is 200 times higher for codeine than for morphine (Chen et al., 1991...

Effects of μ-Opioid Receptor Agonists in Assays of Acute Pain-Stimulated and Pain-Depressed Behavior in Male Rats: Role of μ-Agonist Efficacy and Noxious Stimulus Intensity

“…The m-agonists tested in this study were selected because they vary along a continuum of efficacy to activate G protein signaling pathways coupled to m-receptors (Selley et al, 1997(Selley et al, , 1998Thompson et al, 2004). Despite these differences in efficacy at the receptor level, all six m-agonists were fully effective to block both acid-stimulated stretching and acid-induced depression of ICSS.…”

“…First, a relatively low-intensity noxious stimulus was used (intraperitoneal administration of 1.8% lactic acid in a volume of 1.0 ml/kg), and antinociception was assessed for a range of m-agonists that vary in their relative efficacies at m-receptors. Specifically, effects were examined for methadone, fentanyl, morphine, hydrocodone, buprenorphine, and nalbuphine [listed in order from highest to lowest efficacy as determined by in vitro functional assays of agonist-stimulated 59-O-(3-thiotriphosphate) binding] (Selley et al, 1997(Selley et al, , 1998Thompson et al, 2004). We predicted that, consistent with their clinical effectiveness (Brunton et al, 2011), all m-opioid receptor agonists would block acid effects in both assays.…”

Characterization of 6α- and 6β-N-Heterocyclic Substituted Naltrexamine Derivatives as Novel Leads to Development of Mu Opioid Receptor Selective Antagonists

“…For these experiments, a CHO cell line engineered to express relatively low levels of the MOR (0.4 pmol per mg of membrane protein was used), 21 so that NAP and NAQ would act as pure antagonists and readily allow determination of parallel rightward shifts of the DAMGO curve. This MOR expression level was similar to a previously reported MOR B max values (0.2À0.4 pmol/mg) from multiple regions of mouse brain.…”

In Vivo Activity of Norhydrocodone: An Active Metabolite of Hydrocodone

“…In vitro norhydrocodone has been shown to be moderately lower in potency, with similar efficacy, to hydrocodone at mand d-opioid receptors (Thompson et al, 2004). While binding studies and in vitro pharmacodynamic data raise the possibility that norhydrocodone might possess in vivo activity and contribute to the opioid receptor-mediated effects of hydrocodone, to our knowledge, the in vivo activity of norhydrocodone has not been thoroughly examined.…”

“…While norhydrocodone has been suggested to be an inactive metabolite (Smith, 2009), one in vitro study examining Gprotein activation by N-demethylated codeine congeners compared to the parent molecules demonstrated that norhydrocodone had lower potency but similar efficacy as hydrocodone for m-and d-opioid receptor-mediated G-protein activation (Thompson et al, 2004). Since these data indicate that norhydrocodone activates opioid receptor signaling and, as noted above, is a major metabolite of hydrocodone, we hypothesized that norhydrocodone may possess in vivo activity and might contribute to the pharmacodynamics of hydrocodone.…”