Comparative biotransformation of morphine, codeine and pholcodine in rat hepatocytes: identification of a novel metabolite of pholcodine

Jairaj, M. and Watson, D.G. and Grant, M.H. and Skellern, G.G. (2002) Comparative biotransformation of morphine, codeine and pholcodine in rat hepatocytes: identification of a novel metabolite of pholcodine. Xenobiotica, 32 (12). pp. 979-986. ISSN 0049-8254 (https://doi.org/10.1080/0049825021000017911)

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Abstract

1. Pholcodine (3-morpholinoethylmorphine), a semi-synthetic alkaloid, is widely used as an antitussive agent. 2. Norpholcodine [7,8-didehydro-4,5alpha-epoxy-3-(2-morpholinoethoxy)morphinan-6alpha-ol] (NP) and pholcodine-N-oxide [1(9a)-dehydro-(4aR,5S,7aR,9cS,12S)-4a,5,7a,8,9,9a-hexahydro-5-hydroxy-12-methyl-3-morpholinoethoxy-1H-8,9,c-(iminoethano)phenanthro[4,5-bcd] furan-12-oxide] (PNOX) were identified in incubations of pholcodine with freshly isolated rat hepatocytes by liquid chromatography/electrospray-mass spectrometry (LC/ESI-MS). 3. Synthesized NP and PNOX were characterized by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. 4. N-oxidation was the major metabolic pathway for pholcodine, producing a previously unreported metabolite. 5. The metabolism of morphine and codeine was also determined using freshly isolated hepatocytes. 6. For morphine, 3-glucuronidation was the major metabolic pathway, whilst for codeine it was dealkylation (O- and N-). 7. Neither morphine nor its metabolites were metabolites of pholcodine. 8. This observation supports the hypothesis that the absence of analgesic activity with pholcodine may be due to less O-dealkylation in vivo. 9. Together with the slow biotransformation of pholcodine (k(met) = 0.021 microM min(-1)) in comparison with morphine (k(met) = 0.057 microM min(-1)) and codeine (k(met) = 0.112 microM min(-1)), the results obtained were consistent with its low addiction potential and suggest that its antitussive efficacy is mediated by the parent drug or one of its metabolites other than morphine.

ORCID iDs

Jairaj, M., Watson, D.G. ORCID logoORCID: https://orcid.org/0000-0003-1094-7604, Grant, M.H. ORCID logoORCID: https://orcid.org/0000-0002-7712-404X and Skellern, G.G.;
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