Characterization of the pharmacokinetics, behavioral effects and effects on thermal nociception of morphine 6-glucuronide and morphine 3-glucuronide in horses

  • Heather K. Knych
    Correspondence: Heather K Knych, K L Maddy Equine Analytical Pharmacology Laboratory, University of California Davis, School of Veterinary Medicine, 620 West Health Science Drive, Davis, CA 95616, USA.
    K L Maddy Equine Analytical Pharmacology Laboratory, School of Veterinary Medicine, University of California Davis, Davis, CA, USA

    Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA, USA
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  • Kirsten Kanarr
    K L Maddy Equine Analytical Pharmacology Laboratory, School of Veterinary Medicine, University of California Davis, Davis, CA, USA
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  • Yanbin Fang
    K L Maddy Equine Analytical Pharmacology Laboratory, School of Veterinary Medicine, University of California Davis, Davis, CA, USA
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  • Daniel S. McKemie
    K L Maddy Equine Analytical Pharmacology Laboratory, School of Veterinary Medicine, University of California Davis, Davis, CA, USA
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  • Philip H. Kass
    Department of Population Health and Reproduction, School of Veterinary Medicine, University of California Davis, Davis, CA, USA
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      To describe the pharmacokinetics, behavioral and physiologic effects and effects on thermal thresholds of morphine, morphine 6-glucuronide (M6G) and morphine 3-glucuronide (M3G) following administration to horses.

      Study design

      Randomized balanced crossover study.


      A total of seven University-owned horses, five mares and two geldings, aged 3–6 years.


      Horses were treated with a single intravenous dosage of saline, morphine (0.2 mg kg–1), M6G (0.01 mg kg–1) and M3G (0.03 mg kg–1). Blood was collected prior to (baseline) and at several times post administration. Drug and metabolite concentrations were determined by liquid chromatography–mass spectrometry, and plasma pharmacokinetics were calculated. Behavioral observations and physiologic variables (heart rate, step counts, packed cell volume, total plasma protein and gastrointestinal sounds) were determined at baseline and for up to 6 hours. The effects on thermal nociception were determined and thermal excursion was calculated.


      The volumes of distribution were 4.75–10.5, 0.244–0.295 and 0.215–0.356 L kg–1 for morphine, M6G and M3G, respectively. Systemic clearances were 26.8–39.6, 3.16–3.88 and 1.46–2.13 mL minute−1 kg−1 for morphine, M6G and M3G, respectively. Morphine administration resulted in signs of excitation as evidenced by an increase in step counts and subjective behavioral observations, whereas M6G and M3G, based on the same criteria, appeared to cause sedative-like effects. Significant effects on thermal nociception were observed until 4 hours post morphine administration, 1 hour post M6G administration and at various times post M3G administration.

      Conclusions and clinical relevance

      Results of this study provide additional information regarding the use of morphine in horses. Less locomotor excitation and gastrointestinal adverse effects, compared with morphine, coupled with favorable effects on thermal nociception are encouraging for further study of the pharmacodynamics of both M6G and M3G in horses.


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        • Bartlett S.E.
        • Cramond T.
        • Smith M.T.
        The excitatory effects of morphine-3-glucuronide are attenuated by LY274614, a competitive NMDA receptor antagonist, and by midazolam, an agonist at the benzodiazepine site on the GABAA receptor complex.
        Life Sci. 1994; 54: 687-694
        • Combie J.
        • Dougherty J.
        • Nugent E.
        • Tobin T.
        The pharmacology of narcotic analgesics in the horse. IV. Dose and time response relationships for behavioral responses to morphine, meperidine, pentazocine, anileridine, methadone, and hydromorphone.
        J Equine Med Surg. 1979; 3: 377-385
        • Combie J.
        • Shults T.
        • Nugent E.C.
        • et al.
        Pharmacology of narcotic analgesics in the horse: selective blockade of narcotic-induced locomotor activity.
        Am J Vet Res. 1981; 42: 716-721
        • Dönselmann Im Sande P.
        • Hopster K.
        • Kästner S.
        [Effects of morphine, butorphanol and levomethadone in different doses on thermal nociceptive thresholds in horses].
        Tierarztl Prax Ausg G Grosstiere Nutztiere. 2017; 45 ([in German]): 98-106
        • Duke-Novakovski T.
        • Palacios Jimenez C.
        • Fujiyama M.
        • Beazley S.G.
        Plasma histamine concentrations in horses administered sodium penicillin, guaifenesin–xylazine–ketamine and isoflurane with morphine or butorphanol.
        Vet Anaesth Analg. 2021; 48: 17-25
        • Ekblom M.
        • Gårdmark M.
        • Hammarlund-Udenaes M.
        Pharmacokinetics and pharmacodynamics of morphine-3-glucuronide in rats and its influence on the antinociceptive effect of morphine.
        Biopharm Drug Dispos. 1993; 14: 1-11
        • Faura C.C.
        • Olaso J.M.
        • García Cabanes C.G.
        • Horga J.F.
        Lack of morphine-6-glucuronide antinociception after morphine treatment. Is morphine-3-glucuronide involved?.
        Pain. 1996; 65: 25-30
        • Figueiredo J.P.
        • Muir W.W.
        • Sams R.
        Cardiorespiratory, gastrointestinal, and analgesic effects of morphine sulfate in conscious healthy horses.
        Am J Vet Res. 2012; 73: 799-808
        • Guedes A.G.
        • Rudé E.P.
        • Rider M.A.
        Evaluation of histamine release during constant rate infusion of morphine in dogs.
        Vet Anaesth Analg. 2006; 33: 28-35
        • Hamamoto-Hardman B.D.
        • Steffey E.P.
        • Weiner D.
        • et al.
        Pharmacokinetics and selected pharmacodynamics of morphine and its active metabolites in horses after intravenous administration of four doses.
        J Vet Pharmacol Ther. 2019; 42: 401-410
        • Hamamoto-Hardman B.D.
        • Steffey E.P.
        • Seminoff K.
        • et al.
        Preliminary study of the pharmacokinetics, tissue distribution, behavioral and select physiologic effects of morphine 6-glucuronide (M6G) following intravenous administration to horses.
        Can J Vet Res. 2022; 86: 172-180
        • Hanna M.H.
        • Elliott K.M.
        • Fung M.
        Randomized, double-blind study of the analgesic efficacy of morphine-6-glucuronide versus morphine sulfate for postoperative pain in major surgery.
        Anesthesiology. 2005; 102: 815-821
        • Hanna M.H.
        • Peat S.J.
        • Knibb A.A.
        • Fung C.
        Disposition of morphine-6-glucuronide and morphine in healthy volunteers.
        Br J Anaesth. 1991; 66: 103-107
        • Hasselström J.
        • Säwe J.
        Morphine pharmacokinetics and metabolism in humans. Enterohepatic cycling and relative contribution of metabolites to active opioid concentrations.
        Clin Pharmacokinet. 1993; 24: 344-354
        • Janicki P.K.
        Pharmacology of morphine metabolites.
        Curr Pain Headache Rep. 1997; 1: 264-270
        • Klimas R.
        • Mikus G.
        Morphine-6-glucuronide is responsible for the analgesic effect after morphine administration: a quantitative review of morphine, morphine-6-glucuronide, and morphine-3-glucuronide.
        Br J Anaesth. 2014; 113: 935-944
        • Knych H.K.
        • Steffey E.P.
        • McKemie D.S.
        Preliminary pharmacokinetics of morphine and its major metabolites following intravenous administration of four doses to horses.
        J Vet Pharmacol Ther. 2014; 37: 374-381
        • Labella F.S.
        • Pinsky C.
        • Havlicek V.
        Morphine derivatives with diminished opiate receptor potency show enhanced central excitatory activity.
        Brain Res. 1979; 174: 263-271
        • Lipkowski A.W.
        • Carr D.B.
        • Langlade A.
        • et al.
        Morphine-3-glucuronide: silent regulator of morphine actions.
        Life Sci. 1994; 55: 149-154
        • Osborne R.
        • Joel S.
        • Trew D.
        • Slevin M.
        Morphine and metabolite behavior after different routes of morphine administration: demonstration of the importance of the active metabolite morphine-6-glucuronide.
        Clin Pharmacol Ther. 1990; 47: 12-19
        • Osborne R.
        • Thompson P.
        • Joel S.
        • et al.
        The analgesic activity of morphine-6-glucuronide.
        Br J Clin Pharmacol. 1992; 34: 130-138
        • Ouellet D.M.
        • Pollack G.M.
        Effect of prior morphine-3-glucuronide exposure on morphine disposition and antinociception.
        Biochem Pharmacol. 1997; 53: 1451-1457
        • Pasternak G.W.
        • Bodnar R.J.
        • Clark J.A.
        • Inturrisi C.E.
        Morphine-6-glucuronide, a potent mu agonist.
        Life Sci. 1987; 41: 2845-2849
        • Penson R.T.
        • Joel S.P.
        • Bakhshi K.
        • et al.
        Randomized placebo-controlled trial of the activity of the morphine glucuronides.
        Clin Pharmacol Ther. 2000; 68: 667-676
        • Penson R.T.
        • Joel S.P.
        • Clark S.
        • et al.
        Limited phase I study of morphine-3-glucuronide.
        J Pharm Sci. 2001; 90: 1810-1816
        • Prieto-Lastra L.
        • Iglesias-Cadarso A.
        • Reaño-Martos M.M.
        • et al.
        Pharmacological stimuli in asthma/urticaria.
        Allergol Immunopathol (Madr). 2006; 34: 224-227
        • Reed R.
        • Barletta M.
        • Mitchell K.
        • et al.
        The pharmacokinetics and pharmacodynamics of intravenous hydromorphone in horses.
        Vet Anaesth Analg. 2019; 46: 395-404
        • Romberg R.
        • van Dorp E.
        • Hollander J.
        • et al.
        A randomized, double-blind, placebo-controlled pilot study of IV morphine-6-glucuronide for postoperative pain relief after knee replacement surgery.
        Clin J Pain. 2007; 23: 197-203
        • Shimomura K.
        • Kamata O.
        • Ueki S.
        • et al.
        Analgesic effect of morphine glucuronides.
        Tohoku J Exp Med. 1971; 105: 45-52
        • Smith M.T.
        Neuroexcitatory effects of morphine and hydromorphone: evidence implicating the 3-glucuronide metabolites.
        Clin Exp Pharmacol Physiol. 2000; 27: 524-528
        • Smith M.T.
        • Watt J.A.
        • Cramond T.
        Morphine-3-glucuronide - a potent antagonist of morphine analgesia.
        Life Sci. 1990; 47: 579-585
        • Söbbeler F.J.
        • Kästner S.B.
        Effects of transdermal lidocaine or lidocaine with prilocaine or tetracaine on mechanical superficial sensation and nociceptive thermal thresholds in horses.
        Vet Anaesth Analg. 2018; 45: 227-233
        • Stain F.
        • Barjavel M.J.
        • Sandouk P.
        • et al.
        Analgesic response and plasma and brain extracellular fluid pharmacokinetics of morphine and morphine-6-beta-D-glucuronide in the rat.
        J Pharmacol Exp Ther. 1995; 274: 852-857
        • Thomas J.
        • Corson N.I.
        • Meinhold A.
        • Both C.P.
        Neurological excitation in a 6-week-old male infant after morphine overdose.
        Paediatr Anaesth. 2019; 29: 1060-1061
        • van Dorp E.L.A.
        • Kest B.
        • Kowalczyk W.J.
        • et al.
        Morphine-6beta-glucuronide rapidly increases pain sensitivity independently of opioid receptor activity in mice and humans.
        Anesthesiology. 2009; 110: 1356-1363