Background: Morphine-6-glucuronide (M-6-G) and morphine-3-glucuronide (M-3-G) are the two most important metabolites of morphine. Both are pharmacologically active, however, with different effects. M-6-G has been demonstrated capable of inducing anti-nociception and sedation, and M-3-G may induce behavioural excitation and possibly antagonise anti-nociception. Their impact on pharmacodynamics in patients in long-term treatment with oral morphine remains to be settled.
Methods: Forty-two cancer patients treated with oral sustained-release (SR) morphine were assessed for pain, sedation and other side effects related to morphine treatment. Blood samples were analysed for morphine, M-3-G and M-6-G by high-performance liquid chromatography (HPLC).
Results: Significant correlations were found between the daily dose of SR morphine and plasma morphine (M) (r = 0.535, P < 0.001), plasma M-6-G (r = 0.868, P < 0.001) and plasma M-3-G (r = 0.865, P < 0.001). There was no relationship between plasma morphine, M-6-G, M-6-G/M and pain and sedation scores. Seventy-nine percent of the patients suffered from dryness of the mouth, which was the most frequent side effect observed. Patients in this group had higher plasma morphine and M-6-G concentrations than patients who did not suffer from this side effect.
Conclusion: The plasma concentrations of morphine and its metabolites, M-3-G and M-6-G, are significantly correlated to the daily dose of SR morphine. Although M-6-G has analgesic properties, no associations were found between pain and plasma morphine and morphine metabolites. This may be due to the multitudinous factors affecting the dose-effect relationship. Patients with dryness of the mouth had higher concentrations of morphine and M-6-G than patients without this side effect.