A pharmacodynamic study of morphine and its glucuronide metabolites after single morphine dosing in cancer patients with pain

Eleven morphine naive patients with cancer-related pain were given a single dose of either intravenous morphine (n = 5) or oral morphine (n = 6). Blood sampling was performed over a 24-hr period and serial pain assessments were made using a categorical scale. Plasma samples were analyzed for morphine, morphine-6-glucuronide (M-6-G), morphine-3-glucuronide (M-3-G), and normorphine using high-performance liquid chromatography. In neither the intravenous nor oral group was there a correlation between analgesia duration and the half-lives of morphine and M-6-G. There was no correlation between the time to peak analgesia and time to peak concentration for morphine or M-6-G. There was no significant difference in absolute concentrations of M-6-G or M-3-6 nor in the ratio of M-3-G to M-6-G at peak analgesia versus relapse.