MORPHINE AND MORPHINE-6-GLUCURONIDE PLASMA-CONCENTRATIONS AND EFFECT IN CANCER PAIN

The relationships between plasma morphine and metabolite (M3G and M6G) concentrations and analgesic efficacy were investigated in an open st udy of 39 cancer pain patients receiving chronic oral morphine therapy with either morphine sulfate solution or controlled-release morphine tablets. There were no differences in morphine metabolite kinetics, or analgesic efficacy between equivalent doses of conventional or contro lled(l-release formulations. The increase in morphine plasma concentra tion after a dose (1 hr for normal release, 2 hr for controlled releas e) was correlated significantly with the dose of morphine (r = 0.914, P < 0.001). There was a significant reduction in pain intensity (P < 0 .05) and increase in pain relief (P < 0.001) after the dose of morphin e administration, when compared with the predose score. One-half of th e patients had mild and tolerable adverse effects. Patients were class ified by mean pain relief between doses as having optimal, moderate, o r poor pain control. No simple relationship was found between morphine plasma concentration and pain control. Morphine plus M6G concentratio ns in the ''optimal control'' group (751.2 +/- 194 nmol/L), however, w ere more than twice those found in the ''moderate control'' group (276 .9 +/- 41.9 nmol/L) (P < 0.05), and no patient in the moderate control group had a morphine Plus M6G concentration greater than 405 nmol/L. These results support the importance of M6G in morphine analgesia. For these hospitalized patients, there appeared to be a therapeutic range of morphine plus M6G plasma concentrations for optimal pain control w ith a lower limit of 400 nmol/L predose.