Relative bioavailability of controlled-release oral morphine sulfate during naltrexone blockade

The effect of naltrexone hydrochloride on the bioavailability of 60 mg controlled-release oral morphine sulfate in normal volunteers was determined using a randomized, 2-way crossover, analytically blinded study design. Although naltrexone did not qualitatively alter the concentration-time curve for controlled-release morphine, the area under the plasma morphine concentration-time curve from 0-24 h (AUC0-24) was significantly greater (p < 0.01) for morphine given with naltrexone (265 ng x h/ml) than for morphine given alone (215 ng x h/ml). Compared to morphine given alone, the apparent absorption half-life of morphine was decreased from 0.94-0.58 h (p = 0.01) and Cmax was increased from 28.17 ng/ml to 32.26 ng/ml (p = 0.04) during naltrexone blockade, whereas the Tmax and apparent elimination half-life of morphine were not significantly affected. The minimal differences in morphine bioavailability indicate naltrexone may be useful in comparative bioavailability studies of high-dose opioids in opioid-naive normal volunteers.