Morphine kinetics in cancer patients

Abstract

Oral and i.v. morphine kinetics were studied in 7 patients with cancer who needed continuous treatment with morphine because of severe chronic pain. Single oral (20-30 mg) and i.v. (4 mg) doses were given on separate days, followed by repetitive blood sampling for morphine analysis by gas chromatography. Volume of distribution ranged from 0.95-3.75 l/kg and serum clearance from 5.0-16.1 ml/min per kg. Oral morphine in doses that were more than 5 times the i.v. dose gave concentrations (at 10 and 120 min after dose) between 38 and 112 ng/ml. During the 0.25-8 h period after the oral dose serum concentrations were higher than after the i.v. dose. There was a variation in oral bioavailability of 15-64% and an interindividual variation in terminal half-life from 58-467 min. These data warrant careful adjustment of the oral dose under close supervision of the patient at the onset of therapy.