Morphine kinetics in cancer patients

Area of Science:

• Pharmacology
• Clinical Pharmacy
• Oncology

Background:

• Severe chronic pain in cancer patients often requires continuous opioid therapy.
• Morphine is a common analgesic, but its pharmacokinetic variability necessitates careful dosing.
• Understanding morphine's absorption, distribution, metabolism, and excretion is crucial for effective pain management.

Purpose of the Study:

• To compare the pharmacokinetics of oral and intravenous morphine in cancer patients.
• To determine the oral bioavailability and interindividual variability of morphine.
• To provide data for optimizing morphine dosing strategies in chronic pain management.

Main Methods:

• Seven cancer patients receiving continuous morphine for severe chronic pain were studied.
• Single oral (20-30 mg) and intravenous (4 mg) morphine doses were administered on separate days.
• Serial blood samples were collected for morphine analysis using gas chromatography.

Main Results:

• Volume of distribution ranged from 0.95 to 3.75 L/kg; serum clearance ranged from 5.0 to 16.1 mL/min/kg.
• Oral morphine doses >5x IV dose resulted in concentrations of 38-112 ng/mL at 10 and 120 min.
• Oral bioavailability varied significantly (15%-64%), with interindividual terminal half-life ranging from 58 to 465 min.

Conclusions:

• Oral morphine exhibits substantial pharmacokinetic variability in cancer patients.
• Dosing of oral morphine requires careful, individualized adjustment and close patient supervision.
• These findings highlight the importance of therapeutic drug monitoring for oral morphine therapy.