Background: Conjugation with glucuronic acid represents the major route of biotransformation of morphine. The glucuronides morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) are eliminated via the kidneys. Therefore, chronic renal failure should affect the disposition of M3G and M6G. Numerous patients undergoing long-term continuous ambulatory peritoneal dialysis (CAPD) require pain treatment with morphine. There are only limited data available about the disposition of morphine and its active metabolites M6G and M3G in patients on CAPD. We therefore investigated the pharmacokinetics of morphine and its metabolites in CAPD patients.
Methods: This was a single intravenous dose pharmacokinetic study in 10 CAPD patients (1 female, 9 male, age 31-69 years). Morphine-hydrochloride (Mo) (10 mg) was administered intravenously. Serum, urine, and dialysate samples were collected during 24 h. GC-MS-MS and HPLC-MS methods were used to quantify respectively morphine and morphine glucuronides.
Results: While systemic clearance of morphine (1246+/-240 ml/min) was in the range observed in patients with normal kidney function, both M3G and M6G showed substantial accumulation. The area under the concentration-time curve (AUC) ratio of M3G:Mo (33.4+/-7.1) and of M6G:Mo (12.2+/-3.2) was 5.5 and 13.5 times higher than in patients with normal kidney function. Renal clearances of morphine, M3G, and M6G (morphine 3.0+/-2.5 ml/min; M3G 3.9+/-2.2 ml/min; M6G 3.6+/-2.2 ml/min) and dialysate clearances (morphine 4.1+/-1.3 ml/min; M3G 3.2+/-0.7 ml/min; M6G 3.0+/-0.8 ml/min) were extremely low. Therefore the accumulation of M6G and M3G is readily explained by kidney failure which is not compensated by CAPD.
Conclusion: Accumulation of M3G and M6G is due to the substantially lowered clearance by residual renal function and peritoneal dialysis. In view of the accumulation of potential active metabolites, subsequent investigations have to assess the frequency of side-effects in patients on CAPD.