Developmental pharmacokinetics of morphine and its metabolites in neonates, infants and young children

Background. Descriptions of the pharmacokinetics and metabolism of morphine and its metabolites in young children are scant. Previous studies have not differentiated the effects of size from those related to age during infancy. Methods. Postoperative children 0-3 yr old were given an intravenous loading dose of morphine hydrochloride (100 μg kg-1 in 2 min) followed by either an intravenous morphine infusion of 10 μg h-1 kg-1 (n=92) or 3-hourly intravenous morphine boluses of 30 μg kg-1 (n=92). Additional morphine (5 μg kg-1) every 10 min was given if the visual analogue (VAS, 0-10) pain score was ≥4. Arterial blood (1.4 ml) was sampled within 5 min of the loading dose and at 6, 12 and 24 h for morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). The disposition of morphine and formation clearances of morphine base to its glucuronide metabolites and their elimination clearances were estimated using non-linear mixed effects models. Results. The analysis used 1856 concentration observations from 184 subjects. Population parameter estimates and their variability (%) for a one-compartment, first-order elimination model were as follows: volume of distribution 136 (59.3) litres, formation clearance to M3G 64.3 (58.8) litres h-1, formation clearance to M6G 3.63 (82.2) litres h-1, morphine clearance by other routes 3.12 litres h-1 per 70 kg, elimination clearance of M3G 17.4 (43.0) litres h-1, elimination clearance of M6G 5.8 (73.8) litres h-1. All parameters are standardized to a 70 kg person using allometric 3/4 power models and reflect fully mature adult values. The volume of distribution increased exponentially with a maturation half-life of 26 days from 83 litres per 70 kg at birth; formation clearance to M3G and M6G increased with a maturation half-life of 88.3 days from 10.8 and 0.61 litres h-1 per 70 kg respectively at birth. Metabolite formation decreased with increased serum bilirubin concentration. Metabolite clearance increased with age (maturation half-life 129 days), and appeared to be similar to that described for glomerular filtration rate maturation in infants. Conclusion. M3G is the predominant metabolite of morphine in young children and total body morphine clearance is 80% that of adult values by 6 months. A mean steady-state serum concentration of 10 ng ml-1 can be achieved in children after non-cardiac surgery in an intensive care unit with a morphine hydrochloride infusion of 5 μg h-1 kg-1 at birth (term neonates), 8.5 μg h-1 kg-1 at 1 month, 13.5 μg h-1 kg-1 at 3 months and 18 μg h-1 kg-1 at 1 year and 16 μg h-1 kg-1 for 1- to 3-yr-old children.

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