Biotransformation of drugs occurs mainly in the liver, but also in the kidney, lung, intestine, blood and adrenal. Drug-metabolising enzymes are deficient in neonates or have a low activity. Two groups of biotransforming enzymes exist in the liver: phase 1 reactions such as oxidation, reduction and hydrolysis, and phase 2 reactions like conjugations (glucuronidation and sulphation). Later, between the age of 6 months and puberty, oxidative P450 enzymes have
a two times higher activity than in adults. During puberty, the activity of these enzymes decreases towards normal adult levels. Enzymes involved in glucuronidation, sulphation and acetylation reach their maturity at 3 or 4 years of age. Due to the different activities of the enzyme groups, the biotransformation of drugs can vary a lot at diverse ages.
Example: increased toxicity of chloramphenicol in neonates due to low glucuronidase activity.
Most metabolic activity in neonates is reduced.
The metabolic rate of a drug for a neonate and a small child of 3 years of age can vary greatly.