Objectives: The effect of aging on drug metabolism in humans has not yet been completely described.
Methods: Two hundred twenty-six patients with equal histopathologic conditions were investigated. The cytochrome P450 contents in the liver biopsy samples, the plasma antipyrine clearance rates after oral administration and, as an independent control of vitality, serum testosterone levels were determined.
Results: Cytochrome P450 content in subjects from 20 to 29 years of age was 7.2 +/- 2.6 nmol.gm-1, increased during the fourth decade (+7.2%, p = NS), declined after 40 years (-16%, p < 0.01) to a level that remained unaltered up to 69 years, and declined further after 70 years (-32%, p < 0.001). The antipyrine (phenazone) clearance rate in young subjects was 46.4 +/- 18.5 ml.min-1, remained unaltered during the fourth decade, and declined after 40 years by a rate of 0.34 ml.min-1 per year toward old age (-29%, p < 0.001). The half-life in young subjects was 9.5 +/- 2.0 hours and increased after 30 years toward old age (+26%, p < 0.001). The volume of antipyrine distribution, 0.46 +/- 0.12 L.kg-1 in young subjects, decreased after 30 years (-11%). In line with the testosterone content, the decrease in drug metabolism was equal in both sexes.
Conclusion: This study shows a reduction of in vitro and in vivo drug metabolism with age in humans. The data suggest that at least three age groups--young, middle-aged, and elderly--should be included in the evaluation of the pharmacokinetics of a new drug. The reduction of drug metabolism (-30%) after 70 years of age indicates that care is needed in the prescription of drugs for elderly subjects.