A PHARMACODYNAMIC BASIS FOR CHANGES IN DRUG ACTIVITY ASSOCIATED WITH AGING IN THE ADULT

The information available in the literature suggests that age has an effect on various organs and tissues responsible for drug handling and that these changes are reflected in altered drug activity. It has been suggested that drug absorption is slower and less complete in the older animal and that once absorbed there is delayed distribution of the agent. Plasma levels of the drug may achieve higher levels in the older animal due to the delayed excretion and slower metabolic inactivation. These changes will also result in a longer duration of activity. The response to a drug is dependent not only on the amount of drug made available to and in equilibrium with the receptor site but also on the state of the receptor site itself. There is evidence to suggest that with increasing age there are a fewer number of active receptor sites available, and thus, the sensitivity to a drug may be depressed or enhanced depending on the action of the drug, whether it acts in a positive manner or produces its effect as a result of inhibition. The over-all response to a drug is a reflexion not only of the sum of these influences but of the ability of the organism to adjust to the internal change induced by the drug. Drug effects may be more pronounced in the older animal because of limited homeostatic mechanisms. In the review it was pointed out that these observations are based on very limited experimental data and that additional work is necessary to expand our knowledge of the effect of age on drug activity.