Pharmacokinetics in Geriatric Patients: Effect of Age on Drug Metabolism

Research shows that age does not cause significant differences in CYP450 enzyme levels or activities. The key isozymes—CYP3A, CYP2D6, and CYP2C9—remain relatively stable with aging. However, hepatic drug clearance can be moderately reduced due to decreased liver blood flow and organ size, which affects how efficiently certain drugs are metabolized.

CYP3A substrates like amlodipine and erythromycin show no significant clearance changes in elderly patients. However, atorvastatin's clearance decreases substantially, necessitating dose reduction. This variability suggests that factors beyond enzyme activity—such as hepatic blood flow and liver size—influence drug clearance in geriatric populations.

Phase II drug metabolism reactions—including glucuronidation, sulfation, and acetylation—do not appear to change significantly with age. These conjugation processes remain stable in geriatric patients, meaning age-related alterations in drug metabolism are primarily driven by phase I changes and hepatic structural changes rather than phase II enzyme dysfunction.

Aging reduces both liver blood flow and hepatic organ size, which decreases drug clearance for certain medications. These structural and physiological changes can moderate hepatic drug clearance independent of enzyme activity levels. This is why geriatric patients may require dose adjustments even when CYP450 enzyme levels remain unchanged.

Drugs are metabolized primarily in the liver through phase I and phase II processes. Phase I involves the CYP450 monooxygenase system, while phase II involves conjugation reactions. In geriatric patients, both pathways remain enzymatically active, but reduced hepatic blood flow and liver size compromise overall drug clearance capacity.

Although CYP450 enzyme levels and phase II enzyme activities remain relatively stable with age, hepatic drug clearance can be moderately reduced. This reduction results from decreased liver blood flow and organ size rather than enzyme deficiency. Consequently, drugs like atorvastatin require dose reduction to prevent accumulation and potential toxicity in elderly patients.

Geriatric patients exhibit substantial variability in drug metabolism, which alters both efficacy and safety of pharmacotherapy. This variability stems from individual differences in hepatic blood flow, liver size, and the differential clearance of specific drugs. Some substrates like amlodipine maintain normal clearance while others like atorvastatin show reduced clearance, requiring individualized dosing strategies.