13.6: Pharmacokinetics in Geriatric Patients: Effect of Age on Drug Metabolism

Geriatric patients show significant variation in how their bodies process medications, which can change how effective and safe treatments are. The liver is the primary organ where drug metabolism occurs, involving two main types of chemical reactions: phase I and II. Phase I metabolism is driven by the cytochrome P450 enzyme system, which includes key types such as CYP3A, CYP2D6, and CYP2C9. Research indicates that while aging doesn't notably alter the levels or activity of these enzymes, it can lead to a moderate decrease in the liver's ability to clear drugs.

Specific drugs processed by the CYP3A enzyme, like amlodipine and erythromycin, don't show a significant change in clearance rates in the elderly. On the other hand, atorvastatin, another drug processed by this enzyme, does have a reduced clearance in older adults, suggesting a need to lower its dosage. The reactions involved in phase II metabolism, which include processes like glucuronidation, sulfation, and acetylation, remain stable despite a person's age.

Additionally, the natural changes that come with aging, such as a decrease in liver blood flow and organ size, can affect how efficiently the liver can clear certain medications. This underscores the importance of monitoring and adjusting drug dosages in the aging population to ensure therapeutic effectiveness and avoid potential toxicity.

Geriatric patients exhibit substantial variability in drug metabolism, altering the efficacy and safety of pharmacotherapy.

Drugs are metabolized primarily in the liver by phase I and phase II processes.

Phase I involves the CYP450 monooxygenase system, with some key isozymes being CYP3A, CYP2D6, and CYP2C9.

Studies show that age does not cause significant differences in CYP enzyme levels and activities; however, hepatic drug clearance can be moderately reduced.

For instance, CYP3A substrates such as amlodipine and erythromycin showed no significant clearance changes in older patients. However, atorvastatin’s clearance decreased, necessitating its dose reduction.

Phase II drug metabolism reactions, such as glucuronidation, sulfation, and acetylation, also did not appear to change significantly with age.

Lastly, aging can reduce liver blood flow and size, decreasing drug clearance for certain drugs.