13.10: Pharmacokinetics in Obese Patients: Drug Metabolism and Excretion

Drug metabolism, a critical process in the liver, involves two primary phases: Phase I reactions and Phase II conjugation. Obesity introduces significant alterations in this metabolic process, primarily due to fatty infiltration of the liver, leading to conditions such as nonalcoholic fatty liver disease (NAFLD). This condition can modify the activities of both Phase I and II enzymes, impacting how drugs are metabolized in obese patients.

Phase I metabolism sees variable effects across Cytochrome P450 (CYP) enzymes in obese individuals. For example, CYP3A4 activity typically decreases, affecting the metabolism of drugs like carbamazepine and triazolam. In contrast, enzymes such as CYP2E1 exhibit increased activity, possibly due to the metabolic demands of fatty acids, ketones, and ethanol, substances often found in higher levels of obesity. Other CYP enzymes, including CYP2D6, CYP1A2, CYP2C9, and CYP2C19, also show variable activity changes, influencing the clearance rates of drugs they metabolize.

Phase II metabolism involves conjugation reactions, with uridine diphosphate glucuronosyltransferase (UGT) playing a significant role. Obesity can enhance the clearance of drugs metabolized via UGT and other Phase II enzymes, indicating an altered metabolic capacity in obese patients. Furthermore, obesity's impact extends to liver blood flow, with high-extraction drugs showing increased clearance in obese subjects, suggesting changes in liver blood flow and enzyme activities.

Renal elimination, another crucial drug elimination pathway, is also influenced by obesity. Processes such as glomerular filtration, tubular secretion, and tubular reabsorption change, often increasing drug clearance in obese patients. This comprehensive overview underlines the complex interaction between obesity and drug metabolism, emphasizing the need for a nuanced understanding of pharmacokinetics in obese patients to optimize therapeutic outcomes.

Obesity can influence drug metabolism and elimination.

Many obese patients exhibit fatty liver, which impacts Phase I and II enzyme activities. Phase I enzymes are usually more affected than Phase II enzymes.

For instance, increased activity is observed in a few Phase I enzymes. These enzyme activities can be influenced by fatty acids, ethanol, and genetic polymorphisms.

In contrast, CYP3A4 shows reduced activity in obese patients.

Drugs metabolized by the major Phase II enzyme, UGT, exhibit increased clearance.

The use of high-extraction drugs in obese individuals showed increased clearance, indicating enhanced liver blood flow.

Additionally, obesity can impact renal drug elimination.

Drugs secreted through glomerular filtration and tubular secretion show increased clearance in obese patients compared to the non-obese group. However, certain drugs, such as lithium, demonstrate lower tubular reabsorption, increasing their clearance in obese individuals.