Drug Distribution as One-Compartment Model and Elimination by Nonlinear Pharmacokinetics: Overview

Linear elimination follows first-order kinetics at all doses, where the elimination rate is proportional to drug concentration. Nonlinear elimination occurs when metabolizing enzymes become saturated at higher doses, causing the elimination rate to become independent of concentration. Mixed elimination involves both processes simultaneously, combining first-order and Michaelis-Menten kinetics to describe the saturable process.

At low doses, metabolite formation follows first-order kinetics with constant elimination rates. At higher doses, metabolizing enzymes become saturated, inducing nonlinear metabolite formation where elimination capacity plateaus. This saturation creates a dose-dependent relationship between drug administration and plasma concentration, exemplified by niacin extended-release tablets used to treat hyperlipidemia.

Niacin undergoes extensive first-pass metabolism through multiple parallel pathways that are dosing-rate specific. One pathway involves glycine conjugation to form nicotinuric acid, while another produces nicotinamide adenine dinucleotide and additional metabolites. Multiple doses of niacin extended-release tablets saturate these conjugation pathways, establishing a nonlinear relationship between dose and plasma drug concentration.

A single drug can be metabolized into different metabolites through parallel pathways, each with distinct saturation characteristics. For example, sodium salicylate forms both a glucuronide and a glycine conjugate, with conjugation rates depending on available glycine. These competing pathways create complex elimination profiles where saturation of one pathway shifts metabolism toward others.

Intravenous infusion follows zero-order input with nonlinear elimination, maintaining constant drug delivery while saturable elimination processes occur. Oral administration involves first-order absorption combined with nonlinear elimination. These different input-elimination combinations require distinct mathematical equations to predict drug behavior and plasma concentrations over time.

The elimination equation combines first-order and Michaelis-Menten kinetics, where the first-order rate constant k represents the sum of all first-order elimination processes. The second term signifies the saturable process governed by enzyme kinetics. Together, these components describe mixed elimination where both linear and nonlinear processes occur simultaneously.

Glycine conjugation is a saturable metabolic process where the rate depends on available glycine concentration. When glycine becomes limiting, conjugation capacity plateaus, contributing to nonlinear elimination kinetics. This substrate-dependent saturation exemplifies how metabolic cofactors influence drug elimination and create dose-dependent pharmacokinetic profiles.