Quantitative Pharmacokinetics Drug-Protein Binding Dr. Chalet Tan

Learning Objectives proteins that bind drugs and types of protein binding factors affecting drug-protein binding effect of protein binding on drug distribution Required reading: Tozer & Rowland, Introduction to Pharmacokinetics and Pharmacodynamics, Chapter 5, p83-89.

Relevance of Drug-Protein Binding to Drug Therapy protein-bound drugs are pharmacologically inactive protein-bound drugs does not cross cell membranes, affecting: distribution metabolism excretion Free-form of drug is more important. Pharmacologically relevant.

Types of Drug-Protein Binding reversible weak chemical bonds such as hydrogen bonds or van der Waals forces occurs to most drugs irreversible covalent chemical bonds accounts for certain toxicities of drugs and carcinogens e. g. high doses of acetaminophen Completely reversible between proteins and drugs, because they have very weak bonds, non-covalent. CYP3A4 metabolism of APAP. Leads to reactive metabolites. Binds with proteins and leads to cell death. Irreversible binding DOES occur sometimes.

Major Drug-Binding Proteins in Plasma TABLE 5-3 albumin: binds mostly to acidic drugs a1-acid glycoprotein: binds mostly to basic drugs lipoproteins: binds mostly to neutral drugs Tissue binding is much less available for our evaluation. Know very little about tissue-protein binding. Albumin is most abundant, a1 is 2nd most abundant, 3rd is lipoproteins, (binds to lipophilic drugs). Albumin is most by far, 500-700 vs, 9-23. Know the concentration of albumin in plasma.

The fraction of unbound drug in plasma varies widely among drugs Highly bound drugs are fu <0.1. Fraction unbound is less than 10% in highly bound drugs. fu is a constant in most clinical drugs. Level is much lower than albumin or a1-glycoprotein. [Protein] >>> [drug] Some drugs are close to the albumin. When that happens the fu becomes non-constant. For now, safe to assume FU is a constant. Figure 5-13

Methods for Studying Drug-Protein Binding equilibrium dialysis ultracentrifuge gel chromatography electrophoresis For the ease of chemical analysis, total plasma concentrations of drugs (instead of unbound drug concentrations) are most commonly measured. Cu = C x fu

Drug-Protein Binding Affects Drug Distribution Figure 5-14

Drug-Protein Binding Affects Drug Distribution Ct, drug concentration in tissues. At any moment, drug amount in body can be expressed in these equations. Equation at bottom explaisn to you what contributes to a volume of drug distribution of a certain drug. Explains why V of different drugs vary so much. Different drug can have different access to different PORTION of the same tissue. Complex. Cu = CuT, concentration unbound in plasma = concentration unbound in tissue. at equilibrium, Cu = CuT

Drug-Protein Binding Affects Drug Distribution V: volume of distribution Vp: volume of the plasma VT: volume of the tissue fu : free fraction of the drug in the plasma fuT : free fraction of the drug in the tissue 1 calculation question about this. VT of a drug is dependent on its physicochemical properties.

V and Volumes of Body Water For a 70-kg healthy individual, extracellular plasma 3 L interstitial fluids 13 L intracellular 26 L Total 42 L 3 Liters, VERY important number. Volume of drugs is close to 3 L because it’s very slow, and outside of vasculature. Might not be able to cross membrane, and stay in extracellular space. Never use this equation to calculate volume of distribution. Divide by Co. For 2 compartment model calculate for steady state. You calculate volume using the equations from Dr. D’Souza. Q1: Hydrophilic so not entering cell, use the 13 L of interstitial fluid V rarely correlates to a real volume. V of drugs with MW > 70 KD is close to 3 L. V of small MW drugs that are neither bound in plasma nor in tissue is between 16 – 42 L.

Factors Affecting Drug-Protein Binding physicochemical properties concentration protein displacement by co-administered drugs e. g. increased free warfarin level when co-administered with salicylic acid (displacer) Warfarin therapeutic level is much lower than salicylic acid.

Displacement may occur if: the drug (e. g. warfarin) is highly protein-bound; and the displacer (e. g. salicylic acid) occupies most of the binding site.

Factors Affecting Drug-Protein Binding pathophysiological conditions of the patients Liver is an important organ for protein synthesis. When Liver has problem, plasma-protein level will decrease.

V of propranolol is proportional to its fu in plasma. control patients with chronic hepatic disease Increase in Volume of distribution, leads to increase in fu (fraction unbound in plasma). Figure 5-15

Plasma Protein Binding Determines the Distribution of Many Drugs Table of example drugs that show their fu, and increase in Vd.

V of Some Drugs Does Not Depend on Plasma Protein Binding Sometimes there is no coorelation, unclear relationship. In this case, it must be FuT, the tissues binding. Infer that reason vinblastine has higher Vd than Vincristine, vinblastine must have a higher tissue binding number.

Furosemide and amiodarone have fu between 0. 02-0 Furosemide and amiodarone have fu between 0.02-0.04, an acidic and a basic drug respectively. Yet, the former has a V of 10 L and the latter 7000 L. What contributes to this difference? 18 18