Effect of Protein Binding on PK/PD

Effect of protein binding on PK/PD Clinical pharmacology weekly meeting 24 Feb. 2015 Sean Oosterholt Protein Binding • At therapeutic concentrations in plasma, many drugs exist mainly in bound form. • The most important plasma protein in relation to drug binding is albumin. Albumin binds many acidic drugs and a smaller number of basic drugs – Other plasma proteins include ß-globulin, a-acid glycoprotein and Lipoproteins Function of Protein Binding proteins • Serum albumins are important in regulating blood volume by maintaining the osmotic pressure of the blood compartment – They also serve as carriers for molecules of low water solubility this way isolating their hydrophobic nature. • ß-globulins are a subgroup of globulin proteins produced by the liver or immune system – Mostly involved with transport. • α-acid glycoprotein is an alpha-globulin and act as a carrier of basic and neutrally charged lipophilic compounds • The amount of a drug that is bound to protein depends on three factors: – the concentration of the drug – its affinity for the binding sites – the concentration of protein. • As an approximation, the binding reaction can be regarded as a simple association of the drug molecules with a finite population of binding sites, analogous to drug–receptor binding: 퐷 + 푆 ⇌ 퐷푆 free Binding complex drug site • The usual concentration of albumin in plasma is about 0.6 mmol/l (4 g/100 mL). With two sites per albumin molecule, the drug-binding capacity of plasma albumin would therefore be about 1.2 mmol/L. – For most drugs, the total plasma concentration required for a clinical effect is much less than 1.2 mmol/l, so with usual therapeutic doses the binding sites are far from saturated, and the concentration bound [DS] varies nearly in direct proportion to the free concentration [D]. – Under these conditions the fraction bound: [DS]/([D] + [DS]) is independent of the drug concentration. Some drugs, such as tolbutamide work at plasma concentrations at which the binding to protein is approaching saturation This means that adding more drug to the plasma increases its free concentration disproportionately. • Doubling the dose of such a drug can therefore more than double the free (pharmacologically active) concentration. • It is the unbound drug that is pharmacologically active – Both in metabolism in the liver or any pharmacological effect. • The fraction of drug that is free in aqueous solution can be less than 1%, – small differences in protein binding (e.g. 99.5 versus 99.0%) can have large effects on free drug concentration and drug effect. – Differences are common between human plasma and plasma from species used in preclinical drug testing Conditions in which the plasma concentration of major plasma proteins are altered Conditions Change in concentration Albumin hepatic cirrhosis + burns + nephritic syndrome + pregnancy + a-glycoprotein myocardial infarcts - surgery - trauma - rheumatoid arthritis - Drug-drug interactions involving protein binding Before After % increase in Displacement Displacement unbound fraction Drug A % bound 95 90 % unbound 5 10 +100 Drug B % bound 50 45 % unbound 50 55 +10 Influence of drug binding on pharmacokinetic parameters Volume of Distribution: • When concentrations are measured as “total plasma concentration” the volume of distribution can become very small due to protein binding. Example: Warfarin binds for 99% to plasma Albumin • Plasma Concentrations after a 10mg Warfarin dose – Total = 1 mg/L – Bound = 0.99 mg/L – Unbound = 0.01 mg/L • Apparent Volume – Total = 10 mg/1 mg/L = 10 L – Unbound = 10 mg/0.01 mg/L = 1000 L Volume of distribution • Small volumes of distribution – Warfarin: 10L – Gentamicin: 18L • Low volume of distribution does not necessarily mean plasma protein binding. • Both Warfarin and Gentamicin have low volumes of distribution. – Gentamicin is highly ionized and does not cross membranes very well. – The volume of distribution of Gentamicin is close to the physical volume of extracellular fluid. Clearance • Clearance is depends on the free drug concentration and not on protein binding. • However, only the free fraction of drug can be cleared 푄표푟푔푎푛 ∙ 푓푢 ∙ 퐶푙푖푛푡 퐶푙표푟푔푎푛 = 푄표푟푔푎푛 + 푓푢 ∙ 퐶푙푖푛푡 Importance of protein binding In most cases Clinically not very important – Generally less than one third of the drug in the body in bound to plasma proteins even in the most extreme cases. • A change in unbound fraction from 1% to 10% releases less than 5% of the total amount of drug in the body, and produces at most a %5 increase in pharmacologically active unbound drug at the site of action. – Displacement of drugs from the binding site does not happen very often • Drugs that are “displacers” are few and rarely used therapeutically Exception: • IV bolus dose can cause rapid displacement of substances bound to plasma proteins before redistribution happens • Translating between species or from in vitro to in vivo – Protein binding can have big differences between species in vitro to in vivo minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial that will inhibit the visible growth of a microorganism Dashed line: Control growth Filled squares: Drug without albumin Filled circles: Drug with albumin White squares: drug without albumin in a concentration equivalent to in vivo unbound drug concentrations Discussion Plasma protein binding does not automatically result in clearance restrictions. • Example: Ciclesonide: bound for 99% to proteins but clearance is close to hepatic blood flow – Intrahepatic re-equilibration between bound and free drug can occur so fast that the vast majority of drug can be metabolized as the blood crosses the liver. • Same could be said about the pharmacodynamic effect, if association/dissociation is quick Guidelines • EMA: – “The degree of protein binding of the investigational drug should be determined before phase I. If the investigational drug is extensively protein bound to a specific binding site and present at concentrations saturating the binding sites, the risk of displacement of other drugs known to be subject to clinically relevant displacement interactions could be evaluated in vitro at a time point relevant for the clinical development program. If a clinically relevant interaction is predicted based on in vitro data, an in vivo study measuring unbound concentrations could be considered.” .

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