Transcript Plasma Protein Binding

Plasma Protein Binding
2014.1.24
Jun Min Jung
Overview
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Compounds can bind to albumin, α1-acid glycoprotein, or
lipoproteins in blood.
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Binding reduces free drug in solution for penetration into tissue to
reach the therapeutic target or to the liver and kidney for elimination.
Components of Plasma
Blood plasma Consists of:
Water 90%
Plasma Proteins 6-8 %
Electrolytes (Na+ & Cl-) 1%
Other components:
Nutrients (e.g. Glucose and amino acids)
Hormones (e.g. Cortisol, thyroxine)
Wastes (e.g. Urea)
Blood gases (e.g. CO2, O2)
Plasma Protein Binding Fundamentals
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The interaction of drug molecules with plasma proteins is
electrostatic.
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Binding usually is rapid, average equilibrium time of 20ms.
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Most delivery routes are oral dosing, intravenous injection,
intraperitoneal, intramuscular, subcutaneous, transdermal, transfer
drug
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Intravenous (정맥), intraperitoneal (복강), intramuscular (근육),
subcutaneous (피하), transdermal (피부)
Percent of unbound, compared to rat and dog in human, is 10- and 20- fold lower
resulting lower clearance.
Two plasma proteins that are most
responsible for binding of drug
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Albumin which in human is called HSA (human serum albumin)
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Primary functions are maintaining blood pH, osmotic pressure, and
transport molecules
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AGP (acid glycoprotein)
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Primary function is to carry steroids
Albumin
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Albumin (66.5 kDa), single polypeptide chain having 585 aa with 17 disul
fide bonds, is the most abundant protein (60%) in the blood plasma. (3.5-5.
0 g/dl)
Transport: It can bind and transport many diverse molecules and serve as lo
w-specificity transport protein, which include:
◦ a. Metal ions
◦ b. Free fatty acid
◦ c. Bilirubin
◦ d. Bile acid
◦ e. Hormones
Site I
Site II
AGP (acid glycoprotein)
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Consists 181 aa in a single polypeptide chain and MW of 44kDa
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Very high carbohydrate content (45%) and very acidic isoelectric
point around 3
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I.
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I.
Class I drugs – Warfarin (경구용 항응고제), Diazepam (진정제)
One to three binding sites per molecule, saturable
Class II drugs – Indomethacin (소염제)
Binds moderately to HSA, six bindings per HSA molecule
Class III drugs – Phenytoin(항간질제)
Weak HSA binding, many binding sites per molecule
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I.
Class IV-Digitoxin(심장강화제)
Binds to HSA, not saturable
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Class V-Erythromycin(항생물질)
I.
Binds to HSA, can be saturated
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I.
Class VI-Imipramine(신경진정제)
Binds to HSA, AGP, lipoproteins(HDL, LDL, VLDL)
Consequence of Chirality on PPB
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PPB affects compound distribution, metabolism, and renal
clearance
Propranolol (부정맥 치료제)
Warfarin (경구용 항응고제)
Disopyramide (항부정맥 치료제)
Verapamil (칼슘길항제)
Indacrinone (이뇨제)
PPB Effects

Free Drug Hypothesis
I.
Drug-plasma protein complex cannot permeate through cell membranes by passive
transcellular permeation
II.
Free drug passes through membranes to reach tissues
III. Free drug molecules are available for liver metabolism and renal excretion
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Two complementary factors of PBB
I.
Extent of binding at equilibrium (percent bound or percent unbound in plasma[fu,plasma],
equilibrium dissociation constant Kd)
II.
Rate of association and dissociation (association and dissociation rate constant Kd and Ka)
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If the drug molecules are
1. Highly bound (low % unbound)
2. Tightly bound (slow dissociation)
then effects of PPB can be as follows
I.
Retain drug in plasma
II.
Restrict distribution of drug into target tissue
III.
Decrease metabolism, clearance, prolong (half life)
IV.
Limit brain penetration
V.
Require higher loading doses but lower maintenance doses
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High % bound, slow dissociation Restrictive
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Fast kinetics(high dissociation rate) Permissive
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In short, high binding to plasma protein (high % bound) alone does
not itself determine the consequences of plasma binding
Impact of PPB on Distribution
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PPB can have either ‘Restrictive’ or ‘Permissive’
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PPB also can be restrictive of BBB permeation
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Binding keeps in bloodstream resulting reduced permeation
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Vd – Volume of distribution
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Vplasma – Volume of plasma in the body
Vtissue – volume of tissue in the body
Fu - fraction unbound in the plasma
Fu,t – fraction unbound in the tissue
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Effect of PPB on Clearance & Pharmacology
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Propranolol - >90% PPB, permissive of 90% liver extraction
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Warfarin – 99% PPB, restrictive of 0.3% liver extraction
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PPB reduces vascular receptor occupancy  No diffusion barriers
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For example, angiotensin II decreased by adding albumin
Structure Modification Strategies for PPB
Plasma Protein Binding Methods
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Literature and commercial in silico predictors of plasma protein
binding are available
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Equilibrium dialysis is the gold standard method. Ultrafiltration,
ultracentrifugation, microdialysis, and plasmom resonance methods
are used
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Red blood cell binding is important to measure for some
compounds
Equilibrium Dialysis Method
Ultrafiltration Method
Problems