Protein Binding Dr. Abid Yousaf, PhD Assistant Professor Faculty of Pharmacy University of Central Punjab Lahore. Protein Binding A drug's efficiency may be affected by the degree to which it binds to the proteins within the body. The less bound a drug is, the more efficiently it can traverse cell membranes or diffuse. Common blood proteins that drugs bind to are human serum albumin, lipoprotein, glycoprotein, and α, β‚ and γ globulins. A drug in blood exists in two forms: bound and unbound. Depending on a specific drug's affinity for plasma protein, a proportion of the drug may become bound to plasma proteins, with the remainder being unbound. Types of Protein Binding (A) On the basis of protein type: 1.Plasma Protein Binding, e.g., binding of drug with albumin, globulin, prothrombin, thrombin, fibrinogen etc. 2.Tissue Protein Binding, e.g., binding of drug with lipoproteins, surfaces of RBCs, Cell membranes etc. Acidic drugs, such as NSAIDs, Sulfonamides, warfarin etc., mostly bind to plasma albumins. Basic drugs, such as Antihistamines, Antidepressants, propranolol etc., frequently bind with α1-acid glycoproteins. Types of Protein Binding Types of Protein Binding (B) On the basis of bonding: 1- Reversible Protein Binding: In this case, drug is bound to protein with weak bonding such as hydrogen bonding, van der waal forces etc. A chemical equilibrium will exist between the bound and unbound states, such that: Protein + drug ⇌ Protein-drug complex Types of Protein Binding Particularly, it is the unbound fraction which exhibits pharmacologic effects. It is also the fraction that may be metabolized and/or excreted. For example, the "fraction bound" of the fenofibrate is 99%. This means that of the amount of fenofibrate in the blood, 99% is bound to plasma proteins. The remaining 1% (the fraction unbound) is the fraction that is actually active and may be excreted. 2- Irreversible Protein Binding: In this case, drug is bound to protein with strong bonding such as ionic, covalent or coordinate covalent bond. Protein + drug Protein-drug complex Protein Binding Types of Protein Binding (C) On the basis of specificity: 1- Non-Specific: Some ionized substances become loosely and nonspecifically bound to plasma proteins. Because, at the pH of blood, the proteins carry net positive or negative charges which allow binding with oppositely charged ions. This represents a very less important binding as coulombic forces provide only weak bonds, which can be easily dispersed. Types of Protein Binding 2- Specific: In this case, specific drugs bind to specific proteins at particular binding sites (receptors). This is more important type of binding which involves chemical groups of drugs and specific binding sites on proteins, e.g., drug binding with plasma albumin. The specific binding sites are in every way analogues to drug receptors. Plasma albumin has 4 different binding sites viz. Site I (Warfarin binding site), Site II (Diazepam binding site), Site III (Digoxin binding site) & Site IV (Tamoxifen binding site). kinetics of Protein Binding Suppose reversible drug-protein binding, Ka P + D ⇌ PD complex Kd Rate of a reaction depends upon the concentration of the reactants. This is explained by the Law of Mass Action. Guildberg and Waage gave this law in 1869. It states that “rate of a reaction is directly proportional to the product of the molar concentrations or active masses of the reactants”. kinetics of Protein Binding Thus according to this law: Rate of drug-protein complex formation α [P][D] Rate of drug-protein complex formation = Ka [P][D] & Rate of drug-protein complex dissociation α [PD] Rate of drug-protein complex dissociation = Kd [PD] At equilibrium, Rate of complex formation = Rate of complex dissociation Ka [P][D] = Kd [PD] kinetics of Protein Binding kinetics of Protein Binding kinetics of Protein Binding kinetics of Protein Binding kinetics of Protein Binding kinetics of Protein Binding kinetics of Protein Binding Factors Affecting Protein Binding Protein binding is affected mainly because of drug related and patient related factors. 1- Lipophilicity Highly lipophilic drug tends to localize in adipose tissues. Neutral unionized drugs bind more efficiently to lipoproteins. Lipoproteins tend to bind lipophilic drugs by dissolving them in their lipid core. Factors Affecting Protein Binding 2- Acidic & Basic Nature Acidic drugs, such as NSAIDs, Sulfonamides, warfarin etc., bind more frequently with plasma albumins. On the other hand, basic drugs, such as Antihistamines, Antidepressants, propranolol etc., bind with α1-acid glycoproteins. 3- Concentration of Drug The concentration of drug in the body also influence the protein binding, e.g., as concentration of α1-acid glycoproteins is much less, lidocaine at therapeutic concentration will saturate the binding. Factors Affecting Protein Binding 4- Affinity of drug for a Particular Protein Some drugs will demonstrate high affinity for a particular site, e.g., Digoxin has high affinity for cardiac muscle protein. 5- Number of Binding Sites on Protein Molecule Also number of binding sites on protein play important role in protein binding, e.g., albumin has more number of binding sites compared to α1-acid glycoproteins which has fewer binding sites. Thus, because of multiple binding sites, drug bind to albumin will require more concentration compared to drug binding to α1-acid glycoproteins. Clinical significance of Protein Binding 1- Physiological Significance For example, transferrin (a β-globulin) acts as a vehicle for transfer of Fe++ to bone-marrow. Fe++ are essential for haemoglobin formation, but if not bound to protein, can produce toxic effects. 2- Pharmacological Significance Only free form of the drug (not bound to protein) can produce intended pharmacological effects. The drug bound to protein cannot produce its effects, cannot permeate across membranes for distribution, metabolism & excretion. This may result in accumulation of drug in the body.
