Protein binding of drugs

Protein binding of drugs and the different factors affecting protein binding

1. Protein binding of drugs Mr. Naresh Gorantla , M.Pharm .., ( Ph.D ) Assoc professor, Balaji college of Pharmacy,Anantapuramu . [email protected]

2. C on t e nt s Introduction Mechanisms of protein drug binding Classes of protein drug binding. Binding of drug to blood components. Plasma proteins Blood cells Binding of drug to extravascular tissue protein Factors affecting protein drug binding Significance of protein/tissue binding of drug

3. INTRODUCTION P ha r m a c ol o gi c al ly in a cti ve due t o i ts p h a r m a co k in e ti c a nd Pharmacodynamic inertness. Protein + drug ⇌ Protein-drug complex Protein binding may be divided into: Intracellular binding. Extracellular binding. The phenomenon of complex formation of drug with protein is called as Protein-Drug binding . The proteins are particularly responsible for such an interaction. 3

4. MECHANISMS OF PROTEIN DRUG BINDING : Binding of drugs to proteins is generally of reversible &irreversible. Reversible generally involves weak chemical bond such as: Hydrogen bonds Hydrophobic bonds Ionic bonds Van der waal’s forces. Irreversible drug binding, though rare, arises as a result of covalent binding and is often a reason for the carcinogenicity or tissue toxicity of the drug.

5. 6 Drug Ab s or b ed Free drug in plasma Protein bound drug in plasma Receptor Pharmacologic and Therapeutic response No pharmacologic response, therefore no therapeutic action Figure : Protein-Drug Binding

6. Binding of drug falls into 2 classes: Blood C o mp on ent Plasma p r o t eins Bl o od cells Extra vascular tissues Proteins Fats Bo n es

7. 1 . BINDING OF DRUG TO BLOOD COMPONENTS A. Plasma protein-drug binding: - • • The binding of drugs to plasma proteins is reversible. The extent or order of binding of drug to plasma proteins is: Albumin › ὰ 1-Acid glycoprotein › Lipoproteins › Globulins .

8. 1. B i n d i n g of d r u g t o h u m a n ser u m Al b u m in . It is the m o s t a b u n dant pla s m a pro t e i n (59 % ), having M. W . of 65,000 with large drug binding capacity. Both endogenous compounds such as fatty acid, bilirubin as well as drug binds to HSA. Four diff. sites on HSA for drug binding . Site I: warfarin & azapropazone binding site. Site II: diazepam binding site. Site III: digitoxin binding site. Site IV: tamoxifen binding site . 32 Site 1 Site 2 Site 3 Site 4 Drug binding site on Human Serum Albumin Warfarin binding site Diazapam binding site Digitoxin binding site Tamoxifen binding site

9. 2. Binding of drug to α1 -Acid glycoprotein: (orosomucoid) It has a M.W. 44,000 and plasma conc. range of 0.04 to 0.1 g%. It binds to no. of basic drugs like imipramine, lidocaine, propranolol, quinidine. Ty p e s 3. Binding of drug to Lipoproteins: Lipoproteins are amphiphilic in nature. Mol wt: 2-34 Lacks dalton. Lipid core composed of: Inside: triglyceride & cholesteryl esters. Outside: Apoprotein .

10. 4 . Binding of drug to Globulins It mainly binds to endogenous substances. In plasma several globulins have been identified. ἀ 1 -globulin:- (transcortin) corticosteroid binding globulin. ἀ 2 -globulin :- (ceruloplasmin) it binds vit. A, D, E, K & cupric ions . ᵝ 1 -globulin:- ( transferrin ) it binds to ferrous ions. ᵝ 2 -globulin :- binds to carotinoids. ᵞ -g l o b u l i n :- b i n d s s p e c if i c a lly to anti g ens .

11. B. BINDING OF DRUG TO BLOOD CELLS In blood , 40% of blood cells of which major component is RBC (95%). The RBC is 500 times in diameter as the albumin. The rate & extent of entry into RBC is more for lipophilic drugs. The RBC comprises of 3 components. Ha e m oglo b i n : I t ha s a M . W . of 64,50 Da l . D rugs li k e phenytoin, pentobarbital bind to haemoglobin. Carbonic anhydrase : Carbonic anhydrase inhibitors drugs are bind to it like acetazolamide & chlorthalidone. Cell membrane: Imipramine & chlorpromazine are reported to bind with the RBC membrane.

12. 2 . BINDING OF DRUG TO EXTRAVASCULAR TISSUE PROTEIN S Importance : 1. It increases apparent volume of distribution of drug. 2 . localization of a drug at a specific site in body. Factor affecting: lipophilicity, structural feature of drug, perfusion rate, pH differences. B inding orde r : L i v er › K id n ey s › Lun g s › M u scles Tissue Binding of 1 . L i ver Irrev e r s i ble b i nding of Epoxid e s of Halogenated Hydrocarbon & Paracetamol. 2. L ungs Basic drugs: Imipramine, Chlorpromazine, & AntiHistaminics.

13. C o nt… Tissue Binding of 3.Kidney Metallothionin protein binds to Heavy metals & results in Renal accumulation and toxicity. 4.Skin Chloroqui n e & P h e noth i az i ne binds t o Melanin. 5.Eye Chloroquine & Phenothiazine also binds to Eye Melanin & results in Retinopathy. 6.Hairs Arsenicals, Chloroquine, & Phenothiazine. 7.Bones Tetracycline(yellow d iscoloration of teeth), Lead(replaces Ca & cause brittleness) 8.Fats Lipophilic drugs (thiopental), Pesticides (DDT) 9.Nucleic Acid Chloroquine & Quinacrine.

14. F AC T O R S AFFE C T IN G PR O T E I N DRU G B I N DIN G 1 . D r ug- r ela t ed fa c to r s :- Physicochemical characteristics of drug – Protein binding is directly related to the lipophilicity of drug. An increase in lipophilicity increases the extent of binding. Concentration of drug in the body – The extent of protein-drug binding can change with both changes in drug as well as protein concentration. Affinity of a drug for a particular binding component – Alteration in the concentration of drug substance as well as the protein molecules or surfaces subsequently brings alteration in the protein binding process.

15. 2. Protein related factors :- Physicochemical characteristics of protein or binding agent – Lipoproteins & adipose tissue tend to bind lipophilic drug by dissolving them in their lipid core. Concentration of protein or binding component – The amount of several proteins and tissue components available for binding, changes during disease state. Number of binding sites on the binding agent – Albumin has a large no. of binding sites as compared to other proteins and is a high capacity binding component.

16. 3. Drug interactions :- a) Competition between drugs for the binding site s [ Displacement interactions ] :- A drug-drug interaction for the common binding site is called as displacement interaction. D.I. can result in unexpected rise in free conc. of the displaced drug which may enhance clinical response or toxicity. Even a drug metabolite can affect D.I. If the drug is easily metabolisable or excretable, it’s displacement results in significant reduction in elimination half life.

17. b) Competition between drug & normal body constituents :- The free fatty acids are known to interact with a no. of drugs that binds primarily to HAS. the free fatty acid level increase in, Physiological (fasting) Pathological (diabetes, myocardial infraction ) Pharmacologically induced (after heparin or caffeine administration ) .

18. c. Allosteric changes in protein molecule:- The process involves alteration of the protein structure by the drug or it’s metabolite thereby modifying its binding capacity. e.g. aspirin acetylates lysine fraction of albumin thereby modifying its capacity to bind NSAIDs like phenylbutazone . 4. Patient-related factors : a. Age : Neonates: Low albumin content: More free drug. Young infants: High dose of Digoxin due to large renal clearance. Elderly: Low albumin: So more free drug. b. Intersubject variability: Due to genetics & environmental factors.

19. c. Disease states:- Disease Influence on plasma protein Influence on protein drug binding Renal failure ↓ Albumin content ↓ binding of acidic drugs; neutral and basic drugs are un affected Hepatic failure ↓ Albumin synthesis ↓ binding of acidic drugs; and binding of basic drugs is normal or ↓ depending on AAG levels Inflamatory states i.e,truama surgery etc… ↑AAG levels ↑ binding of basic drugs; neutral and acidic drugs are un affected

20. SIGNIFICANCE OF PROTEIN/TISSUE BINDING OF DRUG 1. Absorption- As we know the conventional dosage form follow first order kinetics. So when there is more protein binding then it disturbs the absorption equilibrium . 2. Distribution- A pr o t e in b o und drug i n p ar t i cu l a r do e s n ot c ro s s t he BBB , t h e placental barrier, the glomerulus. Thus protein binding decreases the distribution of drugs. 3. Metabolism- P r o t e i n b in d ing d e crea s e s t he m e t a bo l i s m of d r ugs & e nh anc e s t h e biological half life. Only unbound fraction get metabolized. e.g. Phenylbutazone & Sulfonamide.

21. 4. Elimination • • • Only the unbound drug is capable of being eliminated. P r ot e i n bin d i ng pr e v en t th e en t r y of dru g t o the m e t a bol i zi ng o r g a n (liver ) & to glomerulus filtration. e.g. Tetracycline is eliminated mainly by glomerular filtration. 5. Systemic solubility of drug Lipoprotein act as vehicle for hydrophobic drugs like steroids, heparin, oil soluble vit. 6 . Drug action- Protein binding inactivates the drugs because sufficient concentration of drug can not be build up in the receptor site for action. e.g. Naphthoquinone

22. 7. Sustain release- The complex of drug protein in the blood act as a reservoir & continuously supply the free drug. e.g. Suramin sodium-protein binding for antitrypanosomal action. 8. Diagnosis- The chlorine atom of chloroquine replaced with radiolabeled I- 131 can be used to visualize- melanomas of eye & disorders of thyroid gland . 9. Drug Targeting- The binding of drugs to lipoproteins can be used for site-specific delivery of hydrophilic drugs.

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