Swati Jain Academy, Indore, MP, India
For A drug moiety there are several tissue components comprising of blood and extravascular tissues available in the body for biological interactions to take place. The molecules indulged in such biological or physiological interactions are said to be macromolecules as they are large in structures and quite complex depending upon the type involved. These macromolecules are proteins, DNA, adipose tissues etc. thus the protein binding process is defined as a phenomenon of complex formation following the interaction of drug moiety and the protein molecule.
The significance of protein binding of drugs is that - As a protein bound drug is neither metabolized nor excreted hence it is pharmacologically inactive due to its pharmacokinetic and Pharmacodynamic inertness. A bound drug always remains confined to a specific tissue or to a particular site with which it possess a greater affinity. The major benefit is the prolonged duration of action of drug as the protein
Usually weak chemical bonds such as ionic bonds, hydrophobic bonds, hydrogen bonds or Vander wall’s forces are involved in protein binding of drugs and thus are defined as a reversible process. Covalent bond formation though being very rare may result into permanent or so called irreversible binding with a great potential of portending adverse effects like carcinogenicity, terratogenecity, tissue or organ toxicity etc. this has been widely observed in the therapeutic combination of chloroform and paracetamol whose metabolites gradually lead to hepatotoxicity.
Factors affecting Protein Drug Binding.
DRUG RELATED FACTORS
(a) Physiochemical characteristics of the drug.
Lipophilicity is the most desirable physiochemical parameter that is perquisite for protein binding to occur. Also an increase in the lipid content of drug moiety eventually enhances the rate as well as extends of protein binding process. As observed in case of intramuscular. Injection of cloxacillin as attributed to greater lipophilicity displays 95% protein binding.
(b) Concentration of drug in the body.
Alteration in the concentration of drug substance as well as the protein molecules or surfaces subsequently brings alteration in the protein binding process.
(c) Drug’s affinity towards protein/tissue.
This factor entirely depends upon the degree of attraction or affinity the protein molecule or tissues have towards drug moieties. For Digoxin has more affinity for cardiac muscles proteins as compared to that of proteins of skeletal muscles or those in the plasma like HSA.
PROTEIN/TISSUE RELATED FACTORS
(a) Concentration of protein/binding component.
This is the most important tissue related parameter to be given priority. As the human serum plasma proteins constitute the major portion of the plasma proteins, a large number of drugs undergo an extensive binding with them as compared to the concentration of other protein molecule
(b) Number of binding sites on the protein
In association to the concentration of proteins molecules available the number of binding sites available in the protein molecules is also significant. Albumin not only possesses large number of binding sites but also has greater potential of carrying out binding process. Numerous drug exhibit multiple site binding with albumin molecules in plasma like fluocloxacillin, ketoprofen, indomethacin etc .
(a) Competitive binding of drugs.
Displacement interactions are predominant ones among these reactions. In case where two or more drugs have same or identical affinity for a same site then they struggle with one another to bind at the same site. Consider a drug I is bound to a specific site on the molecule and if a second drug called as Drug II is administered now, then the drug meaty having greater affinity towards the bound site would effectively displace th e former drug. This phenomenon is said to be Displacement reaction. The drug which is been removed from its binding site is said to be displaced drug while the one that does the displacement is called as displacer.
The best example for such interactions is the competitive protein binding that occurs between Warfarin and phenylbutazone for HSA, as both are potent binders of HAS, where phenylbutazone is displacer while warfarin is displaced. Clinically such reaction acquire importance when the displaced drug (any) is more than 95% bound to plasma proteins, or occupies small volume of distribution even less than that of 0.15 L/Kg. also when the active drug or the administered pharmacological agent possess narrow therapeutic index. Such situation may also develop in case the displacer drug has greater affinity or at the same time the drug/protein concentration ratio is very high and exhibits a very rapid and significant increase in the plasma concentration of drug.
PATIENT RELATED FACTORS
Patients related factors have their own importance after all the drug has to generate its response on to the administered patient. In this numerous parameters are taken into account like Age, diseased state, pharmacokinetic and Pharmacodynamic characteristics. Protein content and its specific type greatly varies with the age factor. As observed the neonates or newly born babies have very low levels of albumins in the plasma thereby resulting in rebound concentration of drug that is primarily bind to albumin is a major shortcoming. As far as elderly patients are concerned the albumin levels goes down while the concentration of AAG is high enough.
Disease states: the alterations in protein content and thereby the rate and extend of protein binding is greatly influenced by the albumin which is the major drug binding protein. This may ultimately lead to hypoalbuminemia which eventually with the pace of time completely impairs the entire protein drug binding process. For such situations the basic pathological conditions of diseases like trauma, burns, renal, cardiac or hepatic failure etc are largely responsible. Pharmacokinetics as well as Pharmacodynamic of drugs greatly influences the distribution, clearance and thus the biotransformation of drugs to a greater extend. Usually an increased potential of toxicity is observed due to increased concentration of free or the unbound drug.
Protein drug binding is divided into two major categories called as:
(A)Binding of drugs to blood components which includes
(B)Binding to extravascular tissues like proteins, fats, bones etc.
There are several blood components for a drug to be exposed to and undergo protein binding process.
I. Plasma- protein drug binding
Blood components mainly plasma proteins and RBC’s are the major portion of the bulk that actually interacts with the drug moieties as soon as they enter the blood systemic circulation. The plasma proteins being in surplus amounts in the blood undergoes major complexation with drug also this reaction or process is usually reversible due to large variety available in types of proteins. The sequence of
(a) Binding of drugs to human serum albumins.
The human serum albumin also abbreviated as HSA is a plasma protein with a molecular weight of 650000 and comprises of 59% of total plasma protein content present in the blood. It is most abundantly present in plasma with a very high potential of binding drugs. It has been widely observed that there is no equilibrium between the concentration of drug and that of HSA, as the administered dose is usually smaller as compared to that of plasma proteins present. Almost all types of drugs whether acidic, basic or neutral drugs undergo significant HSA binding.
Experimental studies have postulated that any drug can bind to numerous protein binding site .In such circumstances the primary site is the major binding site and the other as the secondary site; for example, for dicoumarol site I is the primary site while site II is secondary. Those drugs having affinity for the same binding sites compete with one another. But the
Many substances (generally endogenous compounds) such as tryptophan, saturated fatty acids, unsaturated fatty acids, bile salts or bilirubin etc all exhibit effective albumin binding . This is due to diversity in the structures of proteins,
the structures of free drug moiety and their affinity towards the protein molecule. The different sites are:
Site I: To this specific site a large population of drugs bind like
Site II: This is actually said to be Diazepam binding site. benzodiazepines, medium chain fatty acids, ibuprofen, ketoprofen, etc. bind extensively at the very site. This is so because due to structural changes the following drugs have high and specific affinity for the site. At both the sites I&II many drugs are known to bind.
Site III: This very protein site is called as Digitoxin binding site Site IV: This is referred as Tamoxifen binding site.
At the sites III & IV very few drugs are known to bind.
(b) Drugs Binding to
The acid glycoprotein exists with a 44,000 molecular weight and comprises of 0.04 to 0.1 g% of the total plasma concentration of proteins. It is actually called as the Oromucoid as it mainly binds to basic drugs like Imipramine, Desipramine, lidocaine, Quinidine, etc.
(c) Binding of drugs to lipoproteins
Lipoproteins are those macromolecules present in plasma which portends a greater capacity of forming hydrophobic bonds. The major reason attributed to this is the larger lipid content present in them. But the plasma concentration of lipoproteins is very limited as compared to that of HSA and AAG.
The lipid core of Lipoproteins has their outer core to be made of apoproteins while the internal core is a potpourri containing triglycerides and esters of cholesterol.
The acidic drugs like diclofenac bind extensively to lipoproteins. Similar response are generated by basic drugs like chlorpromazine and neutral drugs like cyclosporine A. Lipophilic drugs (basic) exhibit greater affinity for such type of protein binding.
Lipoprotein binding is predominant when larger quantity of drugs bind to them and also in circumstances revealing lower plasma levels of HSA and AAG. The process of binding involves the dissolution of drug molecules into the lipid core of the protein and hence the binding capacity is in direct relation with the lipid content. This binding of drugs to lipoproteins is noncompetitive in nature. The lipoproteins have molecular weights ranging from 2 to 3 lakhs. This in turn is proportional to the chemical composition of the molecules. They are classified on the basis of their density. The 4 classes of lipoproteins are observed depending upon their variations in density:
1.Very Low Density Lipoproteins (VLDL)
2.Low Density Lipoproteins (LDL)
3.High Density Lipoproteins (HDL).
(d)Binding of drugs to Globulins
Different types of plasma proteins are labeled here below:
II. Binding of drugs to blood cells.
RBC’s are the major blood cells which rates about 40% of total blood. The red blood corpuscles constitute 95% of the total blood cells concentration in the body. The diameter or the width of the RBC’s is 500 times higher than that of plasma proteins. The 3 compartments which being the major portion of red blood cells to which drugs can bind are:
(a)Hemoglobin: this has molecular weight & structural similarity to that of HAS but the concentration is much higher than of albumins in blood. Examples of drugs that bind are phenytoin, pentobarbital etc.
(b)Carbonic Anhydrase: Carbonic anhydrase inhibitors mainly bind to the site like chlorthaizine.
(c)Red Blood cell membrane: basic drugs like imipramine are known to bind to RBC membrane.Both the hydrophilic and lipophilic drugs can enter RBCs but the lipophilic drugs can do to a greater extend.
TISSUE LOCALIZATION OF DRUGS
The tissue binding of drugs are also very significant processes occurring in the body. Unlike HSA, the body tissues constitute100 times that of HAS i.e; about 40% of the total body weight . Multiple tissue drug binding are feasible. Tissue drug binding is very essential and vital process as it assists in enhancing the apparent volume of distribution for drugs as this follows a direct relation with the ratio of concentration of drug in body
Also it results in prolonged duration of action due to increase in half life reason being the localization of drug at a specific site in the tissues. Studies also reveal that a very large population of drugs no matter acidic, basic or neutral undergoes reversible binding whereas the plasma protein drug binding exhibits
Liver> Kidney> Lung> Muscle.lets have an overview of some tissue drug binding.
1.Liver: Irreversible binding of drugs like paracetamol and their epoxide- metabolites to liver tissues results in hepatotoxicity.
2.Lungs: like imipramine, chlorpromazine and antihistamines accumulation of drugs like imipramine, desipramine or other drugs in lungs eventually leads to congestion in heart or may even produce severe lungs cancer.
3.Kidneys: the protein called as metallothion is widely present in kidneys which have a tendency to undergo complexation with heavy metals such as lead, mercury and cadmium. This gradually paves path for the major renal failures or renal toxicity.
4.Skin: Many drugs are known to accumulate in skin with subsequent reaction with melanin which can ultimately result in skin diseases. Drugs such as chloroquine , phenothiazines are usually involved in this.
5.Eyes: the retinal pigments of the eye also contain melanin. Drugs like chlorquine are responsible for retinopathy as these drugs they interact with the melanin present in the retinal pigments.
6.Bones: bones are made up of calcium and most of the antibiotics mainly like tetracycline exhibits extensive binding to bones and teeth. The permanent
SIGNIFICANCE OF PROTEIN BINDING OF DRUGS.
Targeted drug delivery is the major research area in this era of medical and life sciences. The binding of drugs to lipid containing proteins called as lipoproteins is effectively utilized for controlled and
The best requirement of
The protein drug binding also portends an advantage of efficient drug distribution, absorption and finally prolonged duration of action for longer treatment of chronic diseased conditions.
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