GENERAL PRINICPLES OF DRUG ACTION 13 In equilibrium,the rate of the forward reaction of an agonist A reversibly bound to its s proportional to the concentration of A and R,and the proportionality constant is rate of association =k.IAllRI Similarly,the rate of the backward reaction,in which the dissociates aga is proportional to the rate of dissociation=[AR] At equilibrium the rate of the forward reaction equals the rate at which existing AR complexes dissociate i.e.AlR]=ARI.In other words,within a certain period of time the same number of molecules A will bind to and dissociate from the receptor.At equilibrium the dissociation constant K can be described as follows: IARI (ii)Rate theory.Rate theory was proposed by Paton and Rang in 1965.According to this theory the most important factor in determining drug action is th e rate at which drug receptor combination takes place.The rate theory can be explained by the formula Rate of receptor occupation=- (iii)Induced-fit theory.This theory states that after combination,the substrate in duces a change in conformation of the enzyme.leading to an enzymatically active orientation of groups.Ex:Acetylcholine may interact with the regulating protein and alter the normal forces.Macromolecular perturbation theory and activation-aggregasion theories are the ex- tension of induced fit theory
GENERAL PRINICPLES OF DRUG ACTION 13 C-8—N-CHEMI\CHE2-1.PM5 In equilibrium, the rate of the forward reaction of an agonist A reversibly bound to its receptor R is proportional to the concentration of A and R, and the proportionality constant is denoted by k1 rate of association = k1[A][R] Similarly, the rate of the backward reaction, in which the agonist–receptor complex dissociates again, is proportional to the concentration of the AR complex rate of dissociation = k2 [AR] At equilibrium the rate of the forward reaction equals the rate at which existing AR complexes dissociate i.e. k1[A][R] = k2[AR]. In other words, within a certain period of time the same number of molecules A will bind to and dissociate from the receptor. At equilibrium the dissociation constant K can be described as follows: K = k k 1 2 = [A][R] [AR] (ii) Rate theory. Rate theory was proposed by Paton and Rang in 1965. According to this theory the most important factor in determining drug action is the rate at which drug receptor combination takes place. The rate theory can be explained by the formula: Rate of receptor occupation = k2 1 + KA [A] (iii) Induced-fit theory. This theory states that after combination, the substrate induces a change in conformation of the enzyme, leading to an enzymatically active orientation of groups. Ex: Acetylcholine may interact with the regulating protein and alter the normal forces. Macromolecular perturbation theory and activation-aggregasion theories are the extension of induced fit theory
3 Physico-chemical Properties of Organic Medicinal Agents At the most fundamental level,the ability of a chemical compound to elicit a pharmaco logical/therapeutic effect is related to the influence of various physical and chemical(physic chemical)properties of the chemical substance on the biomolecule(s)that it interacts with Among the most pharmacologically influencial physico-chemical properties of organic medici- nal agents are: 1.Solubility 2.Partition coefficient 3.Dissociation constant(pKa) 4.Hydrogen bonding 5.Molar refractivity (MR) 6.Ionization 7.Drug shape 8.Complexation 9.Surface activity 10.Protein binding 11.Bioisosterism SOLUBILITY The solubility of a substance at a given temperature is defined as the concentration of the dissolved solute,which is in equilibrium with the solid solute.Solubility depends on the solute and solvent as well as temperature,pressure,and pH.The solubility of a substance is the ratio of these rate constants at equilibrium in a given solution. The solubility of an organic medicinal agent may be expressed in terms of its affinity/ philicity or repulsion/phobicity for either an aqueous (hydro)or lipid (lipo)solvent. PPT The atoms and molecules of all organic substances are held together by various types of bonds (e.g.London forces,hydrogen bonds,dipole-dipole,etc.).These forces are intricately involved in solubility because it is the solvent-solvent,solute-solute,and solvent-solute inter- actions that govern solubility 14
3 Physico-chemical Properties of Organic Medicinal Agents At the most fundamental level, the ability of a chemical compound to elicit a pharmacological/therapeutic effect is related to the influence of various physical and chemical (physicochemical) properties of the chemical substance on the biomolecule(s) that it interacts with. Among the most pharmacologically influencial physico-chemical properties of organic medicinal agents are; 1. Solubility 2. Partition coefficient 3. Dissociation constant (pKa) 4. Hydrogen bonding 5. Molar refractivity (MR) 6. Ionization 7. Drug shape 8. Complexation 9. Surface activity 10. Protein binding 11. Bioisosterism. The solubility of a substance at a given temperature is defined as the concentration of the dissolved solute, which is in equilibrium with the solid solute. Solubility depends on the solute and solvent as well as temperature, pressure, and pH. The solubility of a substance is the ratio of these rate constants at equilibrium in a given solution. The solubility of an organic medicinal agent may be expressed in terms of its affinity/ philicity or repulsion/phobicity for either an aqueous (hydro) or lipid (lipo) solvent. KSOLUBILITY SOL PPT = k k The atoms and molecules of all organic substances are held together by various types of bonds (e.g. London forces, hydrogen bonds, dipole-dipole, etc.). These forces are intricately involved in solubility because it is the solvent-solvent, solute-solute, and solvent-solute interactions that govern solubility. 14
PHYSICO-CHEMICAL PROPERTIES OF ORGANIC MEDICINAL AGENTS 15 H,0H,0 H.O H,0H,0 The most important intermolecular attractive forces (bonds)that are involved in the solubilization process are; 1.Vander waals attraction (induced dipole).They are weakest intermolecular force (0.5-1.0 kcal/mole)which occur between nonpolar groups (e.g.hydrocarbons).They are highly distance and temperature dependent. electr egative nents ar are str and ele cally betweer ing is a specific exampleo ng and 3.Ionic Bonding.Ionic bond is electrostatic attraction between cations and anions These ionic attractions arecommo in inorganic compounds and salts of organic molecules and are relatively strong(5 kcal/mole). Probably the most important factor in the t their ability to ionize.The degree of ionization of a drug is by far the best predictor of solubility for most compounds.which are acidic or basic. 4.Ion-Dipole Bonding.This is electrostatic force between a cation/anion and a dipole g(1-5k mn and is1o endent.Ion mn ater. e re of both philic d cina agent i n of rophilicfe n organic med es wit 1 e the extent of interaction of organic medicinal e,wh and/or aqueous phases Methods to Improve the Solubility of Drugs 1.Structural Modifications (a)One method to increase solubility of a drug is to alter the chemical structure of the nereaesohbitybyinOTorgnee molecule The addition of polar rboxylic acids.keto nd o ogen bonding and the interaction with water Anothe ion can be to 1 ce thy e s.A od i p ng H The ethyl est r hy the melti the int and in ses solubility. ary amine.The saddition re
PHYSICO-CHEMICAL PROPERTIES OF ORGANIC MEDICINAL AGENTS 15 C-8—N-CHEMI\CHE3-1.PM5 KSOL KPPT H O2 H O2 H O2 H O2 H O2 The most important intermolecular attractive forces (bonds) that are involved in the solubilization process are; 1. Vander Waals attraction (induced dipole). They are weakest intermolecular forces (0.5–1.0 kcal/mole) which occur between nonpolar groups (e.g. hydrocarbons). They are highly distance and temperature dependent. 2. Dipole-Dipole Bonding. These forces occur when electronegative elements are attached to carbon. They are stronger (1.0 to 10 kcal/mole) and occur electrostatically between electron deficient and electron rich atoms (dipoles). Hydrogen bonding is a specific example of this bonding and serves as a prime contributor to hydrophilicity. 3. Ionic Bonding. Ionic bond is electrostatic attraction between cations and anions. These ionic attractions are common in inorganic compounds and salts of organic molecules and are relatively strong (5 kcal/mole). Probably the most important factor in the prediction of water solubility in ionic drugs is their ability to ionize. The degree of ionization of a drug is by far the best predictor of solubility for most compounds, which are acidic or basic. 4. Ion-Dipole Bonding. This is electrostatic force between a cation/anion and a dipole. It is relatively strong (1-5 kcal/mole) and is low temperature and distance dependent. Iondipole bonding is an important attraction between organic medicinal agent and water. Hence, the relative solubility of an organic medicinal agent is a function of the presence of both lipophilic and hydrophilic features within its structure, which serve to determine the extent of interaction of the organic medicinal agent with lipid and/or aqueous phases. Methods to Improve the Solubility of Drugs 1. Structural Modifications (a) One method to increase solubility of a drug is to alter the chemical structure of the molecule. The addition of polar groups like carboxylic acids, ketones and amines can increase solubility by increasing hydrogen bonding and the interaction with water. (b) Another structural modification can be to reduce intramolecular forces. An example of structural modification to enhance solubility by this method is methyl dopa (solubility ~10 mg/ml) and methyl dopate (solubility 10-300 mg/ml) depending on pH. The addition of the ethyl ester to methyldopa reduces the intramolecular hydrogen bond between the carboxylic acid and primary amine. Therefore, this addition reduces the melting point and increases solubility
16 PRINCIPLES OF ORGANIC MEDICINAL CHEMISTRY (c)Use of Co-solvents.Another method to increase solubility is the use of co-solvents. The co-solvents can increase solubility by several orders of magnitude.Some com monly used co-solvents are propylene glycol,polyethylene glycol,ethanol and sorbitol. The addition of a co-solvent can increase solubility of hydrophobic molecules by re- ducing the dielectric constant of the solvent.Some problems with the use of co-sol- vents are precipitation of the drug with dilution of solvent mixture and tissue dam age or pain upon injection.This dilution occurs after administration of the drug into the body. (d)Employing surfactants.Surfactants can also be used to enhance solubility.A surfactant or surface active agent is amphiphilic,meaning it has polar end(the cir- cular head)and a nonpolar (the tail).When a surfactant is placed in water it will form micelles.A nonpolar drug will partition into the hydrophobic core of the micelle and will get solubilized. (e)Complexation.There are many types of complexing agents.Complexation relies or relatively weak forces such as London forces,hydrogen bonding and hydrophobic interactions.As the concentration of complexing agent is increased,so is the solubility, up to a point.In some cases however,the complex can precipitate out from solution as the concentration of complexing agent is increased. Importance of Solubility (a)The concept of solubility is not only imp armacist because it ns the ration of solution before it can be absorbed by the body o or have any biological ac ous and lin e i st establis 10ra0 at and 05 the oili prope agen (hy ofth organi tural featu eous or lipid media. oting w res r PARTITION COEFFICIENT se is als resen city.The east y can ypartit on co equilibrium constant of drug concentrations nhas =drug and for"ionizable"molecules(acids,bases,salts),where alpha(a)is the degree of ionization in aqueous solution.It is basically a constitutive property
16 PRINCIPLES OF ORGANIC MEDICINAL CHEMISTRY C-8—N-CHEMI\CHE3-1.PM5 (c) Use of Co-solvents. Another method to increase solubility is the use of co-solvents. The co-solvents can increase solubility by several orders of magnitude. Some commonly used co-solvents are propylene glycol, polyethylene glycol, ethanol and sorbitol. The addition of a co-solvent can increase solubility of hydrophobic molecules by reducing the dielectric constant of the solvent. Some problems with the use of co-solvents are precipitation of the drug with dilution of solvent mixture and tissue damage or pain upon injection. This dilution occurs after administration of the drug into the body. (d) Employing surfactants. Surfactants can also be used to enhance solubility. A surfactant or surface active agent is amphiphilic, meaning it has polar end (the circular head) and a nonpolar (the tail). When a surfactant is placed in water it will form micelles. A nonpolar drug will partition into the hydrophobic core of the micelle and will get solubilized. (e) Complexation. There are many types of complexing agents. Complexation relies on relatively weak forces such as London forces, hydrogen bonding and hydrophobic interactions. As the concentration of complexing agent is increased, so is the solubility, up to a point. In some cases however, the complex can precipitate out from solution as the concentration of complexing agent is increased. Importance of Solubility (a) The concept of solubility is not only important to a pharmacist because it governs the preparation of solutions as a dosage form but also because a drug must be in solution before it can be absorbed by the body or have any biological activity. (b) Drugs must be in solution to interact with receptors. Drugs have some degree of solubility in both aqueous and lipid compartments. In order for a chemical compound to dissolve in a particular solvent/medium the compound must establish attractive forces between itself and molecules of the solvent. Hence, it is possible to estimate the solubility properties of an organic medicinal agent (hydrophilic vs. lipophilic) by examining the structure of the organic medicinal agent and noting whether its structural features promote affinity for aqueous or lipid media. The ability of a drug to dissolve in a lipid phase when an aqueous phase is also present, often referred to as lipophilicity. The lipophilicity can be best characterized by partition coefficient. Partition coefficient can be defined as the equilibrium constant of drug concentrations for “unionizable” molecules in the two phases: P = [drug] [drug] lipid water and for “ionizable” molecules (acids, bases, salts), where alpha (α) is the degree of ionization in aqueous solution. It is basically a constitutive property.
PHYSICO-CHEMICAL PROPERTIES OF ORGANIC MEDICINAL AGENTS 17 [drughipid P=(1-a)druglater Naturally,the partition coefficient is one of the several physicochemical parameters influencing drug transport and distribution.The contribution of each functional groups and their structural arrangement help to determine the lipophilic or hydrophilic character of the molecule.Partition coefficient majorly influence drug transport characteristics;the way in which the drugs reach the site of action from the site of application (e.g.injection site, gastrointestinal tract,and so forth).Since the blood distributes drugs,they must penetrate and traverse many cells to reach the site of action. Compound Lipid/water partition coefficient Ethanol 0.03 Morphine 0.40 Barbitone 1.40 phenobarbitone 5.90 Phenobarbitone has a high lipid/water partition coefficient of 5.9.Thiopentone sodium has a chloroform/water partition coefficient of about 100,soit is highly soluble in lipid and can easily pass through blood brain barrier. Partition coefficient measurement since partition coefficients are difficult to measure in living systems.they are usually determined in vitro using 1-octanol (n-octanol)as the lipid phase and a phosphate buffer of pH 7.4 as the aqueous phase.The partition coefficient,P is dimensionless. and its logarithm (log P)is widely used as the mea re of lipophilicity.The LogP is dete termined by following meth ods: (i)Log P by shake flask.The shake flask method is the oldest and most tedious way of measuring log Pvalues.The UV absorbance of an aqueous solution is measured before and after being shaken with a known volume of octanol.The method is messy and smelly but is the only method that can be used in cases of very low Log P values.One advantage of the method is that the appearance of compound in the octanol may be checked against the disappearance from the aqueous phase to see if any surface effects have occurred.Some molecules may form effective surfactants.It is very important to pre-saturate the solvents in prolonged shake flask experiments.The experiment must be performed over 3 days or more to ensure equilib- rium is reached.although the actual time taken in doing the experiment is about .5 day. (ii)Log P by HPLC.HPLC may be used to estimate Log P values.Compounds with known Log P's ar cted to a C18 hase hpic colur e.Unk dict Log P.Strictly this e is alid fo nolecules. than Charged far more complex rete tion ple p
PHYSICO-CHEMICAL PROPERTIES OF ORGANIC MEDICINAL AGENTS 17 C-8—N-CHEMI\CHE3-1.PM5 P = [drug] (1 drug] lipid − α water )[ Naturally, the partition coefficient is one of the several physicochemical parameters influencing drug transport and distribution. The contribution of each functional groups and their structural arrangement help to determine the lipophilic or hydrophilic character of the molecule. Partition coefficient majorly influence drug transport characteristics; the way in which the drugs reach the site of action from the site of application (e.g. injection site, gastrointestinal tract, and so forth). Since the blood distributes drugs, they must penetrate and traverse many cells to reach the site of action. Compound Lipid/water partition coefficient Ethanol 0.03 Morphine 0.40 Barbitone 1.40 Phenobarbitone 5.90 Phenobarbitone has a high lipid/water partition coefficient of 5.9. Thiopentone sodium has a chloroform/water partition coefficient of about 100, so it is highly soluble in lipid and can easily pass through blood brain barrier. Partition coefficient measurement Since partition coefficients are difficult to measure in living systems, they are usually determined in vitro using 1-octanol (n-octanol) as the lipid phase and a phosphate buffer of pH 7.4 as the aqueous phase. The partition coefficient, P is dimensionless, and its logarithm (log P) is widely used as the measure of lipophilicity. The LogP is determined by following methods: (i) Log P by shake flask. The shake flask method is the oldest and most tedious way of measuring log P values. The UV absorbance of an aqueous solution is measured before and after being shaken with a known volume of octanol. The method is messy and smelly but is the only method that can be used in cases of very low Log P values. One advantage of the method is that the appearance of compound in the octanol may be checked against the disappearance from the aqueous phase to see if any surface effects have occurred. Some molecules may form effective surfactants. It is very important to pre-saturate the solvents in prolonged shakeflask experiments. The experiment must be performed over 3 days or more to ensure equilibrium is reached, although the actual time taken in doing the experiment is about 0.5 day. (ii) Log P by HPLC. HPLC may be used to estimate Log P values. Compounds with known Log P’s are injected onto a C18 reverse phase HPLC column to create a calibration curve. Unknown compounds are then injected to predict Log P. Strictly this technique is only valid for neutral molecules. Charged molecules have far more complex retention behaviour than simple partition