Partition coefficient
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The partition coefficient is the ratio of concentrations of a compound between two immiscible solvents at equilibrium. For drug delivery, the lipophilic/hydrophilic balance affects the rate and extent of drug absorption, as biological membranes are lipoidal in nature and passively absorbed drugs' rate of transfer is directly related to their lipophilicity. Drug classes are determined based on factors like pKa and LogP values, with class affecting absorption. Parameters like pH, pKa, and lipid solubility of un-ionized drugs influence the gastrointestinal absorption of weakly acidic or basic drugs.
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Partition coefficient
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Physicochemical properties of drug
DEFINITION:
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 bio molecule that it interacts with.
1)Physical Properties
Physical property of drug is responsible for its action 2)Chemical Properties
The drug react extracellularly according to simple chemical reactions like neutralization, chelation, oxidation etc.
Various Physico-Chemical Properties are,
Solubility Partition Coefficient
Dissociation constant Hydrogen Bonding Ionization of Drug Redox Potential Complexation Surface activity Protein binding Isosterism
1. Solubility:
• The solubility of a substance at a given temperature is defined as the concentration of the dissolved solute, which is in equillibrium with the solid solute.
• Solubility depends on the nature of solute and solvent as well as temperature , pH & pressure.
• The solubility of drug may be expressed in terms of its affinity/philicity or repulsion/phobicity for either an aqueous or organic solvent.
The atoms and molecules of all organic substances are held together by various types of bonds (e.g. hydrogen bond, dipole –dipole, ionic bond etc.)
These forces are involved in solubility because it is the solvent-solvent, solute-solute, solvent-solute interactions that governs solubility.
Methods to improve solubility of drugs
1) Structural modification (alter the structure of molecules) 2) Use of Cosolvents (Ethanol, sorbitol,PPG,PEG)
3) Employing surfactants 4) Complexation
Importance of solubility
1. Solubility concept is important to pharmacist because it govern the preparation of liquid dosage form and the drug must be in solution before it is absorbed by the body to produce the biological activity.
2. Drug must be in solution form to interact with receptors.
Physicochemical propeties of drug
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Physiochemical properties of drug
1)Physical Properties
Physical property of drug is responsible for its action
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The drug react extracellularly according to simple chemical reactions like neutralization, chelation, oxidation etc.
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Partition coefficient
- 1. Prepared by- Hasan Hussain
- 2. In the physical sciences, a partition coefficient (P) is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium. The partition coefficient generally refers to the concentration ratio of un-ionized species of compound, whereas the distribution coefficient refers to the concentration ratio of all species of the compound (ionized plus un- ionized).
- 3. For drug delivery, the lipophilic/hydrophilic balance has been shown to be a contributing factor for the rate and extent of drug absorption. Since biological membrane is lipoidal in nature the rate of drug transfer for passively absorbed drugs is directly related the lipophilicity of the molecule.
- 5. Class I ◦ Example: metoprolol, paracetamol Class II ◦ Example: glibenclamide, bicalutamide, ezetimibe, aceclofenac Class III ◦ Example: cimetidine Class IV ◦ Example: Bifonazole, amphotericin B, furosemide, acetazolamide, ritonavir, paclitaxe.
- 6. pKa < 3 is for a strong acid 3<pKa<7 is for a weak acid 7<pKa<11 is for a weak base pKa>11 is for a strong base
- 7. Paracetamol- It has a pKa value of 9.5 and is therefore largely unionized over the physiological range of pH. LogP - 0.46
- 8. Aceclofenac LogP - 2.17 pKa. : 4.7
- 9. Cimetidine is weakly basic with the pKa values reported as 6.93. LogP- 0.4
- 10. Acetazolamide pKa- 7.93 LogP -0.26
- 11. Since majority of the drugs are either weakly acidic or basic, their solution in water contain ionised and unionised species. The drug undergoes dissociation in their aqueous solutions. The extent of ionisation depends upon the pH of the solution. For example a weak acid will ionise mostly in alkaline pH while a weakly basic will ionise in acidic medium. The unionised drug is more lipid soluble and thus readily absorbed.
- 12. Hence gastrointestinal absorption of a drug can be improved if the extent of ionisation is reduced. The absorption of weakly acidic or basic can be influenced by the following: 1. pH at the site of absorption. 2. Dissociation constant 3. Lipid solubility of unionised drugs
- 13. The contents of the stomach have pH ranging from 1-3. Weakly acidic drugs having pKa value less than 4.3 were absorbed faster. Strongly acidic drugs were almost not absorbed. pH of intestinal fluid ranges from 5-8. In case of basic drugs absorption is faster for those having pKa values less than 8.5 than those having pKa values between 9-12
- 15. Thank You