In This Ppt Drug Dissolution Theories Are Mentioned With Detailed Explanation, It Can Be Used As Reference Purpose For Understanding.

1. Theories of Drug Dissolution
Prepared By : Harshvardhan M. Raval

2. Introduction
❑ Except in case of controlled-release formulations, disintegration and deaggregation occur rapidly if it is a
well-formulated dosage form. Thus, the two critical slower rate-determining processes in the absorption
of orally administered drugs are:
1. Rate of dissolution
2. Rate of drug permeation through the biomembrane.

3. ❑ Based on the intestinal permeability and solubility of drugs, Amidon et al developed Biopharmaceutics
Classification System (BCS) which classifies the drugs into one of the 4 groups :

4. Dissolution :
❑ Dissolution Is Called As A Process In Which A Solid Substance Solubilises
In A Given Solvent (Water) I.E. Mass Transfer From The Solid Surface To
The Liquid Phase.
❑ Several Theories To Explain Drug Dissolution Have Been Proposed By
Many Scientists . Some Of The Important Ones Are:
1. Diffusion Layer Model/Film Theory
2. Danckwert’s Model/Penetration Or Surface Renewal Theory
3. Interfacial Barrier Model/Double-barrier Or Limited Solvation Theory.

5. 1. Diffusion Layer Model Theory
❑ This Is The Simplest And The Most Common Theory For Dissolution. Here,
The Process Of Dissolution Of Solid Particles In A Liquid, In The Absence Of
Reactive Or Chemical Forces, Consists Of Two Consecutive Steps:
1. Solution Of The Solid To Form A Thin Film Or Layer At The Solid/Liquid
Interface Called As The Stagnant Film Or Diffusion Layer Which Is Saturated
With The Drug; This Step Is Usually Rapid.
2. Diffusion Of The Soluble Solute From The Stagnant Layer To The Bulk Of
The Solution; This Step Is Slower And Is Therefore The Rate-determining
Step In Drug Dissolution.
3. The Equation To Which Explains The Rate Of Dissolution When The Process
Is Diffusion Controlled And Involves No Chemical Reaction Was Given By
Noyes And Whitney.

6. ❑ The in vivo dissolution is always rapid than in vitro dissolution because the moment the
drug dissolves; it is absorbed into the systemic circulation. As a result, cb = 0, and
dissolution is at its maximum.
❑ To obtain good in vitro-in vivo dissolution rate correlation , the in vitro dissolution
should always be carried under sink conditions. This can be achieved in one or more of
the following ways:
1. Bathing the dissolving solid in fresh solvent from time to time.
2. Increasing the volume of dissolution fluid.
3. Removing the dissolved drug by partitioning it from the aqueous phase of the
dissolution fluid into an organic phase placed either above or below the dissolution
fluid—for example, hexane or chloroform.
4. Adding a water miscible solvent such as alcohol to the dissolution fluid.
5. By adding selected adsorbents to remove the dissolved drug
❑ The in vitro sink conditions are so maintained that cb is always less than 10% of cs.

7. 2. Danckwert’s Model (Surface Renewal Theory)
❑ Danckwert didn’t believed the existence of a stagnant layer and suggested that
turbulence in the dissolution medium exists at the solid/liquid interface.
❑ As a result, the agitated fluid consisting of macroscopic mass of eddies or packets
reach the solid/liquid interface in a random fashion due to eddy currents, absorb
the solute by diffusion and carry it to the bulk of the solution.
❑ Such solute containing packets are continuously replaced with new packets of
fresh solvent due to which the drug concentration at the solid/liquid interface
never reaches Cs and has a lower limiting value of Ci . Since the solvent packets
are exposed to new solid surface each time, the theory is called as surface renewal
theory.
❑ The Danckwert’s model is expressed by equation :

9. 3. Double Barrier Or Limited Solvation Theory
❑ The diffusion layer model and the Danckwert’s model were based on two assumptions:
1. The rate-determining step that controls dissolution is the mass transport.
2. Solid-solution equilibrium is achieved at the solid/liquid interface.
❑ An intermediate concentration can be present at the interface because of solvation
mechanism and is a function of solubility rather than diffusion.
❑ When considering the dissolution of a crystal, each face of the crystal will have a
different interfacial barrier. Such a concept is given by the following equation:

10. ❑ In this theory, the diffusivity D may not be independent of saturation concentration Cs.
❑ Factors Affecting Drug Dissolution and Dissolution Rate :
1. Physicochemical properties of the drug
2. Dosage form factors
❑ Physicochemical properties of the drug : Particle Size and Effective Surface Area of the Drug
Polymorphism and Amorphism
Salt Form of the Drug
Drug pKa and Gastrointestinal pH
Lipophilicity and Drug Absorption
Drug Permeability and Absorption
Stereochemical Nature of Drug

11. ❑ Effect of dosage form on dissolution and absorption :

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