- Dissolution is the physicochemical process by which a solid substance enters the solvent phase to yield a solution. Dissolution (release of the drug from the dosage form) is a key pre-requisite for any orally administered drug to be systemically effective.
- It is a dynamic process.
- Dissolution is defined as the process by which a known amount of drug substance goes in to solution per unit time under standaridized condition.
Important dissolution factors can be identified by Noyes Whitney equation:
dC ‗ D.A Ko/w (Cs-Cb)
dt
v h
Where dC/dt is a rate of drug dissolution at time‘t’
A = surface area of the particle, h=thickness of the stagnant film layer
Cs - Cb=concentration of gradient of diffusion of drug.
K=water/oil partition coefficient.
D=1/N X (VA); D=diffusion coefficient of compound in the medium
Where N= solvent viscosity
A = surface area of the particle, h=thickness of the stagnant film layer
Cs - Cb=concentration of gradient of diffusion of drug.
K=water/oil partition coefficient.
D=1/N X (VA); D=diffusion coefficient of compound in the medium
Where N= solvent viscosity
VA = Solute molecular volume
Intrinsic dissolution rate
Apparatus to measure IDR
- Intrinsic dissolution rate (IDR) of a pure substance is the rate at which it dissolves from a constant surface area whilst the temperature , agitation , pH and ionic strength of the dissolution medium are kept constant.
- Dissolution rate refers to the rate at which the solid dissolves in a solvent.
- IDR≥ 1.0mg/min/sq.cm suggests drug dissolution will not be the rate limitting step .
- IDR ≤0.1mg/min/sq.cm suggests drug dissolution will be the rate limitting step .
Apparatus to measure IDR
- Wood apparatus : rotating disk
- Stationary disk apparatus
Need of Dissolution testing devices
- Solid drugs absorbed only from the solution .
- In vitro test – estimate amount of drug released per unit time.
- In vitro disintegration test not sufficient .
- In vitro dissolution test most reliable predictors of in vivo performance.
- Dissolution - rate limiting factor.
Common conditions
- Simulated gastric and intestinal fluids – 37 deg Celsius
- Fixed speed agitator
- Screen for separation of disintegrated particles from the bulk
USP Apparatus
Classification of dissolution apparatus in different pharmacopeias:
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