Theories of drug dissolution
-
Upload
tanmay93saha -
Category
Health & Medicine
-
view
198 -
download
5
Transcript of Theories of drug dissolution
![Page 1: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/1.jpg)
THEORIES OF DRUG DISSOLUTION
![Page 2: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/2.jpg)
Presented by…….
TANMAY SAHA , TANAYA PALIT & TANUMOY GHOSHB.PHARM, 3RD YEAR, 6TH SEMESTER
ROLL No.- 18601913099 18601913097 18601913100
GURU NANAK INSTITUTE OF PHARMACEUTICAL SCIENCE & TECHNOLOGY
![Page 3: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/3.jpg)
INTRODUCTION:Drug is the single chemical entity present in a medicine
that is used for diagnosis, prevention, treatment/ or cure of disease.
Dissolution is a process in which a solid substance solubilizes in a given solvent i.e. mass transfer from the solid surface to the liquid phase.
Drug dissolution mainly occurs in stomach & intestine.
ENTERIC – COATED TABLETS
ACIDIC DRUGS ARE
USED
![Page 4: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/4.jpg)
![Page 5: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/5.jpg)
DESCRIPTION:There are many theories to explain drug dissolution.
The important ones are ----
1. Diffusion Layer model / Film theory2. Danckwert’s model / Penetration or Surface Renewal
Theory3. Interfacial barrier model / double-barrier or Limited
solvation theory.
![Page 6: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/6.jpg)
Diffusion Layer Model / Film Theory:Simplest & most common theory of dissolution.
This process of dissolution of solid particles in a liquid in absence of chemical forces, consists of two consecutive steps------I. Solution of liquid to form a thin film or layer at the
solid-liquid interface known as Stagnant film or diffusion layer.
This layer is saturated with the drug.
This step is usually rapid.
![Page 7: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/7.jpg)
II. Diffusion of the soluble solute from the stagnant layer to bulk of the solution.
This step is slower
Rate – determining step in drug dissolution.
![Page 8: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/8.jpg)
This theory is controlled by Noyes & Whitney equation:
dC/dt = k ( Cs – Cb )
Where, dC/dt = dissolution rate of the drug k = dissolution rate constant ( first order ) Cs = concentration of drug in the stagnant layer
( also called as the saturation or maximum drug solubility )
Cb = concentration of drug in the bulk of the solution at time t.
![Page 9: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/9.jpg)
Nernst & Brunner incorporated Fick’s first law of diffusion &
modifies the Noyes-Whitney’s equation to:
dC/dt = DAKw/o (Cs-Cb)/VhWhere,
D=diffusion coefficient of the drugA=surface area of the dissolving solidKw/o =water in oil partition coefficient of the drug or intrinsic
dissolution rate constant. Cs-Cb =concentration gradient for drug of diffusion.V=volume of dissolution mediumh=concentration gradient for diffusion of drug
![Page 10: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/10.jpg)
Noyes-Whitney’s equation assumes that the surface area of the dissolving solid remains constants during dissolution which is practically not possible.
To account for the particle size decrease & change in surface area by Hixon & Crowell’s cubic root law of dissolution is used:
W01/3 – W1/3 = Kt
Where,W0 =original mass of the drugW =mass of the drug remaining to dissolve at time tK =dissolution rate constant
![Page 11: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/11.jpg)
Modified Noyes & Whitney’s equation represents first order kinetics, the driving force for which is the concentration gradient (Cs-Cb).
To obtain good in vitro- in vivo dissolution rate correlation , in vitro- in vivo dissolution must be performed under sink condition. That means Cs >> Cb .
dC/dt = K
This equation follows Zero order kinetics. This represents that dissolution rate is constant under sink condition.
![Page 12: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/12.jpg)
![Page 13: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/13.jpg)
Danckwert’s model: Dankwert didn’t approve of existence of stagnant layer &
suggested that turbulence in the dissolution medium exists at solid/liquid interface.Danckwert’s model is expressed by-----------
V dC/dt = dm/dt = A( Cs – Cb )
Where,
m=mass of solid dissolved,
![Page 14: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/14.jpg)
Interfacial barrier model: An intermediate concentration can exist at the
interface as a result of solvation mechanism & is a function of solubility rather than diffusion.
This concept is given by------G = Ki (Cs –
Cb ) Where,
G=dissolution rate per unit area, Ki =effective interfacial transport constant.
![Page 15: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/15.jpg)
Conclusion:The diffusion layer model & Danckwert’s model were based
on---- The rate-determining step that controls dissolution is the mass
transport. Solid-solution equilibrium is achieved at the solid/liquid interface.
The interfacial barrier model can be extendeded to both diffusion layer model & Danckwert’s model
![Page 16: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/16.jpg)
References: D.M.Brahmankar & Sunil B. Jaiswal, Biopharmaceutics &
Pharmacokinetics- A treatise, 2nd Ed., Vallabh Prakashan, Absorption of Drugs, Theories of Drug Dissolution, Page no: 29 -35.
V. Venkateswarlu, Biopharmaceutics & Pharmacokinetics, 2nd Ed., Absorption of a weak acidic drug, Theories of Drug Dissolution, Page no: 33-36
![Page 17: Theories of drug dissolution](https://reader035.fdocuments.in/reader035/viewer/2022081421/588941511a28abde5a8b484f/html5/thumbnails/17.jpg)