Dissolution and its theories

Dissolution is A Qualitative tool to assess the Bioavailability of drugs It refers to the conversion of the solid dosage form to solution form. A disintegration test is performed on dosage forms to ensure that it may not fail Dissolution in the later testing phase. However both Dissolution and Disintegration show ‘S’ Shaped Curve when the graph is plotted between Rate of Dissolution or Disintegration vs Time.

Importance of Dissolution

• It Rate-limiting step for poorly soluble drugs before absorption in the blood.
• Rate of Dissolution helps to understand the extent of Solid Solid and Solid Solvent interactions in the Dosage form.
• Dissolution Rate Helps to predict IVIVC(In vivo In vitro Correlation) of the dosage form.
• Various factors like F1 F2 and Q are used in Dissolution which helps to compare Dissolution profiles.

Theories and Laws Governing Dissolution.

• Dankwart model: It proposes Solvent molecules in solution takes Solute from dosage form towards the solution.
• Interfacial Barrier model: It defines Interfacial transport rather than Diffusion.
• Noye’s Whitney equation: It defines the Rate of Dissolution of Solid in Solvent.
• Convective Diffusion: Dissolution under varying conditions of Flowrates and Heat is studied using this model.
• Higuchi Model: Release of Drug From various Dosage forms like Semisolids, Ointments, Creams, and Gels considering the path taken for drug release is studied using this model.
• Ostwald Frendulich Law: It gives Drug Solubility and Surface Area Co-relation.
• Hixon Crowell Cube root Law: It gives the Surface area volume relationship of the Dosage form. If uniformly sized particles are present in dosage form the dissolution occurs parallel to the plane’s and uniformly.
• Baker Lonsdale model: This model gives the relation between the release of drugs from the circular matrix in a controlled manner example Release of Drugs from microspheres.

Intrinsic Dissolution Rate:

Rate of Dissolution of Active drug substance is termed as Intrinsic dissolution rate. It is often used to determine the chemical purity of active pharmaceutical ingredients.

Apparatus to determine Intrinsic Dissolution Rate.

• USP Wood Apparatus (Rotating Disk System).
• Stationary Disk System.
• Enhancer Cell.
• Diffusion Cell.