Dissolution of Solid Dosage Forms

If drugs are administered in solid dosage forms, they must be dissolved in the GI tract before absorption can take place. For drugs with low solubility and high dose, the dissolution will be slow, and the dissolution rate will be the rate-limiting step for absorption. Factors that affect dissolution will control the whole absorption process.

Noyes-Whitney equation can be used to describe the dissolution rate as following dm D ,

— = A — (Cs — C) C = 0 at sink condition, (4.16)

dt h where dm/dt is the rate of solid dissolution, A is the solid surface area, D is the diffusion coefficient, h is the thickness of unstirred boundary layer, Cs is the drug aqueous solubility, and C is the concentration at h (Fig. 4.7).

For drugs with low solubility, formulation strategies such as microniza-tion (increases A), ionization (increases Cs), solubilization (surfactants), and

disintegrants can be used to enhance dissolution and fraction of drug absorbed (Anderson and Pitman, 1980; Frenning and Stromme, 2003; Schreiner et al., 2005; Jinno et al., 2006).

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