Release rates from topical formulations containing drugs in suspension

Expressions describing release rates from topical formulations of drug suspensions are proposed and compared to the exact solution of the governing differential material balances. These expressions are also compared to the approximate solution proposed by Higuchi more than 35 years ago. For the first time, the quality and limitations of the Higuchi solution are evaluated in detail. Despite its simple form, Higuchi's equation reasonably represents the rate of drug release for most drug concentrations, although it strictly predicts the exact result only when the initial drug concentration is much larger than its solubility limit. A modification of the Higuchi equation is proposed which improves the prediction of the cumulative mass released to within 0.5% for all suspended drug concentrations. We also present simple algebraic expressions for calculating the time required for all of the drug to dissolve, and the fraction of mass released when dissolution is complete.