Drug release from non-disintegrating hydrophilic matrices: sodium salicylate as a model drug

Tablets containing different concentrations of sodium salicylate in a (hydroxyethyl)methylcellulose matrix swelled without disintegration or attrition. The release rate of the drug from the whole tablet was shown to conform with a model diffusional equation for two-sided release from a slab maintaining a constant surface-volume ratio on swelling, and the rate constant was linearly dependent on the drug dosage, as predicted by this equation, when the polymer concentration was kept constant. With varying polymer concentration, correction of the rate constants for their dependence on drug content of the matrix yielded constants dependent on the polymer concentration only. Such rate constants observed a semi-logarithmic relation to polymer content and extrapolated to give reasonable diffusion coefficient values for hypothetical low and high polymer content matrices. The temperature coefficient yielded a high value (9.15 kcal . mol^-1) for the activation energy of the diffusion process, consistent with the energy barrie in a polymer matrix. Such treatment of rate constants and component concentration variables offers a valuable method of correlating formulation and release parameters in non-disintegrating hydrophilic matrices.