Rheological and mechanical properties of pharmaceutical gels. Part II: Medicated systems: relevance to hydration properties and drug release.

Aim of this work was to evaluate the influence of rheological and mechanical properties of medicated gels containing two viscosity grades of either sodium carboxymethylcellulose or polyacrylic acid on hydration properties and drug release. The two polymers were chosen since they produce different semi-solid systems: polymeric colloidal solutions in the case of sodium carboxymethylcellulose and true gels in the case of polyacrylic acid. Salicylic acid and sodium salicylate were used as model drugs for sodium carboxymethylcellulose solutions, benzoic acid and sodium benzoate were employed for polyacrylic acid gels. The relationship previously found (Part I of this study) between the viscoelastic (tg delta) and the gel strength parameters in non-medicated gelified vehicles also exists in medicated gels: the higher the tg delta, the lower the gel strength. The results obtained on isoviscous samples (either colloidal solutions or true gels), containing two viscosity grades of the same polymer, demonstrate that an increase in tg delta and the concomitant decrease in the mechanical resistance of the sample always accompany an improvement in hydration properties. The different rheo-mechanical and hydration properties entrain different release profiles only when drugs are suspended in the vehicle (i.e. in the case of the free acids here considered). In these cases, the limiting step for drug release is likely the preliminary drug dissolution, which is in turn affected by the hydration propensity of the vehicle.