Influence of drug solubility in the vehicle on clinical efficacy of ointments.

In a bilateral paired comparison (randomized double-blind study) 31 dermatitis patients (atopic and contact dermatitis) were tested with two ointments containing 0.0056% betamethasone-17-benzoate. One ointment was applied on each side of the body. The topical formulations differed in their solution capacities for the drug by a factor of about 50 (solution-type: high mutual affinity between drug and vehicle; suspension-type: low affinity). The different antiinflammatory effects were studied visually by assessing five symptoms: erythema, scaling, infiltration, lichenification, and excoriation. On the 5th day, 73% of the patients showed significant differences between the sides in favor of the suspension-type ointment (Wilcoxon test). Blanching tests on 30 volunteers confirmed the result. The in vitro drug release, however, was faster with the solution-type ointment. The efficacy of an ointment can be increased greatly, if the solution capacity for the drug is low, and thus the partition coefficient between the stratum corneum (barrier of the skin) and the vehicle is high. As long as the barrier is not damaged completely, the difference in drug release is not the determining factor for the effect.