The aim of the present work was to extend our previous in-vitro drug release studies using semisolid dermatological bases with non-impregnated cellulose acetate membranes. A comparison of the performances of two apparatuses, the more commonly used Franz cell and the new modified USP (mini paddle with ointment holding cell) systems were applied to this work. Five different semisolid as well as two marketed preparations containing 1% diclofenac sodium were used. Complex, slightly non-linear release curves indicating sink conditions were found. This was explained by the co-diffusion of excipients modifying the characteristics of the membrane and the receiving medium dynamically. Although our test model is, as a rule, not suitable to establish an in-vivo-in-vitro correlation, good qualitative as well as quantitative correlations were found within some types of dermatological bases. The correlation between the results of the two in-vitro methods also depends on the type of semisolids studied. The release curve characteristics and the amount of diclofenac sodium released at 6 h were measured. Their repeatability and reproducibility were calculated. The slopes and Q-values were correlated with in-vivo data. In general, the modified USP method provided more precise results than the Franz cell method.
