A new discriminative criterion for the development of Franz diffusion tests for transdermal pharmaceuticals

PURPOSE. In vitro skin/membrane permeation profiling of topical pharmaceuticals is an important overall quality attribute in the evaluation of product consistency and it is also used for IVIVR (in vitro - in vivo relationship) purposes in product development and change control. Franz diffusion cell (FDC) experiments are emerging as a generally accepted methodology in this field, where the choice of operational conditions requires a data-supported justification towards the discriminating power of the test. A response function is therefore proposed to objectively quantify the discriminating power. METHODS. We evaluated the usefulness of the proposed response function by studying one of the operational conditions, i.e. the influence of receptor medium composition, on the FDC in vitro penetration behaviour of the model compound testosterone formulated in four different topical preparations, using both artificial membranes and dermatomed human skin. A second application is a FDC test system for spilanthol. RESULTS. From the obtained cumulative amount of the active (testosterone or spilanthol) in the receptor fluid versus time curves, the permeability coefficient Kp of the active from each formulation was calculated. The evaluation of the discriminating power of the different media was performed using our new objective response function based upon an equal spread criterion of normalised Kp values. CONCLUSION. The proposed new criterion was found to be useful for the rational design of an in vitro diffusion test for transdermal pharmaceuticals. We demonstrated significant differences in discriminating power between the different media used: (a) for testosterone-containing formulations, it was shown that HPBCD-containing media are more discriminative compared to ethanol- or BSA-containing media; (b) for spilanthol-containing formulations, PBS containing formulations also gave better discriminating results than ethanol-based receptor media