Inter‐ and intralaboratory variation of in vitro diffusion cell measurements: An international multicenter study using quasi‐standardized methods and materials

In vitro measurements of skin absorption are an increasingly important aspect of regulatory studies, product support claims, and formulation screening. However, such measurements are significantly affected by skin variability. The purpose of this study was to determine inter- and intralaboratory variation in diffusion cell measurements caused by factors other than skin. This was attained through the use of an artificial (silicone rubber) rate-limiting membrane and the provision of materials including a standard penetrant, methyl paraben (MP), and a minimally prescriptive protocol to each of the 18 participating laboratories. "Standardized" calculations of MP flux were determined from the data submitted by each laboratory by applying a predefined mathematical model. This was deemed necessary to eliminate any interlaboratory variation caused by different methods of flux calculations. Average fluxes of MP calculated and reported by each laboratory (60 +/- 27 microg cm(-2) h(-1), n = 25, range 27-101) were in agreement with the standardized calculations of MP flux (60 +/- 21 microg cm(-2) h(-1), range 19-120). The coefficient of variation between laboratories was approximately 35% and was manifest as a fourfold difference between the lowest and highest average flux values and a sixfold difference between the lowest and highest individual flux values. Intralaboratory variation was lower, averaging 10% for five individuals using the same equipment within a single laboratory. Further studies should be performed to clarify the exact components responsible for nonskin-related variability in diffusion cell measurements. It is clear that further developments of in vitro methodologies for measuring skin absorption are required.

The effect of beta-cyclodextrins on the permeation of diclofenac from supersaturated solutions

Supersaturation is a very useful method of enhancing the permeation of drugs across membranes such as skin, because unlike other methods, it does not interfere with the ultrastructure of the stratum corneum. Many drugs are able to form inclusion complexes with beta-cyclodextrins (beta-CDs) and this study investigates the anti-nucleating effects of these compounds on supersaturated solutions of diclofenac. The ability of various betaCDs to form inclusion complexes with diclofenac was assessed by measuring their saturated solubilities. Solutions containing hydroxypropyl beta-cyclodextrin (HPbeta-CD, with a molar substitution of 0.9) produced a 7.5-fold increase in the solubility of diclofenac, which suggested that a strong complex was formed between the two compounds. This association was characterized using differential scanning calorimetry. Permeation across silicone membranes of these saturated solutions of diclofenac in the presence of the different betaCDs produced similar flux values suggesting that the overall activity was also similar. The effect of different molar ratios of HPbeta-CD and diclofenac, and the anti-nucleating effect of HPbeta-CD (both on its own and in combination with a known anti-nucleant, hydroxypropylmethyl cellulose (HPMC)) on the diffusion of diclofenac across silicone membranes was investigated. HPbeta-CD appears to have a stabilizing effect on supersaturated solutions of diclofenac as a co-ingredient with HPMC.

The back diffusion of glucose across human skin in vitro

For diabetic patients, blood glucose monitoring is an important part in the management of their disease, however the acquisition of blood requires the use of invasive and often painful methods, and the development of a technique that removes these problems would represent a major advance. The uppermost membrane of the skin, the stratum corneum, has been shown to be the main barrier to percutaneous absorption, but there have been claims that polar water-soluble compounds diffuse across it via aqueous pathways. In this study, skin diffusion cells were used to investigate the back diffusion of tritiated water and the convective transport of 3H-glucose across full thickness human skin after the application of a number of different materials to the stratum corneum. Significant amounts of 3H-glucose back diffused only after complete removal of the stratum corneum by tape stripping, and it is likely that any future attempts to monitor blood glucose levels using non-physical techniques will require a certain degree of damage to the stratum corneum. The extraction through the skin of tritiated water and 3H-glucose after the application of solutions with different osmotic pressures were consistent with the theory that solutions with high osmotic pressures dehydrate the stratum corneum which suggests that passive transport of these radiolabelled molecules through porous pathways was insignificant.