To assess the intrinsic variability of the integrity tests and the effect of the human donor on the results, the overall, the inter-donor and the intra-donor variabilities Rapamycin clinical trial were calculated for TEER, TEWL, TWF and BLUE (Table 8). For TEER, CVs for the overall, the inter-donor and the intra-donor variability were 64%, 45% and 43%, respectively. This implies that the variability of the method, given as the intra-donor variability, is close to the inter-donor variability and therewith covering the donor effect. The same is true for the other integrity tests (TEWL, TWF and BLUE), for which the donor
effect was also close to the method variability. Therewith, a clear separation of human donors based on the integrity test results is hardly possible. Additionally, means and overall variability of the different integrity tests were calculated for full-thickness and dermatomed human skin separately (data not shown).
In general, the values were close within each integrity test. For instance, TWF results were 302 ± 188 ∗ 10−5 cm h−1 (n = 20, CV = 62%) and 248 ± 146 ∗ 10−5 cm h−1 (n = 20, CV = 59%) for dermatomed and full-thickness skin from the same human donors, respectively. This is in line with the previously reported comparability of absorption results through both skin preparation types ( Guth, 2013). Furthermore, the donor effect was consistent over all methods with values ranging from 32% to 48%. These values were also in line with the general donor effect observed for STK38 dermal absorption BGJ398 solubility dmso experiments in vitro being ∼43% ( Southwell et al., 1984). The overall method variabilities determined in this study for four different test
compounds are with CVs of 33% and 45% for maxKp and AD, respectively, in line with the reported variability ranging from 2% to 111% ( Southwell et al., 1984 and van de Sandt et al., 2004). The method variabilities obtained for all five integrity tests, including ISTD, are in the same range (30–57%). The ISTD is advantageous over the ‘solitary’ integrity tests conducted in advance or after an absorption experiment, since outliers or abnormalities observed for the kinetics of the test compound can be interpreted in parallel with the kinetics of the ISTD. For instance, an abrupt increase of absorption of the test compound after the washing procedure is classified as a wash-in effect due to mechanical disruption of the barrier if the ISTD shows a parallel effect, or it is classified as a substance-specific wash-in effect if the absorption of the ISTD is not affected. The latter case – washing increases the test compound absorption – can be relevant for regulatory purposes. In addition, formulation-related barrier impairment could be identified.