, 1988). However, the comparability of water flow through skin tissue in vivo and in vitro is limited. Previous work about TEWL application in vitro indicates that only severe damages can be detected (Netzlaff et al., 2006). The same conclusion is drawn for the current work where no, poor or even inverse correlations were observed Selleck LDK378 between TEER, TEWL or TWF and test compound absorption (Table 7). Yet, the stated general applicability for in vitro testing failed to reflect 14C-mannitol (Lawrence, 1997) and 35sulfur mustard absorption in vitro (Chilcott et al., 2002). A lack of correlation
to highly lipophilic test compounds was reported, too (Levin and Maibach, 2005). Taken together all three standard tests are able to sort out a substantial part of impaired human skin samples in general. Limit values of 2 kΩ, 10 g m−2 h−1 and 4.5 ∗ 10−3 cm h−1 for TEER, TEWL and TWF, respectively, seem appropriate to judge between unwanted use of impaired skin and unnecessary rejection
of skin samples. However, destruction of barrier function during the experiment does not become obvious by these tests and – shown by falsely classified skin – only a rough differentiation is possible. Furthermore, none of the named integrity tests seems universally applicable. Defined ‘applicability domains’ for each integrity test which limits their use to test compounds in PD-0332991 price specific physico-chemical spaces or to specific experimental conditions (in vitro and/or in vivo, human and/or rat skin, excised and/or reconstructed skin etc.)
can help to choose the most indicative test for the relevant case. Moreover, future use of reconstructed human skin for testing of dermal absorption asks for the adjustment of the generated data to human skin based on a prediction model (Schäfer-Korting et al., 2008) which still needs to be set up. For this purpose, the cut-off values need to be adapted as well. Because of the limitations of the standard integrity tests (TEER, TEWL and TWF), two other integrity parameters (ISTD, BLUE) were checked for their ability to correlate with absorption results and explain continuous differences of the skin barrier function. Extreme outliers were clearly identified with BLUE, but correlations to test compound absorption were poor and partly even inverse. Although a general 3-mercaptopyruvate sulfurtransferase applicability of BLUE cannot be ruled out, lack of advantage over established tests makes further investigations redundant. The opposite was true for ISTD. These results were positively and highly correlated with test compound results. The correlation over a wide absorption range of 14C-MCPA (6–100%) to 3H-testosterone as internal reference standard was 0.859 (n = 45). Comparison of results for normal and intentionally damaged rat skin samples suggests under these experimental conditions (rat skin, receptor fluid water) a provisional cut-off value of 35% AD 3H-testosterone ( Fig. 2).