ABSTRACT
Dermal absorption of chemicals is a key factor in non-dietary human risk assessment with agrochemicals. A ring trial exploring the dermal absorption potential of 14C-Caffeine was investigated in six laboratories under GLP conditions using the OECD TG 428-compliant in vitro assay with flow-through cells and dermatomed human skin. Potential sources of variation were reduced by a common protocol, test item and skin source. Particularly, skin samples from the same donors were distributed over two repeats and between labs in a non-random, stratified design. The data show very similar recovery in the various investigated compartments of the assay between laboratories, repeats and donors, which demonstrates that the assay can be robustly and reliably performed. A low intra-lab variability was demonstrated in all 6 laboratories participating. Further, a low inter-lab variability was notable in five out of six laboratories resulting in mean caffeine absorption estimates of 3.93 ± 2.96 % of the applied dose (4 mg/mL) and a mean overall compound recovery of 99.03 ± 2.11 %. In the remaining laboratory presumably due to a Covid-19 driven shipment delay of the skin samples, the skin quality may have been affected since the receptor fluid recovery was substantially higher and to a certain degree did correlate with skin integrity parameters. Overall, the ring trial's results demonstrate that the OECD TG 428-compliant in vitro assay can be robustly and reliably performed in different laboratories. Skin absorption results for caffeine did not present any statistically significant variability, with minor differences in the results across the laboratories (5/6) and indicates the robustness of the methodology and the positive impact of strictly controlling the variables in the study. Understanding the variability in the in vitro dermal absorption assay with human skin will continue contributing to the design and conduct of appropriate regulatory studies, and adding further considerations to the existing OECD dermal guidance notes. This will result in greater confidence in the data utilised from these assays in risk assessments by companies and regulators alike.