The Organization for Economic Cooperation and Development established Test Guidelines (TG) for non-animal testing methods to assess the safety of cosmetic ingredients. TG439 utilizes reconstructed human epidermis to assess skin irritation. TG439 are available for evaluating agents after 4-h exposure, but it is unclear whether those methods can be used to estimate skin irritation after 24-h exposure, which is necessary for evaluating non- or only slightly irritating substances, such as cosmetic ingredients, and for pharmaceutical approval in Japan. In this report, we assessed the performance of the EpiDerm^TM SIT (skin irritation test) as an alternative method for predicting slight irritation after 24-h exposure. We tested 40 substances listed by the Skin Irritation Test Working Group of the Japanese Society for Alternatives to Animal Experiments. Evaluation of water-soluble substances had 92% accuracy. Evaluation of oil-soluble substances exhibited only 62% accuracy, 18% sensitivity, and 93% specificity. Although the assay produced a high percentage of false-negative results for oil-soluble substances, positive predictivity was high for all substances. A positive result in the EpiDerm^TM SIT indicates a strong possibility of skin irritation. These results suggest that the EpiDerm^TM SIT is useful for assessing the potential of substances as irritants after 24-h exposure.

Many medical professionals have shown an interest in the clinical use of essential oils (EOs) for their analgesic and sedative effects. However, the main side effects of EOs are skin irritation and sensitization, and their frequency and severity have not been clarified. This paper describes the results of the comparison of skin sensitization reactions of three essential oils containing linalool using an in vitro sensitization test (human cell line activation test, h-CLAT). The h-CLAT method quantifies the expression levels of monocyte and cluster of differentiation (CD) cell surface markers (CD86 and CD54) in a THP-1 human monocytic leukemia cell line after EO exposure. The test oils were bergamot EO (BEO), Ho leaf EO (HLEO), and thyme linalool EO (TLEO). The dose-finding assay showed the estimated concentration required to elicit 75% cell viability (CV75) of TLEO to be 96.3 µg/mL while values were not obtained for BEO and HLEO at up to 1000 µg/mL. All EOs induced skin sensitization as revealed by CD54 values. Therefore, skin sensitization induced by EO mixtures could be confirmed using h-CLAT.