Percutaneous penetration/dermal absorption of hair dyes in vitro

This paper presents an in vitro technique to analyse percutaneous penetration and dermal absorption of hair dyes, topically applied to excised pig skin. Representative examples are given by the radio-labelled hair dyes p-phenylenediamine and bis-(5-amino-1-hydroxyphenyl)-methane. Both compounds were assessed under simulated use conditions and were analysed in representative formulations including the specific conditions for oxidation hair dyes. To be able to differentiate between topically adsorbed and systemically available amounts, the bioavailability of the hair dyes is defined as the amount penetrated and/or remaining in the exposed skin after removing the stratum corneum. Less than 1% of the assessed topically applied dyes was found to be bioavailable in the presence of hydrogen peroxide, typically added to oxidation hair dyes prior to applications. Compared with published results and unpublished in-house in vivo data, the level of confidence was high. Owing to in-house experience over about 5 years in using excised pig skin for measurements of percutaneous penetration and dermal absorption of hair dyes, the technique was found to be successful and appropriate to reduce the number of test animals normally used for toxicological assessments. The essentials of this technique are actually recommended by the SCCNFP (The Scientific Committee on Cosmetic Products and Non Food Products intended for Consumers) for the safety evaluation of cosmetic ingredients, particularly for hair dyes. The corresponding OECD guideline as well as the guidance document has been drafted and is currently in discussion on the level of the national coordinators.