Percutaneous penetration and metabolism of 2-nitro-p-phenylenediamine in human and fuzzy rat skin

2-Nitro-p-phenylenediamine (2NPPD) is a dye used in semipermanent and perma nent (tinting color) hair dye formulations. National Toxicology Program tox icology and carcinogenesis testing of 2NPPD has raised concerns about its s afety. Therefore, we initiated in vitro studies to measure absorption and m etabolism of 2NPPD in human and fuzzy rat skin and rat jejunal tissue. Inte stinal tissue metabolism of 2NPPD was compared to skin metabolism since tox icology data from oral 2NPPD studies will be used for future safety assessm ent purposes. Absorption was measured over 24 h by using flow-through diffu sion cells with a receptor fluid consisting of Hepes-buffered Hank's balanc ed salt solution, Dosing vehicles were applied to skin and intestine in the diffusion cells for 30 min. 2NPPD metabolites were determined by high-perf ormance liquid chromatography methodology. In human skin, the percentages o f total applied dose absorbed (receptor fluid + skin) over 24 h were 9.2 +/ - 5.7 (mean +/- SD) and 9.5 +/- 3.2 for the ethanol and semipermanent vehic les, respectively, with approximately 3% remaining in skin. In rat skin, th e percentages of total applied dose absorbed over 24 h were 9.3 +/- 1.2 (me an +/- SE), 6.9 +/- 1.2, and 4.2 +/- 0.1 for the ethanol, semipermanent, an d permanent formulation vehicles, respectively, with approximately 3% remai ning in skin. In rat intestinal tissue, the percentage of total applied dos e absorbed over 24 h was 10.9 +/- 1.2, with approximately 5% remaining in t he tissue. In human and rat skin, 2NPPD was metabolized to triaminobenzene and N4-acetyl-2NPPD. 2NPPD was also metabolized to a sulfated 2NPPD metabol ite in rat skin, but not in human skin, 2NPPD was extensively metabolized i n both human and rat skin with ethanol application; metabolism was not as e xtensive with a semipermanent formulation application. In rat intestinal ti ssue, 62% of 2NPPD was metabolized upon absorption to triaminobenzene and N 4-acetyl-2NPPD. Differences in the metabolic profiles (proportion of each m etabolite formed) were found between the skin and intestinal tissue. These results suggest that 2NPPD is rapidly absorbed and extensively metabolized in both skin and intestinal tissue. The extent of metabolism and the metabo lic profile were found to be species-, tissue-, and dosing vehicle-dependen t. Metabolism information will be useful in predicting the extent of 2NPPD and/or 2NPPD metabolite systemic absorption relative to a dermal exposure, which will improve the health hazard assessment of 2NPPD.