Reduction of azo dyes during in vitro percutaneous absorption.

The azoreduction of phenylazo-2-naphthol (Sudan I), 5-(phenylazo)-6-hydroxynaphthalene-2-sulfonic acid [aniline subsidiary color of FD&C Yellow No. 6 (ANSC)], and phenylazophenol [Solvent Yellow 7 (SY7)] in skin during percutaneous absorption was measured and the contributions of cytosolic and microsomal reductions were characterized. By using a series of azo dyes with a common U-14C-labeled phenylazo moiety, percutaneous absorption and metabolism were measured in vitro in flow-through diffusion cells with Sencar mouse, hairless guinea pig, and human skin. Azoreductase assays using subcellular fractions from skin of all species were used to examine intracellular rates and distribution for the series of dyes. The absorption of the lipophilic dyes Sudan I and SY7 from a finite surface dose of 5 micrograms/cm2 was extensive in all species. In mouse, 32.8 +/- 2.8% of the applied Sudan I dose and 64.1 +/- 3.3% of the applied SY7 dose were absorbed in 24 hr. In the hairless guinea pig, 57.6 +/- 5.9% of the applied Sudan I dose and 67.8 +/- 4.6% of the applied SY7 dose were absorbed in 24 hr. Human skin was least permeable, with 26.4 +/- 6.7% of the applied Sudan I dose and 36.1 +/- 4.5% of the applied SY7 dose absorbed in 24 hr. Sudan I and SY7 were extensively reduced in skin of all species during percutaneous absorption (29.5 and 26.5%, respectively, of the absorbed dose in human skin and greater than 50% of the applied dose in other species). ANSC was the least absorbed, with 5% or less penetrating. SY7 was preferentially reduced in the skin cytosol of all species, whereas Sudan I was equally reduced in the skin cytosol and microsomal fractions. The site of azoreduction in the cell may affect the metabolic fate of the liberated arylamine. The extensive azoreduction observed during percutaneous absorption may modulate the toxicities of these compounds and must be considered when effective doses are determined for quantitative risk assessments from dermal exposures.