Comparison of the metabolism of 10 chemicals in human and pig skin explants

Abstract

Skin metabolism is important to consider when assessing local toxicity and/or penetration of chemicals and their metabolites. If human skin supply is limited, pig skin can be used as an alternative. To identify any species differences, we have investigated the metabolism of 10 chemicals in a pig and human skin explant model. Phase I metabolic pathways in skin from both species included those known to occur via cytochrome P450s, esterases, alcohol dehydrogenases and aldehyde dehydrogenases. Common Phase II pathways were glucuronidation and sulfation but other conjugation pathways were also identified. Chemicals not metabolized by pig skin (caffeine, IQ and 4-chloroaniline) were also not metabolized by human skin. Six chemicals metabolized by pig skin were metabolized to a similar extent (percentage parent remaining) by human skin. Human skin metabolites were also detected in pig skin incubations, except for one unidentified minor vanillin metabolite. Three cinnamyl alcohol metabolites were unique to pig skin but represented minor metabolites. There were notable species differences in the relative amounts of common metabolites. The difference in the abundance of the sulfate conjugates of resorcinol and 4-amino-3-nitrophenol was in accordance with the known lack of aryl sulfotransferase activity in pigs. In conclusion, while qualitative comparisons of metabolic profiles were consistent between pig and human skin, there were some quantitative differences in the percentage of metabolites formed. This preliminary assessment suggests that pig skin is metabolically competent and could be a useful tool for evaluating potential first-pass metabolism before testing in human-derived tissues.

Keywords: comparison; cosmetics chemicals; human; pig; skin metabolism.

Figure 1

2‐AAF metabolism in pig and human skin. A, Pig skin. B, Human skin. Radiochromatograms of the medium collected at 24 h from pig and human skin incubations. C, Radiochromatogram showing the impurities of radiolabeled 2‐AAF present in medium from 24 h incubations without skin. 2‐AAF, 2‐acetyl aminofluorene [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 2

7‐EC metabolism in pig and human skin. A, Pig skin. B, Human skin. Radiochromatograms of the medium collected at 24 h from pig and human skin incubations. 7‐EC, 7‐ethoxycoumarin; HC, hydroxycoumarin [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 3

Resorcinol metabolism in pig and human skin. A, Pig skin. B, Human skin. Radiochromatograms of the medium collected at 24 h from pig and human skin incubations. R, resorcinol [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 4

4‐A‐3‐NP metabolism in pig and human skin. A, Pig skin. B, Human skin. Radiochromatograms of the medium collected at 24 h from pig and human skin incubations. 4‐A‐3‐NP, 4‐amino‐3‐nitrophenol [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 5

Propyl paraben metabolism in pig and human skin. A, Pig skin. B, Human skin. Radiochromatograms of the medium collected at 24 h from pig and human skin incubations. There was a slight difference in the retention times of the peaks between the samples shown due to a new column being used for the human samples. To confirm that metabolites were the same in pig and human skin, medium samples were co‐injected on the new column. I and VI, 4‐hydroxybenzoic acid glucuronide conjugate; X, 4‐hydroxybenzoic acid; XII, propylparaben sulfate conjugate [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 6

Cinnamyl alcohol metabolism in pig and human skin. A, Pig skin. B, Human skin. Radiochromatograms of the medium collected at 24 h from pig and human skin incubations. B, benzoic acid; I, 3‐hydroxy‐3‐phenylacrylic acid; II, benzoic acid glucuronide; III, OH‐cinnamic acid‐cysteine glucuronide; IV, 3‐hydroxy‐3‐phenylpropanoic acid with OH‐cinnamyl alcohol‐glutathion (or 3‐hydroxypropanoic acid‐glutathion); V, benzoylglycine; VI, cinnamic acid; VII, cinnamic acid glucuronide [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 7

Vanillin metabolism in pig and human skin. A, Pig skin. B, Human skin. Radiochromatograms of the medium collected at 24 h from pig and human skin incubations with metabolite: I, vanillyl alcohol glucuronide + protocatechuic aldehyde glucuronide; II, vanillic acid glucuronide + vanillyl alcohol sulfate; III, vanillin glucuronide; IV, protocatechuic aldehyde; V, vanillic acid [Colour figure can be viewed at wileyonlinelibrary.com]