Effect of Skin Metabolism on Dermal Delivery of Testosterone: Qualitative Assessment using a New Short-Term Skin Model

Abstract

The skin is a metabolically active organ expressing biotransformation enzymes able to metabolize both endogenous molecules and xenobiotics. We investigated the impact of metabolism on the delivery of testosterone through the skin with an ex vivo pig ear skin system as an alternative model for human skin. Penetration, absorption and metabolic capabilities were investigated up to 72 h after application of [¹⁴C]-testosterone doses of 50-800 nmol on either fresh or frozen skin, with the latter model being metabolically inactive. Testosterone absorption and metabolite production were monitored by radio-HPLC and gas chromatography-mass spectrometry. Testosterone absorption through frozen skin was much lower, irrespective of the dose of testosterone applied, compared to fresh skin. Using fresh skin samples, >95% of the radioactivity recovered in culture media, as well as the skin itself, corresponded to metabolites. These results were compared with the metabolic data obtained from other in vitro systems (liver and skin microsomes). The present work leads to the conclusion that most of the enzymatic activities expressed in liver fractions are also expressed in pig and human skin. The metabolic activity of the skin can modulate the biological activity of pharmaceuticals (and xenobiotics). Consequently, it can also greatly affect transdermal drug delivery.

Comparison of the Skin Penetration of 3 Metabolically Stable Chemicals Using Fresh and Frozen Human Skin

“…This is in contrast to the findings of Hewitt et al [5], who reported that the passage of the larger-molecular-weight and lipophilic chemicals fluoxypyr esters from the SC into the epidermis required cleavage of the ester to a more hydrophilic metabolite. Likewise, benzo(a)pyrene also requires metabolism to more hydrophilic metabolites before it can penetrate the epidermis and dermis [6,7]. With this in mind, the penetration and metabolism of some chemicals known to be metabolized by dermal metabolizing enzymes should be determined using fresh human skin.…”

“…The reason for lower penetration of certain chemicals (e.g., benz(a)pyrene) in frozen skin is mainly due to inactivation of metabolizing enzymes [6,7], which is why our studies were specifically designed to compare chemicals that were not metabolized by dermal enzymes. For all 3 chemicals, the amounts recovered in the epidermis and dermis were very low and represented less than 2% of the applied dose; therefore, the statistical differences determined in these layers for BA were considered not to be biologically relevant.…”

“…However, there are conflicting reports as to whether the freezing process affects the penetration of chemicals. The outcomes of the comparisons appear to depend on the physicochemical properties of chemicals tested [4,5,6,7], the skin tissues used (which vary between full-thickness [8], dermatomed [9,10] and isolated epidermis layers, i.e., 100 μm thick [11,12]), the length of storage (30-60 days [13] vs. a few hours [9]), and the parameters by which they were compared, e.g., distribution across skin layers, penetration coefficients and cumulative amounts in the receptor fluid (RF). Considering the varied reports of comparisons between fresh and frozen skin, before embarking on the main testing of 50 chemicals, we first evaluated whether the storage of human skin affected the penetration of model chemicals.…”

Standardization of an in vitro Model for Evaluating the Bioavailability of Topically Applied Compounds on Damaged Skin: Application to Sunscreen Analysis

“…Porcine skin is readily available from slaughterhouses, and its histological and biochemical properties have been repeatedly shown to be similar to human skin [9,10]. Therefore, in addition to human skin, we have also evaluated the effect of skin damage on a pig skin model, previously developed using skin explants from domestic pig ears [11,12]. …”

Comparison of the metabolism of 10 cosmetics‐relevant chemicals in EpiSkin™ S9 subcellular fractions and in vitro human skin explants

“…Differences for two chemicals were attributed to a lack of appropriate cofactor in S9 incubations and protein binding influencing chemical uptake in skin explants. Metabolism is a key step that can modulate skin absorption of topically applied chemicals and thus greatly affect skin bioavailability, as reported by Jacques et al for benzo[a]pyrene and testosterone (Jacques, Perdu, Duplan, et al, 2010;Jacques et al, 2014). approaches, reference methods to identify toxicants were based on animal tests that inherently integrate bioavailability; however, these are now banned according to the 7th Amendment to the Cosmetics Directive (EU, 2003).…”