Safety Assessment in Skin Care
related data. These methods all refer to
liver related metabolism:
• Microsomes-cellular organelles con-
taining metabolic enzymes;
• Liver suspended cells; and
• Three-dimensional human hepato-
A study describing testosterone metabolism in vitro using fresh and frozen
pigskin as well as human skin microsomes
was published in 2014. In this paper, scientists concluded that the absorption of
the steroid through frozen skin was lower
when compared to fresh skin. Using fresh
skin more than 95% of the applied dose
has been recovered in the study and corresponded to the compound metabolites.
The metabolites were indicative of oxidative enzymes. Microsomes are vesicles
that are formed in the lab from broken endoplasmic reticulum of debris of mechanically broken cells. This process separates
cytochrome P450 enzymes and fragmented endoplasmic reticulum. Microsomes
are considered a valuable tool for investigating compound metabolism in vitro.
It is clear that existing experimental models
and methodologies for the assessment of
skin metabolism are insufficiently validated.
An attempt to assess the utilization of fresh
full-thickness human skin explants as a
model was conducted and published in The
American Society for Pharmacology and
Experimental Therapeutics (ASPET) Journal
in 2015. Here, too, scientists determined that
the whole-skin metabolic activity was sig-
nificantly lower when compared to whole-
liver, but certainly relevant and important.
They demonstrated several metabolic
reactions in skin including glucuronida-tion, sulfation, N-acetylation, catechol
methylation, and glutathione conjugation.
When frozen, the skin lost its glucuroni-dation activity.
Compounds that demonstrated metabolism in this study were triclosan, indomethacin, diclofenac sodium, minoxidil
and estradiol. A limiting factor in using
skin explants from human donors to study
skin metabolism is the great variability between them, which can be affected by age,
lifestyle, ethnicity and other factors. In this
specific study variability was from 1.4- to
13.0-fold depending on the test substrate.
A joint 2010 publication by MatTek
and Procter & Gamble describes gene
expression analysis of metabolisms in
the 3D Epiderm in vitro model. The team
conducted a comparison of the expression of 139 genes encoding for xenobiotic
metabolizing enzymes in this model that
is composed of human-derived skin cells.
In this analysis, 87% of the genes were
consistent when compared to human skin.
This observation points toward some
similarities in metabolic function. The
study was conducted comparing Epiderm
models derived from four donors and
demonstrated consistency in the model
with donors’ variability. The enzymes expressed in this model were mainly phase
2 metabolic enzymes that are characteristic to skin metabolism.
In summary, while the scientific community agrees that skin metabolism is of
importance and can, in some cases, significantly affect product performance, there is
no clear assessment path. Although there
are testing options, none has been validated or extensively studied. In addition,
such studies require intense, potentially
costly, sensitive analytical capabilities.
A key question to be answered in the
chain of product development is the
amount of compound penetrated into
and through the skin as reflected in skin
absorption studies. If a significant amount
penetrates the circulation, liver metabolism may be a relevant endpoint to be
tested. If the compound resides in skin
living tissue, skin metabolism should be
considered. One of the reasons for finding no correlation between in vitro and
in vivo assays may be skin metabolism
since if the compound is metabolized in
the skin it may exhibit an activity that is
very different from that observed in vitro.
As always, the need to conduct skin
metabolism should be determined on a
case by case basis in the overall assessment
of risk. •
• Jacques C. et al. Effect of skin metabolism on dermal delivery of testosterone: qualitative assessment using a new short-term skin
model. Skin Pharmacol Physiol (2014) 188-200.
• Manevski N. et al. Phase 2 metabolism in human
skin: skin explants show full coverage of glucuronida-tion, sulfation, n-acetylation, catechol methylation and
glutathione conjugation. ASPET. 43 (2015) 26-139.
• Hu T. et al. Xenobiotic metabolism gene expression in the Epiderm in vitro human epidermis model
compared to human skin. Toxicology In Vitro 24 (2010)
If a compound concentrates in the skin tissue it may, within 30 days of
total skin renewal in healthy individuals, leave the skin via exfoliation.