About Sébastien Grégoire
From analytical chemistry to skin biology, Sébastien Gregoire supports the development of innovative topical products through skin absorption filter. His 25 years of experience in Pharmaceutical and cosmetic industry allows him to assist development of topical products from early in silico stage to ultimate in vivo proof. Author and co-author of more than 50 articles covering all aspects of skin bioavailability, Sébastien Grégoire participated in international collaboration through Cosmetic Europe programs. His freelance activity founded in 2024 addresses any questions related to topical exposure including analytical support.
How is cutaneous bioavailability defined?
The bioavailability reflects the ability of the chemical to reach the systemic circulation after a given route of administration. Obviously, it includes the ability of the chemical to pass the barrier (gut, lung, skin…) and local metabolism. From a pharmacokinetic point of view, bioavailability is defined using pharmacokinetic (PK) data. For a given route of exposure, the bioavailability is the ratio of AUC (Area Under the Curve) of the route of administration divided by the AUC of Intravenous route, corrected for dose ratio if necessary.
For cosmetics, topically applied, PK data is usually not available. Another definition is used to measure skin bioavailability. It is defined as the fraction of parent chemical found in the viable part of the skin (e.g. Viable Skin + Dermis + Receptor Fluid) after topical exposure to in-vitro skin. Of course, such a definition implies that the skin is metabolically active for chemical metabolized in the skin. Otherwise, the skin bioavailability is overestimated. In the same way, if radiolabeling is used with simple scintillation counting and metabolism or chemical degradation takes place, the skin bioavailability is overestimated as it includes parent and metabolites/degradation products.
Skin bioavailability has another level of difficulty compared to the other routes of administration. The skin bioavailability depends on the dose but also on the duration of the exposure. For this reason, skin bioavailability is measured over 24 hours, or for rinsed product such as hair dye, after 30 min exposure time followed by 23.5 h post exposure.
Thus, skin bioavailability is the sum of ADME process occurring locally in the skin.
Skin is a complex organ with different diffusion pathways. What are they? How are they differentiated?
Skin is complex organ made of different layers. Among the different layers, Stratum Corneum (SC) is defined as the rate limiting step. Modelled as a simple homogenous membrane in the Potts & Guy relationship. This assumption does not reflect the real structure of the SC, which is a composite membrane typically describes as a brick-and-mortar structure. According to its physic-chemical properties, a chemical can pass through the lipid domain and/or through the corneocyte. The situation is even more complicated. Indeed, the corneocyte is surrounded by a cornified envelope and a lipid envelope. Corneocytes are connected one to each other by corneodesmosomes [1] (Sjövall et aL 2024). These sub-compartments can contribute to the chemical diffusion.
As a first intention, lipophilic chemical passes preferentially through the lipid domain. Nevertheless, corneocytes could have a significant contribution to the diffusion of lipophilic chemicals. Hydrophilic chemicals pass through the skin more than expected based simply on the lipophilic properties of the SC. Appendages such as hair follicle is another route of absorption through the skin. Many studies support its contribution. Sebastia-Saez et Al quantified recently the contribution of transfollicular route to the skin permeability [2]. It represents up to 50% for hydrophilic chemicals and decreases for lipophilic chemicals. Lipophile chemicals can also pass through the hair follicle, but their permeability is limited with possible accumulation in sebaceous gland as observed on MALDI imaging [3]. Contribution of hair follicle is not enough to explain the skin permeability of hydrophilic chemicals. Existence of a polar pathway was proposed. It could be related to imperfections in the SC lipid layers. Such imperfections could affect the integrity of the frozen skin submitted to freeze-thaw cycling, which would rather affect the hydrophilic chemicals [4].
This brief description of different diffusion pathways in the skin points out the complexity to model the skin bioavailability. It can explain also the difference on skin permeability according to the anatomical site, such as forehead or scalp compared to back or forearm [5, 6].
Which methods can be used to characterize these pathways?
In vivo evaluation should be the most appropriate to measure effectively the skin bioavailability. Nevertheless, it addresses some difficulties. The method must be none invasive, excluding possibility of biopsies. Tape stripping can be used to monitor skin absorption. It is recognized for bioequivalence of topical products. Using appropriate protocol, kinetic information can be obtained by tape striping. However, it does not provide quantitative information on skin bioavailability. Raman spectroscopy and related methods have been greatly improved over the last years, thanks to data treatment and laser performance allowing the development of SRS, CARS. These spectroscopic methods provide concentration profile in vivo in the skin. The main limitation is related to the lack of specificity of spectroscopic method which can be overcome using labeling using deuterated chemical or partially with data treatment.
Obviously, measurement of skin bioavailability requires an ex vivo skin set up on a diffusion cell. Using an appropriate analytical method, the chemical is then quantified in the different skin layers. The measurement of skin permeability or skin bioavailability does not provide any information about the pathway used by the chemical to pass through the skin.
Combination of models and methods has to be used to define properly the diffusion pathway, firstly to investigate contribution of transfollicular route, the model must have follicles. Abdominal skins obtained from surgery have lack of follicle. For such study, inner face of pig ear skin is a suitable model. Pig skin is the only animal recognized as a good surrogate of human skin. Difference between pig and human skin could be due to the contribution of hair follicle. Different methods can be used to demonstrate the transfollicular route: hair follicle plugging, differential stripping, differential Sampling by micro-biopsy or imaging methods (such as MALDI, fluorescence). Each method has its pros and cons. The method is selected according to the information requested. If the information is about the ability of the chemical to reach sebaceous gland through the hair follicle, imaging method is appropriate. If the contribution of the transfollicular route to the overall permeability is searched, the plugging method has to be used. Whatever the method used, such study is unusual and requires specific and dedicated experiments.
To define precisely the diffusion pathway in the SC, method with enough high spatial resolution has to be used. Indeed, thickness of lipid layer between corneocyte is within 100 nm range and thickness of corneocyte is about 1 µm. Method such as SIMS does not have enough high resolution, typically with the µm range. Thus, it cannot be localized precisely lipid domain. An alternative is to use 3D-SIMS allowing the possibility to visualize at the nm range the distribution within the SC [1]. Spectroscopic technic such as CARS can also visualize the diffusion pathway in a confocal way [7].
A balance between question addressed, methods performance and outcome, models characteristics, study cost and timing allow to define properly the most appropriate approach.
References
- Sjövall, P. et al. Scientific Reports 14, 18681 (2024)
- Sebastia-Saez, D., Lian, G. & Chen, T. Pharmaceutical Research 41, 567–576 (2024)
- Grégoire, S. et al. Advanced Drug Delivery Reviews 153, 137–146 (2020)
- Abdayem, R. et al. Experimental Dermatology 24, 972–974 (2015)
- Bormann, J. L. and Maibach, H. I. Cutaneous and Ocular Toxicology 39, 213–222 (2020)
- Wargniez, W. et al. Pharmaceutical Research 39, 1935–1944 (2022).
- Chen, X. et al. Journal of Controlled Release 200, 78–86 (2015).
Contact
Sébastien Grégoire – CEO Consulting
sg@sebastiengregoire-consulting.fr
