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Created on 09.29
Unveiling the human skin barrier: Measuring the transdermal water loss (TEWL) value across common skin areas
Unveiling the human skin barrier: Measuring the transdermal water loss (TEWL) value across common skin areas
In the "Technical Guidance Principles for In Vitro Release (IVRT) and In Vitro Transdermal (IVPT) Studies of Locally Acting Chemical Generic Drugs (Trial)" issued by the CDE in June 2025, it is recommended to use ex vivo human skin as an IVPT skin model.
In its "Draft Industry Guidelines for In Vitro Permeation (IVPT) Studies Submitted with ANDA Applications for Topical Drug Products" issued in October 2022, the FDA stated: "A reasonable acceptance criterion for skin barrier integrity (limit value) may be set as follows: the transdermal water loss (TEWL) barrier test value for skin on the human trunk or thigh should not exceed approximately 15 g/m²/h (i.e., ≤15 g/m²/h); if this limit value is adopted, skin specimens with a TEWL value>15 g/m²/h shall be considered non-compliant. Skin specimens that fail the barrier integrity test may not be used in drug administration studies, but may be used as non-administered control skin specimens."
In Appendix 2 of the "Guideline on the Quality and Equivalence of Locally Applied, Locally Acting Cutaneous Products" issued by EMA in September 2024 – specifically regarding the selection of skin models for IVPT testing guidelines – it is stated: "Prior to each test, skin integrity should be assessed. The selected skin integrity testing method and its acceptance criteria must be specified; furthermore, the acceptance criteria proposed for all parallel tests shall be reasonable and consistent."
The varying skin absorption rates across different body regions are influenced by factors such as stratum corneum thickness, hydration level, and lipid composition. The FDA recommends using ex vivo human skin samples – specifically skin from the trunk and thighs – for these assessments; trunk skin primarily refers to the skin on the chest, abdomen, back, and pelvic area. We measured the transdermal water loss values for these common human skin sites using the VAPOSCAN Transdermal Water Loss Measuring Device (ASCH, Japan) in a room with an ambient temperature of 22.5°C and a relative humidity of 52% (Table 1).
Conclusion and Discussion:
The minimum TEWL value measured across 13 different skin sites was observed on the back (5.5 g/m²/h), while the maximum value was recorded on the palms (30.9 g/m²/h); the mean TEWL value was 10.0 g/m²/h and the median was 7.6 g/m²/h. The TEWL values were predominantly concentrated within the range of 5–10 g/m²/h, with a standard deviation (SD) of 6.7. In contrast, the mean TEWL values for the trunk and thigh skin were 6.4 g/m²/h, indicating a smaller variability and more consistent data. It is recommended that pharmaceutical formulation developers, when conducting In Vitro Permeation Testing (IVPT) using ex vivo human skin models, determine the barrier integrity threshold based on the specific characteristics of the target skin sites.
References:
1. CDE: Technical Guidance Principles for In Vitro Release (IVRT) and In Vitro Transdermal (IVPT) Study Techniques for Locally Acting Chemical Generic Drugs (Trial Version), June 2025
2. FDA: In Vitro Release Test Studies for Topical Drug Products Submitted under ANDAs – October 2022
3. EMA: Guideline on the quality and equivalence of locally applied, locally acting cutaneous products – September 2024
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