A Stable Formula Still Needs Skin Data
Content and stability tests describe the final formula before use. Skin distribution, retention, release, and tolerability answer what happens after application.
Two sets of measurements
A topical formula can be uniform, contain the intended amount of active, and remain stable in storage. Those results describe the product before application. They do not show where the active is distributed in skin or how long it remains there.
The intended tissue determines the next test. A product designed to act near the surface and one aimed at a deeper local compartment require different distribution and retention readouts. The question is not simply whether something crosses skin, but whether the measured behavior fits the intended use. [1]
A carrier name is not a delivery result
Vesicles illustrate why formulation details matter. In a 2006 laboratory study using rabbit ear skin, researchers compared deformable liposomes and ethosomes carrying ketotifen. They tested whether the drug was inside the vesicles, outside them, or both. Delivery behavior changed with the carrier and the drug’s loading state; empty ethosomes did not improve delivery of non-entrapped ketotifen in that model. The finding concerns that formulation and experiment, not every vesicle or active. [2]
Another study examined ethosomes in miniature-pig skin. Electron microscopy did not show visible damage to the stratum corneum, while spectroscopy and thermal measurements indicated changes in its lipid organization. An intact-looking structure and a measurable molecular change can coexist. Neither observation establishes clinical tolerability of a finished product. [3]
Test the formulation that will be used
Carrier studies help frame a development question; they cannot substitute for measurements on the intended final formula. A change in vehicle, excipients, or manufacturing conditions can change the system being evaluated. Teams therefore need to check active stability and release alongside skin-layer distribution, local retention, and relevant tolerability under defined conditions. [1]
In-formula quality tells a team whether a product can be made and stored as intended. In-skin measurements show what happens after application. The two sets of data support different decisions, and the final formulation must be evaluated against its own delivery goal.
Sources
- Natsheh H, Touitou E. Molecules (2020), review of phospholipid vesicles and skin delivery.
- Elsayed MMA, et al. International Journal of Pharmaceutics (2006), ketotifen vesicle study in rabbit ear skin.
- Niu XQ, et al. International Journal of Pharmaceutics: X (2019), ethosome mechanism study in miniature-pig skin.