This editorial clarifies the evidence boundary between local skin deposition and systemic absorption. It outlines validation experiments to distinguish carrier effects from active ingredient efficacy, ensuring accurate formulation assessment without overgeneralizing in vitro data.
Defining the Evidence Boundary
Topical formulations operate through two distinct pathways: local deposition within the stratum corneum and systemic absorption into the bloodstream. Local deposition involves the retention of active ingredients in the epidermis to exert targeted effects, while systemic absorption refers to the penetration of these ingredients into circulation, potentially causing broader physiological impacts.
Current research predominantly relies on ex-vivo skin models to measure penetration rates. However, these models lack a circulatory system, meaning they cannot directly measure systemic absorption. Consequently, data from these models reflect only local penetration capabilities and cannot substitute for in vivo pharmacokinetic studies, which are essential for assessing systemic exposure risks.
Carrier Material Specificity
Carrier materials such as hydrogels and liposomes influence local deposition and penetration by altering the release kinetics of active ingredients and their interaction with the skin. For instance, stimuli-responsive hydrogels can release drugs under specific conditions like temperature or pH changes, thereby enhancing local retention. This mechanism is highly dependent on the material's specific properties and the experimental conditions under which it is tested.
The efficacy of these carriers is not universally transferable. For example, the penetration-enhancing effects of ionic liquid hydrogels observed in in vitro studies have not been fully validated in animal models for long-term safety or clinical efficacy. Furthermore, results from one carrier type, such as ethanolic lipid vesicles for essential oils, cannot be directly extrapolated to other systems like hydrogels for drug delivery without independent verification.
Biological Targeting and Attribution
The biological effects of local deposition must be interpreted in the context of specific target mechanisms. In conditions like psoriasis, local drugs aim to inhibit inflammatory pathways such as the IL-23/Th17 axis. However, the carrier itself does not directly determine target activation; rather, it indirectly influences this process by increasing the local concentration of the active ingredient to a threshold level.
Attributing biological effects to the active ingredient requires distinguishing it from the carrier's influence. For example, when assessing the antioxidant effects of essential oils encapsulated in liposomes, it is crucial to exclude the carrier's own impact on enzyme activity. Only by isolating these variables can researchers accurately attribute observed effects to the active components rather than the delivery system.
Validation and Failure Signals
A robust validation plan requires distinct experiments to isolate variables. First, Franz diffusion cell studies should measure cumulative recovery in the receptor fluid to assess transmembrane transport, using blank and positive controls. Second, separate experiments must quantify active ingredient concentration in the stratum corneum to evaluate local deposition, comparing different carrier types while keeping other parameters constant.
Predefined failure signals are essential for objective assessment. If the cumulative recovery in the receptor fluid does not increase significantly over time, or if the stratum corneum concentration remains below the detection limit, the formulation is deemed ineffective for that specific endpoint. Additionally, process parameters like temperature and stirring speed must be controlled to ensure carrier stability, with particle size and polydispersity index serving as key quality metrics.
Editorial and use boundary
Evidence is limited to in vitro models and specific carrier studies; no clinical efficacy or safety claims are made.