Impact of porcine skin processing on in vitro permeation testing: implications for reliable evaluation of topical metformin delivery

Porcine ear skin is widely used as a surrogate for human skin in in vitro permeation testing (IVPT) due to its structural and physiological similarities. However, variability in tissue sourcing represents a major but underrecognized source of error, as commercial processing steps such as scalding and dehairing can compromise skin barrier integrity. In this study, we investigated how porcine skin sourcing affects drug permeation using a topical 6% metformin lotion formulated with glycerol or glycerol plus propylene glycol. Permeation was evaluated using Franz diffusion cells over 48 h, and barrier structure was assessed histologically. Although functional barrier integrity testing (e.g., transepidermal water loss or electrical resistance) was not performed, structural assessment provided direct evidence of barrier disruption. Skin obtained from a butcher shop exhibited markedly increased permeation, with 20-60-fold higher flux within 2 h and cumulative permeation reaching ∼80% of the applied dose at 48 h. In contrast, skin procured directly from a slaughterhouse maintained structural integrity and barrier function, with cumulative permeation remaining below 30%. Histological analysis demonstrated loss of the stratum corneum and viable epidermis in processed skin, whereas these layers remained intact in unprocessed tissue. These findings demonstrate that commercially processed porcine skin can substantially overestimate drug permeation, thereby compromising the reliability and translational relevance of IVPT data. These results underscore the need for routine barrier integrity verification and standardized reporting of skin sourcing and handling conditions in IVPT studies. Standardized tissue sourcing, transparent reporting, and barrier integrity verification are essential to ensure physiologically meaningful evaluation of topical and transdermal drug delivery systems.