Comparative human and porcine skin permeation profiles of novel metformin lotion formulations

Metformin (Met), a highly hydrophilic drug, exhibits anti-inflammatory and regenerative effects beyond its antidiabetic role. However, its extreme polarity limits passive diffusion through the stratum corneum, necessitating optimized formulations for effective transdermal delivery. This study compared Met permeability through human and porcine ear skin using three lotion formulations containing different permeation enhancers: (1) 6% Met + glycerol (Gly); (2) 6% Met + Gly and propylene glycol (PG); and (3) 10% Met + PG and Transcutol^® (PG + T). In human skin, Gly and PG formulations showed minimal permeation, whereas PG + T markedly increased cumulative permeation and flux, confirming the critical role of Transcutol^®. Porcine skin displayed consistently higher permeability across all formulations (Gly < PG < PG + T), indicating that it may overestimate human permeability to hydrophilic drugs. Despite limited transdermal flux, Gly- and PG-based lotions produced measurable Met accumulation within the epidermis and dermis, suggesting potential for topical use. Overall, these results emphasize the importance of enhancer selection-particularly Transcutol^®-in improving dermal delivery of hydrophilic compounds and caution against relying solely on porcine skin as a human surrogate.