A novel pre-clinical trial platform for evaluating candidate hypo- or hyperpigmentation-inducing agents identifies menthol as a UV-free tanning agent ex vivo

Patients that suffer from hypo- or hyperpigmented areas of the skin often have to endure great psychological distress. The development of topical agents that efficiently modulate hypopigmentation (e.g. in melasma, ephelides, lentigines) or hyperpigmentation (e.g. repigmentation of hypopigmented lesions, “UV-free tanning”), is a major challenge for dermatological research, since available preclinical in vitro models inadequately reflect the complex neuroectodermal-mesodermal interplay that modulates epidermal melanogenesis. Therefore, we investigated if the ex vivo human skin organ culture model could serve as preclinical assay for the qualitative and quantitative analysis of candidate pigmentation agents. We cultured full-thickness human skin ex vivo under serum-free conditions and either topically applied, or supplemented the medium with, the hyperpigmentation inducing agents [Nle4,D-Phe7]-α- melanocyte-stimulating hormone (NDP-MSH, 1μM), forskolin (25μM) or Dimethylsulfoxid (1%) for 3 days. NDP-MSH, forskolin and DMSO resulted in a significant increase in melanin content, significantly increased pre-melanosome formation, and significantly enhanced intraepidermal tyrosinase activity in situ. Interestingly, application of the hypopigmentation inducing tyrosinase inhibitors, kojic acid and 4- butylresorcinol, prevented melanogenesis induced by topical DMSO or systemic forskolin application.