Psoralens are materials that make the skin more sensitive to UV light. They are called photosensitizing agents and they found in plants or manufactured artificially. Psoralens are taken as pills (systemically) or can be applied directly to the skin, by soaking the skin in a solution that contains psoralen. The psoralens allow to lower the dose of the UVA energy. When they are combined with exposure to UVA in PUVA, they are highly effective at clearing psoriasis andvitiligo. Like UVB light treatments, the reason in case of vitiligo is that they increase the sensitivity of the Melanocyte, the cell that manufactures the skin color, to UV light. The melanocyte has sensors that see the UV light and tells it to manufacture the skin brown color. The color protects the body from the harming UV light. It can also be a connection with the skin's immune response.
Psoralen is belongs to Furocoumarine species, psoralen derivate from phototoxic agents, it can not produce melanin directly. It will produce the photochemical effect when absorbe the UV light. This medication is the early application in treating vitiligo. There are few points of the mechanism of this medication to treat the vitiligo. The first one is protopic will increase the efficacy of the UV light to skin. The density of the melancytes will increase in the epidermis. The protopic will make melanin oxidize. And make it spread to other part of skin. Secondly, It will destroy sulfhydryl compounds of skin because inflammatory reaction. Stimulating the activity of the tyrosinase, it leads to the formation and transportation of the melanin in time. Thirdly, stimulate the keratin to form cells and release some melanin cells factors, making the rest of the melancytes inside epidermis or follicle hair regenerate. Fourth, prevent the vitiligo relevant antibody expressing, so the melancytes will not be destroyed.