Psoralens are naturally occurring compounds, classified as furocoumarins, found in various plants. These organic chemicals are potent photosensitizers, meaning they remain inert until exposed to long-wave ultraviolet light (UVA), when they become biologically active. This activation is the basis for their therapeutic applications and their ability to cause unwanted skin reactions.
Natural Sources of Psoralens
Psoralens are produced by many common plants, often as a defense mechanism against fungi. They are found in the Umbelliferae family, which includes vegetables like celery, parsnips, and parsley. The Rutaceae family of plants, known for citrus fruits, also contains these compounds; limes, lemons, and bergamot oranges are notable sources.
Figs, from the Moraceae family, are another source of psoralens. The concentration of these chemicals within a plant can increase when it is under stress from disease, bruising, or fungal infection. This variability means the potential for a skin reaction can change depending on the plant’s condition.
Therapeutic Uses and Mechanism
Psoralens are used in a medical procedure called PUVA (Psoralen + UVA) to manage skin disorders involving the overproduction of cells. The treatment involves ingesting a measured dose of a psoralen medication, like methoxsalen, or applying it topically. The targeted skin area is then exposed to a specific dose of UVA radiation.
Once activated by UVA light, psoralen molecules insert themselves into the DNA of skin cells. They form chemical cross-links between the DNA strands, which inhibits cell replication and division. For a condition like psoriasis, where skin cells multiply too rapidly, this process slows the formation of plaques. This makes PUVA an option for severe psoriasis that has not responded to other treatments.
This therapy is also applied to treat vitiligo, a condition where the skin loses its pigment-producing cells (melanocytes). PUVA therapy is thought to increase the sensitivity of any remaining melanocytes to UV light, stimulating them to produce melanin and repigment the skin. It is also used in treating cutaneous T-cell lymphoma, a type of skin cancer, by inducing the death of malignant cells. Long-term use of PUVA is associated with an increased risk of developing skin cancers, including squamous cell carcinoma and melanoma, due to its DNA-altering nature.
Phytophotodermatitis
Accidental exposure to psoralen-containing plants followed by sun exposure can lead to phytophotodermatitis. This is not an allergic reaction but a direct toxic and inflammatory response. It occurs when psoralens from plant juices on the skin are activated by sunlight’s UVA radiation.
This condition has earned descriptive names like “margarita burn” or “lime disease” (not to be confused with Lyme disease), from handling limes for drinks in the sun. Another example, “celery rash,” can affect grocery workers or gardeners who handle plants like parsnips or celery. The reaction is specific to the areas where the plant substance made contact with the skin.
Initial symptoms appear 24 to 48 hours after exposure and include redness, stinging, and itching, which can progress to painful blistering. A distinct feature is the post-inflammatory hyperpigmentation that follows the initial inflammation. As the blisters and redness subside, the affected skin becomes deeply pigmented, leaving dark patches that can persist for weeks or months. Prevention involves washing skin thoroughly with soap and water after handling these plants, especially before sun exposure.