PUVA Therapy (Psoralen and Ultraviolet A Therapy)

PUVA is an acronym for psoralen and ultraviolet A. PUVA refers to the interaction of long wavelength ultraviolet light (320-400 nm) with a pharmaceutical molecule of plant origin, psoralen, producing a type of "photodynamic chemotherapy." PUVA is useful in treating a number of human diseases.

Psoralens are a family of plant chemicals that are able to absorb light in the ultraviolet A (UVA) spectrum (320 nm-400 nm). Once this absorption has occurred, these chemicals become activated and can react within the body producing phototoxicity. Without exposure to the correct wavelengths of light, psoralens are not biologically active. Although human tissues are relatively resistant to the effects of ultraviolet A light alone, this changes dramatically when psoralens are present. The most obvious change is a marked increase in ultraviolet A sensitivity manifested as burning of the skin (a phototoxic burn). It is hypothesized that PUVA produces reactive oxygen atoms that lethally damage the DNA molecule and/or alters the chemical receptors of skin cells (epidermal cells) and certain immune cells (T-lymphocytes).

Two forms of the psoralen molecule are currently medically useful: 8-methoxypsoralen (8-MOP, Oxsoralen, Methoxsalen) and 5-methoxypsoralen (not FDA approved). 8-MOP is the only psoralen available in the United States by prescription. 8-MOP can be given orally or applied topically. The use of topical of 8-MOP is uncommon because of the high likelihood of severe sunburns using this technique. The oral dose of 8-MOP that is prescribed for each patient depends upon the individual's weight.

About 60-90 minutes after ingesting a weight-adjusted dose the 8-MOP, the patient's skin is exposed to fluorescent bulbs emitting UVA light energy. The precise amount of light exposure is specified by a physician in units of energy. The length of time of the exposure depends on the intensity of light discharged by the bulbs, which can vary significantly depending on the age of the bulbs. Modern PUVA boxes usually contain a device that monitors the amount of light energy to which the patient is exposed and will automatically end the treatment when the correct dose is attained. Initially, exposures generally last about 30 seconds and later can extend to as long as 20 minutes, depending on the patient's pigmentation and tolerance as well as the intensity of the UVA source. Exposures ought to occur no more often than every 48 hours in order to avoid burning. Generally, the amount of light to which the patient is exposed is gradually increased over the course of 30 treatments. Ultimately, the number of treatment exposures will depend on the patient's response to treatments. If the patient's disease is confined to palms and soles, there are UVA sources designed to treat such areas.

The PUVA bath is more popular in Europe than the U.S. The patient is immersed in a bathtub containing water to which psoralen has been added. After a specified period, the patient is exposed to UVA. As in standard PUVA, the light exposure is gradually increased.

Although not strictly PUVA, there is a treatment termed "extracorporeal photophoresis" whereby a patient's white blood cells are extracted from the bloodstream and then exposed to psoralen, followed by UVA, and then added back into the bloodstream. This is a rather expensive treatment option and is only used under exceptional conditions in a few institutions.

PUVA can potentially cause skin cancer, just as does natural ultraviolet light. In addition, because of the intensity and duration over which it may be given, patients are at greater risk to develop squamous cell skin cancers and melanomas in treated skin than would otherwise be the case. In addition, excessive aging of the skin manifest as excessive lentigines with atrophy (poikiloderma) is likely to occur in the areas treated. Although rare, serious burns are possible because of inadvertent over dosage of UVA. Patients must limit their environmental exposure to sunlight for 24 hours after taking psoralens.

There are a number of diseases where PUVA is of proven benefit, including psoriasis (but not psoriatic arthritis), mycosis fungoides (cutaneous T-cell lymphoma), graft versus host disease, and vitiligo. Occasionally, PUVA is also used to treat atopic dermatitis, chronic itching, and certain types of photodermatitis.