Drug Interaction Report

MONITOR: Concomitant use of methoxsalen with other known photosensitizing agents may increase the risk of photosensitivity reactions. These agents include anthralin, coal tar or coal tar derivatives, griseofulvin, hypericin extracts (e.g., St John's Wort), fluoroquinolones, phenothiazines, retinoids, halogenated salicylanilides (bacteriostatic soaps), sulfonamides, sulfonylureas, tetracyclines, thiazides, and certain organic staining dyes such as methylene blue, toluidine blue, rose bengal, and methyl orange.

MANAGEMENT: Caution is advised and pharmacologic response to methoxsalen therapy should be carefully monitored if concomitant use of other photosensitizing agents cannot be avoided. Patients should be advised to avoid sun exposure, even through window glass or cloud cover, for at least 8 hours after methoxsalen ingestion and during the 24 hours following photochemotherapy or photopheresis treatment. Protective devices should be used if sun exposure cannot be avoided, such as a hat and gloves and/or sunscreens containing ingredients that filter out UVA radiation (e.g., benzophenone and/or PABA esters). Sunscreens must be applied to all areas that might be exposed to the sun, including the lips. For the treatment of psoriasis, sunscreens should not be applied to areas affected by psoriasis until after treatment in the UVA chamber. In addition, UVA-absorbing wrap-around sunglasses should be worn during daylight for 24 hours after methoxsalen ingestion and during the 24 hours following photochemotherapy or photopheresis treatment to prevent the irreversible binding of methoxsalen to proteins and DNA components of the lens, which can lead to formation of cataracts. The glasses should be worn any time patients are exposed to direct or indirect sunlight, whether they are outdoors or exposed through a window. Because erythema and/or burning due to photochemotherapy and sunburn due to sun exposure are additive, patients should not sunbathe for 48 hours after photochemotherapy.

GENERALLY AVOID: The ingestion of foods containing photosensitizing components (e.g., limes, figs, parsley, parsnips, rue (Ruta graveolens), mustard, carrots and celery) may increase the risk of photosensitivity and severe burning during methoxsalen therapy. Two cases of photosensitivity involving rue and a soup containing celery, parsley, and parsnip have been reported in PUVA patients.

MANAGEMENT: Patients who are undergoing PUVA treatment and taking methoxsalen should be advised to avoid eating large quantities of these foods.

GENERALLY AVOID: Concurrent use of ethanol and phenothiazines may result in additive CNS depression and psychomotor impairment. Also, ethanol may precipitate dystonic reactions in patients who are taking phenothiazines. The two drugs probably act on different sites in the brain, although the exact mechanism of the interaction is not known.

MANAGEMENT: Patients should be advised to avoid alcohol during phenothiazine therapy.