METHOXSALEN

METHOXSALEN
(meth-ox'a-len)
8-MOP, Oxsoralen, Uvadex
Classifications: psoralen; pigmenting agent;
Therapeutic:pigmenting agent

Pregnancy Category: C

Availability

10 mg capsules, 20 mcg/mL solution; 1% lotion

Action

Plant derivative with strong photosensitizing effects: used with ultraviolet-A light (UVA) in therapeutic regimens called PUVA (P-psoralen). After photoactivation by long wavelength, UVA, methoxsalen combines with epidermal cell DNA causing photodamage (cytotoxic action).

Therapeutic Effect

Photodamage inhibits rapid and uncontrolled epidermal cell turnover characteristic of psoriasis. Results in an inflammatory reaction with erythema. Strongly melanogenic.

Uses

With controlled exposure to UVA to repigment vitiliginous skin and for symptomatic treatment of severe disabling psoriasis that is refractory to other forms of therapy.

Unlabeled Uses

(PUVA therapy) mycosis fungoides.

Contraindications

Sunburn, sensitivity (or its history) to psoralens, diseases associated with photosensitivity (e.g., SLE, albinism, melanoma or its history); invasive squamous cell cancer; cataract; aphakia; previous exposure to arsenic or ionizing radiation; pregnancy (category C), lactation. Safety (oral) in children is not established.

Cautious Use

Hepatic insufficiency; GI disease; chronic infection; treatment with known photosensitizing agents; immunosuppressed patient; cardiovascular disease; lactation. Safety (lotion) in children <12 y is not established.

Route & Dosage

Idiopathic Vitiligo
Adult: Topical Apply lotion 1–2 h before exposure to UV light once/wk

Psoriasis
Adult: PO Give 1.5–2 h before exposure to UV light 2–3 times/wk: <30 kg, 10 mg; 30–50 kg, 20 mg; 51–65 kg, 30 mg; 66–80 kg, 40 mg; 81–90 kg, 50 mg; 91–115 kg, 60 mg; >115 kg, 70 mg

Administration

  • Note: Methoxsalen therapy with UV light (PUVa therapy) should be done under the complete control of a physician with special competence and experience in photochemotherapy.
Oral
  • Give with milk or food to prevent GI distress.
  • Maintain consistent time relationship between food–drug ingestion. Food digestion and absorption appear to affect drug serum levels.
Topical
  • Only small (<10 cm2), well-defined areas are treated with lotion. Systemic treatment is used for large areas.
  • Apply lotion with cotton swabs, allow to dry 1–2 min, then reapply. Protect borders of the lesion with petrolatum and sunscreen lotion to prevent hyperpigmentation.
  • Use finger cots or gloves to apply lotion and prevent photosensitization and burned skin.
  • Apply sunscreen lotion to the skin for about one third of the initial exposure time during PUVA therapy until there is sufficient tanning. Do not apply to psoriatic areas before treatment.
  • Store lotion and capsules at 15°–30° C (59°–86° F) in light-resistant containers unless otherwise directed by manufacturer.

Adverse Effects (≥1%)

CNS: Nervousness, dizziness, headache, mental depression or excitation, vertigo, insomnia. Special Senses: Cataract formation, ocular damage. GI: Cheilitis, nausea and other GI disturbances, toxic hepatitis. Skin: Phototoxic effects: severe edema and erythema, pruritus, painful blisters; burning, peeling, thinning, freckling, and accelerated aging of skin; hyper- or hypopigmentation; severe skin pain (lasting 1–2 mo), photoallergic contact dermatitis (with topical use), exacerbation of latent photosensitive dermatoses, malignant melanoma (rare). Body as a Whole: Transient loss of muscular coordination, edema, leg cramps, systemic immune effects, drug fever.

Interactions

Drug: Anthralin, coal tar, griseofulvin, phenothiazines, nalidixic acid, sulfonamides, bacteriostatic soaps, tetracyclines, thiazides compound photosensitizing effects. Food: Food will increase peak and extent of absorption.

Pharmacokinetics

Absorption: Variably from GI tract. Peak: 2 h. Duration: 8–10 h. Distribution: Preferentially taken up by epidermal cells; distributes into lens of eye. Elimination: 80–90% in urine within 8 h. Half-Life: 0.75–2.4 h.

Nursing Implications

Assessment & Drug Effects

  • Schedule a pretreatment ophthalmologic exam to rule out cataracts; repeat periodically during treatment and at yearly intervals thereafter.
  • Lab tests: Monitor CBC, kidney and liver function, and antinuclear antibody tests during oral therapy.
  • Fair-skinned patients appear to be at greatest risk for phototoxicity from PUVA therapy (see ADVERSe EFFECTS).
  • Be aware that repigmentation is more rapid on fleshy areas (i.e., face, abdomen, buttocks) than on hands or feet.

Patient & Family Education

  • Expect that effective repigmentation may require 6–9 mo of treatment; periodic treatment usually is necessary to retain pigmentation. If, after 3 mo of treatment, there is no apparent response, drug is discontinued.
  • Avoid additional exposure to UV light (direct or indirect) for at least 8 h after oral drug ingestion and UVA exposure.
  • Understand intended treatment schedule: After topical application, the initial sunlight exposure is limited to 1 min, with subsequent gradual and incremental exposures by prescription.
  • Avoid additional UV light for 24–48 h after topical application and UVA exposure.
  • Wear sunscreen lotion (with SPF 15 or higher) and protective clothing (hat, gloves) to cover all exposed areas including lips, to prevent burning or blistering if sunlight cannot be avoided after the treatment.
  • Do not sunbathe for at least 48 h after PUVA treatment. Sunburn and photochemotherapy are additive in the production of burning and erythema.
  • Wear wraparound sunglasses with UVA-absorbing properties both indoors and outdoors during daylight hours for 24 h. Do not substitute prescription sunglasses or photosensitive darkening glasses; they may actually increase danger of cataract formation.
  • Alert physician to appearance of new psoriatic areas, flares, or regressed cleared skin areas during treatment and maintenance periods.

Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug

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