Each ml. of Oxsoralen Lotion contains 10 mg methoxsalen in an inert vehicle containing alcohol (71% v/v), propylene glycol, acetone, and purified water.
Methoxsalen is a naturally occurring substance found in the seeds of the Ammi majus (Umbelliferae) plant and in the roots of Heracleum Candicans. It belongs to a group of compounds known as psoralens or furocoumarins. The chemical name of methoxsalen is 9-methoxy-7H-furo(3, 2g) (1)-benzopyran-7-one. It has the following structure:
II. CLINICAL PHARMACOLOGY
The exact mechanism of action of methoxsalen with the epidermal melanocytes and keratinocytes is not known. Psoralens given orally are preferentially taken up by epidermal cells (Artuc et al, 1979). 1 The best known biochemical reaction of methoxsalen is with DNA. Methoxsalen, upon photoactivation, conjugates and forms covalent bonds with DNA which leads to the formation of both monofunctional (addition to a single strand of DNA) and bifunctional adducts (crosslinking of psoralen to both strands of DNA) (Dall'Acqua et al, 1971). 2 Reactions with proteins have also been described (Yoshikawa et al, 1979). 3
Methoxsalen acts as a photosensitizer. Topical application of this drug and subsequent exposure to UVA, whether artificial or sunlight, can cause cell injury. If sufficient cell injury occurs in the skin an inflammatory reaction will result. The most obvious manifestation of this reaction is delayed erythema which may not begin for several hours and may not peak for 2 to 3 days or longer. It is crucial to realize that the length of time the skin remains sensitized or when the maximum erythema will occur is quite variable from person to person. The erythematous reaction is followed over several days or weeks by repair which is manifested by increased melanization of the epidermis and thickening of the stratum corneum. The exact mechanics are unknown but it has been suggested melanocytes in the hair follicles are stimulated to move up the follicle and to repopulate the epidermis. (Ortonne, et al, 1979). 4