<p>The combination treatment regimen of Psoralen (P) and UVA (Ultraviolet radiation of 320-400 nm wavelength) is commonly known as PUVA. Skin reactivity to UVA radiation is enhanced by the ingestion of Methoxsalen. The drug reaches its maximum bioavailability 1-3 hours after oral administration and may last for up to 8 hours. Methoxsalen is reversibly bound to serum albumin and is also preferentially taken up by epidermal cells. In both mice and man, Methoxsalen is rapidly metabolized. Approximately 95% of the drug is excreted as a series of metabolites in the urine within 24 hours.<br />
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The exact mechanism of action of Methoxsalen with the epidermal melanocytes and keratinocytes is not known. The best known biochemical reaction of Methoxsalen is with DNA. Methoxsalen, upon photoactivation, conjugates and forms covalent bonds with DNA (both monofunctional and bifunctional). Reactions with proteins have also been described.</p>