About Methoxsalen Topical Psoralen ,Photosensitizing agent, Antipsoriatic. Mechanism of Action of Methoxsalen Methoxsalen is 8-methoxy-psoralen. Methoxsalen selectively inhibits the synthesis of deoxyribonucleic acid (DNA). At high concentrations it suppresses cellular RNA and protein. It binds preferentially to the guanine and cytosine moieties of DNA, leading to cross-linking of DNA and inhibits DNA synthesis and DNA function. 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 (cross linking of Psoralen to both strands of DNA). Reactions with proteins have also been described. PUVA promotes melanogenesis in normal skin. It increases pigmentation from transfer of melanosomes to keratinocytes.