A study of the relationship between photosensitizing and therapeutic activity of 4,5',8-trimethylpsoralen, and its major metabolite 4,8-dimethyl, 5'-carboxypsoralen.

The molecular basis for the clinically observed differences in the skin photosensitizing activity and therapeutic effectiveness of the topically applied and orally administered drug trimethylpsoralen (TMP) was investigated. TMP, when tested topically, is a very potent photosensitizing and therapeutically effective furocoumarin in the treatment of psoriasis. When administered orally, however, it is significantly less photosensitizing and therapeutically a less effective drug than the commonly used furocoumarin 8-methoxypsoralen. This decreased reactivity of oral TMP is attributable to its poor solubility and rapid in vivo metabolic transformation to several inactive (nonphotosensitizing) metabolites, one of which is referred to as 4,8-dimethyl,5'-carboxypsoralen (DMeCP). The supporting evidence has been obtained by: (a) isolation of the urinary metabolite DMeCP and subsequent comparison of its properties with the synthetically prepared DMeCP and its methyl ester; (b) examining the dark and photochemical interactions of TMP, DMeCP, and DMeCP methyl ester with DNA and determining their ability to form interstrand cross-links with DNA; and (c) studying the inhibition of DNA and RNA synthesis in Ehrlich ascites tumor cells and the killing of bacteria and T2 bacteriophages. The structure-activity relationship of TMP and DMeCP also has been examined in normal human subjects and in patients with psoriasis. The order of topical therapeutic effectiveness in terms of ability to clear psoriasis plaques appeared to be: TMP greater than 8-MOP greater than DMeCP methyl ester greater than DMeCP. The data also suggest the methyl ester of DMeCP to be an interesting nonphotosensitizing furocoumarin that photoconjugates to DNA better than 8-MOP and is therapeutically effective in psoriasis.