Résumé : Psoralens such as 8-methoxypsoralen and 4,5',8-trimethylpsoralen (TMP) are used in photochemotherapy for the treatment of a variety of epidermal proliferative diseases. Sequential treatments of the skin with psoralens plus ultraviolet light in the range of 320-400 nm (UVA light), referred to as PUVA therapy, results in the suppression of abnormal keratinocyte growth. With the recognition that the psoralens are phototoxic and carcinogenic, presumably due to their ability to intercalate into DNA and photo cross-link pyrimidine bases following UVA light activation, it is clear that the development of biologically active analogs lacking this activity would be of significant therapeutic benefit. Towards this goal we have characterized active 4'- and 5'-pyridinium derivatives of 4',5'-dihydro-TMP (H2TMP), a psoralen analog that does not form DNA cross-links. These analogs, which are charged at physiological pH and cannot penetrate cells, are unique in that they retain biological activity as inhibitors of keratinocyte cell growth when activated by UVA light. However, they do not appear to cross-link or damage DNA as determined by plasmid DNA unwinding and nicking experiments, in intact cells using fluorescent analysis of DNA unwinding assays, and by thymidine uptake studies. Reverse transcription-polymerase chain reaction and western blotting demonstrated that, unlike TMP and H2TMP, when activated by UVA light, the pyridinium derivatives were not inhibitors of transcription since interferon-gamma-inducible nitric oxide synthase mRNA and protein in the keratinocytes were unaffected. Taken together, our data suggest that uptake of the compounds by the cells and DNA cross-link formation are not required for growth inhibition. These findings further support the model that the cell membrane is an important target for the psoralens.