Present aspects concerning the molecular mechanisms of photochemotherapy with psoralens.

The various photophysical and photochemical events that can be produced by psoralens after their excitation by UV-A radiation are reviewed, with particular reference to their possible significance for induction of photobiological effects. A close correlation has been found between the covalent photoaddition of furocoumarins to the pyrimidine bases of DNA and the inhibition of DNA synthesis inside living cells; this fact underlines the importance of this photoaddition for the antiproliferative effects exerted by furocoumarins. A good correlation has also been found between the formation of cross-linkages in DNA and the induction of erythema on skin; monofunctional furocoumarins, like methyl-angelicin derivatives, are unable to induce erythema, although they have very strong antiproliferative properties.