Photophysical, photochemical and photobiological properties of pyrrolocoumarins; a new class of photoactive compounds

With the aim of finding new photoactive compounds that may reduce the side effects of 8-methoxypsoralen photochemotherapy we report on some photophysical, photochemical and photobiological properties of recently synthesized pyrrolocoumarins, in particular 4-methyl-N-ethyl-pyrrolo[3,2-g]coumarin (PCNEt) which has an absorption maximum in the UV-A (320-400 nm). Laser (347 nm) flash photolysis studies showed triplet transients that were quenched by O2 and by ground state PCNEt. Singlet minus triplet spectra were broad (350-550 nm) and, at 700 nm, indicated solvated electron and radical production. PCNEt complexes with DNA in the dark and photobinds to thymine but does not form DNA cross-links. PCNEt was phototoxic in yeast with an action spectrum similar to its absorption spectrum. PCNEt showed photohaemolytic activity but was not phototoxic on guinea pig skin. These data suggest that PCNEt may photosensitize via several mechanisms: direct DNA photobinding, photodynamic action, or photoproduction of radicals.