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The antiproliferative activity of the classic photochemotherapeutic agents, that is bifunctional psoralens (8-methoxypsoralen, 8-MOP; 5-methoxypsoralen, 5-MOP; 4,5',8-trimethylpsoralen, TMP) as well as of those more recently proposed, that is, monofunctional furocoumarins (methylangelicins and 3-carbethoxypsoralen, 3-CPs) is ascribed to their capacity to photo-induce lesions in DNA. In this connection while bifunctional agents photo-induce both mono- and bifunctional (inter-strand cross-linkages) damages in the macromolecule both in vitro and in vivo, monofunctional furocoumarins photodamage DNA only through monofunctional lesions. To have a direct and precise comparison between the classic agents (8-MOP, 5-MOP, TMP) and those recently proposed (4,5'-dimethylangelicin, 4,5'-DMA; 3-CPs), we have made a comparative study of the capacity of the various compounds to form the dark complex with DNA as well as of their capacity to induce mono- and bifunctional damages in the macromolecule. A comparative study on the antiproliferative activity in terms of capacity to inhibit DNA and RNA synthesis in Ehrlich ascites tumor cells as well as of the capacity to inhibit epidermal DNA synthesis in mice is also reported. The emerging picture can give a precise comparison in terms of both photobinding to DNA, as well as antiproliferative and skin-phototoxic activities between the classic agents 8-MOP, 5-MOP, TMP and the new monofunctional agents 4,5'-DMA and 3-CPs.

In order to study the relationship between certain photophysical and photochemical properties of furocoumarins and their photobiological activities, the quantum yields of the formation of excited triplet states, the capacity to generate singlet oxygen in vitro and the effect of oxygen on photoinduced cell killing and on the induction of cytoplasmic "petite" mutations in Saccharomyces cerevisiae were determined for a series of 4'-methylangelicins and 3-carbethoxypsoralen (3-CPs). The capacity to generate singlet oxygen, in good agreement with the photophysical data, correlates well with the oxygen effect observed on the lethality of Saccharomyces cerevisiae shown by the various compounds and followed the ranking order 3-CPs greater than 4',4-dimethylangelicin greater than 4'-methylangelicin greater than 4',5-dimethylangelicin. On the other hand the oxygen effect on the induction of cytoplasmic "petite" mutations on yeast appears much less pronounced than that shown on survival. Concerning other photobiological properties previously studied no correlation was observed between the capacity to generate singlet oxygen in vitro and the skin photosensitizing activity of the compounds. Moreover, the generation of singlet oxygen by furocoumarins does not appear as the main cause for the photoinduction of skin tumors in mice.

Nine furothiocoumarins corresponding to psoralen, 8-methylpsoralen, 3-carbethoxy-8-methylpsoralen, 8-methoxypsoralen, pseudopsoralen, angelicin, isopseudopsoralen, allopsoralen and pseudoisopsoralen were synthesized by treating furocoumarins with phosphorus pentasulfide. Photobiological studies on haploid yeast cells (Saccharomyces cerevisiae) revealed that the furothiocoumarins exert some photoactivity on cell survival and the induction of mitochondrial damage. In most cases, the furothiocoumarins were less active than their furocoumarinic counterparts and exhibited a preference for a monofunctional type of action. A certain photochemotherapeutic activity can be suggested.

In this paper the photochemical and photobiological properties of 8,8-desmethyl-xanthyletine (homopsoralen) have been studied. This drug forms a molecular complex with DNA undergoing intercalation between two base pairs of the macromolecule. By subsequent irradiation (365 nm) the compound photobinds covalently to the macromolecule showing an initial DNA-photobinding rate higher than that of 8-methoxypsoralen. In the photoreaction with DNA the drug effectively forms inter-strand cross-linkages. Its ability to generate singlet oxygen when irradiated with ultraviolet light is markedly higher than that of 8-MOP. Homopsoralen when irradiated in water solution undergoes effective photolysis. The drug shows a lower antiproliferative activity on diploid yeast (D 7) than 8-MOP and an almost parallel mutagenic activity. Also the gene convertogenic activity, determined on the same yeast strain, is lower than that of 8-MOP. Homopsoralen does not show any skin phototoxicity on guinea-pig skin. Taking into account its DNA-photobinding capacity, its antiproliferative activity, its reduced genotoxicity and lack of skin-phototoxicity homopsoralen may deserve further studies for evaluating its photochemotherapeutic activity.

The metabolism of 8-methoxypsoralen (8-MOP) by the yeast Saccharomyces cerevisiae has been investigated in order to determine if this cell system could provide a simple model to study the metabolism of new photosensitizing drugs in vitro. 8-MOP was found to be rapidly metabolized by S. cerevisiae in non growing conditions. A metabolite was detected which exhibits a structure similar to that of a metabolite previously isolated in dog and man. Ethanol showed a strong inducing effect on 8-MOP metabolization.

The mutagenic activity of photoadditions induced by 4,5'-dimethylangelicin (4,5'-DMA), 3-carbethoxypsoralen (3-CPs), angelicin, 8-methoxypsoralen (8-MOP), 5-methoxypsoralen (5-MOP) and 4,5',8-trimethylpsoralen (TMP), was studied in haploid yeast cells at the nuclear and cytoplasmic levels. With regard to the induction of cell killing 4,5'-DMA and 3-CPs were about 5 to 6 times more active than 8-MOP probably due to an efficient repair of 4,5'-DMA or 3'CPs induced monoadditions in DNA. 4,5'-DMA was about 5-fold less active than angelicin, reflecting the different photoreactivities of the compounds towards DNA. In accord with its monofunctional activity, 4,5'-DMA was less efficient on the induction of nuclear reversions but more efficient on the induction of cytoplasmic "petite" mutants per viable cell than 8-MOP and the other bifunctional compounds. In contrast to 3-CPs and angelicin, the reversion induction by 4,5'-DMA followed a 2 hit kinetics. For the induction of CanR forward mutants per viable cell 4,5'-DMA was more efficient than 3-CPs and approached the activity of 8-MOP; angelicin, 5-MOP and TMP were slightly more efficient than 8-MOP, whereas 3-CPs was clearly less efficient. Thus, the monofunctional furocoumarins exert different genotoxic effects when compared on a survival basis. Cell killing effects in the presence and in the absence of oxygen were also evaluated.