Synthesis, photobiological activity and photoreactivity of methyl-thieno-8-azacoumarins, novel bioisosters of psoralen

4,4′-dimethyl-thieno-8-azacoumarin (6) and 4,4′,5′-trimethyl-thieno-8-azacoumarin (8) were synthesised. Their photobiological activity was tested on human tumour cell lines. Interestingly, for 6, a photocytotoxic ability higher in HL-60, comparable in HeLa cells, with respect to that of the well-known drug 8-methoxypsoralen (8-MOP), was demonstrated. The covalent photoaddition to DNA occurs by means of the molecular mechanism already demonstrated for furocoumarins. However, it is noteworthy that no skin phototoxicity appears.