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Some heteropsoralens, obtained by replacing one or both the intracyclic oxygen atoms with sulphur and/or selenium, were studied. In preliminary tests, these compounds showed strong photobiological activity, in some cases more than two orders of magnitude higher than that of psoralen. Heteropsoralens containing sulphur undergo intercalation inside duplex DNA, showing evident affinity for the macromolecule; when selenium replaces furan oxygen, the psoralen isoster also undergoes intercalation but with lower efficiency, while psoralen isosters in which pyrone oxygen is replaced by selenium practically do not intercalate. Parallel behaviour was also observed for DNA photobinding and crosslink formation. The cycloadduct between furan selenium and pyrone sulphur isoster and thymine was isolated and characterized. The capacity of the various psoralen isosters to generate singlet oxygen and superoxide radical anion was studied. For the former the yield varies markedly for the various compounds, while for the latter the yield is similar for all compounds.

In this paper a theoretical study, concerning molecular mechanics optimised structures, obtained by quantum mechanics as well as molecular mechanics calculations was carried out with the aim of correlating the theoretical model of the interactions between azapsoralens and DNA with the data experimentally obtained. The theoretical model suggests that both furan-side and pyrone-side double bonds may be involved in the cycloaddition with pyrimidines (although the cycloaddition at the level of furan is preferred), and is in line with the capacity of these compounds to form inter-strand cross-links. Moreover, concerning the theoretical intercalation model calculations on 3,4,4',5'-tetramethylazapsoralen intercalated inside a polynucleotide, they suggest a cis-syn arrangement between furan-side of the intercalated ligand and the above situated thymine, with which, under light activation, a cycloadduct may take place, having a cis-syn steric arrangement. Also this datum is in agreement with the cis-syn regio and stereochemistry of the isolated 4,4',5'-trimethylazapsoralen-thymine cycloadduct. Finally, from theoretical data, the role of nitrogen seems not important: in fact only small differences were found with the corresponding methylpsoralens so that the small differences observed may be mainly attributed to steric rather than to electronic effects. In general a good correlation between the theoretical model and the experimental data was observed.

With the aim to obtain new bis-intercalating agents in DNA a series of compounds was prepared linking two identical tricyclic moieties (two 4,5'-8-trimethylpsoralen (TMP)) or different (one TMP and one pyrido[3,2-b]quinoline) through a hydrocarburic aminated flexible chai. Bis-psoralen-amines as well as psoralen-pyrydoquinolin-amines were obtained. For comparison the corresponding mono-psoralen-amines were also prepared condensing a TMP moiety with various hydrocarburic aminated chains. Melting profiles of the complexes between bis-psoralen-amines or psoralen-pyridoquinolin-amines and DNA evidence two different thermal transitions, which can be correlated with bis-intercalation which takes place, at least in part, inside duplex DNA. On the other hand mono-psoralen-amines evidenced only one thermal transition, in line with mono-intercalation. Bis-intercalating agents complexed with DNA, under UVA irradiation photoconjugate covalently to the macromolecule, even if to a lower extent in comparison with mono-intercalating agents. Moreover these bis-intercalating agents in the photoreaction with DNA form inter-strand cross-links; also in this case bis-intercalating agents are less active than mono-intercalating agents.

7-Methyl- (1 a) and 4,7-dimethylallopsoralen (1 b), prepared about ten years ago and till now considered non-photosensitizing agents, have been re-investigated for studying their photochemical and photobiological properties. They form a molecular complex with DNA in the ground state, undergoing intercalation between two base pairs of the macromolecule. By successive irradiation with U.V.-A they covalently photobind to DNA inducing only the formation of mono-adducts. They are not able to induce erythema on guinea pig skin, but show evident antiproliferative activity on various biological substrates. Compound (1 b) shows both photochemical and photobiological properties at a degree higher than that of compound (1 a).

Biological and pharmacological effects of psoralens are correlated to their capacity to photoreact with pyrimidine bases of native DNA and in particular to form inter-strand cross-linkages. To determine possible prefenrential sites for the photoreaction the Authors have studied the capacity of psoralen to form complexes and to photoreact with various polynucleotides. The Authors have shown that specific sites exist in DNA for the photochemical interaction with psoralen; these sequences which can be considered as specific receptors for photobiological activity of psoralen are represented by alternate sequences in every strand of A and T, corresponding to poly d(A--T) - poly d(A--T). These receptor sites show a high capacity for intercalation and successive photoreaction with psoralen.

TMP (4,5',8-trimethylpsoralen) usually employed in PUVA therapy and TMA (4,6,4'-trimethylangelicin), phototherapeutic agent under clinical and biological investigation, show an inhibitory effect of competitive type on the low Km cyclic AMP phosphodiesterase present in rat liver. The two drugs exhibit Ki values of 135 and 320 microM, respectively.