Synthesis and photobiological properties of 4,8-dimethyl-5'-carboxypsoralen: a major metabolite of 4,5',8-trimethylpsoralen.

Recently a major metabolite of 4,5',8-trimethylpsoralen (TMeP) (a photochemotherapeutic agent), was isolated from the urine of mice and human volunteers receiving the drug orally; it was identified as 4,8-dimethyl-5'-carboxypsoralen. The synthesis of this compound has been carried out to obtain a distinct confirmation of the structure of the urinary metabolite and to study its photochemical and photobiological properties. The results obtained showed that this interaction and photoreaction with DNA are very poor; this fact can be correlated with the presence of the ionizable carboxylic group that undergoes a repulsion by the phosphate residues of the macromolecule. This hypothesis is confirmed by the higher interaction and photoreaction with DNA of the 4,8-dimethyl-5'-carboxypsoralen methyl ester in which, of course, the ionizable character is no more present. In connection with this very low photoreacting capacity with DNA, the synthesized metabolite proved lacking of photosensitizing effects on human and guinea pig skin. This fact provides an explanation of the very low photosensitizing properties of TMeP when given orally in contrast with the high activity after topical application.

DNA receptor sites for the intercalation of nogalamycin.

The intercalation of the planar chromophoric moiety of nogalamycin between two base pairs of duplex DNA has been evidenced by means of low-dichroism measurements. The possible presence of specific binding sites for mogalamycin on DNA has been suggested by studies on the denaturation and renaturation of DNA complexed with nogalamycin. A clear evidence was obtained by investigating the interaction of nogalamycin with polydeoxyribonucleotides containing known, regularly repeating sequences, used as model compounds. The results obtained with these polymers and the DNA suggest that the segment containing both purine (A,G) anf pyrimidine (T,C) bases in alternate sequences is the preferential receptor site on the DNA. A decreasing affinity is exhibited by poly d(A--T)-poly d(A--T), poly d(G--C)-poly d(G--C) and poly dG-poly dC segments, in the order. The poly dA-poly dT sequence appears to be closed to the interaction of nogalamycin.

Contribution of monofunctional adducts formed by furocoumarins with DNA to the inhibition of nucleic acids synthesis.

In addition to the bifunctional adducts (cross-linkages), that furocoumarins on radiation at 365 nm form in DNA, monofunctional adducts also proved able to inhibit the nucleic acid synthesis in Ehrlich ascites tumor cells.

Receptor sites of DNA for the photoreaction with psoralen.

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.

Studies on the photosensitizing properties of angelicin, an angular furocoumarin forming only monofunctional adducts with the pyrimidine bases of DNA.

The bioligical photosensitizing properties of furocoumarins are due to the formation of adducts with the pyrimidine bases of DNA under irradiation with long wavelength ultraviolet light. The greatest importance is attributed to the difunctional adducts, which form cross-linkings between the 2 strands of DNA. As angelicin, photoreacting with DNA, forms only monofunctional adducts, and therefore no cross-linkings, its photosensitizing properties have been studied in order to evaluate the ability of monofunctional adducts to produce biological effects. The results obtained studying the inhibition of DNA, RNA and protein synthesis in Ehrlich ascite tumor cells after irradiation in the presence of angelicin and psoralen (for a comparison), and the inhibition of the ability of identically treated cells to transmit the tumor showed a remarkable ability of monofunctional adducts to produce biological effects.