Structure-activity studies on the mechanism of action of miracil D.

To investigate the nature of the complex formed between DXA and Miracil D (l-diethylaminoethylamino-4-methyl-lOthiaxanthenone, Lucanthone, Nilodin), a series of derivatives of this carcinostatic, schistosomicidal, and bacteriostatic com pound have been examined in three test systems: the growth of intact Bacillus subtilis, the heat-denaturation profile of na tive DNA, and the subcellular DNA-directed RNA polymerase from Escherichia coli. The derivatives differed from the parent compound by modifications of the diamino side chain, the pres ence of other substituents on the heterocyclic ring system, or in the ring system itself. In the series of congeners in which only the side chain dif fered from Miracil D, substitution of a methyl group for hy drogen on the proximal nitrogen or absence of the terminal nitrogen was accompanied by loss of activity in all three test systems; a variety of other substitutions were compatible with activity. In the 6-chloro-Miracil D series, however, alkyl sub stitution on the proximal nitrogen of the side chain did not abolish activity. Derivatives with alterations in the 4-substituent of Miracil D, or modifications in the ring system itself, included a number of compounds which stabilized the DNA helix against heat denaturation but did not inhibit growth of B. subtilis significantly. On the basis of these data, the concept is proposed that the proximal nitrogen of the side chain of Miracil D interacts with a phosphate residue of the DNA backbone and that the hetorocyclic ring system interacts, probably by intercalation, with base residues in DNA. Correlations of the activities of the derivatives in these test systems with anticancer and schistosomicidal potencies are dis cussed.