Correlation between electron-donating ability of a series of 3-nitroflavones and their efficacy to inhibit the onset and progression of aberrant crypt foci in the rat colon.

A series of five 3-nitroflavones were tested for their ability to inhibit the formation of colon aberrant crypt foci (ACF) induced by a s.c. injection of azoxymethane (C2H6N2O) in rats. Our aim was to relate the electron-donating effects of the 3-nitroflavones as characterized by their Hammett substitution constants with their efficacy in inhibiting ACF. In a first assay (initiation, protocol A) the 3-nitroflavone as well as the 4'-substituted nitro-, methoxy-, fluoro-, and hydroxy-3-nitroflavones were continuously present in the diet. In a second assay (postinitiation, protocol B) they were given for a period of 4 weeks after the last azoxymethane injection. The different substituents of the 3-nitroflavones at the 4'-position spanned a spectrum of Hammett constants (sigma(p+)), going from +0.79 for the electron-withdrawing group, NO2, to -0.92 for the electron-donating group, OH. For both protocols the percentages of inhibition plotted versus the Hammett substitution constants showed a linear correlation, the most efficacious ACF inhibition being produced by the molecules with the most electron-donating substituents. Moreover, the nitroflavones were not only chemoprotective during initiation of the ACF, but also therapeutic in the postinitiation progression assay. The above correlations may be of predictive value in the search for new chemoprotective agents. The overall molecular mechanism of the inhibition of ACF by the 3-nitroflavones under study appears to involve redox reactions.