Effects of flavone 6-substitutions on GABAA receptors efficacy

Flavones have been studied for their activities via benzodiazepine site on the type-A γ-aminobutyric acid (GABA(A)) receptors, for which knowledge on structure-efficacy relationships has been rather limited in comparison to that on structure-affinity relationships. The present study focused on flavone 6-substitution, implied in previous studies being relevant to efficacy. Structure analogs, each varying only at position 6, were compared, including 6-fluoroflavone, 6-chloroflavone, 6-bromoflavone, and 2'-hydroxyflavone analyzed in the present study, as well as 6,2'-dihydroxyflavone reported earlier. Radio-ligand binding assays, whole-cell patch-clamp, and mouse behavioral experiments were performed. In consistent with a previous report, the present whole-cell patch-clamp and animal behavior experiments demonstrated 6-bromoflavone to be a positive modulator at GABA(A) receptors acting through flumazenil-sensitive high-affinity benzodiazepine site. In contrast, the other two 6-haloflavones were both neutralizing modulators. In vitro electrophysiological and in vivo animal experiments showed that 2'-hydroxyflavone was a neutralizing modulator, different in efficacy from its structural analog, 6,2'-dihydroxyflavone, a negative modulator of GABA(A) receptors. The fact that flavone analogs differing only at position 6 showed drastically different pharmacological properties clearly points to 6-substitution being an important determinant of efficacy. The results suggest that a large width of the first atom on the 6-substituent favors a high binding affinity of the 6-substituted flavone, whereas a large overall volume of the 6-substituent favors positive modulator activity, which could be modified by, e.g., 2'-hydroxyl substitution. These findings have contributed to the understanding of quantitative structure-efficacy relationships for flavones acting at GABA(A) receptors, and hence facilitation of flavone-based drug development.