Interaction of indole derivatives with monoamine oxidase A and B. Studies on the structure-inhibitory activity relationship.

Indole and isatin (2,3-dioxindole) analogues were studied as inhibitors of MAO-A and B. They exhibited reversible and competitive MAO inhibition. Three dimensional structures of the compounds tested were constructed and minimized using PC-based molecular graphic software. The QSAR analysis revealed the requirement of co-planar structure of substituents at C2 and C3 of indole ring for selective MAO-A inhibition, whilst type of bond was less essential. The presence of hydroxy group at C5 of isatin increased selectivity of MAO-A inhibition, however simultaneous insertion of substituents into both positions of indole ring (5-hydroxy-2-phenylindole) led to a decrease of MAO-A inhibition. The planar molecules demonstrating potent MAO-A inhibition have the average sizes 7 A in length and 6 A in width. The MAO B inhibition also depended on the sizes of planar molecules however distribution of electron density in the molecules was another precondition for the selective inhibition of the enzyme.