Prenylated Flavanones Isolated from Flowers of Azadirachta indica (the Neem Tree) as Antimutagenic Constituents against Heterocyclic Amines

Four prenylated flavanones were isolated from the methanol extract of the flowers of Azadirachta indica (the neem tree) as potent antimutagens against Trp-P-1 (3-amino-1,4-dimethyl-5H-pyrido[4,3b]indole) in the Salmonella typhimurium TA98 assay by activity-guided fractionation. Spectroscopic properties revealed that those compounds were 5,7,4′-trihydroxy-8-prenylflavanone (1), 5,4′-dihydroxy7-methoxy-8-prenylflavanone (2), 5,7,4′-trihydroxy-3′,8-diprenylflavanone (3), and 5,7,4′-trihydroxy3′,5′-diprenylflavanone (4). All isolated compounds were found for the first time in this plant. The antimutagenic IC50 values of compounds 1-4 were 2.7 ( 0.1, 3.7 ( 0.1, 11.1 ( 0.1, and 18.6 ( 0.1 IM in the preincubation mixture, respectively. These compounds also similarly inhibited the mutagenicity of Trp-P-2 (3-amino-1-methyl-5H-pyrido[4,3-b]indole) and PhIP (2-amino-1-methyl-6phenylimidazo[4,5-b]pyridine). All of the compounds 1-4 strongly inhibited ethoxyresorufin Odealkylation activity of cytochrome P450 1A isoforms, which catalyze N-hydroxylation of heterocyclic amines. However, compounds 1-4 did not show significant inhibition against the direct-acting mutagen NaN3. Thus, the antimutagenic effect of compounds 1-4 would be mainly based on the inhibition of the enzymatic activation of heterocyclic amines.