Structure-activity relationship in potentially anti-tumor promoting benzalacetone derivatives, as assayed by the epstein-barr virus early antigen activation

The in vitro anti-tumor promoting activities of antimutagenic benzalacetone (4-phenyl-3-buten-2-one), its monosubstituted derivatives and related compounds, cinnamaldehydes and cinnamic acids, were evaluated by determining the inhibitory effect on Epstein-Barr virus early antigen (EBV-EA) activation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) in Raji cells. In this short-term assay, benzalacetone, which is the basic structure of dehydrozingerone (one-half analog of curcumin) inhibited the EBV-EA activation; the IC(50) value, the molar ratio of benzalacetone to TPA needed for inhibiting 50% of positive cells activated with 32 pmol TPA, was 129. IC(50) values of 2- and 4-methoxybenzalacetones were about one-half of that of benzalacetone and the methoxy compounds were more effective than hydroxybenzalacetones. IC(50) values of chloro- and trifluoromethyl-benzalacetones were higher than that of benzalacetone, indicating that these compounds are weaker inhibitors. In addition, the position of a substituent on the benzene ring affected the inhibitory effect. In benzalacetone derivatives substituted by a hydroxy-, methoxy-, chloro- or trifluoromethyl group, the 2-substituted derivatives exhibited the strongest inhibitory effect, followed by the 3- and the 4-substituents. Cinnamic acid derivatives also decreased the inhibitory effects in the same order. In the side chain of benzalacetone, the terminal group adjacent to the carbon-carbon double bond also affected the inhibitory effect. The conversions of the methylketone to aldehyde and carboxyl groups, i.e., cinnamaldehyde and cinnamic acid, increased the inhibitory effect: the IC(50) values were about one-third of that of benzalacetone. beta-Methyl styrene, which in the side chain has no carbonyl group adjacent to the double bond, inhibited the EBV-EA activation at the concentration of about one-third of that of benzalacetone, indicating that the carbonyl group negatively affects the inhibitory effect. This agreed with the previous observation between isoeugenol and dehydrozingerone, 4-hydroxy-3-methoxy derivatives of beta-methyl styrene and benzalacetone, respectively. The mechanism of the EBV-EA activation inhibition was discussed by being compared with the inhibition of mutagenesis for which the unsaturated bonded-carbonyl system is necessary.