The aromatase inhibitory activity of compounds 1-11 was investigated by the method previously described by Stresser et al. (Stresser et al., 2000), with minor modifications (Prachayasittikul et al., 2014). The assay was performed with a Gentest kit using enzyme CYP19 and DBF as a fluorometric substrate. DBF was dealkylated by aromatase and then hydrolyzed, yielding the fluorescein product. Briefly, 100 µL of cofactor containing 78.4 µL of 50 mM phosphate buffer (pH 7.4), 20 µL of 20x NADPH-generating system (26 mM NADP+, 66 mM glucose-6-phosphate and 66 mM MgCl2), and 1.6 µL of 100 U/mL glucose-6-phosphate dehydrogenase was pipetted into a 96-well plate and preincubated in a water bath (37 °C) for 10 min. The reaction was initiated by adding 100 µL of an enzyme/substrate mixture containing 77.3 µL of 50 mM phosphate buffer (pH 7.4), 12.5 µL of 16 pmol/mL CYP19, 0.2 µL of 0.2 mM DBF and 10 µL of tested compound or 10 % DMSO as a negative control or ketoconazole/letrozole as a positive control. After the incubation at 37 °C for 30 min, the reaction was terminated by adding 50 µL of 2.2 N NaOH. The fluorescence signal was recorded using an excitation wavelength of 490 nm and an emission wavelength of 530 nm with a cutoff of 515 nm. The percentage of inhibition (% inhibition) was calculated using Equation 1. Compounds with greater than 50 % inhibition were determined to be active compounds and were further diluted and assayed in triplicate. IC50 values were determined by plotting concentrations versus % inhibition.