The aromatase inhibitory activity of the quinone compounds (1-11) are summarized in Table 1(Tab. 1). The compounds were categorized according to their aromatase inhibitory activities (IC50) as highly active (IC50 < 1 µM), moderately active (1 µM < IC50 < 10 µM), weakly active (IC50 > 10 µM) and inactive (% inhibition ≤ 50 % at 12.5 µM) (Prachayasittikul et al., 2014). The results indicate that compounds 1 and 4 are the most potent compounds, affording equivalent IC50 values of 0.5 ± 0.3 and 0.5 ± 0.4 µM, respectively. Both compounds (1 and 4) exhibited more potent aromatase inhibitory activity than did the reference drug, ketoconazole, as indicated by their IC50 values, which were 5.2-fold lower than the value obtained for ketoconazole (IC50 = 2.6 ± 0.7 µM). Compounds 3 and 11 were found to be inactive, and the rest of the tested compounds exhibited moderately active (2, 6, 7, 8, 9 and 10) to weak (5) activities. The order of aromatase inhibitory activity was as follows: 1 ≈ 4 > 8 > 6 > 10 > 9 > 7 > 2 > 5 >> 3 and 11. A detailed discussion regarding the structure-activity relationships of the tested compounds is provided in Supplementary information.