According to the diversity in molecular

According to the GSK2656157 in molecular skeletons, aromatase inhibitors can be categorized into two types: steroidal and non-steroidal blockers [6]. Steroidal AI (e.g., exemestane in Fig. 1) derived from the substrate androstenedione interacts with aromatase through chemical actions, resulting in an irreversible binding process of the species; while non-steroidal AI (e.g., anatrozole and letrozole in Fig. 1) binds to enzyme through non-covalent interactions, resulting in a reversible binding process. Nowadays, aromatase inhibitors (AIs) constitute the first-line drugs for ER-positive breast cancer in postmenopausal women. The third generation of AIs (exemestane, anatrozole, and letrozole) was approved by Food and Drug Administration as first-line therapy for hormonally-responsive breast cancer in postmenopausal women, since they had been proven to be superior to tamoxifene known as a representative of selective estrogen receptor modulators (SERMs) [7]. However, with the broad applications of AIs in clinical practices, some unexpected problems are gradually shown up, such as non-responses to some of patients, resistance to AI treatment and inhibition of some CYP450 enzymes [8]. Hence, there are urgent needs to discover and develop new generation of AIs to overcome the defects. 2-Phenyl indole, an estrogen-mimicking chemical structure, has been used widely in drug designs for treatment of estrogen-related diseases [9], [10], [11]. Some SERMs (e.g., bazedoxifene and pipendoxifene, Fig. 2) are based on 2-phenyl indole skeletons. D-15414 (Fig. 2), a 2-phenyl indole analogue, is a non-steroidal estrogen with fairly high binding affinity for estrogen receptors [12]. Thus, we are reasonable to postulate that 2-phenyl indole derivatives may enter into the substrate-binding pocket of aromatase, because they are mimics of estrone and estradiol, the products of aromatization reactions, with high binding affinity to aromatase enzyme. On the other hand, imidazole and 1,2,4-triazole are nitrogen-containing heterocyclic rings that can coordinate with the heme of aromatase [13]. Based on the knowledge above, a series of novel non-steroidal aromatase inhibitors were designed by introducing the azole group at the 3 position in the 2-phenyl indole framework (Fig. 3A). The indole moiety accounts for fitting the binding site of aromatase, whereas the azole moiety chelates the iron atom in the heme existing in the active site of aromatase. Since our designed molecules are structurally similar to letrozole (Fig. 3B), it is possible for the azole substituted 2-phenyl indole to have certain bioactivity in inhibiting aromatase. To study the impact of the modified 2-phenylindoles on aromatase activity, we synthesized twenty imidazole or 1,2,4-triazole substituted 2-phenyl indoles (namely, compounds 8a-t (Table 1)), and tested their inhibitory activities against aromatase in our lab. To explore the molecular mechanism of inhibitory action, an integrated computational strategy was then applied to investigate the binding of letrozole and compound 8o to aromatase.
Results and discussion
Conclusion The use of aromatase inhibitor or ER modulator as adjuvant therapy has been the mainstay of treatment for postmenopausal women with ER-positive early-stage breast cancer. Persistent risk of tumor recurrence remains a clinical and scientific challenge. Therefore, it is worthwhile to prevent tumor recurrence by developing an alternative strategy with better efficacy. In this work, we strived to develop novel non-steroidal AIs based on a 2-phenylindole scaffold to suppress the bioactivity of aromatase, and three compounds 8o, 8c and 8e were proved experimentally to be more powerful than letrozole does. Compound 8o is considered to be the most potent AI due to its lowest value (14.1 nM) of IC50/aromatase. Additionally, results of MCF-7 cell activity assay revealed that compound 8o exhibited much lower cytotoxicity (IC50/MCF-7: 325 μM) as compared to letrozole (IC50/MCF-7: 4.73 μM), possibly indicating a better safety profile for compound 8o. Furthermore, we computationally studied the binding structures of letrozole and compound 8o in aromatase, providing significant impact on the AIs' efficacy. The discovery of new highly potent AIs and understanding their molecular mechanism of inhibitory action is central to further improving therapeutic options for ER-positive breast cancer.