Design, synthesis and biological evaluation of novel bis(indolyl)-tetrazine derivatives as anti-breast cancer agents.

A series of novel bis(indolyl)-tetrazine derivatives were designed and synthesized to develop potential anti-breast cancer agents. The compounds were characterized by spectral analysis using 1H NMR, 13C NMR spectroscopy, and HRMS. Further, the structure of one of the derivatives 5b was confirmed by single crystal X-ray diffraction technique. The compounds exhibited good to moderate anti-proliferative activity against MCF-7 and MDA-MB-231 cell lines, showing IC50 values of 7.57-22.52 μM and 10.08-21.49 μM, respectively. However, only four compounds, 5b, 5f, 5i, and 5j were found to be active against the T-47D cell line. Particularly, compounds 5b and 5f demonstrated the most promising anti-proliferative activity as compared to standard drug bazedoxifene (IC50 = 12.78 ± 0.92 μM), with IC50 values of 5.11 ± 0.16 and 4.69 ± 0.51 μM, respectively against estrogen receptor-alpha (ER-α) dominant (ratio of ER-α/ER-β is 9/1) T-47D cell line. Further, compound 5b exhibited binding affinity towards ER-α with an IC50 value of 1729 ± 24 nM when assessed for its affinity towards ER-α through a competitive ER-α binding assay. The Western blot analysis confirmed that compound 5b reduced the ER-α protein's expression, impeding its subsequent transactivation and signalling pathway within T-47D cells. Current findings imply that compound 5b, which exhibits significant ER-α antagonistic activity, can be a potential lead compound for developing anti-breast cancer agents.