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.