Elucidation of Structure-Activity Relationships in Indolobenzazepine-Derived Ligands and Their Copper(II) Complexes: The Role of Key Structural Components and Insight into the Mechanism of Action

Indolo[3,2-d][1]benzazepines (paullones), indolo[3,2-d][2]benzazepines, and indolo[2,3-d][2]benzazepines (latonduines) are isomeric scaffolds of current medicinal interest. Herein, we prepared a small library of novel indolo[3,2-d][2]benzazepine-derived ligands HL¹-HL⁴ and copper(II) complexes 1-4. All compounds were characterized by spectroscopic methods (¹H and ¹³C NMR, UV-vis, IR) and electrospray ionization (ESI) mass spectrometry, while complexes 2 and 3, in addition, by X-ray crystallography. Their purity was confirmed by HPLC coupled with high-resolution ESI mass spectrometry and/or elemental analysis. The stability of compounds in aqueous solutions in the presence of DMSO was confirmed by ¹H NMR and UV-vis spectroscopy measurements. The compounds revealed high antiproliferative activity in vitro in the breast cancer cell line MDA-MB-231 and hepatocellular carcinoma cell line LM3 in the low micromolar to nanomolar concentration range. Important structure-activity relationships were deduced from the comparison of anticancer activities of HL¹-HL⁴ and 1-4 with those of structurally similar paullone-derived (HL⁵-HL⁷ and 5-7) and latonduine-derived scaffolds (HL⁸-HL¹¹ and 8-11). The high anticancer activity of the lead drug candidate 4 was linked to reactive oxygen species and endoplasmic reticulum stress induction, which were confirmed by fluorescent microscopy and Western blot analysis.