Indolo[2,3-e]benzazocines and indolo[2,3-f]benzazonines and their copper(ii) complexes as microtubule destabilizing agents

Abstract

A series of four indolo[2,3-e]benzazocines HL¹–HL⁴ and two indolo[2,3-f]benzazonines HL⁵ and HL⁶, as well as their respective copper(II) complexes 1–6, were synthesized and characterized by ¹H and ¹³C NMR spectroscopy, ESI mass spectrometry, single crystal X-ray diffraction (SC-XRD) and combustion analysis (C, H, N). SC-XRD studies of precursors Vd, VIa·0.5MeOH, of ligands HL⁴ and HL⁶·DCM, and complexes 2·2DMF, 5·2DMF, 5′·ⁱPrOH·MeOH provided insights into the energetically favored conformations of eight- and nine-membered heterocycles in the four-ring systems. In addition, proton dissociation constants (pK_a) of HL¹, HL² and HL⁵, complexes 1, 2 and 5, overall stability constants (log β) of 1, 2 and 5 in 30% (v/v) DMSO/H₂O at 298 K, as well as thermodynamic solubility of HL¹–HL⁶ and 1–6 in aqueous solution at pH 7.4 were determined by UV–vis spectroscopy. All compounds were tested for antiproliferative activity against Colo320, Colo205 and MCF-7 cell lines and showed IC₅₀ values in the low micromolar to sub-micromolar concentration range, while some of them (HL¹, HL⁵ and HL⁶, 1, 2 and 6) showed remarkable selectivity towards malignant cell lines. Ethidium bromide displacement studies provided evidence that DNA is not the primary target for these drugs. Rather, inhibition of tubulin assembly is likely the underlying mechanism responsible for their antiproliferative activity. Tubulin disassembly experiments showed that HL¹ and 1 are effective microtubule destabilizing agents binding to the colchicine site. This was also confirmed by molecular modelling investigations. To the best of our knowledge, complex 1 is the first reported transition metal complex to effectively bind to the tubulin-colchicine pocket.