Development of Novel Bis(indolyl)-hydrazide–Hydrazone Derivatives as Potent Microtubule-Targeting Cytotoxic Agents against A549 Lung Cancer Cells

The biological significance of microtubules makes them a validated target of cancer therapy. In this study, we have utilized indole, an important pharmacological scaffold, to synthesize novel bis­(indolyl)-hydrazide–hydrazone derivatives (NMK-BH compounds) and recognized NMK-BH3 as the most effective one in inhibiting A549 cell proliferation and assembly of tissue-purified tubulin. Cell viability experiments showed that NMK-BH3 inhibited proliferation of human lung adenocarcinoma (A549) cells, normal human lung fibroblasts (WI38) and peripheral blood mononuclear cells (PBMC) with IC50 values of ∼2, 48.5, and 62 μM, respectively. Thus, the relatively high cytotoxicity of NMK-BH3 toward lung carcinoma (A549) cells over normal lung fibroblasts (WI38) and PBMC confers a therapeutic advantage of reduced host toxicity. Flow cytometry, Western blot, and immunofluorescence studies in the A549 cell line revealed that NMK-BH3 induced G2/M arrest, mitochondrial depolarization, and apoptosis by depolymerizing the cellular interphase and spindle microtubules. Consistent with these observations, study in cell free system revealed that NMK-BH3 inhibited the microtubule assembly with an IC50 value of ∼7.5 μM. The tubulin–ligand interaction study using fluorescence spectroscopy indicated that NMK-BH3 exhibited strong and specific tubulin binding with a dissociation constant of ∼1.4 μM at a single site, very close to colchicine site, on β-tubulin. Collectively, these findings explore the cytotoxic potential of NMK-BH3 by targeting the microtubules and inspire its development as a potential candidate for lung cancer chemotherapy.