Synthesis and Structure of Arene Ru(II) N^∧O‑Chelating Complexes: <i>In Vitro</i> Cytotoxicity and Cancer Cell Death Mechanism

A panel of six new structurally related organometallic arene Ru­(II) complexes of general composition [(η⁶-benzene)­Ru­(L)­Cl] (<b>1</b>–<b>3</b>) and [(η⁶-<i>p</i>-cymene)­Ru­(L)­Cl] (<b>4</b>–<b>6</b>) (L = dimethylaminobenzhydrazones) have been designed and synthesized in search of new ruthenium anticancer drugs. The identities of the synthesized complexes have been well-established by elemental analysis and various spectral (FT-IR, UV–vis, NMR, and HR-MS) methods. The solid-state molecular structures of the ruthenium complexes were determined with the help of X-ray crystallography and confirms the presence of a pseudo-octahedral geometry around ruthenium. Furthermore, cytotoxicity of the complexes has been unveiled with the aid of MTT assay against A549 (lung carcinoma), LoVo (colon adenocarcinoma), HuH-7 (hepato cellular carcinoma) along with the noncancerous 16HBE (human lung bronchial epithelium) cells and compared with the effect of the standard drug cisplatin. Interestingly, complexes <b>4</b>, <b>5</b>, and <b>6</b> which contain a <i>p</i>-cymene moiety induce a remarkable decrease of cell viability against all the cancer cells tested. The capacity corresponding to the inhibition of A549 cells proliferation was analyzed by 5-ethynyl-2-deoxyuridine (EdU) incorporation assay and indicated a notable effect of <i>p</i>-cymene counterparts <b>4</b>, <b>5</b>, and <b>6</b> over cisplatin. Further studies such as AO-EB (acridine orange–ethidium bromide) staining, flow cytometry, and Western blot analyses on cell death mechanism signified that the cytotoxicity was associated with apoptosis in cancer cells. This clearly suggests that <i>p</i>-cymene-capped Ru­(II) complexes are also one of the propitious cancer therapeutic candidates and are worthy of further investigations.