Abstract
The toxicity (IC50) of a series of mononuclear ruthenium complexes containing bis[4(4′-methyl-2,2′-bipyridyl)]-1,n-alkane (bbn) as a tetradentate ligand against three eukaryotic cell lines—BHK (baby hamster kidney), Caco-2 (heterogeneous human epithelial colorectal adenocarcinoma) and Hep-G2 (liver carcinoma)—have been determined. The results demonstrate that cis-α-[Ru(Me4phen)(bb7)]2+ (designated as α-Me4phen-bb7, where Me4phen = 3,4,7,8-tetramethyl-1,10-phenanthroline) showed little toxicity toward the three cell lines, and was considerably less toxic than cis-α-[Ru(phen)(bb12)]2+ (α-phen-bb12) and the dinuclear complex [{Ru(phen)2}2{μ-bb12}]4+. Fluorescence spectroscopy was used to study the binding of the ruthenium complexes with human serum albumin (HSA). The binding of α-Me4phen-bb7 to the macrocyclic host molecule cucurbit[10]uril (Q[10]) was examined by NMR spectroscopy. Large upfield 1H NMR chemical shift changes observed for the methylene protons in the bb7 ligand upon addition of Q[10], coupled with the observation of several intermolecular ROEs in ROESY spectra, indicated that α-Me4phen-bb7 bound Q[10] with the bb7 methylene carbons within the cavity and the metal center positioned outside one of the portals. Simple molecular modeling confirmed the feasibility of the binding model. An α-Me4phen-bb7-Q[10] binding constant of 9.9 ± 0.2 × 106 M−1 was determined by luminescence spectroscopy. Q[10]-encapsulation decreased the toxicity of α-Me4phen-bb7 against the three eukaryotic cell lines and increased the binding affinity of the ruthenium complex for HSA. Confocal microscopy experiments indicated that the level of accumulation of α-Me4phen-7 in BHK cells is not significantly affected by Q[10]-encapsulation. Taken together, the combined results suggest that α-Me4phen-7 could be a good candidate as a new antimicrobial agent, and Q[10]-encapsulation could be a method to improve the pharmacokinetics of the ruthenium complex.
Highlights
Due to the increasing resistance of bacteria, Gramnegative species, to the range of drugs currently in clinical use there is significant interest in developing new antimicrobial agents (Boucher et al, 2009)
As encapsulation in cucurbit[n]urils {macrocyclic host compounds composed of n glycoluril monomeric units, Q[n]–see Figure 3 (Lagona et al, 2005; Kim et al, 2007; Isaacs, 2009)} can potentially decrease the toxicity of a drug and modulate the drugs’ ability to bind serum proteins, (Wheate et al, 2004; Jeon et al, 2005; Li et al, 2013b) we have examined the ability of α-Me4phen-7 to form an inclusion complex with Q[10] and determined the subsequent effect on the toxicity of the ruthenium complex toward eukaryotic cells and its ability to bind human serum albumin (HSA)
The toxicities of the [Ru(phen′)(bbn)]2+ complexes against three eukaryotic cell lines {baby hamster kidney (BHK), human epithelial colorectal adenocarcinoma (Caco-2) and human hepatocellular carcinoma (Hep-G2)} were determined and compared to the values obtained for the dinuclear complex Rubb12, [Ru(Me4phen)3]2+ {designated (Me4phen)3, the most active of the mononuclear complexes originally identified by Dwyer and co-workers} and the control anticancer agent cisplatin
Summary
Due to the increasing resistance of bacteria, Gramnegative species, to the range of drugs currently in clinical use there is significant interest in developing new antimicrobial agents (Boucher et al, 2009). Previous studies from our group have examined the antimicrobial properties of di-, tri- and tetra-nuclear polypyridylruthenium(II) complexes in which the metal centers are linked by the bis[4(4′-methyl-2,2′-bipyridyl)]1,n-alkane ligand (“bbn”; see Figure 1) (Li et al, 2011, 2016; Gorle et al, 2014). These oligonuclear ruthenium complexes showed excellent activity against Gram-positive bacteria, and maintained the activity against current drug-resistant strains such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) (Gorle et al, 2016). The oligonuclear complexes showed variable activity to Gram-negative strains, with a number of species being essentially resistant to the ruthenium complexes (Gorle et al, 2016)
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