Eukaryotic Cell Toxicity and HSA Binding of [Ru(Me4phen)(bb7)]2+ and the Effect of Encapsulation in Cucurbit[10]uril

Biyun Sun; Ian F Musgrave; Anthony I Day; Kirsten Heimann; F Richard Keene; J Grant Collins

doi:10.3389/fchem.2018.00595

Eukaryotic Cell Toxicity and HSA Binding of [Ru(Me₄phen)(bb₇)]²⁺ and the Effect of Encapsulation in Cucurbit[10]uril

Front Chem. 2018 Nov 30:6:595. doi: 10.3389/fchem.2018.00595. eCollection 2018.

Authors

Biyun Sun¹, Ian F Musgrave², Anthony I Day¹, Kirsten Heimann^{3

4}, F Richard Keene^{5

6}, J Grant Collins¹

Affiliations

¹ School of Physical, Environmental and Mathematical Sciences, University of New South Wales, Australian Defence Force Academy, Canberra, ACT, Australia.
² Discipline of Pharmacology, Adelaide Medical School, University of Adelaide, Adelaide, SA, Australia.
³ College of Medicine and Public Health, Flinders University, Adelaide, SA, Australia.
⁴ College of Science and Engineering, James Cook University, Townsville, QLD, Australia.
⁵ Department of Chemistry, School of Physical Sciences, University of Adelaide, Adelaide, SA, Australia.
⁶ Australian Institute of Tropical Health and Medicine/Centre for Molecular Therapeutics, James Cook University, Townsville, QLD, Australia.

Abstract

The toxicity (IC₅₀) of a series of mononuclear ruthenium complexes containing bis[4(4'-methyl-2,2'-bipyridyl)]-1,n-alkane (bb_n) 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.

Keywords: HSA binding; cucurbit[10]uril; cytotoxicity; ruthenium complexes; supramolecular chemistry.