Investigation of the binding of cis/trans-[MCl4(1H-indazole)(NO)]− (M = Ru, Os) complexes to human serum albumin

• Cis/trans-[M(Cl)4(1H-indazole)(L)]− (M: Ru/Os; L: NO/CO) complexes were studied
• Moderately strong binding of the mononitrosyl complexes to human serum albumin
• Binding to both drug sites, I and II, of albumin
• Fatty acid displacement by the Ru mononitrosyl complex observed by EPR spin labeling
• Rapid decomposition of the monocarbonyl complex in aqueous solution

Overall binding affinity of sodium or indazolium cis/trans-[MCl4(1H-indazole)(NO)] (M = Ru, Os) complexes towards human serum albumin (HSA) and high molecular mass components of the blood serum was monitored by ultrafiltration. HSA was found to be mainly responsible for the binding of the studied ruthenium and osmium complexes. In other words, this protein can provide a depot for the compounds and can affect their biodistribution and transport processes. In order to elucidate the HSA binding sites tryptophan fluorescence quenching studies and displacement reactions with the established site markers warfarin and dansylglycine were performed. Conditional stability constants for the binding to sites I and II on HSA were computed showing that the studied ruthenium and osmium complexes are able to bind into both sites with moderately strong affinity (logK’ = 4.4–5.1). Site I is slightly more favored over site II for all complexes. No significant differences in the HSA binding properties were found for these metal complexes demonstrating negligible influence of the type of counterion (sodium vs indazolium), the metal ion center identity (Ru vs. Os) or the position of the nitrosyl group on the binding event. Electron paramagnetic resonance spin labeling of HSA revealed that indazolium trans-[RuCl4(1H-indazole)(NO)] and long-chain fatty acids show competitive binding to HSA. Moreover, this complex has a higher affinity for site I, but when present in excess, it is able to bind to site II as well, and displace fatty acids.

Hydrolytic stability of a series of mononitrosyl and monocarbonyl complexes: cis/trans-[MCl4(1H-indazole)(NO)]− (M = Ru, Os) and trans-[RuCl4(1H-indazole)(CO)]− was investigated. Interaction of the mononitrosyl compounds with human serum albumin was characterized by fluorometry and separation methods, as well as by electron paramagnetic resonance spectroscopy using a spin labeled fatty acid.Figure optionsDownload as PowerPoint slide