Design of Ruthenium N-(1-alkylpyridin-4(1H)-ylidene)amide (PYA) Carbonyl Complexes as pH-Triggered Carbon Monoxide Releasing Molecules

Abstract

The odourless and colourless toxic gas carbon monoxide (CO) has long been termed a ‘silent killer’, and has been extensively studied in the fields of physiology, biochemistry, medicine, pharmacology and toxicology. This has led to the finding that CO is an essential signalling molecule in humans and plays an important role in numerous physiological processes. Given this role of endogenous CO, the biological significance and therapeutic potential of exogenously applied CO has been widely studied. More recently, therapeutic alternatives to gaseous CO have been explored given that there are several limitations of using gaseous CO as a therapeutic agent, especially its toxicity. In particular, there has been a considerable amount of research carried out on carbon monoxide releasing molecules (CORMs). An issue with most of the known CORMs is their lack of tissue specificity. Therefore, the focus of this study was to design a novel CORM that can selectively release CO when triggered by a specific stimulus. To introduce the desired selectivity, an N-(1-alkylpyridin-4(1H)-ylidene amide (PYA) ligand was coordinated to a Ru centre bearing three CO co-ligands. Protonation of the PYA ligand results in concomitant labilisation of CO due to the change in electron-donating properties of this particular ligand type. Subtle changes in pH around the physiological range (pH 6.4-7.4) were hypothesised to be sufficient to selectively trigger the CO release in slightly acidic environments, such as heart tissue after cardiac arrest or in solid tumours. Based on these design principles, pH-CORM-1 [Ru(CO)3Cl(N-(1-benzylpyridin-4(1H)-ylidene)picolinamide)][Ru(CO)3Cl3] and also the related non-pH-responsive analogues [1-benzyl-4-(picolinamide)pyridinium][Ru(CO)3Cl3] (1) and [Et4N][Ru(CO)3Cl3] (2) were designed, synthesised, and assayed on their CO releasing ability and biological activity. The release of CO was tested spectrophotometrically using the myoglobin assay at various pH values and in dependence of the temperature. The results clearly show a pH dependent CO release for pH-CORM-1, i.e., the lower the pH the quicker, while the CO release from 1 and 2 was nearly pH independent. Increasing the temperature from 20 to 37°C was accompanied by higher rate constants for all compounds. This proof-of-principle study shows that the pH dependent release of CO from pH-CORM-1 is due to the inherent properties of the complex, specifically the PYA ligand. Preliminary investigations into the anticancer activity of the compounds support further studies in relevant biological models considering the pH-dependent properties of pH-CORM-1.