Polymer encapsulation of ruthenium complexes for biological and medicinal applications

Abstract

Some Ru complexes have extremely promising anticancer or antibacterial properties, but their poor H 2 O solubility and/or low stability of many Ru complexes in aqueous solution under physiological conditions and/or metabolic/biodistribution profile prevent their therapeutic use. To overcome these drawbacks, various strategies have been developed to improve the delivery of these compounds to their target tissues. The first strategy is based on physical encapsulation of Ru complexes in carriers, such as polymeric micelles, microparticles, nanoparticles and polymer-lipid hybrids, which enabled the delivery and controlled release of the active Ru drug candidate. The second strategy involves covalent conjugation of the ruthenium complex to a polymer to give a prodrug that can be converted to the active drug at a more controllable rate. In this Review, we provide an overview of recent developments in polymer encapsulation of Ru complexes for biological and medicinalapplications, and place particular Here, the presence of Ru centre enable 3D geometries not available with organic chemistry. For example, the Ru(II) complex DW1 and its enantiomer DW2 mimic the shape of the alkaloid staurospaurine, and complexes thus also exhibit impressive kinase inhibition activity (targeting glycogen synthase kinase 3 (GSK3) signalling) and cytotoxicity in human melanoma cancer cells 38. The biological properties of Ru complexes-such as bioactivity, cellular uptake and intracellular distribution-depend, among others, on chemical properties such as electronegativity, chemical hardness, stereochemistry and net charge of the Ru centre. Ru drug candidates often feature strongly bound ligands, such as amine, phosphine, π-bound arenes and cyclopentadienyl derivatives. These are complemented by weakly bound ligands such as Cl − or RCO 2 − , which can be displaced by chelators or simply when an excess of competitive ligands is present. As is the case for reduced Pt derivatives, the lower-valent Ru(II) and Ru(III) complexes have favourable ligand exchange kinetics with O-donor and N-donor ligands. Ru complexes have several other advantages for biological and medicinal applications, including their usual low (or zero) toxicity to healthy tissues and their distinct mode of action. Indeed, they operate through different pathways to most Pt-based drugs, which typically only interact with DNA (BOX 1). Ru(II) and Ru(III) complexes have similar ligand exchange kinetics to the Pt(II) complexes used as antineoplastic drugs 39. For small ligands such as H 2 O, the ligand exchange rate on the Ru centre is on the order of hours, which is similar to the timescale of cell division in many cell types 40. Furthermore, Ru can bind biomolecules responsible for Fe solubilization and transport in plasma, including serum transferrin and albumin. Rapidly dividing cells, such as cancer cells, require more Fe, which leads to an upregulation of the number of transferrin receptors at the cell surface 41. Consequently, Ru complexes have been proposed to preferentially targ...

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