Mass spectrometry as a powerful tool to study therapeutic metallodrugs speciation mechanisms: Current frontiers and perspectives

Metal-based compounds form a promising class of therapeutic agents, whose mechanisms of action still need to be elucidated, and that are in general prone to undergo extensive speciation in physiological environment. Thus, determination of the fate of the metal compounds in complex biological systems, contributing to their overall pharmacological and toxicological profiles, is important to develop more rationalised and targeted metal-based drugs. To these aims, a number of spectroscopic and biophysical methods, as well as analytical techniques, are nowadays extensively applied to study the reactivity of metal complexes with different biomolecules (e.g. nucleic acids, proteins, buffer components). Among the various techniques, molecular mass spectrometry (MS) has emerged in the last decade as a major tool to characterise the interactions of metallodrugs at a molecular level. In this review, we present an overview of the information available on the reactivity of various families of therapeutic metallodrugs (mainly anticancer compounds based on Pt, Ru, Au and As) with biomolecules studied by different MS techniques, including high-resolution ESI-, MALDI- and ion mobility-MS among others. Representative examples on the potential of the MS approach to study non-covalent interactions are also discussed. The review is organized to present results obtained on samples with different degrees of complexity, from the interactions of metal compounds with small model nucleophiles (amino acids and nucleobases), model peptides/oligonucleotides, target proteins/nucleic acids, to the analysis of serum, cell extracts and tissue samples. The latter requiring combination of proteomic methods with advanced MS techniques. Correlations between molecular reactivity of metallodrugs and biological activity are hard to establish, but differences in the reactivity of metallodrugs to biomolecules and their different adducts, as revealed by MS methods, may indicate differences in their modes of action. Overall, the knowledge offered by MS methods on metallodrugs speciation is invaluable to establish new rules and define new trends in the periodic table aimed at rationalizing the behavior of metal compounds in complex living systems.