Mass spectrometry techniques for imaging and detection of metallodrugs

Undoubtedly, metallomic approaches based on mass spectrometry have evolved into essential tools supporting the drug development of novel metal-based anticancer drugs. This article will comment on the state-of-the-art instrumentation and highlight some of the recent analytical advances beyond routine, especially focusing on the latest developments in inductively coupled plasma-mass spectrometry (ICP-MS). Mass spectrometry-based bioimaging and single-cell methods will be presented, paving the way to exciting investigations of metal-based anticancer drugs in heterogeneous and structurally, as well as functionally complex solid tumor tissues.     

Methods to identify protein targets of metal-based drugs

Metal-based anticancer agents occupy a distinct chemical space due to their particular coordination geometry and reactivity. Despite the initial DNA-targeting paradigm for this class of compounds, it is now clear that they can also be tuned to target proteins in cells, depending on the metal and ligand scaffold. Since metallodrug discovery is dominated by phenotypic screenings, tailored proteomics strategies were crucial to identify and validate protein targets of several investigative and clinically advanced metal-based drugs. Here, such experimental approaches are discussed, which showed that metallodrugs based on ruthenium, gold, rhenium and even platinum, can selectively and specifically target proteins with clear-cut down-stream effects. Target identification strategies are expected to support significantly the mechanism-driven clinical translation of metal-based drugs.     

Unusual DNA binding modes for metal anticancer complexes

DNA is believed to be the primary target for many metal-based drugs. For example, platinum-based anticancer drugs can form specific lesions on DNA that induce apoptosis. New platinum drugs can be designed that have novel modes of interaction with DNA, such as the trinuclear platinum complex BBR3464. Also it is possible to design inert platinum(IV) pro-drugs which are non-toxic in the dark, but lethal when irradiated with certain wavelengths of light. This gives rise to novel DNA lesions which are not as readily repaired as those induced by cisplatin, and provides the basis for a new type of photoactivated chemotherapy. Finally, newly emerging ruthenium(II) organometallic complexes not only bind to DNA coordinatively, but also by H-bonding and hydrophobic interactions triggered by the introduction of extended arene rings into their versatile structures. Intriguingly osmium (the heavier congener of ruthenium) reacts differently with DNA but can also give rise to highly cytotoxic organometallic complexes.     

Insights into metalloproteins and metallodrugs from electron paramagnetic resonance spectroscopy

Metal ions play an important role in diverse biological processes, and much of the basic knowledge derived from studying native bioinorganic systems are applied in the synthesis of new molecules with the aim of diagnosing and treating diseases. At first glance, metalloproteins and metallodrugs are very different systems, but metal ion coordination, redox chemistry and substrate binding play essential roles in advancing both of these research fields. In this article, we discuss recent metalloprotein and metallodrug studies where electron paramagnetic resonance spectroscopy served as a major tool to gain a better understanding of metal-based structures and their function.     

A novel phenotype-based approach for systematically screening antiproliferation metallodrugs

Ruthenium (Ru) derivatives have less toxicity and higher water-solubility than cisplatin, giving them great potential as antitumor metallodrugs. In this study, zebrafish were employed as a whole-organism model to screen new Ru compounds for anti-cell proliferation activity. After soaking fish embryos in cisplatin and five Ru derivatives, [Ru(terpy)(bpy)Cl]Cl, [Ru(terpy)(dppz)OH₂](ClO₄)₂, [Ru(terpy)(tMen)OH₂](ClO₄)₂, [Ru(terpy)(Me₄Phen)OH₂](ClO₄)₂, and Ru(bpy)₂Cl₂, only cisplatin and [Ru(terpy)(bpy)Cl]Cl-treated embryos displayed obvious phenotypic effects, such as fin-reduction. After further modification of [Ru(terpy)(bpy)Cl]Cl's main structure and the synthesis of two structurally related compounds, [Ru(terpy)(dcbpyH₂)Cl]Cl and [Ru(terpy)(dmbpy)Cl]Cl, only [Ru(terpy)(dmbpy)Cl]Cl exhibited fin-reduction phenotypes. TUNEL assays combined with immunostaining techniques revealed that treatment with cisplatin, [Ru(terpy)(bpy)Cl]Cl, and [Ru(terpy)(dmbpy)Cl]Cl led proliferating fin mesenchymal cells to undergo apoptosis and consequently caused fin-reduction phenotypes. Furthermore, [Ru(terpy)(bpy)Cl]Cl was able to activate the P53-dependent and independent pathways, and induced human hepatoma cells to undergo apoptosis. In summary, it was concluded that the zebrafish model was effective for the screening of phenotype-based antiproliferation metallodrugs.     

Inhibiting prolyl isomerase activity by hybrid organic–inorganic molecules containing rhodium(II) fragments

A small molecule containing a rhodium(II) tetracarboxylate fragment is shown to be a potent inhibitor of the prolyl isomerase FKBP12. The use of small molecules conjugates of rhodium(II) is presented as a general strategy for developing new protein inhibitors based on distinct structural and sequence features of the enzyme active site.