Advancing Metal-Based Anticancer Drugs with ICP-MS and Imaging MS Techniques

Metallomics approaches based on mass spectrometry have become increasingly important in the support of developing metal-based anticancer drugs. This area is a key focus for Gunda Koellensperger and her colleagues at the University of Vienna (Austria) and they recently published an article discussing this state-of-the-art instrumentation, as well as highlighting recent analytical advances, focusing especially on the latest developments in inductively coupled plasma-–mass spectrometry (ICP-MS). It is their belief that these methods will lead to fruitful explorations of metal-based anticancer drugs and their interactions with functionally complex solid tumor tissues. Koellensperger recently spoke to Spectroscopy about this work.

You published a recent paper (1) highlighting some of the recent methodological developments for the analysis of metal-based anticancer compounds in different (pre-) clinical settings, with a focus on metallomics mass spectrometry approaches and imaging methods. Why did you decide to focus on those approaches in your research?

I am an analytical chemistry enthusiast; however, I was always interested in testing the potential of cutting-edge methods in important applications—bridging the gap between method development and routine. Cancer research focusing on metal-based drugs is of paramount importance. Nowadays, these therapeutics belong to the most widely used standard chemotherapy regimens. At the same time, there are still extensive research activities in the field, aiming to improve metal-based therapy regimes and develop new drugs. In fact, a great share of current clinical cancer-related studies worldwide involves metal-based drugs. Most recently, promising trials for their combination with immunotherapies have emerged. However, there are still many open, and surprisingly unaddressed, questions, requiring the implementation of top-notch analytical methods in preclinical and clinical settings. Anticancer metal drugs not only exert cytotoxic activities against malignant cells, but also have a complex impact on the diverse cellular components of the innate and acquired immune systems. While in former times platinum drugs were considered immunosuppressive, during the last few years it became clear that these compounds might exert immune-stimulating activities. As a consequence, research has shifted in recent times from a “cancer-cell centered” to a “tumor-microenvironment centered” perspective, requiring that we understand and thus analyze the interactions of complex cell populations at the single-cell level. Making progress in single-cell analysis in this context is thus important and timely. Metallomics methods are an integral part in this exiting endeavor. In the future, our group would like to contribute to an improved understanding of determinants of the cancer–immune interplay.

The paper advocates ICP-MS for the analysis of single cells. What are the benefits of using that technique in the performance of that task over other analytical techniques?

The field of single-cell analysis is, at the moment, one of the most exciting fields of analytical method development. Single-cell analysis, in the best case, provides information-rich data for a single cell, considering that the cell status, function, and adaptation undergo dynamic changes, and are all determined by the spatial arrangement of cells in the tissue; therefore, temporally and spatially resolved measurements of thousands of cells at the single-cell level are needed to make progress. Elemental imaging based on laser ablation ICP-MS (LA-ICP-MS) has the potential of meeting all these requirements. Innovative labeling strategies paved the way to multiparametric and multiplexing assays. The design of low-dispersion ablation cells enabled excellent spatial resolutions in the sub-1-µm range. Evidently, in the field of metal anticancer drugs, the advantage of measuring cellular drug accumulation in a quantitative manner is key.

Specifically, the techniques of LA-ICP-MS, imaging mass cytometry (IMC),matrix-assisted laser desorption/ionization-mass spectrometry imaging (MSI), and nanoprobe secondary-ion mass spectrometry (nano-SIMS) are discussed in your paper, with each technique having specific benefits. Briefly, can you discuss the benefits of each? Where would you choose one technique over the other?

Each of the mentioned techniques can image tissues in their cellular heterogeneity. Nano- SIMS is unrivaled in terms of spatial resolution, allowing sub-cellular resolution. Thus, in metal-based drugs research, a detailed picture of cellular accumulation is achieved. Different cellular accumulation sites can be distinguished. Drug targets, such as the nucleus of DNA, or different organelles can be imaged. These could provide key data supporting a hypothesized mode of action. Moreover, depending on which ion gun is used in nano-SIMS (and which metal is addressed), metal isotope measurement can be accompanied by 13C and 15N tracer measurement. For example, when studying platinum-based drugs using isotopically enriched ligands, it is possible to infer whether metal and ligand are co-localized within the cell. This way, drug chemistry within the cell is revealed. Finally, tracer studies using isotopically enriched nutrients enable parallel metabolic experiments, showing for example whether cells depend on fatty acids or whether glycolysis is preferred. Thus, it is possible to show metal uptake along with metabolic activity of cells. As a major drawback, this instrumental platform is only rarely available, and falls short regarding analytical throughput and manageable study sizes, when compared to other mass spectrometric imaging techniques. Among MS platforms, matrix-assisted laser desorption/ionization (MALDI) shows the poorest spatial resolution. To the best of my knowledge, typically spatial resolutions in the 3–5 µm range are reported, as compared to 1–2 µm (or even sub-1-µm) in LA-ICP-MS. However, it has its undisputed role in imaging lipids or proteins in cells. In the field of metal drug research, some interesting studies regarded imaging of the drug. In this specific application, molecular mass spectrometry potentially reveals whether intact drugs are accumulating in cells, or whether ligand exchange or other reactions occur in cellular environments. More recently, exciting multi-modal MS imaging strategies have emerged, combining LA-ICP-MS and MALDI. These studies are still rather method-oriented; however, I would like to see more of this type of data integration or fusion studies combining different imaging modalities in the future. Finally, I would say that ICP-MS-based imaging approaches are the best solution for quantitative imaging of metallodrugs, offering at the same time the opportunity to obtain a multi-parametric assay on cell type, state, and function. Imaging mass cytometry already has, and will have, a major role in advancing cancer immunology and ultimately improve therapy options.

Can you briefly summarize the conclusions that you have come to so far in your research?

We need to measure more, to go beyond proof-of-principle studies, to have a real impact. My group aims at carrying out large studies focusing on the tumor microenvironment and metal drugs, both established drugs and candidate drugs. We would like to build open-source databases. The idea is to deploy an atlas of tumor tissues and metal-based drugs. I think making data available is a major prerequisite to progress as well.

What are your next steps in this work?

Combining different modalities of cellular imaging and measurement!

References

(1) S. Theiner, A. Schoeberl, A. Schweikert, B.K. Keppler, and G. Koellensperger, Curr. Opin. Chem. Biol. 61(4), 123-134 (2021). https://doi.org/10.1016/j.cbpa.2020.12.005

Gunda Koellensperger

Gunda Koellensperger is a professor of Analytical Chemistry at the University Vienna, Austria. Currently, she is the head of the Department of Analytical Chemistry, vice dean of the Faculty of Chemistry, and vice chair of the Vienna metabolomics platform. She is an expert in methods based on mass spectrometry, multidimensional chromatography, and stable isotopes, with more than 170 publications. Her research envisages the development of innovative omics-type analytical tools in the interdisciplinary fields of metallomics and metabolomics