By Haoyu Li
Desorption electrospray ionisation (DESI) technique, invented by Takás et al in 2004, allowed ambient mass spectrometry (MS) analysis under room conditions.1 It offered a solution to the issue faced in traditional MS where samples had to be introduced into a vacuum or into an inaccessible region closely coupled with a vacuum system.1 In brief, DESI uses a beam of charged solvent droplets that is formed by pneumatically assisted electrospray. The beam is directed onto a tissue surface and performs simultaneous localised extraction and ionisation of molecules. The residue from the solvent droplets, which carries the resultant ions, are then fed into a mass spectrometer for analysis. Mass spectrometry imaging (MSI) has emerged as a powerful tool because it is able to directly track the distribution and relative abundance of many different chemical species in a tissue of interest.2 The specimen surface is firstly overlayed by a grid, divided into pixels. Molecular ions are then extracted from each pixel and analysed with MS. With information from MS, localised relative abundance of different chemicals can be visualised, usually in the form of a heatmap.
When MSI is coupled with DESI, it offers an ambient, non-destructive method to rapidly identify, measure, and map hundreds of metabolites from an unmodified sample, with minimum risk of masking or redistribution of the signal ex vivo, due to the non-invasive nature of the workflow.2 Banerjee et al provides a detailed methodology and clear diagram of the DESI-MS workflow in their work on prostate cancer.3 Examples of chemical distribution heatmaps are also available in the Results section of this work. Other MSI imaging techniques such as secondary ion mass spectrometry (SIMS) and matrix assisted laser ionization MS (MALDI-MS) fall outside the scope of this article, but more information can be found in reference (4).
Initially, DESI-MSI was mainly used in the discovery of biomarkers for cancer diagnosis because of its ability to track hundreds of chemical species at once. However, DESI-MSI has more recently proven itself useful in drug delivery systems (DDSs) and metabolism analysis studies. Before DESI-MSI, visualization of drug distribution in the tissue relies on either radiolabelling or fluorescent labelling.2 The former is expensive and raises safety concerns regarding radiation and, hence, is only able to monitor drugs at sub-therapeutical doses.2 Meanwhile, fluorescence labelling is a less expensive alternative but tissue autofluorescence can often interfere with the measurements. Additionally, the labelling process requires modification to the active drug molecule2. Unfortunately, neither method could distinguish the original drug molecule from its metabolites. On the other hand, DESI-MSI can monitor many chemical species all at once, tracking the original drug and all its metabolites whilst also distinguishing between them. No modification to the drug molecule means that it can be dosed at therapeutically relevant concentrations without problems such as radiation safety in radiolabelling, or modification-driven changes of pharmacokinetics in fluorescent labelling. These advantages give DESI-MSI the potential to develop new DDSs. For instance, Lamont et al was able to show significant accumulation of an anti-cancer candidate and its metabolites in fibrotic tissue lesions, which led to the exclusion of the compound.5 This was only possible because of DESI-MSI’s high sensitivity and resolution. Bäckström et al compared inhaled vs intravenous DDSs of bronchodilator by looking at the distribution of the drug after the two administration routes.6 It was found that the inhaled drug was able to be retained in the bronchial epithelium and subepithelium, and its levels were more than 30-fold higher in both regions than the deuterium counterpart administered intravenously.
One big aspect of drug studies is whether the drug can cross the blood brain barrier (BBB). This highly selective semipermeable boarder poses as a great challenge in the development of drugs that are intended to be delivered to the brain. Commonly, BBB permeability is assessed by in vivo cerebral microdialysis, which measures unbound drug concentrations in the extracellular fluid of the brain.2 Such technique is technically demanding and lacks detailed localization information. DESI-MSI’s straight-forward workflow and high resolution thus comes in handy as an alternative to investigate BBB permeability. Vallianatou et al demonstrated the ability of DESI-MSI to accurately visualize drug localization in the brain by inhibiting multidrug resistance 1 protein (MDR1).7 Prior to MDR1 inhibition, the MDR1 substrate, loperamide, showed restricted entry into the brain and localised primarily in the ventricles. After inhibition, the level of loperamide significantly increased in various brain structures – especially the grey matter. Propranolol, which is not a MDR1 substrate, was used as control and exhibited no significant differences in distribution before and after the inhibition. Through this experiment, the group was able to explore DESI-MSI’s potential in BBB permeability investigations with promising results for future implications.
Another advantage of DESI-MSI comes from the simplicity of overlaying it with histopathological, immunohistochemical, and fluorescence staining images. DESI-MSI can detect even the smallest alterations in metabolites and lipids and these alterations are often products of enzymatic activity from pathophysiological processes.2 These enzymes, their larger molecular weight cofactors, and corresponding genes (which can be visualized using the methods listed previously in this paragraph) are all potential targets for drug inhibition to reverse the pathological phenotype.2 Therefore, DESI-MSI provides additional depth in information in terms of not only detecting potential targets, but also testing the effectiveness of newly developed treatments. For example, Shroff et al was able to show using DESI-MSI that the glutaminolysis pathway was a druggable target in MYC-driven kidney cancer.8 Inhibition of glutaminase prevented MYC-driven renal tumorigenesis in vivo, by disabling glutamate’s entry into the Krebs cycle. Gerbig et al investigated colorectal adenocarcinoma largely driven by KRAS.9 The study was able to pinpoint the exact location of mutations in the KRAS gene using the DESI-MSI data. This provided crucial insight in the prediction of therapeutic sensitivity for KRAS cancer because mutations in the KRAS causes the tumour to be less responsive to epidermal growth factor receptor (EGFR)-targeted monoclonal antibody therapy.
In summary, DESI-MSI has proven itself useful multiple times in the field of biomedical research. The ability to produce multiplex data using an untargeted methodology has received increasing attention as the idea of multi-omics grew more popular. Perhaps soon, this technique will be added to the day-to-day toolkit for the next generation of pharmaceutical investigators.
- Takáts Z, Wiseman JM, Gologan B, Cooks RG. Mass spectrometry sampling under ambient conditions with desorption electrospray ionization. Science. 2004 Oct 15;306(5695):471-3.
- Soudah T, Zoabi A, Margulis K. Desorption electrospray ionization mass spectrometry imaging in discovery and development of novel therapies. Mass Spectrom Rev. 2021 Oct 13.
- Banerjee S, Zare RN, Tibshirani RJ, Kunder CA, Nolley R, Fan R, Brooks JD, Sonn GA. Diagnosis of prostate cancer by desorption electrospray ionization mass spectrometric imaging of small metabolites and lipids. Proc Natl Acad Sci USA. 2017 Mar 28;114(13):3334-3339.
- Takáts Z, Strittmatter N, McKenzie JS. Ambient Mass Spectrometry in Cancer Research. Adv Cancer Res. 2017;134:231-256.
- Lamont L, Eijkel GB, Jones EA, Flinders B, Ellis SR, Porta Siegel T, Heeren RMA, Vreeken RJ. Targeted Drug and Metabolite Imaging: Desorption Electrospray Ionization Combined with Triple Quadrupole Mass Spectrometry. Anal Chem. 2018 Nov 20;90(22):13229-13235.
- Bäckström E, Hamm G, Nilsson A, Fihn BM, Strittmatter N, Andrén P, Goodwin RJA, Fridén M. Uncovering the regional localization of inhaled salmeterol retention in the lung. Drug Deliv. 2018 Nov;25(1):838-845.
- Vallianatou T, Strittmatter N, Nilsson A, Shariatgorji M, Hamm G, Pereira M, Källback P, Svenningsson P, Karlgren M, Goodwin RJA, Andrén PE. A mass spectrometry imaging approach for investigating how drug-drug interactions influence drug blood-brain barrier permeability. Neuroimage. 2018 May 15;172:808-816.
- Shroff EH, Eberlin LS, Dang VM, Gouw AM, Gabay M, Adam SJ, Bellovin DI, Tran PT, Philbrick WM, Garcia-Ocana A, Casey SC, Li Y, Dang CV, Zare RN, Felsher DW. MYC oncogene overexpression drives renal cell carcinoma in a mouse model through glutamine metabolism. Proc Natl Acad Sci USA. 2015 May 26;112(21):6539-44.
- Gerbig S, Golf O, Balog J, Denes J, Baranyai Z, Zarand A, Raso E, Timar J, Takáts Z. Analysis of colorectal adenocarcinoma tissue by desorption electrospray ionization mass spectrometric imaging. Anal Bioanal Chem. 2012 Jun;403(8):2315-25.