15
By comparison, MSI takes a conventional tissue section and coats it with a matrix, which extracts molecules from the tissue, but retains the spatial relationships found in the underlying tissue. Following preparation, the sample is measured in the spectrometer. The result is spatially resolved mass spectra. Because the laser only probes the matrix crystals which are on the surface of the section, the underlying cellular features are not disrupted and can be taken through a standard histological staining routine so that a high-quality histology image can be captured. By merging this scan with the molecular information from the MALDI mass spectrometer a histology-directed analysis of the tissue is possible.
Recent work by M. Reid Groseclose et al (ref) evaluated the additional information that MALDI MSI could provide over and above LC-MS in a nephrotoxicity study on the anti- cancer drug Dabrafenib (DAB) in rats. This work was in support of developing DAB for use in paediatric patients. Previous studies had identifi ed some unexpected adverse kidney effects in juvenile rats. These effects had not been seen in adult studies [3].
Initially, MALDI MSI could determine the distribution of DAB and its metabolites in the kidney (Figure 1). Subsequent analysis of tubular deposits seen in the fi rst experiments, provided the chemical composition at these locations and triggered a more complete risk assessment for paediatric treatment with Dabrafenib than would have been possible using LC-MS analysis alone.
Figure 2. Ion path mounted on optical bench with TOF/TOF unit. The principles of MALDI Imaging (MALDI IMS)
Importantly, expanding the performance envelope of a MALDI system requires a fast laser. The cutting-edge systems of today imply a 10 kHz laser, but accommodating a 10 kHz laser can raise many technical and mechanical issues that need to be solved. For example, traditional instruments reposition the sample relative to a fi xed laser spot. The mechanics of moving a sample quickly and precisely enough to keep up with a fast laser are challenging. However, by reversing the traditional thinking and moving the laser relative to a fi xed sample position, the potential of the 10 kHz laser has been fully realised.
In addition, developments in detector design, notably, incorporation of the academically acclaimed dynamically harmonised ParaCell™, developed by Professor Eugene Nikolaev and co-workers at the Russian Academy of Sciences in Moscow, into solariXTM
XR
system. This innovative design stabilises the ion cyclotron resonance signal over a broad mass range. Mass spectra can be acquired with 10 million resolving power when high-throughput is not required. The instrument also provides benchmark performance at faster acquisition rates, resolving power is greater than 250,000 at m/z 400 in one second at 7T. This and other recent improvements create a nearly ideal and yet unmatched analyser for complex mixtures like those found in MALDI imaging.
The challenge that fast analysis poses for control, capture and processing routines is also a signifi cant consideration when an instrument is working at the speeds required for routine analysis. Unlike classical MALDI instruments, new-generation systems now use strategies such as parallel computing threads, each fast enough to keep up with the speed of the analysis.
Summary
MALDI Imaging Mass Spectrometer Experimental Workfl ow
The MALDI imaging experiment is initiated by mounting a tissue section onto a target, applying a matrix and rastering a laser across the surface of the tissue. At each discrete location within a virtual grid the laser is fi red, and a mass spectrum is acquired. By plotting the ion intensities as a function of the x and y coordinates on the tissue, ion images are generated.
New Technology & Advanced Tools in MALDI Technology
MALDI has become an established and powerful technique for drug discovery and instrument manufacturers have switched their focus to certain key system components of the system. These have become the subject of intensive development as companies look to advance performance, reduce costs and streamline workfl ows for researchers. The goals for a user may be specifi c to each application. For example, speed, throughput and cost per sample in HTS vs. resolution and sensitivity in imaging applications. However, improving and optimising the critical components for an individual application can deliver improvements for all.
At the heart of a system is a high speed laser operating at 10 kHz. Stability is critical for both spatial and mass resolution such that improving the precision of the optical bench which carries the ion optics and the necessary lenses, refl ectors and detector modules, performance can be signifi cantly upgraded. The confi guration of a typical TOF/TOF optical bench is featured in Figure 2.
Having emerged in basic research and subsequently proven its value in routine uHTS and MSI applications, forward-thinking researchers are investing in the latest instrumentation and expanding the application of the technique, anticipating additional insights and a deeper understanding of a candidate compound early in the development process. In order to facilitate a rapid route to market for new, safe and effi cacious drugs pharmaceutical companies are seeking out innovative approaches and technologies [5]. MALDI technology is already making an impact in small molecule R&D, and many industry observers believe we will see this grow signifi cantly over the coming years.
References 1. Pharmaprojects 2016. Pharma R&D Annual Review of 2016
2. Peter Marshall, Melanie Leveridge, Carl Haslam, Gabriella Clarke, Jessica Chandler, Adrian Dunn, Neil Hardy, Michelle Pemberton, 2016. Ultra High Throughput Drug Discovery Screening by MALDI-TOF Mass Spectrometry –Exceeding One Million Samples per Week (Poster)
3. M. Reid Grosclose et al, 2015. Imaging MS in Toxicology: An Investigation of Juvenile Rat Nephrotoxicity Associated with Dabrafenib Administration. J. Am. Soc. Mass Spectrom. (2015) 26:887:898. DOI: 10.1007/s13361-015-1103-4 (Internet)
https://www.ncbi.nlm.nih.gov/pubmed/25804893 [Accessed 15/12/2016]
4. Groseclose, M.R., Laffan, S.B., Frazier, K.S. et al. J. Am. Soc. Mass Spectrom. (2015) 26: 887. doi:10.1007/s13361-015-1103-4
5. Castellino S, Groseclose MR, Wagner D. 2011. MALDI imaging mass spectrometry: bridging biology and chemistry in drug development (Internet)
https://www.ncbi.nlm.nih.gov/pubmed/22074284/ [Accessed 15/12/2016]
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