Mass spectrometry imaging in drug distribution and drug metabolism studies – Principles, applications and perspectives

New mass spectrometry imaging (MSI) techniques are gaining importance in various stages of the drug discovery and development chain as well as in more fundamental studies of drug delivery. The advantage of MSI is that for an imaging technique, the selectivity is very high and as a label-free technique, it enables simultaneous and compound-specific imaging of drugs and their metabolites. In addition to that, a vast number of endogenous metabolites, which may serve as tissue biomarkers, can be imaged simultaneously, providing a very extensive characterization of the sample. The present review focusses on the application of MSI for imaging of drugs and their metabolites, primarily is studies of drug distribution and drug metabolism. The first half provides a quite general introduction to MSI with the main focus on MALDI-MSI and DESI-MSI, which are the two most commonly applied MSI techniques for pharmaceutical applications. It also has sections where the necessary sample preparation, including cryo-sectioning, is discussed, along with sections on data analysis and quantitative MSI, which in particular for pharmaceutical applications plays an increasingly significant role. The second half of the paper presents examples of pharmaceutical applications of MSI. The chapter does not contain an exhaustive list of all published studies, but should be considered a curated list of examples where MSI has contributed with information which cannot readily be obtained by conventional analytical techniques. The examples are chosen to represent diverse fields within drug delivery (local and systemic) and drug metabolism with examples from different organs and from whole-body studies. The final section presents a broader look at ongoing trends within MSI and some thoughts on where MSI is heading in the years to come.