Taking MALDI to the Next Level

I think it’s fair to say that MALDI-MS imaging has been a transformative technique when it comes to understanding the molecular makeup and regional heterogeneity of biological tissues. Its label-free nature and ability to differentiate compounds by molecular weight make it suitable for many applications – from drug discovery to pathology and biomedical research.

And yet, it’s also fair to argue that MALDI imaging faces inherent challenges – namely, ion suppression, sensitivity at higher spatial resolutions, and quantification dynamic range –which technological advances have failed to address. Until now.

Full disclosure up front: I work for Bruker Daltonics and – as its Global MS Imaging Market Manager – I have a clear interest in the continued rise of MALDI. However, I truly believe that new technology now on the market will be game changing when it comes to the applicability of MALDI. Take, for example, the lower sensitivity when increasing resolution, we adapted MALDI-2 – laser-based post-ionization (PI) that increases the sensitivity of MALDI imaging by up to three orders of magnitude (dependent, of course, on the sample, matrix, and analyte). The upside of a sensitivity boost? Opening the door to studying compounds typically inaccessible to traditional MALDI, including many smaller molecules, such as lipids, vitamins, and glycans (1), as well as expanding the number of molecular channels available for untargeted tissue subtyping.

The downside? More ion signals result in more complex spectra. Here, a second recent innovation for MALDI – trapped ion mobility spectrometry (TIMS) – proves invaluable by adding the dimensionality of Collisional Cross Section (CCS) to separate the complex signals by CCS as well as m/z. Using TIMS we routinely observe dozens of near-isobaric ion signals from tissue cleanly separated by CCS and m/z that would otherwise not be resolved by m/z alone. An additional benefit for TIMS is that CCS is an intrinsic property of the molecule and therefore can be used in combination with m/z to more confidently identify the compound.

In my view, the powerful combination of MALDI-2 and TIMS could increase adoption of MS imaging in a number of application areas – but (bio)pharmaceutical research has much to gain in particular. By integrating the analysis of tissue, cells, and body fluids for a deeper understanding of disease mechanisms, researchers can develop better biomarkers, monitor more drug compounds and their metabolites, and work towards truly personalized medicine.

For example, the additional sensitivity of MALDI-2 is particularly significant for targeted drug and drug metabolite imaging for Drug Metabolism and Pharmacokinetics (DMPK) studies. As demonstrated in one study, a serial dilution of five pharmaceutical compounds – caffeine, chloroquine, rosuvastatin, reserpine and BI-YYY (a new drug compound) (2) were spotted onto control tissue and analysed using MALDI and MALDI-2. Results showed lower detection limits for MALDI-2 for all five compounds. In particular, the peak intensity of BI-YYY was enhanced by a factor of 300 by MALDI-2.

As part of the same study, liver and kidney tissue sections from rat dosed with chloroquine were imaged to compare MALDI vs MALDI-2. Chloroquine and metabolite images were more intense using MALDI-2. Given this degree of enhancement, MALDI-2 has the potential to deliver new distribution information from previously undetected metabolites as well as providing lower limits of detection for target compounds – both vitally important to DMPK studies. In addition, hundreds of other signals are available in the same datasets to explore pharmacodynamics.

In short, the adaptation of MALDI-2 onto the timsTOF technology yields a synergistic pairing. MALDI-2 produces ion signals in greater abundance and complexity, whilst TIMS has the capacity to resolve the additional signals with intrinsic CCS values that improve confidence of molecular identifications. Together, timsTOF fleX with MALDI-2 promise to and broaden the scope of imaging studies of small (bio)molecules that were previously undetectable with traditional MALDI imaging.