Enhanced Sensitivity Using MALDI Imaging Coupled with Laser Postionization (MALDI-2) for Pharmaceutical Research

Abstract

Visualizing the distributions of drugs and their metabolites is one of the key emerging application areas of matrix-assisted laser desorption/ionization–mass spectrometry imaging (MALDI-MSI) within pharmaceutical research. The success of a given MALDI-MSI experiment is ultimately determined by the ionization efficiency of the compounds of interest, which in many cases are too low to enable detection at relevant concentrations. In this work we have taken steps to address this challenge via the first application of laser-postionisation coupled with MALDI (so-called MALDI-2) to the analysis and imaging of pharmaceutical compounds. We demonstrate that MALDI-2 increased the signal intensities for 7 out of the 10 drug compounds analyzed by up to 2 orders of magnitude compared to conventional MALDI analysis. This gain in sensitivity enabled the distributions of drug compounds in both human cartilage and dog liver tissue to be visualized using MALDI-2, whereas little-to-no signal from tissue was obtained using conventional MALDI. This work demonstrates the vast potential of MALDI-2-MSI in pharmaceutical research and drug development and provides a valuable tool to broaden the application areas of MSI. Finally, in an effort to understand the ionization mechanism, we provide the first evidence that the preferential formation of [M + H] + ions with MALDI-2 has no obvious correlation with the gas-phase proton affinity values of the analyte molecules, suggesting, as with MALDI, the occurrence of complex and yet to be elucidated ionization phenomena.

Evaluation of lipid coverage and high spatial resolution MALDI-imaging capabilities of oversampling combined with laser post-ionisation

“…An innovative option to overcome this decreased ionisation efficiency is through the use of laser post-ionisation combined with MALDI-MSI (so-called MALDI-2). MALDI-2 has been demonstrated to enable an up to two order of magnitude increase in sensitivity for lipids and other molecular classes from biological tissues [25][26][27]. Recently, MALDI-2 has been combined with transmission mode MSI to enable spatial resolutions as low as 600 nm under oversampling conditions [28], but it has not yet been evaluated for oversampling using conventional front-side MALDI laser introduction.…”

Imaging Isomers on a Biological Surface: A Review

“…is modi cation allows a lateral resolution as low as ∼10 µm. 20) Post-ionization e ionization e ciency of MALDI is estimated to be <10 −4 relative to generated neutrals, 21,22) or even signi cantly lower depending on the compound class, which limits the sensitivity of this technique as many molecules are not detected. A method to boost the MALDI ion yield is a postionization strategy.…”

“…Barré et al also used MALDI-2 to enhance sensitivity and signal intensity of various pharmaceutical compounds. 22) In addition, it was shown that ibuprofen, which is not detectable with regular MALDI, was detectable as a radical cation using MALDI-2. It is hypothesized that the ionization mechanism in MALDI-2 involves resonant two-photon ionization of the matrix.…”

“…21) Nevertheless, a full understanding of the MALDI-2 ionization, just like the regular MALDI ionization process, remains elusive. 21,22,24) Aside from MALDI, also other ionization techniques can bene t from post-ionization. Liu et al combined DESI with post-photoionization by using a portable krypton lamp for secondary ionization of the desorbed neutrals.…”

“…As an alternative to chemical derivatization, also post-ionization strategies can be used to enhance the ionization of nonpolar compounds such as cholesterol. 22,23,25) In addition to cholesterol, also imaging of other steroid hormones have been reported by enhancing the sensitivity. Recently, Cobice et al have introduced Girard reagent T (GirT) as on-tissue derivatization and enabled the detection of steroids with MALDI.…”