Qualitative and Quantitative MALDI Imaging of the Positron Emission Tomography Ligands Raclopride (a D2 Dopamine Antagonist) and SCH 23390 (a D1 Dopamine Antagonist) in Rat Brain Tissue Sections Using a Solvent-Free Dry Matrix Application Method

Abstract

The distributions of positron emission tomography (PET) ligands in rat brain tissue sections were analyzed by matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI). The detection of the PET ligands was possible following the use of a solvent-free dry MALDI matrix application method employing finely ground dry α-cyano-4-hydroxycinnamic acid (CHCA). The D2 dopamine receptor antagonist 3,5-dichloro-N-{[(2S)-1-ethylpyrrolidin-2-yl]methyl}-2-hydroxy-6-methoxybenzamide (raclopride) and the D1 dopamine receptor antagonist 7-chloro-3-methyl-1-phenyl-1,2,4,5-tetrahydro-3-benzazepin-8-ol (SCH 23390) were both detected at decreasing abundance at increasing period postdosing. Confirmation of the compound identifications and distributions was achieved by a combination of mass-to-charge ratio accurate mass, isotope distribution, and MS/MS fragmentation imaging directly from tissue sections (performed using MALDI TOF/TOF, MALDI q-TOF, and 12T MALDI-FT-ICR mass spectrometers). Quantitative data was obtained by comparing signal abundances from tissues to those obtained from quantitation control spots of the target compound applied to adjacent vehicle control tissue sections (analyzed during the same experiment). Following a single intravenous dose of raclopride (7.5 mg/kg), an average tissue concentration of approximately 60 nM was detected compared to 15 nM when the drug was dosed at 2 mg/kg, indicating a linear response between dose and detected abundance. SCH 23390 was established to have an average tissue concentration of approximately 15 μM following a single intravenous dose at 5 mg/kg. Both target compounds were also detected in kidney tissue sections when employing the same MSI methodology. This study illustrates that a MSI may well be readily applied to PET ligand research development when using a solvent-free dry matrix coating.

Quantification in MALDI-MS imaging: what can we learn from MALDI-selected reaction monitoring and what can we expect for imaging?

“…Another study correlated the MALDI-MSI response with quantitative LC-MS/MS performed on adjacent tissue sections and, after harvesting from rats, several single doses of olanzapine at different concentrations [14]. Goodwin et al also employed MALDI-MSI to map commonly employed PET ligands in rat brain tissue sections [15]. However, the on-tissue approach does not account for droplet dispersion on the tissue and does not reflect the actual analyte concentration per gram of tissue (i.e., signal recorded expressed versus amount [in mol or g] spotted).…”

“…Several papers report the normalization of all spectra against the total ion current (TIC) and prove its suitability to reduce the influence of inhomogeneous matrix coating, that is critical in MALDI-MS imaging experiments [14,15,19,20]. However, if one background ion (interference) dominates in one of the groups, the TIC normalization can lead to incorrect results [21].…”

MALDI Imaging Mass Spectrometry—A Mini Review of Methods and Recent Developments

“…Make sure to wait long enough between the spraying rounds for the tissue to fully dry. Dry matrix can be applied for the analysis of lipids or drugs, [26][27][28] however the results for peptides and proteins have not yet been satisfactory. A large number of publications focus on the different matrices and matrix application strategies, 3) and we strongly recommend the beginner in IMS to read in to this particular part of the experiments.…”

Matrix-Assisted Laser Desorption/Ionization (MALDI) Imaging Mass Spectrometry (IMS): A Challenge for Reliable Quantitative Analyses

“…4). Unlike the aforementioned approach which utilizes the combination of two di erent types of mass spectrometry, Goodwin et al 56) made an attempt to obtain quantitative data directly from the MALDI TOF spectra. ese authors set up to compare the signal intensities from the tissue section collected a er an intravenous injection of the target compound versus those obtained from quantitation-control spots of the compound which was applied to vehicle-control tissue sections.…”