Matrix‐assisted laser desorption/ionization imaging mass spectrometry for direct measurement of clozapine in rat brain tissue

Abstract

Matrix-assisted laser desorption/ionization hyphenated with quadrupole time-of-flight (QTOF) mass spectrometry (MS) has been used to directly determine the distribution of pharmaceuticals in rat brain tissue slices which might unravel their disposition for new drug development. Clozapine, an antipsychotic drug, and norclozapine were used as model compounds to investigate fundamental parameters such as matrix and solvent effects and irradiance dependence on MALDI intensity but also to address the issues with direct tissue imaging MS technique such as (1) uniform coating by the matrix, (2) linearity of MALDI signals, and (3) redistribution of surface analytes. The tissue sections were coated with various matrices on MALDI plates by airspray deposition prior to MS detection. MALDI signals of analytes were detected by monitoring the dissociation of the individual protonated molecules to their predominant MS/MS product ions. The matrices were chosen for tissue applications based on their ability to form a homogeneous coating of dense crystals and to yield greater sensitivity. Images revealing the spatial localization in tissue sections using MALDI-QTOF following a direct infusion of (3)H-clozapine into rat brain were found to be in good correlation with those using a radioautographic approach. The density of clozapine and its major metabolites from whole brain homogenates was further confirmed using fast high-performance liquid chromatography/tandem mass spectrometry (HPLC-MS/MS) procedures.

Imaging MALDI MS of Dosed Brain Tissues Utilizing an Alternative Analyte Pre-extraction Approach

“…As demonstrated with the previous cobimetinib experiments, tissue that was washed with ammonium citrate and presprayed with cyclohexane prior to matrix application yielded optimal extraction with a~8-fold gain in overall average signal intensity ( Table 2). As a result, a more ubiquitous distribution of clozapine was observed ( Figure 5e) and is in line with previously reported autoradiography data, which detected clozapine distributed throughout the brain [20]. Ammonium formate, a more commonly used wash buffer [7,8], was also tested to determine if it can yield the same signal gains as ammonium citrate.…”

MALDI In-Source Decay, from Sequencing to Imaging

“…For small molecules analysis, two main classes may be distinguished -exogenous drugs and endogenous metabolites. Exogenous drugs have been studied in MALDI-MSI far later than proteins (since 2004) for different targets such as olanzapine (antipsychotic) [85][86][87], imatinib [87], vinblastine [88], and banoxantrone [89] (cancer drugs). Among endogenous metabolites, lipids such as glycerophospholipids and glycosphingolipids or simple lipids like cholesterol, diacylglycerols, or triacylglycerols have been extensively investigated [90].…”

Probing the Relationship Between Detected Ion Intensity, Laser Fluence, and Beam Profile in Thin Film and Tissue in MALDI MSI

“…Soltwisch et al employed a combination of four peptides within their recent study [7], but no analysis of the effect of fluence on these molecules individually or of this increased complexity on the observed trends was explicitly presented. A 2006 study by the Korfmacher group showed the detected ion response to laser energy for clozapine and norclozapine in dosed rat brain tissue, but no measurement of the size of the irradiated area was carried out, nor was this performed in a spatially resolved (imaging) mode, meaning the spatial distribution of the drug within the tissue cannot be discounted as a significant contributor to the trends seen [10]. Lagarrigue et al discussed the spectral differences resulting from different size irradiated areas and fluence regimes in a tissue imaging context but no energy measurements were presented to accompany these data [11].…”