Exemplifying the Screening Power of Mass Spectrometry Imaging over Label-Based Technologies for Simultaneous Monitoring of Drug and Metabolite Distributions in Tissue Sections.

Mass spectrometry imaging (MSI) provides pharmaceutical researchers with a suite of technologies to screen and assess compound distributions and relative abundances directly from tissue sections and offer insight into drug discovery-applicable queries such as blood-brain barrier access, tumor penetration/retention, and compound toxicity related to drug retention in specific organs/cell types. Label-free MSI offers advantages over label-based assays, such as quantitative whole-body autoradiography (QWBA), in the ability to simultaneously differentiate and monitor both drug and drug metabolites. Such discrimination is not possible by label-based assays if a drug metabolite still contains the radiolabel. Here, we present data exemplifying the advantages of MSI analysis. Data of the distribution of AZD2820, a therapeutic cyclic peptide, are related to corresponding QWBA data. Distribution of AZD2820 and two metabolites is achieved by MSI, which [(14)C]AZD2820 QWBA fails to differentiate. Furthermore, the high mass-resolving power of Fourier transform ion cyclotron resonance MS is used to separate closely associated ions.

Mapping drug distribution in brain tissue using liquid extraction surface analysis mass spectrometry imaging.

Liquid extraction surface analysis mass spectrometry (LESA-MS) is a surface sampling technique that incorporates liquid extraction from the surface of tissue sections with nanoelectrospray mass spectrometry. Traditional tissue analysis techniques usually require homogenization of the sample prior to analysis via high-performance liquid chromatography mass spectrometry (HPLC-MS), but an intrinsic weakness of this is a loss of all spatial information and the inability of the technique to distinguish between actual tissue penetration and response caused by residual blood contamination. LESA-MS, in contrast, has the ability to spatially resolve drug distributions and has historically been used to profile discrete spots on the surface of tissue sections. Here, we use the technique as a mass spectrometry imaging (MSI) tool, extracting points at 1 mm spatial resolution across tissue sections to build an image of xenobiotic and endogenous compound distribution to assess drug blood-brain barrier penetration into brain tissue. A selection of penetrant and "nonpenetrant" drugs were dosed to rats via oral and intravenous administration. Whole brains were snap-frozen at necropsy and were subsequently sectioned prior to analysis by matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) and LESA-MSI. MALDI-MSI, as expected, was shown to effectively map the distribution of brain penetrative compounds but lacked sufficient sensitivity when compounds were marginally penetrative. LESA-MSI was used to effectively map the distribution of these poorly penetrative compounds, highlighting its value as a complementary technique to MALDI-MSI. The technique also showed benefits when compared to traditional homogenization, particularly for drugs that were considered nonpenetrant by homogenization but were shown to have a measurable penetration using LESA-MSI.

Pragmatic approaches to determine the exposures of drug metabolites in preclinical and clinical subjects in the MIST evaluation of the clinical development phase.

The recent stream of regulatory guidelines on the Safety Testing of Drug Metabolites by the FDA in 2008 and the ICH in 2009 and 2012 has cast light on the importance of qualifying metabolite exposure as part of the safety evaluation of new drugs and has provided a much needed framework for the drug safety researcher. Since then, numerous publications interpreting the practicalities of the guidelines have appeared in the literature focusing on strategic approaches and/or adaptation of modern analytical methodologies, e.g., NMR and AMS, for the identification and quantification of metabolites in the species used in preclinical safety assessments and in humans. Surprisingly, there are few literature accounts demonstrating how, in practice, a particular strategy or analytical method has been used to qualify drug metabolites during the safety evaluation of a drug during clinical development. At the same time as the initial FDA and ICH guideline releases, the neuroscience therapy area of AstraZeneca had a number of projects in clinical development, or approaching this phase, which gave the authors a scaffold upon which to build knowledge regarding the safety testing of drug metabolites. In this article, we present how the MIST strategy was developed to meet the guidelines. Pragmatic approaches have evolved from the experience learned in various projects in DMPK at AstraZeneca, Södertälje, Sweden. Our experience dictates that there is no single strategy for qualifying the safety of drug metabolites in humans; however, all activities should be tied to two unifying themes: first that the exposure to drug metabolites should be compared between species at repeated administration using the relative method or a similar one; and second that the internal regulatory documentation of the metabolite qualification should be agnostic to external criteria (guidelines), indication, dose given, and timing.

Conductive carbon tape used for support and mounting of both whole animal and fragile heat-treated tissue sections for MALDI MS imaging and quantitation.

Analysis of whole animal tissue sections by MALDI MS imaging (MSI) requires effective sample collection and transfer methods to allow the highest quality of in situ analysis of small or hard to dissect tissues. We report on the use of double-sided adhesive conductive carbon tape during whole adult rat tissue sectioning of carboxymethyl cellulose (CMC) embedded animals, with samples mounted onto large format conductive glass and conductive plastic MALDI targets, enabling MSI analysis to be performed on both TOF and FT-ICR MALDI mass spectrometers. We show that mounting does not unduly affect small molecule MSI detection by analyzing tiotropium abundance and distribution in rat lung tissues, with direct on-tissue quantitation achieved. Significantly, we use the adhesive tape to provide support to embedded delicate heat-stabilized tissues, enabling sectioning and mounting to be performed that maintained tissue integrity on samples that had previously been impossible to adequately prepare section for MSI analysis. The mapping of larger peptidomic molecules was not hindered by tape mounting samples and we demonstrate this by mapping the distribution of PEP-19 in both native and heat-stabilized rat brains. Furthermore, we show that without heat stabilization PEP-19 degradation fragments can detected and identified directly by MALDI MSI analysis.

The significance of ambient-temperature on pharmaceutical and endogenous compound abundance and distribution in tissues sections when analyzed by matrix-assisted laser desorption/ionization mass spectrometry imaging.

RATIONALE: Mass spectrometry imaging has proven to be a complementary assay to the traditional labeled-compound studies employed in drug research and development. However, there has been limited examination of the technical limitations of the technique with respect to small molecule stability in samples. METHODS: Raclopride dosed rat brain tissue sections (single dose i.v. 2 mg/kg) were allowed to warm to room temperature for 0 to 5 min prior to either a solvent-based wet matrix-assisted laser desorption/ionization (MALDI) matrix or a solvent-free dry MALDI matrix being applied. Subsequent MS imaging analysis was at a spatial resolution of 200 µm, performed using a MALDI TOF/TOF (Ultraflex II, Bruker Daltonics). RESULTS: MALDI-MS has been used to monitor the time-dependent appearance and loss of small molecule abundance in tissue sections brought rapidly to room temperature for short periods of time. The abundances of a range of markers were seen to vary across the time course, both increasing and decreasing. The intensity of some markers changed significantly within 1 min. Importantly, the abundance of raclopride was seen to decrease over the 5-min time period examined. CONCLUSIONS: The results strongly indicate that considerable care is required to allow comparison of both pharmaceutical and endogenous compounds between MALDI-MSI experiments and also has implications for the standard practice of thaw-mounting multiple tissue sections onto MALDI-MS targets during MSI experiments.

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.

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.

The cardiolipin-cytochrome c interaction and the mitochondrial regulation of apoptosis.

While many studies have focused on cytochrome c release from mitochondria, little attention has been given to the specific interaction between cardiolipin (CL) and cytochrome c, the breaching of which likely represents a critical event in the initiation of mitochondrially mediated apoptosis. Mounting evidence suggests that a decrease in the level of CL affects cytochrome c binding to the inner membrane, thus leading to higher levels of soluble cytochrome c in the mitochondrial intermembrane space. Among the factors known to affect CL levels are thyroid status, plasma concentrations of free fatty acids, Ca2+ dysregulation, and reactive oxygen species (ROS). These factors, especially Ca2+ and ROS, have long been recognized as triggers of cell death and, more recently, as modulators of mitochondrially mediated apoptosis. In this review, we discuss the significance of the disruption of the CL-cytochrome c interaction for cytochrome c release and apoptosis.

Cardiolipin is not required for Bax-mediated cytochrome c release from yeast mitochondria.

Cardiolipin (CL) is an inner mitochondrial membrane phospholipid that contributes to optimal mitochondrial function and is gaining widespread attention in studies of mitochondria-mediated apoptosis. Divergent hypotheses describing the role of CL in cytochrome c release and apoptosis have evolved. We addressed this controversy directly by comparing the spontaneous- and Bax-mediated cytochrome c release from mitochondria isolated from two strains of Saccharomyces cerevisiae: one lacking CL-synthase and therefore CL (DeltaCRD1) and the other, its corresponding wild type (WT). We demonstrated by liquid chromatography-mass spectrometry that the main yeast CL species [(16:1)2(18:1)2] differs in fatty acid composition from mammalian CL [(18:2)4], and we verified the absence of the yeast CL species in the DeltaCRD1 strain. We also demonstrated that the mitochondrial association of Bax and the resulting cytochrome c release is not dependent on the CL content of the yeast mitochondrial membranes. Bax inserted equally into both WT and DeltaCRD1 mitochondrial membranes under conditions that lead to the release of cytochrome c from both strains of yeast mitochondria. Furthermore, using models of synthetic liposomes and isolated yeast mitochondria, we found that cytochrome c was bound more "loosely" to the CL-deficient systems compared with when CL is present. These data challenge recent studies implicating that CL is required for Bax-mediated pore formation leading to the release of proteins from the mitochondrial intermembrane space. In contrast, they support our recently proposed two-step mechanism of cytochrome c release, which suggests that CL is required for binding cytochrome c to the inner mitochondrial membrane.