Anal Bioanal Chem (2011) 401:65–73 DOI 10.1007/s00216-011-4990-7
PAPER IN FOREFRONT
Mass spectrometry imaging with high resolution in mass and space (HR2 MSI) for reliable investigation of drug compound distributions on the cellular level Andreas Römpp & Sabine Guenther & Zoltan Takats & Bernhard Spengler
Received: 19 January 2011 / Revised: 4 April 2011 / Accepted: 6 April 2011 / Published online: 26 April 2011 # Springer-Verlag 2011
Abstract Mass spectrometry (MS) imaging is a versatile method to analyze the spatial distribution of analytes in tissue sections. It provides unique features for the analysis of drug compounds in pharmacokinetic studies such as label-free detection and differentiation of compounds and metabolites. We have recently introduced a MS imaging method that combines high mass resolution and high spatial resolution in a single experiment, hence termed HR2 MS imaging. In the present study, we applied this method to analyze the spatial distribution of the anti-cancer drugs imatinib and ifosfamide in individual mouse organs. The whole kidney of an animal dosed with imatinib was measured at 35 μm spatial resolution. Imatinib showed a well-defined distribution in the outer stripe of the outer medulla. This area was analyzed in more detail at 10 μm step size, which constitutes a tenfold increase in effective spatial resolution compared to previous studies of drug compounds. In parallel, ion images of phospholipids and heme were used to characterize the histological features of the tissue section and showed excellent agreement with histological staining of the kidney after MS imaging. Ifosfamide was analyzed in mouse kidney at 20 μm step size and was found to be accumulated in the inner medulla region. The identity of imatinib and ifosfamide Andreas Römpp and Sabine Guenther equally contributed to this paper. Published in the special issue MALDI Imaging with Guest Editor Olivier Laprévote. Electronic supplementary material The online version of this article (doi:10.1007/s00216-011-4990-7) contains supplementary material, which is available to authorized users. A. Römpp : S. Guenther : Z. Takats : B. Spengler (*) Institute of Inorganic and Analytical Chemistry, Justus Liebig University, Giessen, Germany e-mail:
[email protected]
was confirmed by on-tissue MS/MS measurements. All measurements including mass spectra from 10 μm pixels featured accurate mass (≤2 ppm root mean square) and mass resolving power of R=30,000. Selected ion images were generated with a bin size of Δm/z=0.01 ensuring highly specific information. The ability of the method to cover larger areas was demonstrated by imaging a compound in the intestinal tract of a rat whole-body tissue section at 200 μm step size. The described method represents a major improvement in terms of spatial resolution and specificity for the analysis of drug compounds in tissue sections. Keywords Mass spectrometry imaging . Drug compounds . Accurate mass . High-resolution mass spectrometry
Introduction Mass spectrometry (MS) imaging has become a widely used analytical technique due to its ability to visualize the distribution of a large variety of analytes [1–5]. Numerous applications have been published including the imaging of lipids [6], peptides [7, 8], and proteins [9]. MS imaging has also been applied to detection of drug compounds [10, 11]. Its ability for label-free detection and differentiation between compound and metabolites are major advantages compared to autoradiography, which is the classical method to investigate the spatial distribution of drug candidates [12]. The spatial resolution of MS imaging measurements of drug compounds is typically in the range of 200 to 500 μm, which is sufficient to analyze whole-body sections [11, 13] or individual rat organs [14], but insufficient to resolve detailed features in mouse organs. The highest resolution reported so far for drug compounds was 100 μm [11, 15].
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Mass analyzers employed are typically time-of-flight (TOF) systems such as QTOF [13], TOF/TOF [11, 15], or ion mobility TOF instruments [16]. The use of triple-quadrupole [17] and ion trap [18] mass spectrometers was also reported. Compounds are usually detected in MS/MS mode in order to increase the specificity of the measurement and to prevent interference of matrix cluster ions which are typically abundant in the low mass range [11]. Accurate mass measurements can be used for direct identification without the need for separate fragmentation of each analyte, in many cases. Mass spectrometers with mass accuracies in the low parts per million range were routinely used in many bioanalytical applications. The highest mass accuracy and mass resolution is obtained by Fourier transform (FT) ion cyclotron resonance mass spectrometers [19, 20]. Another type of FTMS is the recently introduced orbital trapping system [21]. The use of these high-quality mass analyzers in matrix-assisted laser desorption/ionization (MALDI) imaging is limited to very few examples so far [7, 22, 23]. These measurements showed, that the full complexity of biological samples can only be resolved with high mass accuracy and high mass resolution analyses. The only FTMS imaging measurement of drug compounds reported so far was performed at 350 μm spatial resolution [14]. Recently, we introduced a method combining for the first time the capabilities of FTMS with spatial resolution in the cellular range [23]. We termed this method HR2 MS imaging because of its ability to obtain high resolution in mass and space. Analytes can be detected with sub-parts per million (ppm) mass accuracy, and MS/MS experiments can be performed in order to verify identification. The obtained MS images are highly specific and show excellent correlation to histological features on the cellular level. Applications include the analysis of phospholipids [23] and neuropeptides [24] at a spatial resolution of 5 μm. In the present study, we applied our method to the detection of the anticancer drugs imatinib and ifosfamide in mouse kidney sections. The resulting MS images provide detailed information on spatial distribution and represent the first mass spectrometric imaging analysis of drug compounds at 10 μm spatial resolution. This constitutes a tenfold increase in effective spatial resolution compared to previous studies and makes detailed histological feature in mouse organs accessible for the first time.
Experimental Samples Mouse tissue samples were obtained from Semmelweis University, Budapest, Hungary. CD1-nude mice (9 weeks old) received a single oral dose of 750 mg/kg imatinib and
A. Römpp et al.
375 mg/kg ifosfamide, respectively. Animals were sacrificed 2 h after administration by neck disruption. Kidneys were resected and immediately snap-frozen in liquid nitrogen. Tissue samples were cut in sections of 20 μm thickness with a cryotome (HM 525 cryostat, Thermo Scientific, Dreieich, Germany) at −20 °C. The sections were thaw-mounted on glass slides. The mounted samples were stored at −80 °C until analysis. For measurements, tissue sections were brought to room temperature in a desiccator (30 min) to avoid condensation of humidity on the sample surface. Optical images of tissue sections were taken before sample preparation with an Olympus BX-40 microscope (Olympus Europa GmbH, Hamburg, Germany). No washing steps were applied prior to matrix application. A solution of the matrix 2,5-dihydroxybenzoic acid (DHB; 98% purity, Aldrich, Germany) in concentrations of 30 mg/mL was prepared in acetone/water (0.1% TFA) 1:1 v/v. The matrix solution was applied with a specially designed pneumatic sprayer [25]. Tissue sections were analyzed immediately after matrix application. All required ethical permissions were obtained for the animal experiments from the respective institutional ethical committee. Instrumentation All experiments were performed using a home-built atmospheric pressure scanning microprobe MALDI (APSMALDI) imaging source attached to a linear ion trap/FT orbital trapping mass spectrometer (LTQ Orbitrap Discovery, Thermo Scientific GmbH, Bremen, Germany). The setup of the AP-SMALDI imaging source is given in detail elsewhere [26]. A nitrogen laser (1=337 nm, LTB MNL106, LTB, Berlin, Germany) operating at a repetition rate of 60 Hz was used for desorption/ionization in this study. The laser beam was focused by the centrally bored objective lens to an optical diameter of 8.4 μm (1/e2 definition). The effective diameter of the ablation spot varies with chosen laser energy [27] and was always less than 10 μm for experiments with 10 μm pixel size. The laser was slightly defocused (and laser energy increased for compensation) for experiments with 20 and 35 μm pixel size in order to increase irradiation area and thus ion yield. Ions from 30 laser pulses were accumulated in the linear ion trap for each mass spectrum. The target voltage was set to 4.3 kV. The step size of the sample stage was set to the desired pixel size (10, 20, 35, and 200 μm). The LTQ Orbitrap instrument was operated in positive-ion mode and in the normal mass range mode (m/z=100–1,000). MS imaging measurements were performed using the Orbitrap detector with a mass resolving power of 30,000 at m/z= 400 in profile mode (i.e., mass peaks were not centroided for data reduction). Automatic gain control (AGC) was disabled during the measurement, and ion injection time was
Mass spectrometry imaging with high resolution in mass and space
manually set to 650 ms. A mass accuracy of
