Mass spectrometry imaging with high resolution in mass and space (HR(2) MSI) for reliable investigation of drug compound distributions on the cellular level.

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 HR(2) 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 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 ∆m/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.