Elsevier

Journal of Proteomics

Volume 75, Issue 16, 30 August 2012, Pages 4999-5013
Journal of Proteomics

Review
Mass spectrometry imaging for drug distribution studies☆

https://doi.org/10.1016/j.jprot.2012.07.028Get rights and content

Abstract

Since its introduction mass spectrometry imaging (MSI) has proven to be a powerful tool for the localization of molecules in biological tissues. In drug discovery and development, understanding the distribution of both drug and its metabolites is of critical importance. Traditional methods suffer from a lack of spatial information (tissue extraction followed by LCMS) or lack of specificity resulting in the inability to resolve parent drug from its metabolites (whole body autoradiography). MSI is a sensitive and label-free approach for imaging drugs and metabolites in tissues. In this article we review the different MSI technologies that have been applied to the imaging of pharmaceuticals. Recent technical advances, applications and current analytical limitations are discussed.

This article is part of a Special Issue entitled: Imaging Mass Spectrometry: A User’s Guide to a New Technique for Biological and Biomedical Research.

Graphical abstract

Highlights

► Mass Spectrometry Imaging is a powerful tool for localizing compounds and metabolites. ► MSI technologies are label-free enabling metabolites to be resolved from the parent drug. ► Cellular level spatial resolution has been demonstrated. ► Recent advancements include full quantitation and high throughput imaging.

Introduction

In drug discovery and development, detailed understanding of pharmacological activity, toxicological effects and distribution is required. For a pharmaceutical compound to exert its desired effects it must reach biological receptors at the target site, but undesired accumulation of compound or metabolites in tissues can lead to strong toxicological effects [1], [2]. Compound or metabolite levels measured in plasma often do not accurately represent the levels present within organs or organ sub-compartments and therefore cannot be relied upon for understanding of efficacy or toxicology of drugs within the body [3].

Mass spectrometry imaging (MSI) has been proven to provide complementary information to the traditional methods utilized in drug distribution studies. As such, its use in both industrial and academic pharmaceutical research has increased rapidly since its introduction.

The purpose of this review is to describe the current role of MSI within drug discovery and development. Here we highlight a few key examples of pharmaceutical compound and metabolite MSI studies and discuss recent developments in both MSI instrumentation and sample preparation processes.

Section snippets

The need for MS imaging in drug discovery and development

MSI has rapidly emerged as a valuable technology for the localization of drugs and metabolites in biological tissues. Although initially demonstrated for the localization of peptides and proteins [4], [5], it was soon applied to low molecular weight analytes and has several key advantages over traditional imaging and analytical methods used in pharmaceutical research, most importantly the technology being completely label free. However, careful consideration of the methodology is vital as drugs

Imaging modalities

A number of differing MSI technologies have been applied to analyze the distribution of drugs and/or metabolites in biological tissues. A concise summary is shown in Table 1.

MALDI

The most widely applied ionization technique in MSI of drugs and metabolites is Matrix-Assisted Laser Desorption/Ionization (MALDI) [6], [8], [9], [10]. In a standard MALDI MSI experiment the tissue section is coated with an UV-absorbing matrix that is typically applied in solution. The analytes are extracted from the tissue by the solvent and become co-crystalized with the matrix as the solution dries. Ionization of the analytes occurs upon subsequent irradiation of the sample with a UV laser.

SIMS

In Secondary Ion MSI (SIMS-MSI), a primary ion beam is directed at the tissue surface resulting in the sputtering of both neutral and charged secondary species from the tissue. It has the primary advantage of achieving submicrometer spatial resolution, which cannot currently be matched by competing MALDI and Desorption Electrospray Ionization (DESI) modes [47], [48]. However, it suffers from an excessively hard ionization, which can result in severe in-source fragmentation, ultimately causing

DESI and LAESI

Desorption Electrospray Ionization (DESI) MS was first demonstrated for the imaging of bioanalytes in tissues in 2006 [59]. In the DESI ionization process, electrosprayed charged solvent droplets (optimized for the analyte/s of interest) are directed at the tissue surface. The impact of the droplet stream results in the desorption of ions from the surface by electrostatic and pneumatic sources. Desorbed gas-phase ions are then transferred into the mass spectrometer by use of an ion transfer

LDI and NIMS

In its simplest form, Laser Desorption Ionization (LDI) can be considered as MALDI without the requirement to coat the sample with a UV absorbing matrix. Typically a UV or IR laser is utilized to ionize chromophores within the analyzed substance. Non-volatile analytes up to 2000 Da have been successfully ionized from metal surfaces [68]. However, such analytes require the presence of a chromophore, which absorbs, and subsequently becomes ionized by, the radiation within the wavelength bands of

Key considerations in sample preparation

In a typical ADME animal experiment applying ex-vivo molecular imaging, the compound is dosed and the animal is sacrificed after an allotted time period. To obtain whole animal distribution information the entire animal (typically a rodent) is sectioned. Alternatively, organs may be extracted or biopsies taken, which are then either homogenized or sectioned using a cryostat. The sectioning process itself is more difficult for whole body tissues and requires, besides a bigger cryostat, an

Strategies for compound and metabolite identification

Conclusive identification of drugs and metabolites in tissue can prove problematic due to the presence of many interfering peaks from endogenous species and (where utilized) matrix within the low mass range. In many cases a drug standard is available to aid method development. Standards may also be available for major metabolites. This enables a targeted imaging approach to be used, maximizing analytical sensitivity.

Quadrupole and ion trap mass analyzers in particular suffer from low mass

Spatial resolution

Spatial resolution requirements are an important consideration in the design of a drug or metabolite imaging experiment and are typically selected based upon the type of tissue being imaged. For whole body imaging experiments of rats or mice, a spatial resolution of 500 μm can often provide a decent overview of the compound and metabolite distribution within the major organs and tissues of interest, whilst maintaining an acceptable data acquisition time. Higher resolution images can be acquired

Quantitation and signal normalization

One important consideration in any drug or metabolite imaging experiment is how representative the recorded ion signal recorded is of the actual amount of compound or metabolite present in the tissue. Ion suppression effects are the main reason for the inhomogeneous loss in signal intensities, which occur in many tissue types. Ion suppression effects are predominantly caused by competing endogenous species such as lipids, or by the presence of the highly abundant salts in most biological

High-throughput MS imaging

MSI, whilst typically providing spatial data on drug distribution within hours rather than the days required for WBA, can still be a relatively time consuming process. The incorporation of continuously rastering high repetition rate lasers such as Nd:YAG [16], [44] and subsequently Nd:YVO4 [83], [98] and Nd:YLF [99] into MALDI ionization sources has vastly decreased the image acquisition time. Whole body rodent imaging of a compound and several metabolites can now be conducted in as little as

Hybrid SIMS

As previously mentioned in this article, SIMS MS imaging suffers from difficulties in absolute identification of the secondary ions produced. However, recent instrument developments have been used to overcome this difficulty. Tandem MS of secondary ions has been demonstrated in different QTOF mass spectrometers with an increased resolving power of up to 15,000 [101] mass accuracy in the low ppm range and maximum achievable spatial resolution of 1 μm [102]. Coupling the SIMS ion source to an

Conclusion and perspectives

Since its initial applications in pharmaceutical imaging, MSI (in particular MALDI-MSI) has gained acceptance as a valuable tool for drug and metabolite localization studies. The ability to directly and rapidly map the distribution of drugs without the necessity for radiolabeling is of clear benefit. However, probably the greatest value offered by MS imaging is the ability to acquire images of both parent drug and multiple metabolites in a single imaging experiment.

The ability to produce

Acknowledgements

The authors thank Dieter Staab, Gregory Morandi and Nicole Ehrehardt (Novartis Institutes for BioMedical Research) for scientific discussion, experimental design and acquisition of the whole-body MALDI MS images shown in Fig. 2. In addition we thank Laura E. Via and Danielle Weiner (National Institute of Allergy and Infectious Disease) for providing tissue biopsies used to generate Fig. 6.

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    This article is part of a Special Issue entitled: Imaging Mass Spectrometry: A User’s Guide to a New Technique for Biological and Biomedical Research.

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