Mass spectrometry imaging of therapeutics from animal models to three-dimensional cell cultures.

In the process of searching for new medicines to improve the quality of human life, detailed understanding of a drug’s absorption, distribution, metabolism and excretion (ADME) has fundamental significance in determining the efficacy or toxicological properties of the pharmaceutical substance. To understand a drug’s distribution, imaging approaches are needed. Autoradiography1, positron emission tomography (PET)2, magnetic resonance imaging (MRI)3 and fluorescence imaging4 are commonly used approaches to examine the spatial distribution of a drug in laboratory animals. These traditional imaging methods require either radioisotopes, magnetic or imaging probes. In autoradiography methods, a radiolabel is attached to the pharmaceutical of interest prior to dosing. In the experiment, the radiolabel is imaged, not the compound itself. Thus, one intrinsic limitation of this technology is the inability to distinguish the localization of a parent drug and its potential metabolites. There are two types of radiography commonly used to image drugs and their metabolites. Macroautoradiography (MARG) is a histological technique that entails a detailed spatial imaging of radioactivity in tissues at the cellular level (typical spatial resolution 10 µm)5. By contrast, whole body autoradiography (WBA), with typical spatial resolutions of 100 µm, has the ability to show the distribution of radiolabeled substances in all organs of an intact animal carcass4. Due to its more limited spatial resolution, WBA is often combined with liquid chromatography mass spectrometry (LC-MS)6, which is capable of identification and quantification of the compound and metabolites from tissue extracts, to gain specific distribution information. Both PET and MRI require the use of labels. Radioactive isotope labels are used in the PET technique. For MRI analysis, drugs typically need to contain 19F or be enriched in 13C to be visualized in vivo as the sensitivity depends on the magnetic properties of the monitored nucleus7. Though PET and MRI can be employed to perform drug-imaging studies, they suffer from millimeter spatial resolution as well as low specificity and limited molecular information. Other labeled imaging techniques, such as fluorescent microscopy, require a drug molecule to be modified with a chemical tag, which can affect its pharmacological properties, drug uptake, and localization within the body. MSI is an interesting alternative used in a growing number of ADME studies and pharmaceutical compound investigations. Many excellent MSI reviews describing drug studies have been published in recent years8–14. MSI possesses nanometer scale spatial resolution. It provides the chemical specificity of mass spectrometry to allow direct mapping of a parent drug or prodrug without any labeling. MSI also enables simultaneous detection of the active and toxic metabolites of a pharmaceutical compound to gain a better understanding the efficacy and toxicity properties of the drug. Briefly, in MSI experiments, sectioned biological specimens are placed in a mass spectrometer and an ordered array of mass spectra are acquired from a raster of positions across the sample surface. Ion density maps can then be reconstructed for any selected peak of a particular mass from the dataset in which each pixel consists of a mass spectrum or multiple MS/MS transitions. Various mass spectrometry ionization techniques have been developed for MSI in biological samples. Due to space limitations, a few ionization techniques will be discussed, but only matrix-assisted laser desorption/ionization (MALDI) will be described in detail below. The novel application of MALDI-MSI in 3D cell cultures as a platform for drug evaluation will then be introduced.