MRI Contrast Agents for Pharmacological Research

The advent of the molecular imaging era has offered to pharmacologists very powerful tools for drug discovery and development, in vivo evaluation of pharmacokinetic properties, andmonitoring drug efficacy (Hargreaves, 2008; Nairne et al., 2015). In fact, molecular imaging technologies provide minimally invasive procedures to visualize, characterize, and quantify biological processes occurring at cellular/subcellular level (Weissleder and Mahmood, 2001), thus overcoming the poor clinical translatability often exhibited by in vitro/ex-vivo experimental models. The continuous advances in biomedical imaging technologies may significantly boost the development of novel and more effective drugs, and accelerating the selection of lead compounds, with important time and costs benefits for healthcare. In vivo imaging of drug delivery and release, as well as monitoring of the therapeutic outcomes, represent the base of personalized medicine, thus allowing patients to be successfully addressed to the more effective therapeutic regime. Overall, the use of molecular imaging procedures aimed at supporting any therapeutic intervention (including surgery) falls within the scopes of theranosis (Lammers et al., 2011). Focusing on pharmacological therapies, a typical theranostic procedure requires the design of an imaging-traceable agent, whose structure and properties are suitably tailored to the aims of the examination. Imaging drug-delivery allows the assessment of the accumulation of the drug at the biological target, thus helping the selection of the more appropriate treatment. To get accurate information, the imaging agent should have the same physico-chemical properties of the drug. This requirement can be successfully met by labeling pharmaceuticals (organic molecules, peptides, proteins, radiochelates) with PETor SPECT-traceable radioisotopes, because of the minimal structural perturbation caused by the introduction of commonly used radionuclides (e.g., 18F, 11C, 123I, 68Ga, 111In; Baum et al., 2010; Gains et al., 2011; Gomes et al., 2011; Witzig et al., 2013; Wynendaele et al., 2014). On the other hand, when the drug is loaded into a nanocarrier, also the other available imaging modalities (CT, MRI, NIRF, US, PAI) can be used to visualize the delivery of the pharmaceutical. The imaging probe can be loaded in the carrier alone or together with the drug. The first option is preferable for drug selection, the second one for monitoring therapies. Among the imaging technologies, MRI is an excellent choice because combines exquisite spatial resolution, no limits in tissue penetration, and a vast portfolio of probes and contrast modalities that allows the design/selection of the best agent for any theranostic application.