Advanced imaging technology can provide anatomical and functional assessment (e.g., conventional anatomical and functional imaging) as well as visualization of biological processes at the cellular and molecular level (e.g., molecular imaging). Both disciplines utilize various imaging modalities such as radionuclide, optical, magnetic resonance, computed tomographic, and ultrasound technology. Selecting the most effective imaging strategy requires a determination of whether the imaging system can meet the necessary requirements for spatial and temporal resolution, sensitivity, and penetration depth for visualization of the imaging target. details the advantages and limitations of different imaging modalities and can serve as a guide for selecting the most appropriate strategy for the chosen indication.
Comparison of modalities for conventional and molecular imaging
While conventional anatomical and functional imaging can provide an overall road map for cell delivery as well as an assessment of the effects of stem cell therapy, molecular imaging can be used to track stem cells in vivo and to study their potential mechanistic benefits. In molecular imaging, imaging probes are used to target the biological process of interest. These imaging probes consist of a carrier (i.e., a cell, nanoparticle, and microbubble), which structurally binds a ligand designed to recognize the molecular target and a signal element to generate a detectable signal (). The ideal imaging probe should have the following important properties: 1) high imaging specificity for tracking the desired biological process, 2) high imaging sensitivity for detection by available imaging modalities, 3) minimal cellular toxicity, and 4) minimal systemic toxicity.
Fundamental concepts in molecular imaging of stem cell therapy
In general, there are two main labeling approaches, each with its unique advantages and disadvantages: 1) direct labeling with radionuclides or iron nanoparticles and 2) reporter gene/probe labeling. Using the direct labeling approach, contrast agents (e.g., signal elements) either bind to cell surface proteins or are transported into the target cell by diffusion, endocytosis, or active transport (e.g., radiolabeled indium oxine and superparamagnetic iron oxide particles) (). In contrast, reporter gene/probe labeling requires cell transfection or transduction with a reporter gene that produces specific proteins (i.e., membrane transport, surface receptor, and intracellular storage proteins as well as intracellular enzymes) that can take up exogenously administered contrast agents. By far the most widely used reporter genes are firefly luciferase (Luc) and herpes simplex virus thymidine kinase (HSV-tk) and their mutants. After delivery of their respective substrates, these enzymes catalyze a chemical reaction that produces a detectable signal ().
Direct and reporter gene labeling for molecular imaging
The major advantage of reporter gene/probe labeling, especially for in vivo
cell tracking, is that cells must be viable with intact protein synthesis machinery in order to produce a detectable signal. In contrast, the signal produced by direct labeling with radioisotopes can be diluted by cell division or dissipate after radioactive decay and/or may persist despite cell death due to the engulfment of dead cells by macrophages.6
Iron labeling by MRI, for example, can remain in the injected site long after cell death, providing erroneous information on the long-term fate of cells despite its superiority in cellular localization.6
Reporter gene/probe imaging is thus better suited for in vivo
monitoring of cell viability. In one of the first clinical applications of reporter imaging, the positron emission tomography (PET) reporter probe HSV-tk was used to track and monitor “suicide gene” therapy for gliomas and hepatocellular carcinomas.7, 8
More recently, Yaghoubi et al demonstrated that reporter gene imaging could track the fate of exogenously administered, genetically modified, and therapeutic cytolytic T cells in patients with glioblastoma.9
However, widespread application has been slowed by safety concerns, such as the potential risk of immunogenicity and tumorgenicity caused by random reporter gene integration, as well as limited sensitivity due to reporter gene/probe slicing.10