Each imaging modality has its advantages and disadvantages in terms of sensitivity, tissue penetration, spatial resolution, and clinical potential [22
]. Optical imaging is mostly applicable to preclinical studies where light penetration is less of an issue than in patients. BLI cannot be used in human studies while tracking of labeled cells with MRI and/or PET may potentially be performed in patients. Combination of various imaging modalities can give complementary information. As a matter of fact, many of the reporter gene-based studies of pluripotent stem cells (e.g. ESCs) incorporated multiple reporter genes [24
]. For example, fluorescent genes (e.g. GFP) can facilitate cell sorting, BLI (with luciferases) can enable in vivo long term monitoring of these cells in a quantitative manner in small animal models, and PET can allow for more clinically relevant, highly sensitive detection of the injected cells and/or the daughter cells. Future development and validation of various iPSC labeling/tracking techniques will further strengthen the arsenal for iPSC-based therapy of various diseases.
Safety of cell labeling is always a major concern in clinical studies since introduction of foreign substances (e.g. image labels or genes) may cause unpredictable alterations in cells. Based on the available literature data, labeling of stem cells with magnetic nanoparticles appears to be safe and is in active clinical development [46
]. For indirect cell labeling, one of the most intensively studied reporter genes, HSV1-tk, is also a suicide gene which adds an extra layer of control to ensure safety [48
]. The ideal imaging approach for tracking iPSCs or their derivatives in patients requires the imaging tag(s) to be non-toxic, biocompatible, and highly specific to reduce perturbation to these cells. Much future effort will be required before this can become a reality and clinical routine.
IPSC-based therapy has tremendous therapeutic potential. However, numerous questions still remain unanswered. Non-invasive imaging techniques have proven to be of great value in preclinical and clinical studies for tracking transplanted stem cells, and will continue to guide the development of future cell-based therapies. With non-invasive imaging techniques, we will eventually be able to determine which cell type is preferable for a given disease (e.g. which cell types to use for iPSC generation and which cell types to transplant) as well as choose the right delivery methods of the cells (e.g. intravenous, intracoronary, or local injection). Detection and correction of iPSC misbehavior (e.g. teratoma formation) is also an important task for imaging. For example, studies have shown that two of the most widely used PET tracers in the clinic, 2-deoxy-2-18
F-FDG, which images glucose metabolism [49
]) and 3'-deoxy-3'-18
F-FLT, which detects cell proliferation [50
]), failed to detect human ESC-derived teratomas since the growth rate of teratomas is quite slow [51
]. However, another tracer, 64
Cu-DOTA-RGD4 which binds to integrin αv
], enabled non-invasive visualization of the teratomas with PET (). This and other studies suggested that imaging integrin αv
expression, instead of imaging the metabolic activity and/or cellular proliferation, may have potential clinical applicability in monitoring the tumorigenicity after stem cell transplantation [51
Figure 5 Bioluminescence imaging (BLI) and positron emission tomography of human ESC-derived teratoma. The photograph and BLI shows teratoma formation by human ESCs at the right flank and tumor formation by a control cell line at the left flank of mice. Specific (more ...)
The requirement for iPSC tracking techniques depends on the clinical scenario. In some cases, only short term tracking is needed while in other cases, long term survival and proliferation would also need to be monitored. For example, transplantation of iPSCs or their derivatives in heart diseases needs to be targeted to the ischemic but not viable myocardium, if the therapy is aimed at improving vascularization. Effective treatment of many diseases may benefit from repeat dosing, where non-invasive imaging can inform clinical decisions regarding the need for repeat dosing and direct where repeat doses are needed. MRI, radionuclide-based imaging techniques, and reporter gene-based approaches will each have their own niches in imaging of iPSCs. Rather than identifying and optimizing one technique applicable for all clinical scenarios, it is probably more appropriate to optimize how a certain imaging technique/modality can be best used to serve the purpose in a specific situation. The continued evolvement of non-invasive imaging techniques will undoubtedly contribute to significant advances in understanding iPSC biology and mechanisms of action. The various imaging modalities, complementary rather than competitive, have the same ultimate goal: personalized medicine for patients. In few other scenarios can such “personalized medicine” be better illustrated than the use of patient-specific iPSCs.