Stem cells hold great potential for treating a variety of human diseases, but more information is needed on how they will function once administered to patients for regenerative medicine purposes. If imaging techniques can be developed that allow the monitoring of these transplanted stem and progenitor cells over time once injected into the body, this would provide a very powerful tool to determine the fate of the cells. This proposal specifically addresses new ways to optimize the use of positron emission tomography (PET), an imaging technology currently used in the human clinical setting, for this purpose. PET is an imaging technique that produces a three-dimensional image by detection of a tracer or label that has been introduced into the body. In these studies we plan to optimize these imaging techniques when a special tag or label is attached to individual stem and progenitor cells, and address the sensitivity of the scanner for their detection. An important goal is to improve the ability to detect the small quantities of cells that may be used, and ensure that the images obtained can accurately identify the number of cells at any given location. Several factors limit the ability to detect the cells reliably in current PET scanners. These limitations include an inherent background signal which alters the ability to accurately identify the cells injected. In addition, processing of the information obtained by the PET imaging system has not been optimized for monitoring transplanted cells where the cell quantity may be very small and the tag or label used to find the cells may be difficult to detect. Further, the methods used to place the tag or label on the cells needs to balance the requirements for obtaining good imaging information without damaging the cells or altering their capabilities in the labeling process. Our plan is to investigate these crucial issues and current roadblocks in the context of stem cell imaging with the goal of substantially advancing the imaging field for stem cell therapies. We will use model systems to ensure that the techniques and applications developed and proposed for human use are safe and do not cause harm to patients of all age groups. These studies will focus on optimizing PET imaging techniques for cell quantification and identification, and develop and refine new ways to monitor cells, including those differentiated from human embryonic stem cells, for short and extended periods of time.
This proposal meets the objective of CIRM RFA 08-02 by providing new ways to overcome current gaps and roadblocks for sensitive imaging methods that would allow the detection of stem and progenitor cells administered to patients for regenerative medicine purposes. These new tools and technologies will serve the State of California and its citizens by providing reliable techniques for any scientist or physician to assess their ideas and new cell transplant protocols before considering use in human patients. Once tested and shown to be effective, these same imaging techniques can then be used in human patients. While stem cells, particularly human embryonic stem cells, have tremendous potential for treating a variety of human diseases, many questions remain about their safety and outcome once they are used. If imaging techniques can be developed that allows the monitoring of the cells over time, this would provide a very powerful tool to determine the fate of the cells after injection into patients. The results of these studies will fill a critical need and substantially advance the regenerative medicine field for a host of human diseases for which there are currently few therapeutic options.
This proposal is focused on the optimization of small animal positron emission tomography (microPET) for in vivo tracking of transplanted human stem and progenitor cells in non-human primates. With this technology, cells are labeled by radioimmunoconjugates detectable via PET. The microPET scanner has previously been used to detect moderate levels of radiolabel successfully and the applicant proposes to optimize the technique for detection of extremely low label levels. The applicant proposes that these improvements may allow the labeling of stem and progenitor cells without affecting viability or function. The proposal’s first aim focuses on improving the sensitivity of the scanning technology by adjusting a wide array of technical parameters to support both cell identification and quantification. The second aim is to improve the methods of radiolabeling cells by testing new labels and optimizing radioactivity levels and incubation times. If successful, this proposal would enhance the sensitivity of PET imaging of transplanted cells while reducing the amount of radioactivity required.
The reviewers agreed that this proposal addresses an important roadblock in stem cell research (ability to track transplanted cells) and would have broad applications if successful. They were enthusiastic about the technical aspects of the proposal, the preliminary data and the PI’s track record. They did raise some concerns about feasibility but expressed confidence in the assembled research team.
The reviewers appreciated the potentially broad impact of this proposal. Development of sensitive, non-invasive, in vivo imaging of transplanted stem cells is a major roadblock to clinical stem cell therapy. The optimization of microPET technology proposed in this application would benefit preclinical research but could also lead to translatable improvements in human PET. One reviewer wondered why the technical experiments described in the first aim haven’t been done before, as they would clearly benefit many types of PET research.
The reviewers commented that this is a well-designed research plan with clearly defined aims and milestones. Two reviewers raised questions about feasibility, specifically whether the applicant would achieve great enough sensitivity to detect cells that might migrate away from the injection site. One reviewer noted the high level of radiolabel required for detection by microPET in preliminary data and wondered if this could be lowered enough to ensure functional integrity of the labeled cells. However, the preliminary data included in the application was sufficient to persuade the reviewers that the research plan is indeed feasible, if ambitious.
The reviewers were unanimous in their praise of the research team. They noted that the applicant has a strong track record in the field and has assembled a well-coordinated, interdisciplinary team. One reviewer commented that if this project can be done, this is the team to do it. It was noted that the applicant is funded by both the NIH and CIRM for related activities, but noted no overlap of budget or research effort.
Overall, this is a strong, well-written proposal from a highly qualified research team with the potential for broad impact in the field.