Early Translational I
The proposed research project aims to solve a key bottleneck in the use of human embryonic stem cells, and induced pluripotent stem cells for the regeneration and replacement of diseased or damaged tissues. This bottleneck is the potential of the transplanted stem cells to develop into a type of tumor called a teratoma, or teratocarcinoma. It is essential to overcome this obstacle before stem cell therapy becomes acceptable for human use. Stem cells and cancer cells have some common properties. Both can replenish themselves indefinitely, and can potentially grow in different parts of the body. Before they are administered to patients, stem cells must be forced in the laboratory to turn into more mature cells that are programmed to become neurons, heart cells, beta cells of the pancreas, and other differentiated cell types. The mature cells, unlike the stem cells, do not grow indefinitely, but rather can replace a specific function that is defective in disease. But what happens if some of the stem cells remain immature, despite our best efforts? They may begin to grow out of control, with potentially disastrous consequences. Hence, we must have a way to eliminate any residual embryonic stem cells before they are administered to patients. We also need safe and effective drugs that can prevent teratoma formation in humans, and treat the disease. The team of investigators working on this research project aim to exploit a fundamental difference between embryonic stem cells and more mature cells. The survival of the embryonic stem cells depends upon the continued activity of an enzyme called mTOR. In contrast, the inhibition of mTOR does not cause the death of mature cells. One of our industry collaborators has developed an ultrapotent drug that inhibits mTOR, and is well tolerated when tested in mice. In preliminary studies, this agent destroyed embryonic stem cells at concentrations that did not harm mature cells. This compound was more effective than rapamycin, which partially inhibits mTOR. We have also discovered a new drug that blocks a biochemical pathway called Wnt, that embryonic stem cells need to interact with their environment. This agent may have the ability to restrain the growth of embryonic stem cells in the body, without injuring important tissues and organs. With these simple but powerful drugs, the project team aims to develop a practical protocol to purge any residual embryonic stem cells or induced pluripotent stem cells from tissue culture, without injuring the function of the mature, differentiated stem cells that are needed for treatment. The investigators also aim to demonstrate that the mTOR and Wnt inhibitors can effectively prevent teratoma formation in vivo, and can destroy any tumors that may develop. The interactive group of scientists and physicians that are participating in this proposal have decades of experience and success in bringing molecules from the laboratory to the clinic.
Statement of Benefit to California:
Stem cell therapy has the potential to revolutionize the treatment of many common diseases that afflict the citizens of the State California. Alzheimer’s disease, diabetes, heart failure, anemia and arthritis are just a few of the illnesses that could potentially be treated. Responding to this need, the citizens of the State of California have dedicated billions of dollars to human stem cell research. For the investment to realize expectations, the supported scientists and physicians must develop stem cell remedies that are not only efficient, but are also devoid of dangerous side effects. Stem cells can regenerate themselves indefinitely. For this reason, stem cells have the potential ability to grow and spread in the body, causing tumors called teratomas or teratocarcinomas. Indeed, teratoma formation has been observed in experimental models. Therefore, it is absolutely essential that we develop tools to prevent and to treat this potentially life-threatening complication of stem cell therapy. Otherwise the deployment in the clinic of the new stem cell treatments could be set back for years, at enormous cost. Every effort must be made to assure that future clinical trials of stem cell therapies offer adequate safeguards to patients. The team of investigators in the proposed CIRM Early Translational Research Grant aim to produce a highly effective drug regimen, that can substantially reduce the possibility of teratoma formation, and that can eliminate any teratomas that occur. The research focuses on drugs, rather than large molecules, because drugs are more easily integrated into the practice of medicine. Preliminary experiments strongly suggest that this goal can be achieved within a few years. The benefits of the proposed research to the State of California and to its citizens will be multiple. It will accelerate the pace of stem cell research, by helping to overcome a major obstacle to clinical trials. It will reduce the cost of stem cell therapeutic development, by providing a clear and rapid protocol for safety studies. Of greatest importance, it will protect the citizen-patients of the State of California, whose disease burden we aim to lessen.
This proposal is focused on a bottleneck, the risk of teratoma formation from undifferentiated human embryonic stem cells (hESC) in preparations of differentiated progeny intended for cell therapy. The key concept is that inhibition of mTOR (mammalian target of rapamycin), a signaling component that is required for survival of ESC, may selectively eliminate undifferentiated hESC without harming differentiated progeny or host cells. In aim 1, the Principal Investigator (PI) proposes to compare an existing inhibitor of mTOR, rapamycin, with a new inhibitor, generated by a pharmaceutical collaborator. Rapamycin potently inhibits the activity of one of the two known mTOR complexes, whereas the new compound potently inhibits both complexes. The PI will assess the ability of these two compounds to deplete undifferentiated hESC in vitro without disrupting committed stem or progenitor cell biology, focusing on neural and hematopoietic cells. In aim 2, the PI will study the in vivo efficacy of these inhibitors to prevent teratoma formation or to kill existing teratomas in rodents. The goal of aim 3 is to determine the effects of the inhibitors on human iPS cells. Aim 4 is based on the known role of WNT signaling in stem cell biology, and will establish whether the effects of mTOR inhibitors can be enhanced when used in combination with a new WNT inhibitor. Reviewers agreed that this project focuses on a central safety issue, and that the identification of a single compound or group of compounds that have the ability to selectively eliminate cells with teratoma forming potential, in vitro and in vivo, would be extremely beneficial to the field. Reviewers found the strategy to leverage specific information on the growth characteristics of ESC towards their elimination interesting, and the PI identified relevant molecular pathways to be targeted. Some reviewers felt that the four aims are well designed and fairly straightforward. However, reviewers raised several issues that severely diminished their enthusiasm for this proposal. They expressed serious concern about the selectivity of the novel mTOR inhibitor. Although rapamycin is already used clinically for immune suppression after organ transplants, the more broadly acting novel inhibitor is likely to have effects on the biology of non-ESC. Critical preliminary data regarding its selectivity toward ESC is lacking, and outcomes and toxicities in addition to the ones proposed, need to be considered in the animal studies. If the novel inhibitor causes death of proliferating progenitors the project will fail. Further concern was voiced about the WNT inhibition approach because that could have undesirable side effects as well. In addition to concerns about off-target effects, reviewers also criticized the targeted effectiveness of the drugs, and found that the goal to reduce tumor size or incidence from transplanted cells by 80% to 95 % is insufficient, since recipients would still be expected to develop teratomas. The preliminary data indicate that the novel mTOR inhibitor causes widespread cell death in hESC cultures, but the PI never states that all of the cells are killed by this treatment. Finally, reviewers pointed out that the PI is relying too heavily on very few drugs/small molecules and that identification of clinically relevant compounds will depend on the development of additional molecular structures. Overall, this is a great application for studying mTOR signaling, but the proposed project is not ready for translation. The PI is an expert in mTOR and other growth regulation signaling pathways, is highly productive and well funded by the NIH for basic research, and therefore is highly qualified to conduct the proposed research. However, he/she does not seem to have stem cell biology expertise. One collaborator provides expertise in small molecules, although no further drug development was proposed. Two additional collaborators provide expertise in neural differentiation and for hematopoietic assays, and the applicant institution’s community provides ample expertise in stem cell biology and has outstanding resources available for carrying out these experiments. Reviewers felt that although the PI has assembled an excellent team of investigators, neither the requested number of personnel nor the total budget is justified. In summary, the reviewers agreed that the PI addresses an important bottleneck, the minimization of teratoma forming potential from pluripotent stem cell-derived cell preparations, and as such the proposed research has the potential for high impact on the stem cell field. However, the feasibility of developing a clinically relevant compound was called into question, based on the potential for significant off-target effects and the questionable effectiveness of the compounds under investigation.