Peripheral Vascular Disease (PVD) is a condition that affects 10 million adults in the U.S., and when advanced can contribute to stroke, heart attacks, leg pain, deficient wound healing, and ulcers. Current therapy, when a long segment of artery or vein is affected, calls for stenting or bypass surgery to be performed (250,000 per year for PVD). This usually requires autologous vein collection, which may have post-surgical complications. Synthetic grafts have been constructed with various biomaterials, but usage is limited to larger diameter applications. Thus, there is clearly an unmet clinical need that affects a significant number of patients, and a possible role for stem cell-based therapy in tissue engineering of small-diameter grafts could address this problem.
Reviewers recognized the importance and plausibility of the target, but were mixed in their evaluations of maturity of the concept. The intent of making engineered tubular scaffolds that would be a template for mesenchymal stem cell (MSC)- or embryonic stem cell (ESC)- derived progenitor cells would offer an attractive alternative to current treatments. This concept evolves from the PI’s recent study demonstrating proof-of-principle in a rodent model to generate a small diameter vascular graft using MSCs seeded in an engineered tubular scaffold. Reviewers noted that while this evidence is encouraging, the absence of studies in large animals suggests that the concept may be more than 5 years from clinical studies, and therefore not sufficiently mature for this award. A differing opinion from another reviewer stated that the concept is ambitious and logical, and if it succeeds could result in clinical trials within 5 years. Several reviewers felt that the proposal should have focused on the MSCs strategy, which was not only more mature, but could also potentially overcome immune rejection issues. Other strengths of the proposal included the acknowledgement of the need for safety studies, and the PI’s willingness to seek collaborations with other laboratories and industry.
Reviewers agreed that the PI is an established scientist whose work in bioengineering of vascular tissues and whose role in the stem cell arena are well-respected. His/her two associates are also highly respected in bioengineering and cerebrovascular research respectively, and additional disciplines needed for representation on the team have been identified. The PI has been collaborating with investigators from bioengineering, vascular surgeons, industrial partners and regulatory experts. His team has won several invention competition awards, and the scaffold technology has been licensed to industry for product development. However, leading such a large team and large scale of study may prove to be challenging.
The general planning is still at a very early stage; therefore it was difficult to gauge the feasibility of the process. Problem formulation and interim goals are well-reasoned. An overall organizational chart was presented, but not much detail was provided regarding the specifics of the planning process, or recruitment of the additional experts. Planning activities mentioned in the proposal included: a workshop to allow team members to present research and define goals and milestones, assembly of subgroups and identification of leaders, and monthly teleconferences.
Discussion by the panel highlighted the application’s relative novelty. Strengths were felt to be the PI and early collaborators, and the proof-of-concept evidence in rodent models. Most panelists felt that a focused proposal on MSC-derived scaffolds (versus hESC-derived scaffolds) could potentially overcome the main criticisms of the concept, immune rejection and insufficient scientific maturity.
Peripheral Vascular Disease (PVD) is a condition that affects 10 million adults in the U.S., and when advanced can contribute to stroke, heart attacks, leg pain, deficient wound healing, and ulcers. Current therapy, when a long segment of artery or vein is affected, calls for bypass surgery to be performed (250,000 per year for PVD). This usually requires autologous vein collection, which creates its own set of problems (additional surgery complications, difficulty in finding sufficient healthy tissue in some patients). Synthetic grafts have been constructed with various biomaterials, but usage is limited to larger diameter applications. Thus, there is clearly an unmet clinical need that affects a significant number of patients, and a possible role for stem cell-based therapy in tissue engineering of small-diameter grafts to address this problem.
Reviewer One Comments
The target of this plan is peripheral vascular disease, which afflicts about 10 million adults in the US. The disease has multiple causality, including atherosclerosis and aneurysm and can result in stroke, gangrene and a variety of disabling symptoms. Current state-of-the-art treatment is stenting and/or bypass surgery. As such, the target is an important and plausible one. The intent of making nanofiber tubular scaffolds that would be a template for MSC- or ESC- derived progenitor cells would offer an attractive alternative to current treatments. Research by this and other groups has shown that MSCs implanted in vascular grafts are replaced by host cells within a matter of weeks. This would obviate the need for long-term immunosuppression and is an attractive option. A clear plan is presented which includes:
1. Characterizing the functions of MSC-seeded vascular grafts in a porcine model, studying anti-thrombogenic properties, need for immunosuppression, the signaling of MSCs to host vascular cells, and differentiation and fate of implanted MSCs. Cells will be tracked by histological analysis and nanoparticle labeling/magnetic resonance imaging.
2. Characterizing the functions of ESC-derived vascular grafts as in (1).
3. Developing serum-free medium with well-defined factors for both MSCs and ESC-derived cells.
4. Developing bioreactors with large culture surface area and efficient mass transport to scale up cell production.
5. Optimizing scaffold materials for graft construction.
6. Establishing new MSC and/or ESC lines (if effective in 1 and 2, above) with GMP standard.
7. Constructing stem cell-seeded vascular grafts with GMP standard.
The plan is ambitious and logical, and if it succeeds could result in clinical trials within 5 years.
Dr. Li is an established scientist whose work in bioengineering of vascular tissues and whose role in the stem cell arena are well-respected. His associates, Drs. Chien and Young also are highly respected in bioengineering and cerebrovascular research respectively. Within the framework provided by these three individuals a path from bench research to clinical translation can be clearly charted. A steering committee for the effort has been identified including the above individuals plus George Daley and Michael West, the latter of whom brings important linkages to industry. Beyond this, although the disciplines needed for representation on the team have been identified, the individuals to fulfill these needs have not been named.
The planning approach is the weak link in this proposal. The need for assembling a team is identified, the need for monthly conferences is noted as is the need for partnering with appropriate labs and companies. Collaborators will be sought with expertise in quality control, regulation and conduct of clinical trials. However, little is in place at this level.
Reviewer Two Comments
• Concept / Significance: The PI proposes to develop a tissue-engineered small diameter vascular graft based on a nanofibrous scaffold and expanded MSCs and ESCs. If successful, this project will provide new methods for treating vascular diseases in peripheral, heart and brain tissue. The goal is highly significant.
• Evidence and maturity: This concept evolves from the PI’s recent study demonstrating proof-of-principle in a rat model to generate a small diameter vascular graft using MSCs seeded in a nanofibrous tubular scaffold. The PI has identified the major challenges in moving this technology to clinical trial treatment of peripheral vascular diseases, and laid out the major milestones for the future “Disease Team Award” proposal. The development milestones are focused.
The PI is an associate professor in bioengineering; is an expert in vascular tissue engineering; has extensive research experience in scaffold design and the use of MSCs for vascular tissue engineering. He has been leading NIH projects on “tissue-engineered nanofibrous vascular graft” and “mechanobiology of MSCs”. He has been collaborating with investigators from bioengineering, vascular surgeons, industrial partners and regulatory experts. However, leading such a large team and large scale of study may prove to be challenging.
The general planning is still at a very early stage; therefore it is difficult to gauge the feasibility of planning. The current steering committee consists of three members. The PI specified the development of the stem cell source as a major goal of this concept/project, including establishing a new cell line, developing serum-free medium, and GMP expansion and derivation. These efforts themselves can be a separate study in itself. The feasibility of these studies remains to be evaluated.
Reviewer Three Comments
This proposal describes the construction of allogeneic cellular vascular grafts, using adult third-party mesenchymal stem cells (MSC) and/or derivatives from clinically-qualified embryonic stem cell lines, seeded onto nanofibrous scaffolds, to treat PVD. There are several aspects to the proposal: 1)design and construction of appropriate nanofibrous tubular scaffold material that cells can integrate into and create the correct tissue architecture; 2) define the optimal source(s) of stem cells in which there are several important non-overlapping criteria. Challenges with the approach are noted. EC and SMC do not grow well in culture. The application does not addresss the potential immune response to allogeneic tissue.
Preliminary data are presented in the application: 1) Candidate scaffolds from both native and synthetic materials have been generated, and have demonstrated some self-remodeling capabilities; 2) MSC grow well and appear to be somewhat immune privileged; and 3) ESC, in murine system, can generate EC and SMC
This is clearly a multi-disciplinary project, requiring different types of engineering and biological expertise, to create these living, artificial vascular grafts.
Strengths in the application include a good listing of the gaps before clinical applications can proceed, and the types of expertise needed to form a multidisciplinary team. Specifically, the applicant acknowledges the need for large animal studies. Another strength is the applicant’s willingness to establish partnerships with other laboratories and industry close some of the gaps in obtaining new cell lines, gaining access to GMP facilities, and conducting large animal model testing.
Weaknesses include the fact that MSC culture technology is already fairly mature, and therefore some aspects of this proposal appear to be attempting to recreate the wheel. The application only briefly mentions iPS, which might be preferred “matched” stem cell source. Finally, the Concept section of the proposal exceeded the word limit and was cut off.
Dr. Li has demonstrated expertise in vascular tissue regeneration, engineering and biomaterials, and has 15 years experience in translating basic research to product development in industry. Dr. Li already has several NIH grants that address some of the basic science issues associated with this proposal: “Tissue engineered nanofibrous vascular grafts”, “Mechanobiology of MSC”, “Nanofibrous scaffolds for Vascular Regenerations”. His team has won several invention competition awards, and the scaffold technology has been licensed to industry for product development, and it is unclear what impact this licensing arrangement might have on clinical development or the new technology. There appears to be a strong team already in place.
Dr. Li has already recruited two excellent consultants and steering committee members, Dr. Young who is the director of Cerebrovascular Research from UCSF, and Dr. Chien from UCSD who has expertise in vascular bioengineering and high-throughput screening. Additional stem cell biologists, George Daley (current President of ISSCR) and Michael West (founder of Geron and ACT), will serve on the Advisory committee. An overall organizational chart is presented, but not much detail was provided regarding the specifics of the planning process. Planning activities mentioned in the proposal included: a workshop to allow team members to present research and define goals and milestones; subgroups assembled and leaders identified; and monthly teleconferences.
Reviewer Four Comments
The application seeks to develop cellular vascular grafts for peripheral vascular disease, with a specific emphasis on mesenchymal stem cells and human ES cells. There are a number of strengths, including the prevalence and importance of the disease target, the logic of cell therapy combined with tissue engineering in this setting, a highly accomplished and credible PI, and the relative novelty of the proposal. However, by comparison with more competitive applications in this cycle of review, the proposal suffers from being premature. The applicant’s exciting work with MSC-seeded vascular grafts in small mammals has yet to be tested in large ones, and therefore the time horizon for a human trial within 5 years must be viewed as excessively optimistic. (This issue is especially germane given the pivotal claim that immunosuppression would only need to be brief, since host MSCs replace the grafted ones quickly, is only known at present to be true for rats.) Use of hESC-derived vascular progenitors is likewise viewed with high enthusiasm from the standpoint of preclinical biology, but as unready for the terms of this grant competition.
The Applicant is Associate Professor of Bioengineering at Berkeley. There is a very strong track record of NIH funding for the underpinning work on induced differentiation of MSCs and on nanofibrous vascular grafts. The consultants including Shu Chien and William Young are very well-chosen for their aspects as are the external advisors. However, team members have not yet bee recruited in several key areas acknowledged as important, including ramp-up of production under GMP conditions, immunosuppression, quality control and assurance, and in vivo imaging. Hence, the multi-disciplinary collaborative network essential for this project to succeed is not yet fully implemented, but should be addressed during the planning phase.
The problem formulation and interim goals are well-reasoned. Other strengths include the outstanding Engineering environment for the optimization of scaffold materials and bioreactors. Some aspects are problematic, such as the nanoparticle labeling and MRI tracking, a method that does not distinguish live cells from dead ones, after grafting. In addition, because the transplantation barrier to allogeneic cells remains an unsolved problem. Because there is not a convincing case that the clinical need for allogeneic vascular grafts is sufficient to justify systemic immunosuppression in humans, this project might have benefited from better use of “lower hanging” alternatives to hESCs, such as mesoangioblasts, telomerized EPCs, and other autologous cells.