The emerging multidisciplinary field of regenerative engineering is devoted to the repair, regeneration, and replacement of damaged tissues or organs in the body. To accomplish this it uses a combination of principles and technologies from disciplines such as advanced materials science, developmental and stem cell biology, immunology, physics, and clinical translation. The term “regenerative engineering” reflects a new understanding of the use of tissue engineering for regeneration and also the growing number of research and product development efforts that incorporate elements from a variety of fields. Because regenerative engineered therapies rely on live cells and scaffolds, there are inherent challenges in quality control arising from variability in source and final products. Furthermore, each patient recipient, tissue donor, and product application is unique, meaning that the field faces complexities in the development of safe and effective new products and therapies that are not faced by developers of more conventional therapies. Understanding the many sources of variability can help reduce this variability and ensure consistent results.
Members of the Forum on Regenerative Medicine at the National Academies of Sciences, Engineering, and Medicine became especially inter-
1 This workshop was organized by an independent planning committee whose role was limited to identification of topics and speakers. This Proceedings of a Workshop was prepared by the rapporteurs as a factual summary of the presentations and discussion that took place at the workshop. Statements, recommendations, and opinions expressed are those of individual presenters and participants and are not endorsed or verified by the National Academies of Sciences, Engineering, and Medicine, and they should not be construed as reflecting any group consensus.
ested in the sources of variability that can affect a regenerative product’s success or failure (e.g., the variability of patient responses across and within clinical trials of the same intervention). Other sources include differences in various donor and recipient characteristics, such as genetics, underlying disease states, and inflammation, as well as variations in the collection, processing, storage, handling, dosing, and delivery of the cells used in the therapies. By understanding these sources of variability, scientists, clinicians, regulators, and others may be better prepared to account for, manage, and reduce these variables when moving a regenerative engineering product into development for the purpose of providing patients with safe and effective therapies.
The Forum on Regenerative Medicine hosted a public workshop on October 18, 2018, in Washington, DC, to explore the various factors that must be taken into account in order to develop successful regenerative engineering products. The agenda for the workshop was developed by an independent planning committee.2 Invited speakers and participants discussed factors and sources of variability in the development and clinical application of regenerative engineering products, characteristics of high-quality products, and how different clinical needs, models, and contexts can inform the development of a product. Speakers also discussed approaches to reducing variability and ensuring consistent, high-quality products and thereby improving patient outcomes. Specific workshop objectives are listed in Box 1-1.
Martha Lundberg, a program director in the Division of Cardiovascular Sciences at the National Heart, Lung, and Blood Institute and a workshop co-chair, said that over the previous several months members of the forum had discussed how clinical trials of regenerative therapies in certain disease areas had shown minimal benefit for patients. There is inherent variability in regenerative engineering products, and this makes demonstrating efficacy difficult, she said. During these discussions, Claudia Zylberberg, the chief executive officer of Akron Biotech, helped the forum formulate the idea of examining sources of variability and how understanding and mitigating sources of variability is important when trying to demonstrate that a product has efficacy. The variability has different sources, including genetic differences and variations in patient response and immunogenicity, donor tissues, and the scaling up and scaling out of the manufacturing process. Lundberg concluded by saying that with the diverse group of participants and expertise at the workshop, there are opportunities for developing ideas about how the field can consider these sources of variability and what can be done about them in the context of developing more consistent and effective regenerative products.
To set the stage for discussion, Guillermo Ameer, the director of the Center for Advanced Regenerative Engineering at Northwestern University, and Cato Laurencin, the chief executive officer of the Connecticut Convergence Institute for Translation in Regenerative Engineering and the director of the Raymond and Beverly Sackler Center for Biomedical, Biological, Engineering and Physical Sciences at the University of Connecticut, provided a brief background on the field of regenerative engineering and the impact of variability on regenerative engineering products. The origins of regenerative medicine, Ameer said, can be traced back to ancient Greece (around 330 BC), when Aristotle observed the regeneration of tails by lizards (Jessop et al., 2016). The field of tissue engineering began about 30 to 35 years ago, when the term “tissue engineering” was first used by Y. C. Fung, a pioneer in bioengineering, Laurencin said. In the 1990s, Ameer said, scientists began to elucidate the capabilities of embryonic stem cells, including their ability to differentiate into any type of cell in the body, and over the past several decades researchers have discovered how to reprogram differentiated adult cells (e.g., from skin or blood samples) to behave as pluripotent stem cells (cells capable of giving rise to other types of cells). About a decade ago, Laurencin and others began to consider how to move the field forward so as to be able to address the grand challenges in tissue engineering.
Regenerative Engineering: A Convergence of Fields
The current attention to regenerative engineering stems from a common interest by those in the engineering and health care fields in moving from symptomatic treatment to curative treatment, Ameer said. The grand challenge is how to use the principles of regenerative medicine to engineer cures (e.g., replace damaged tissue in the heart or transplant compatible
beta cells to secrete insulin). One approach to meeting this challenge is a “convergence” of research efforts across the life sciences, physical sciences, and engineering. Convergence brings together very disparate or separate disciplines that do not generally interact with each other. Ameer referred participants to a 2014 National Academies report that discusses this transdisciplinary integration of programs as a way to begin to solve society’s grand challenges (NRC, 2014) and to the work of Sharp and colleagues at the Massachusetts Institute of Technology. Sharp et al. have called convergence the “third revolution” in biomedicine (after the molecular biology revolution and the genomics revolution) (Sharp et al., 2011). The field of regenerative engineering is a convergence field, Ameer said, and he listed several other examples of such fields, including nanotechnology, synthetic biology, cyber-physical systems (e.g., self-driving cars), and oncofertility. These fields have integrated disparate disciplines to create a new workforce, he said.
Tissue engineering can be defined as “the application of biological, chemical, and engineering principles toward the repair, restoration, or regeneration of living tissues using biomaterials, cells, and factors alone or in combination,” Laurencin said. In 2012 the editors of Science Translational Medicine published a special issue on biomaterials in which Laurencin first defined the updated phrase “regenerative engineering” (Laurencin and Khan, 2012). While those who work in regenerative engineering might use slightly different definitions, Laurencin said, the constant theme of the field is “the convergence of advanced materials science, stem cell science, physics, and developmental biology, and clinical translation toward the regen-
eration of complex tissues, organs, or organ systems” (see Box 1-2). The critical element for moving forward is the deep integration of these different technologies, he said. As an example of this integration, he referred to a recent publication on axolotl salamander limb regeneration in which the authors discuss the potential of their findings in the axolotl for the future of regenerative engineering (Gerber et al., 2018).
From his perspective as a chemical engineer and a member of the Regenerative Engineering Society’s board, Ameer said he sees regenerative engineering as “the convergence of advances in material sciences, stem cell and developmental biology, physical sciences, and clinical translation to develop scalable and reliable tools that enable the regeneration or reconstruction of tissues and organs.” Regenerative engineering enables regenerative medicine, he continued. Ameer listed several ongoing activities that are supporting the development of this new concept. The Regenerative Engineering Society publishes the Regenerative Engineering and Translational Medicine Journal and hosts a range of events, including a “Rock Stars of Regenerative Engineering” conference.3 Ameer also noted that the fall 2018 meeting of the Materials Research Society will include regenerative engineering sessions.4
As an example of this convergence of disciplines in practice, Ameer described efforts at his institution, the Center for Advanced Regenerative Engineering at Northwestern University.5 This new initiative is a partnership among the McCormick School of Engineering at Northwestern and more than 40 other members, including the Feinberg School of Medicine (Northwestern University), the Pritzker School of Medicine (University of Chicago), Lurie Children’s Hospital, NorthShore University HealthSystem, Shirly Ryan Ability Lab, the U.S. Army Institute of Surgical Research, industry partners that can facilitate bringing innovations to the public, and other partners that have unique resources, such as the ability to work with large animal models.
There are a number of signals that the field of regenerative engineering is growing, Laurencin said. Institutes are creating new faculty positions in regenerative engineering, for instance, and the past year has seen the publication of a textbook on regenerative engineering6 and multiple
6 The textbook Regenerative Engineering by Laurencin and Khan is available at https://www.crcpress.com/Regenerative-Engineering/Laurencin-Khan/p/book/9781439814123 (accessed January 8, 2019).
symposia on regenerative engineering.7 This is a new field that is bringing together these different areas and creating a new language, Laurencin said. The National Science Foundation has embraced convergence as one of its “10 big ideas” for the future, he added, and the Burroughs Wellcome Fund and the Raymond and Beverly Sackler Centers have also emphasized the need for convergence. Lastly, the University of Connecticut has partnered with the Advanced Regenerative Manufacturing Institute to better understand how variability in manufacturing can affect patient outcomes. Together, Laurencin said, these groups can combine their various fields of expertise toward the goal of implementing regenerative medicine.
Examples of Regenerative Engineering Approaches
Ameer shared several brief examples of current research in regenerative engineering to illustrate both the potential of the field to advance medicine and the challenges of developing and translating regenerative engineering products.
Smart Scaffolds for Bladder Regeneration
Children born with spina bifida have numerous lower peripheral nervous system complications, Ameer said. To preserve renal function in these children, bladder augmentation with intestinal tissue is sometimes used as a last resort. However, there are significant side effects and functional issues associated with building bladder out of intestinal tissue. The vision for the future, Ameer said, is that it will be possible to use autologous progenitor cells seeded onto a synthetic scaffold to create a tissue that has the elastic properties of the original bladder tissue (Sharma et al., 2013). This approach has been tested in a non-human primate model (baboon), and the animals were recovering well at 6 weeks post-surgery, urinating normally without catheters, he said.
Autologous Endothelial Cells for Tissue Revascularization
Ameer discussed the work of Jiang and colleagues on the use of autologous endothelial cells for tissue revascularization in patients with peripheral artery disease (work not yet published). He explained that the
7 The Regenerative Engineering Symposium (hosted by the Regenerative Engineering Society) was held on October 27–28, 2018. For more information, see https://www.aiche.org/conferences/regenerative-engineering-society-event/2018. A Symposium on Regenerative Engineering (hosted by Northwestern University) was held on May 31, 2018. For more information, see https://regenerative-engineering.northwestern.edu/news-events/symposium.html (accessed January 8, 2019).
functionality of the autologous induced pluripotent stem cells (iPSCs) that are differentiated into endothelial cells varies depending on the health and disease state of the patient. For example, markers of inflammation are highly expressed in patients with peripheral artery disease and even more so in those patients who also have concurrent diabetes. Potentially, he said, functionality can be restored with precision medicine approaches.
4D Printed Bioresorbable Scaffolds
Another area of regenerative engineering is the development of 4D printed scaffolds that can be designed according to a patient’s specific needs (descriptions of this work have not yet been published). For example, Ameer said, scaffolds can be engineered to be sponge-like so that they can be press fit into bone, where they will then become hardened like bone. The cells loaded onto the scaffold will thus have a mechanical environment that more closely mimics that of the surrounding tissue. This type of strategy can help to reduce variability, Ameer added.
Islet Transplantation to the Omentum
One experimental approach to treating diabetes is to transplant pancreatic islets into a patient in order to facilitate insulin production (work not yet published). In this procedure, Ameer said, islets are harvested from the pancreas of a donor and introduced into the liver of the recipient. The procedure is marginally successful, with 50 to 70 percent of patients having to resume insulin injections, Ameer said. The failure of the transplants is due, in part, to the presence of reactive oxygen species in the body that can significantly affect islets. Regenerative engineering approaches are being used to develop materials that can be used to deliver the islets to the desired target tissue (in this case, the omentum) and that are intrinsically antioxidant and thus prevent the islets from being exposed to reactive oxygen species.
Factors That Impact Variability
Ameer offered his thoughts on the factors that can lead to variability in regenerative engineering products and that need to be understood in order to successfully manufacture cell and tissue products for regenerative medicine. Among the factors that affect the cells and tissue used in the products are the characteristics of the donor source, such as specifics of the donor’s immune system, disease stage, medications, age, sex, ethnicity, and biologic heterogeneity. Other questions to be addressed, Ameer said, are how the products are characterized (e.g., the markers selected) and
preserved. Factors that affect the procurement, processing, manufacturing, and storage of cell and tissue sources and products include preservation agents, enzymatic digestion of the tissue, cell separation (e.g., gradients), gene transduction, and cryopreservation. Ameer said that it will be important to partner with industry to move technology, standards, and methods forward in order to keep pace with advances in science. There are also factors that affect patient selection, he said, and the specific inclusion/exclusion criteria are critical to the design of a clinical trial for a product. Other questions related to clinical trial design, he said, include when the donor cells were collected, when it might not be viable to collect cells, or at what stage of disease a patient should receive the cells. Finally, there are factors that affect the delivery and the targeting of the product in the patient. Similar to the factors that could affect the cell and tissue source, examples of factors that affect delivery and targeting include the immune system, disease state, medications, age, sex, ethnicity, and biologic heterogeneity.
This proceedings summarizes the presentations and discussions that took place at the workshop. The workshop’s purpose was to explore the range of variability associated with regenerative engineering products. Ameer provided a brief background on regenerative engineering as well as a preview of the discussions. Following the background presentations just described, invited experts and participants considered three case examples that illustrated sources of variability associated with regenerative engineering products (Chapter 2). The workshop then delved further into the issues of variability across patients receiving regenerative engineering products, including the immune system of the patient (Chapter 3) and variability associated with the donor tissues and cells used in regenerative engineering products (Chapter 4). The focus then shifted to addressing issues of variability and ensuring quality in manufacturing, including topics such as raw materials, quality by design, and tools and techniques to minimize product variability (e.g., preservation techniques) (Chapter 5). The next session discussed developing appropriate metrics and outcome measures for clinical trials to facilitate regulatory review and approval as well as regulatory programs that are available to help bring these products through development (Chapter 6). In the final session of the workshop, panelists reflected on the day’s discussions (Chapter 7).