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Practical applications of biologics: Searching for clear indications
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In recent years, there has been increasing interest in strategies to augment the biology of soft tissue healing. Improved understanding of the cellular and molecular mechanisms of tissue degeneration and the limitations in healing and regeneration of musculoskeletal connective tissues have suggested the need for approaches to augment the basic biology of tissue repair. Advances in the understanding of stem cell biology, the role of cytokines and inflammatory mediators in tissue healing and degeneration, gene transfer techniques and the discovery of novel biomaterials suggest tremendous potential to augment the biology of soft tissue repair. The challenge is to continue well-designed translational research studies to bring these techniques to clinical fruition.
In this Orthopedics Today Round Table, I have asked several individuals with expertise in connective tissue healing to comment on the current status and future potential of various approaches for augmentation of healing.
Scott A. Rodeo, MD
Moderator
Roundtable Participants
-
Moderator
- Scott A. Rodeo, MD
- New York City
- Jason L. Dragoo, MD
- Redwood City, Calif.
- Martha M. Murray, MD
- Boston
- Regis J. O'Keefe, MD, PhD
- Rochester, N.Y.
- George F. Muschler, MD
- Cleveland
Scott A. Rodeo, MD: It is well-established that the current platelet-rich plasma (PRP) formulations have had variable results at best. What are potential avenues to improve the efficacy of PRP? What will the “next generation” of PRP look like?
Jason L. Dragoo, MD: Simply put, we have had variable clinical results because we have not studied which PRP formulation to use with each orthopedic indication. The first approach to improve results is to study which formulation works better for a certain medical indication. For example, there is now both basic science and level 1 clinical evidence to suggest that the use of PRP formulations containing polymorphonuclear white blood cells (WBC) for intra-articular therapy leads to worse results than excluding these cells in the PRP formulation. More studies are needed to identify the correct formulation for treatment of tendinopathy and ligament injury.
The results of PRP therapy may be improved in the future by: 1) the removal of growth factors that mitigate the intended clinical effect; 2) concentrating factors in the blood other than platelets, such as alpha-2 macroglobulin; and 3) adding a progenitor cell source that can respond to the growth factor cues in the PRP.
Martha M. Murray, MD: Any biologic therapy, including PRP, needs to be tailored to the clinical situation for which it will be used. For ACL healing, we have found that all of the cells in the blood are important for tissue healing, even the red blood cells which are typically removed when making PRP. As our best results have been using autologous whole blood to stimulate tissue repair, I envision that whole blood will be our “next generation” of PRP.
The avenues to improve the efficacy of PRP beyond the current ability of whole blood involve understanding not only the biology of the cells we are implanting as PRP, but also the host site into which we are putting the PRP. Understanding the local environment in terms of target cells and their ability to respond to stimuli, whether the biologic agent is injected in between a tissue plane or into a joint cavity, and what the local vasculature looks like are just three factors which will influence the effectiveness of any biologic therapy. In addition to local factors, it is highly likely that factors specific to the individual patient will also need to be taken into account. For example, is the patient in a catabolic or an inflammatory state? Factors at the local, paracrine and endocrine levels will need to be more clearly defined to further optimize the performance of PRP in any given clinical application. Until we have a better understanding of these factors, we may struggle to create a biologic therapy that is more effective than simple whole autologous blood.
George F. Muschler, MD: The use of soluble factors or factors that can be released from platelets has the appeal of being readily translated into clinical practice. Methods for preparation are inexpensive. They are also less burdened by regulatory concerns about safety. However, it is important to remember that PRP does not bring new agents to the table, and probably does not change the concentration of these agents from that normally found in healing wounds. The most important feature of PRP preparation is that processing may remove components of blood or marrow aspirates that may inhibit the biological processes that are desired, such as red cells and nucleated cells.
While the clinical results of initial preparations of PRP have been disappointing, there remain opportunities to process autogenous blood or marrow in a manner that may have more profound effects on local biology by making more selective processing steps. Removing some growth factors and cytokines but not others may change the biological milieu. Similarly, removing some nucleated cells but not others could also be a means of improving biological effects.
Regis J. O’Keefe, MD, PhD: A major issue continues to be the variability of PRP preparations. Several studies have shown the number of platelets and the concentration of growth factors varies when different commercial products are used to obtain PRP for injection. Moreover, multiple PRP preparations using a single commercial product results in different intra-individual results over time. A major factor is whether the PRP preparation includes leukocytes or eliminates the WBC population. Studies have established that WBC-containing preparations lead to increased inflammation, but it is not clear whether the increased inflammation is a positive or negative for tissue healing.
Studies are needed to understand the functional activity of individual PRP components and the manner in which these various components modulate biological activity. Eventually, PRP products will move toward a biologic approach with standardized concentrations of growth factors, blood components and cells.
Rodeo: Do you think connective tissue progenitor cells have any real potential in orthopedic applications given current FDA regulations that limit the ability to use cell-sorting and ex-vivo culturing methods? Will the ability to “manipulate” cells ex-vivo and then reimplant then lead to a material improvement in outcomes?
Dragoo: Maybe. Although harvesting of autologous tissue containing progenitors cells is relatively easy, releasing the cells from the tissue so that they can contribute to tissue regeneration is difficult. Improved methods to release cells from the native tissue with FDA-approved physical and chemical separation techniques will be required. In addition, there is now some evidence to suggest that currently approved technologies may be combined to achieve improved clinical results. Arnold Caplan’s laboratory has shown that the platelet-derived growth factor (PDGF-BB), abundantly found in most PRP preparations, can be used to release the perivascular mesenchymal stem cells (pericytes) from tissue. Studies from Choi and colleagues have confirmed that the combination of adipose-derived stem cells and PRP leads to clinical improvement of osteoarthritis symptoms. These pivotal studies establish proof of concept that will allow larger studies to determine the true efficacy of these treatment regimens.
Murray: I think connective tissue progenitor cells have great potential, particularly if they can be recruited to migrate from elsewhere within the body to the desired site of action. Focusing on the in-situ recruitment of connective tissue progenitor cells to the zones of injury is likely critical to the success of any tissue engineering technology. The in-situ recruitment approach allows for cells to migrate into a wound site as the local nutritional environment allows, thus avoiding the transplant shock that may result in cell loss when going from the carefully controlled culture conditions ex-vivo back into a hypoxic wound site with scarce nutrition in-vivo.
Muschler: The term “connective tissue progenitor” (CTP) is used to describe the heterogeneous mix of cells that are present in native tissues that can be activated to proliferate and to give rise to progeny that differentiate to express one or more connective tissue phenotypes. As such, by definition, CTPs represent an essential element in the formation of any new connective tissue or tissue healing response, whether bone, cartilage, muscle, fibrous tissue, tendon, ligament or blood. FDA regulations do not, of course, limit the potential application of connective tissue progenitors. FDA regulations simply help to define a reasonable burden of proof that the scientific and clinical community should be prepared to provide in order to meet the standards of safety and efficacy that the public, by law, has a right to expect.
In any wound, it is the cell populations that are present, and the biological and physical environment in which they find themselves that determine the outcome. We have become increasingly aware that the concentration, prevalence and biological potential of CTPs in different tissues varies greatly from one tissue to another. We are only beginning to have the tools that enable us to compare tissue sources and processing methods. As we learn more the speed and efficiency with which we can harvest, concentrate and select specific subsets of CTPs, and to transplant cells in a manner that enhances their survival and performance, will inevitably improve.
O’Keefe: Animal studies clearly establish that tissue regeneration requires the response of CTPs. This is a well-organized process whereby cells in the local environment respond to injury signals by undergoing proliferation with expansion of the progenitor cell pool. Once a critical mass of cells is established, these cells undergo tissue specific differentiation, formation of an appropriate matrix, and restoration of the original tissue. This process occurs reliably in some tissues, such as bone where fractures spontaneously heal. Other tissues, like articular cartilage and the ACL, have less regenerative potential. While we have not yet learned how to harness these cells in tissue engineering applications to regenerate skeletal tissues, this avenue holds tremendous promise. However, these decisions need to be informed by evidence-based medicine derived from carefully performed blinded, placebo-based clinical trials. Stem cell “therapy” is an area of medicine where patients desperate for treatment are vulnerable.
A note from the editors
Look for part 2 of this Round Table in the November issue of Orthopedics Today.
For more information:
Scott A. Rodeo, MD, can be reached at Hospital for Special Surgery, 535 E. 70th St., New York, NY 10021; email: rodeos@hss.edu.
Jason L. Dragoo, MD, can be reached at Stanford University Medical Center, Department of Orthopaedic Surgery, Division of Sports Medicine, 450 Broadway St., Pavilion A, Redwood City, CA 94063; email: jdragoo@stanford.edu.
Martha M. Murray, MD, can be reached at Children’s Hospital of Boston, Hunnewell 219 Ortho Surg, 300 Longwood Ave., Boston, MA 02115; email: martha.murray@childrens.harvard.edu.
Regis J. O’Keefe, MD, PhD, can be reached at University of Rochester Medical Center, 601 Elmwood Ave., Box 665, Rochester, NY 14642; email: regis_okeefe@urmc.rochester.edu.
George F. Muschler, MD, can be reached at Cleveland Clinic Foundation, 9500 Euclid Ave., Desk A-40, Cleveland, OH 44195; email: muschlg@ccf.org.
Disclosures: Dragoo and O’Keefe have no relevant financial disclosures; Murray receives research support from the National Institutes of Health (NIH), the National Football League Players Association, and the Children’s Hospital Orthopaedic Surgery Foundation; Muschler is a consultant to the NIH, FDA, Smith & Nephew and Fortus and receives funding from the NIH, Department of Defense, Harvest Technologies, Medtronic, Fortus and DSM Biomedical; Rodeo receives research support from the Arthroscopy Association of North America, AOSSM/MTF Meniscus Transplantation Grant and the Arthritis Foundation, and is a consultant for Rotation Medical, Cytori and Pluristem.