Regenerative Medicine — What Happened & What’s Next, Part 3: Stem Cells
Everything you always wanted to know but were afraid to ask…
At last we arrive at the new NEW discipline in regenerative medicine — STEM CELLS! The stem cell (SC) approach is thought to be superior to delivery of a single growth factor due to cells’ ability to respond to their environment, perform cell signaling, and act as mini bioreactors, generating a complex array of growth factors which in turn create the milieu necessary for the cells to grow, divide, and make the kinds of cells the tissue needs (aka differentiation and proliferation). The regenerative pathways are complex, and the chances of producing the proper mix of biologic signals are potentially higher if cells are used. For example, it was once believed that growth of new blood vessels simply required administration of vascular endothelial growth factor (VEGf). But it was then discovered that additional factors, both biological and structural were required to progress from the beginnings of an arteriole to a functioning vessel. Similarly, administration of nerve growth factor alone has not been shown to regenerate functional neurons, likely because many important cellular-mediated events need to occur, including proper axonal guidance to the correct muscle tissue and formation of mature neuromuscular junctions into neurons.
Stem cells have been touted as potential cures for a diverse array of conditions involving nearly all major organ systems, and there have been early clinical studies in each of these areas: neurologic (spinal cord injury, Parkinson’s, ALS), cardiovascular (infarct repair, stroke, vascular grafts), orthopedic (bone and cartilage repair), metabolic (artificial pancreas) and gastrointestinal (Crohn’s/ulcerative colitis).
SC come in a variety of flavors: Embryonic and placental-derived –first discovered but least studied; SC from umbilical cords; SC from human exfoliated deciduous teeth, differentiating into odontoblasts, and surprisingly, neurons, osteoblasts, adipocytes, and endothelial cells; adult/multipotent — mesenchymal (MSC, isolated from bone connective and marrow tissue), the most widely studied, which can become bone, cartilage, ligament/tendon, and adipose cell types; and created just 10 years ago, the newest SC on the block, induced pluripotent stem cells (iPSC, pluripotent=ability to become nearly any kind of cell) — generated through introduction of specific genetic factors in fibroblasts (or less commonly in MSCs). IPSCs pluripotency gives them the fantastic plasticity of embryonic SC but without the controversy; they can be coaxed into nearly every major cell type including cardiac, kidney, neurological and hepatic. IPSCs have been expanded in vivo to form organ-like tissues in three dimensions, and the first clinical trial (using iPSCs to treat macular degeneration) began in 2015 at the RIKEN Centre in Japan.
MSCs are further along in development than iPSCs as much of the basic biology, including the reprogramming protocols (the specific set of detailed instructions and genetic modifications) needed to achieve pluripotency while also avoiding excessive iPSC growth remains to be worked out. Early efforts to transplant cells for regenerative purposes were stymied by host immune responses. It is a happy coincidence then that MSCs, in addition to not having the typical traits that alert the immune system to their foreign-ness, have natural immunosuppressive properties, further adding to their appeal. Thus, many labs are exploring diverse arrays of biologic and physiologic factors and cell culture conditions that will drive differentiation of MSCs towards specific cell types, and several approaches have progressed into the clinic (discussed in the next installment of this series).
Despite this progress, SCs as a therapeutic class require further R&D to address several limitations. These include difficulties in producing sufficient numbers of uniform cells (many populations become heterogeneous following expansion in vitro); limited proliferation or differentiation potential; and dysfunctional immunomodulatory effects (nicely summarized in a recent review by Burke et al, Clin Transl Med. 2016). Further, the ability of the cell to make more of itself and determine the cell type it ultimately will be (cell fate) is also influenced by environmental stimuli as evidenced by the fact that certain cellular programs or processes can be modulated by non-genetic (epigenetic) factors. How are these and other shelf-life-related factors such as freeze-thaw cycles and storage conditions accounted for in commercial SC products? And because living cell populations can change when kept alive in culture conditions over time, how do we track and understand the significance of those changes? Beyond these issues, there is also the host’s immune status to contend with, as recent discoveries revealed that host immune cells such as macrophages and T-cells play a role in supporting normal cell homeostasis, including stem cells’ regenerative abilities and engraftment (Biernaskie et al, Scientist 2016; Mao and Mooney, PNAS 2015). Furthermore, the epithelial cells of each target organ have their own unique expression patterns of growth factors, adhesion molecules and chemokines, all of which influence regeneration.
Other factors influencing the success of stem cell therapy are the source of the stem cells, the timing of delivery, and the local environment into which the cells are delivered. Companies and academic labs developing (allogeneic) SC therapies generally work with a limited donor pool. This is particularly relevant because there are important biological differences in cells from different sources (human donors), a fact which, along with a lack of standardized assays for cell potency and activity, diminishes our ability to predict the behavior and therefore therapeutic potential of these cells. There is also no consensus as to when in the disease process (and this will vary depending on the exact disease or injury) it is optimal to administer SC. Regarding the local environment, the tissue that is to be repaired has often been subject to inflammation, (chronic or acute, depending on when the initial damage occurred and the exact disease process in question) which makes for an engraftment environment that is potentially less than receptive.
Fortunately, we’ve discovered that in many cases the biological effects of SC are due to immunomodulation and trophic activities (Caplan and Dennis, J Cell Biochem, 2006) and oddly enough, not the engraftment and subsequent creation of new tissue by the SCs. This powerful effect, known as paracrine activity, is thought to explain the early clinical benefits attributed to stem cell therapy. One of the first observations of this effect was seen in the attenuation of fibrotic processes in cardiac infarct and heart failure patients in early trials of autologous SCs (patients were treated with stem cells isolated from their own tissues). This effect has likely led to the generally positive, though modest, clinical outcomes seen in these patients that were treated with various SC (including peripheral blood and marrow derived MSCs), none of which are known to differentiate into cardiomyocytes (reviewed by Jeevanantham et al, Methods Mol Biol, 2013).
The abundance of these unknowns and the challenges of making cell products as consistent and predictable as conventional drugs means that the vast majority of the later-stage development work is with autologous SCs, accounting for the majority of the ~300 stem-cell related clinical trials registered to date [1, 2]. This approach is lower risk as no foreign cellular and genetic materials are introduced into the patient. However, this approach is also highly time consuming (taking weeks or months for the right patient cells to be identified and expanded in cell culture), customized per patient, and therefore expensive. So, the autologous SC approach is slow, expensive and impossible to scale up — companies offering these therapies would be organized as service businesses rather than product development businesses; and the allogeneic (donor cell) approach carries many uncertainties and potential risks. Hence, other approaches are being investigated. These include using specific sequences of drugs to induce regenerative changes in damaged tissue (fascinating work at Gladstone Institutes); using biomaterials loaded with growth factors to attract the patient’s own stem cells to the site where needed; and using devices that can make up for cells’ deficits. The most advanced example in this latter category is the use of automated minimally invasive pumps which deliver insulin in response to automatic measurements of patient’s blood glucose, obviating the need to have the patient’s pancreas re-grown or replaced (at least according to Medtronic!).
That there are often several paths to the desired therapeutic effect raises strategic questions that investors in this space, and in biotech in general often ask — when is it better to have a cell vs mechanical vs drug or other therapy? This of course depends on the specific clinical performance of each product, relative to its development costs and likely price point. It will be a great day when we have several cell therapies approved for use, allowing the market — patients and payors in the healthcare system — to chose the approaches that deliver the highest sustained clinical benefit with the fewest side effects and inconveniences for the money.
In the next installment, we’ll take a look at the later-stage stem cell products in development and analyze the companies and the diseases / conditions they believe to be the best demonstration of the potential of stem cells.
For all things regenerative, be sure to visit the RegenMed Network.
 Great review of cell/stem cell trials provided by Dr. Alexey Bersenev: http://celltrials.info/