Researchers at the University of Sydney have used human stem cells to make pain-killing neurons that provide lasting relief in mice, without side effects, in a single treatment.
The next step is to perform extensive safety tests in rodents and pigs, and then move to human patients suffering chronic pain within the next five years.
If the tests are successful in humans, it could be a major breakthrough in the development of new non-opioid, non-addictive pain management strategies for patients, the researchers said.
“We are already moving towards testing in humans,” said Associate Professor Greg Neely, a leader in pain research at the Charles Perkins Centre and the School of Life and Environmental Sciences.
“Nerve injury can lead to devastating neuropathic pain and for the majority of patients there are no effective therapies. This breakthrough means for some of these patients, we could make pain-killing transplants from their own cells, and the cells can then reverse the underlying cause of pain.”
Published today in the peer-reviewed journal Pain, the team used human induced pluripotent stem cells (iPSC) derived from bone marrow to make pain-killing cells in the lab, then put them into the spinal cord of mice with serious neuropathic pain. The development of iPSC won a Nobel Prize in 2012.
If the tests are successful in humans, it could be a major breakthrough in the development of new non-opioid, non-addictive pain management strategies for patients, the researchers said.
“Remarkably, the stem-cell neurons promoted lasting pain relief without side effects,” co-senior author Dr Leslie Caron said. “It means transplant therapy could be an effective and long-lasting treatment for neuropathic pain. It is very exciting.”
John Manion, a PhD student and lead author of the paper said: “Because we can pick where we put our pain-killing neurons, we can target only the parts of the body that are in pain. This means our approach can have fewer side effects.”
The stem cells used were derived from adult blood samples.
The total cost of chronic pain in Australia in 2018 was estimated to be $139.3 billion.
One of the major achievements in the development in modern medicine is the discovery of stem cells. Stem cells are attracting attention as a key element in future medicine, satisfying the desire to live a healthier life with the possibility that they can regenerate tissue damaged or degenerated by disease or aging.
Development of cell therapy and regenerative medicine using stem cells is expanding the medical industry and businesses as well as increasing the understanding of the nature of the cell itself. Stem cell medicine brings a new paradigm to modern medicine which has relied heavily on medicine or surgery.
Intravenous infusions of bone marrow cells in 1957 from a healthy donor to a leukemia patient following radiation and chemotherapy would come to be considered a kind of hematopoietic stem cell therapy .
Since this historical event, leukemia has been successfully treated with stem cell therapy. Today, treatment with stem cells has received increasing attention as a solution to overcome the limitations of conventional treatment and medicine for intractable diseases. Stem cell therapy has been tried for various diseases, such as Lou Gehrig’s disease, Burger’s disease, spinal cord injury, Parkinson’s disease, and intractable osteoarthritis [2–6].
Recently, stem cell therapy has been introduced in the field of treatment for chronic intractable pain syndromes. The number of clinical or preclinical reports in which stem cell therapy was applied for the treatment of chronic pain has been growing [7,8].
In this review, the emerging opportunities of stem cell therapy for pain treatment are discussed as well as the limitations of stem cell treatments, and ethical and legal issues as well.
Stem cells in pain medicine
Recently, stem cells transplantation has been frequently applied to the treatment of pain as an alternative or promising approach for the treatment of severe osteoarthritis, neuropathic pain and intractable musculoskeletal pain which does not respond to conventional medicine.
Stem cell-based therapies have been realized to be a potential treatment option for articular cartilage repair in patients with knee osteoarthritis [6,30], neuropathic pain [31,32], and intervertebral disc disease .
Degeneration and inflammation of the cartilage that covers the joint surface is the main cause of pain in osteoarthritis.
The cartilage of the articular surface reduces the friction of the joint motion and acts as a cushion against weight loading. While chondrocytes occupy only 1% to 5% of cartilage volume, they produce collagen, proteoglycans, and hyaluronan, which are components of the extracellular matrix, and maintain cartilage structure and physical properties .
However, as the cartilage has no blood vessels and nerves, cartilage regeneration is difficult once the cartilage has been damaged or undergone degenerative changes.
Some that limit the regeneration of, and recovery from, damaged cartilage. Chondrocytes within the cartilage can migrate very slowly to the adjacent lesion.
Even if bone marrow-derived stem cells can help cartilage regenerate, it is only possible when the bone marrow is exposed, or, in other words, when the entire layer of cartilage and chondral bone is damaged.
The evidence for the contribution of stem cells, which have migrated from the synovial membrane or fluid, for cartilage regeneration, is not definitive.
For one of the attempts to overcome the limitation of the natural regeneration of cartilage in osteoarthritis, autologous chondrocyte transplantation has been reported in 1994 .
The implantation of MSCs has also been reported. Repaired tissues treated with MSCs appeared to have better cell arrangement, subchondral bone remodeling, and integration with surrounding cartilage than did repaired tissues generated by chondrocyte implantation .
While adipose tissues, the synovial membrane, and the umbilical cord would be one of the sources, bone marrow-derived MSCs have been known as the standard sources of adult stem cells for the treatment of knee osteoarthritis.
The therapeutic modalities applied for osteoarthritis include surgical intervention or arthroscopy, tissue engineering, and intra-articular injection of cultured stem cells.
These modalities would be applied individually or in combination. Microfractures are made artificially under arthroscopy by awls which are used to make holes through the subchondral bone plate to become focal full-thickness cartilage defects.
This procedure is intended to allow the migration of stem cells in the bone marrow to reach the cartilage defect site. In this case, the cartilage produced by this procedure tends to become less durable fibrous cartilage in comparison to the innate hyaline cartilage .
The scaffolds that provide mechanical support for cells and the extracellular matrix can be used for culturing stem cells. This scaffold also needs surgery through which the cultured scaffold could be implanted in the joint [43,44].
For the convenience of the clinical use of stem cells, intra-articular injection of cultured cell therapy would be a minimally invasive and potentially efficient method for knee osteoarthritis. Intra-articular-injected autologous MSCs increased the knee cartilage volume and improved the pain scores .
MSCs, as an intra-articular injection adjuvant to the arthroscopic debridement procedure, resulted in better outcomes than the debridement alone .
Both increased interleukin 6 (IL-6) and tumor necrosis factor-alpha (TNF-α) in the synovial membrane and increased TNF-α and IL-1β in the vastus lateralis were closely related with the pain and muscular atrophy of osteoarthritis patients .
A precursor of inflammatory cytokines in the serum or synovial fluid was associated with osteochondritis . Due to MSCs having immunomodulatory functions and the characteristic of homing to injured sites, it is effective in treating trauma or inflammatory pain.
Intra-articular cell therapy for osteoarthritis with autologous culture-expanded stem cells showed only four serious adverse events in 844 procedure reviews including one case of infection on the bone marrow aspiration site and one pulmonary embolism; two tumors, not at the site of injection, were reported as unrelated.
The main adverse events related to the procedure were increased pain, swelling, and dehydration after bone marrow aspiration . Safety reports on 227 cases of intra-articular MSC injection showed 7 cases of probable procedure-related complications and 3 cases of possible stem cell-related complications, all of which were either self-limited or were remedied with simple therapeutic measures. There was no cancer-related adverse event reported during the two years of follow-up .
Many of the systemic reviews have reported that the treatment of knee osteoarthritis with intra-articular injections of stem cells showed favorable results in which the therapy can reduce knee pain and improve physical function and cartilage quality [34,51,52]. However, there have been reports which did not have optimistic or promising point of views [53,54].
Some reports emphasized that evidence of efficacy remains limited because of poor study design, a high risk of bias, large heterogeneity, and a wide confidence interval in the estimate of the effects. In addition, rehabilitation programs followed by stem cell injections played an important role in reducing pain .
To date, there have been no reliable and convincing clinical human studies with a high level of evidence conducted on the application of intra-articular stem cell injection to knee arthritis. Intra-articular injection therapy using stem cells suffers from a lack of evidence of efficacy in both functional improvements and cartilage repair.
Treatment of neuropathic pain is a clinical challenge, as the pathogenesis is very complicated. The pathology of neuropathic pain involves the entire nervous systems, including the peripheral nerve, dorsal root ganglion, spinal cord, and brain.
The main idea of applying stem cells to neuropathic pain is based on the ability of stem cells to release neurotrophic factors, along with providing a cellular source for replacing the injured neural cells, which make them ideal candidates for modulating and possibly reversing intractable neuropathic pain.
Hofstetter et al.  and Fischer et al.  confirmed the relief of pain and recovery of motor function by directly administering neural stem cells to a spinal cord injury model. Stem cells would migrate to the injured site, which is called the homing of stem cells [58–60]. Thanks to the homing properties of stem cells, relieving neuropathic pain can be achieved even by intravenous injection .
The stem cells given intravenously were apt to be trapped in the lung when passing through; therefore, only a small amount of the stem cells can survive and move to the injured site .
From such a point of view, selecting the intrathecal route and targeting the pain pathway of the dorsal root ganglion or spinal cord directly looks like the obvious method for stem cell delivery in a spinal cord injury model .
During the early phase in the research of stem cells, the focus has been directed toward the regeneration of tissues.
Recently, the focus has been more on the side of the paracrine effect, which is known to participate in tissue repair by stimulating surrounding cells to be recruited and by suppress the inflammation responses .
Both the neurotrophic factors and neuroinflammatory cascades caused by immune and glial cells also play an important role in the development of neuropathic pain [31,66–69]. When the balance between both factors is destroyed, and the inflammatory side becomes dominant, neuropathic pain is more likely to occur.
Significant increases in IL-1β and IL-6, but not TNF-α, in the cerebrospinal fluid of complex regional pain syndrome patients, which indicates the activation of the neuroimmune system, as compared to controls, was reported .
Various stem cells including human mesenchymal stem or stromal cells, are known to secrete neurotrophic factors and anti-neuroinflammatory cytokines which have neuroprotective and even regenerative effect [64,71–75]. With these paracrine effects, stem cells inhibit the hazard of the inflammatory cytokines .
Neurotrophic factors, especially nerve growth factor (NGF) and glial cell line-derived neurotrophic factor help the injured nerve restore itself in maintaining the function of a nerve, promoting regeneration, and regulating neural plasticity in response to injury . MSCs reduce the secretion of inflammatory cytokine in T-cells such as IL-1β or TNF-α . In addition to the paracrine effects, intrathecal administration of MSCs reduces the reactive oxygen species and pain behavior in neuropathic rats .
(1) Diabetic peripheral neuropathy
The pathology of diabetic peripheral neuropathy initiates from the destruction or obstruction of peripheral vessels. Consequently, decreased blood flow ends up causing nerve damage. The stem cells that secrete neurotrophic factors and paracrine inducing neovascularization should be an effective therapy for diabetic peripheral neuropathy [79–82]. In a diabetic neuropathic pain animal model, transplantation of MSCs improved the blood circulation and nerve conduction velocity. Neurotrophic factors such as NGF, neurotrophin-3 protein, vascular endothelial growth factor, and basic fibroblast growth factor are reported to be involved as attributable factors [83,84].
There have been three reports on diabetic neuropathy in an animal model. Stem cells were administered intramuscularly to the hind leg. Subjects were observed for 2 to 16 weeks and showed improvement in nerve conduction velocity through the paracrine actions of growth factors secreted by MSCs [80,83,84].
MSCs, differentiated into anti-inflammatory cells, attenuated pain behaviors of streptozotocin-induced diabetes in a rat model [85,86]. A report said that patients with type I diabetes who received MSCs did not need analgesics after the dramatic pain reduction at two months, blood flow was recovered after six months, painlessness after nine months, and all tissues with infection and necrosis were recovered .
Spinal cord injury
Patients with spinal cord injury suffer from desperate and intractable pain. Reduced neurotrophic factors caused by disrupted inhibitory pathways and the production of proinflammatory cytokines would be attributable to neuropathic pain [88–90].
In an animal model of spinal cord injury, stem cell therapy reduced pain by differentiating into glial cells and releasing trophic factors. That is, stem cells contribute pain medicine as small analgesic biopumps in addition to supplying cellular sources of tissue regeneration. When the neural stem cells were injected intrathecally into the spinal cord injury rat model, they would have an analgesic effect as small biopumps releasing inhibitory neurotransmitters, such as gamma-aminobutyric acid or glycine .
Other animal studies reported that the transplantation of MSCs for the treatment of spinal cord injury produced gait improvement and evidence of histological regeneration of the nerve [92,93]. In a meta-analysis of an animal model , the efficacy of neural stem/progenitor cell transplantation was higher in transection and contusion models than in compression ones. The shorter the interval between injury and treatment, the better the functional recovery and sensory condition.
Immunosuppressive drugs used for avoiding rejection negatively affected motor function recovery. Scaffold use could boost efficacy on motor function recovery. However, other reports said that the neural stem cells rather increase the pain of spinal cord injury. Neural stem cells survived and differentiated into a predominately astrocytic population; however, the locomotor function was not improved and significant forelimb thermal and mechanical allodynia were observed .
A clinical case of a patient with an incomplete spinal cord injury at the T12-L1 level and a crush fracture in the L1 vertebral body was administered several doses of allogeneic MSCs intrathecally and intravenously. The patient reported a marked decrease of neuropathic pain, an improvement in muscle strength, an increased dermatomal sensation, and a recovery of urological and sexual functions .
Chronic constriction injury (CCI)
Intravenous administration of bone marrow-derived mononuclear cells reduced neuropathic pain in a sciatic nerve CCI model . Systemic administration of human MSCs attenuated the neuropathic pain in a spared nerve injury mouse model. The human MSCs were mainly able to home in on the spinal cord and pre-frontal cortex of neuropathic mice. It reduced the level of IL-1β and IL-17 and increased the anti-inflammatory cytokine IL-10 and the activity of macrophages .
Seven studies involving a sciatic nerve injury model were reviewed. In 4 of them, stem cells were given intravenously. For the other 3, stem cells were placed directly on the sciatic nerve, the L4 dorsal root ganglion, and the lateral ventricle of the cerebrum, respectively. Administration to the lateral ventricle was intended to allow observation of the influence of supraspinal regulation of neuropathic pain. Observations were made for 1 to 90 days after stem cell implantation; reports said that most of the allodynia and hyperalgesia had decreased [31,61,97–101].
Intervertebral disc disease
The expression of TNF-α and IL-8 in the nucleus pulposus of degenerated discs was much higher than that of herniated discs. That would be a reason why the level of pain is more severe in patients with a degenerated disc .
When patients with degenerative disc disease were treated with autologous expanded bone marrow MSCs injected into the nucleus pulposus, the pain and disability were improved, and were comparable to spinal fusion surgery, although disc height was not recovered .
A systemic review of autologous MSC injections for the regeneration of intervertebral discs was conducted on a total of 98 patients and 122 treated levels in seven studies . Bone marrow-derived MSCs harvested from the iliac crest represented the most common type of injected cell.
Patients with fractures of the trabecular bone and intervertebral discs with a complete radial fissure were excluded because the non-integrity of the annulus may allow the injected stem cells to escape.
Patients with low back pain due to initial intervertebral disc degeneration and low-stage radiological degeneration were eligible for the stem cell infiltration. The average Oswestry Disability Index and visual analogue scale scores improved at the one-year follow-up. Quantitative improvements, such as T2-weighted magnetic resonance image scans, protrusion sizes, and disc bulges also improved.
The dose of stem cells
Adequate dosages of stem cells have not been well established. For clinical applications, however, more research on stem cell types, doses, safety, and implantation rates is needed. When the neuronal or adipose stem cells are given in a mouse CCI model repetitively, they have an analgesic effect in a dose-dependent way .
In high concentration of human MSCs, dopaminergic neurons in a rodent Parkinson’s disease model were well preserved, and neurogenesis was enhanced . In a study with a Parkinson’s disease animal model, both mechanisms of human MSCs, mediated by neurotrophic paracrine effects and differentiation to neuronal cells, may work in the neuroprotective process even though only 1.7% of injected human MSCs survive .
To compare and discuss the effect of a lot of clinical studies, it is important to develop internationally standardized methods for MSC production. To be accepted as a standard treatment, stem cell therapy should be evidence-based, legally compliant, and cost-effective treatment and should have excellent clinical outcomes .
Even though various differentiation capacities of stem cells are reported, there is not enough knowledge nor sufficient technique to control the differentiation into desired tissues in vivo [106–108].
However, differentiation techniques to targeted cells by biological factors or physical stimuli are being developed along with the discovery of stem cell populations and the advancement of culture technology. Even though stem cells are still in the very early stages of clinical use, the future of stem cells is very bright with the help of accumulating evidence and technologies.
University of Sydney