SC therapy is viewed as a promising option for ALS because these cells potentially target several of the putative pathogenic mechanisms involved in the onset and progression of the disease, although explicit target engagement for each cell product needs further validation9. Here, we designed a meta-analysis to examine individual progression as a measure of efficacy, using the ALSFRS-R change, which is considered to be linear for the majority of the illness24 Overall, and with the limitations that are discussed below, we found that, based on current evidence, cell therapy has a transient positive effect on the progression of the disease, as measured by the ALSFRS-R. In the available controlled studies that we reviewed (76 patients receiving MSCs or MNCs), there was also a trend for a better progression in patients receiving cell therapy. In those studies, the administration of MSCs was intrathecal17 or intrathecal + intramuscular25, and intracortical for MNCs26.
In our meta-analysis, the effect of the SC therapy on ALSFRS-R was not huge when considering the monthly score but, if sustained, the cumulative effect could represent a meaningful clinical change at 6 months and later. For context, note that the difference at 6 months for the FDA-approved antioxidant edaravone over placebo was 2.41 points in a selected cohort27, or ~0.41 points per month. Additional considerations need to be made because (1) some studies reported a very short follow-up, (2) patients included in some of these trials were in an advanced stage (and this precluded us to analyze the effect on survival) and, (3) long-term changes, although encouraging, need to be interpreted with caution and are more difficult to associate with the intervention due to loss of the most severe patients during the study. Notwithstanding, the clinical data supports a positive effect of the intervention on the progression of the symptoms, if transient.
We also examined the effect on FVC, as a value less likely to be biased. However, and unlike ALSFRS-R, the progression of FVC has been shown to follow distinct trajectories28, so the comparison of pre- and post-treatment FVC slopes maybe less informative with respect to the effect of the intervention. Also, the slope calculation was less accurate as there were fewer values reported and fewer studies reporting values. Nevertheless, we cannot ignore that, overall, there was a worsening of FVC values after the intervention that was pronounced early on. The most plausible interpretation is that the intervention disrupts a delicate respiratory functional balance, being, in that regard, not different from other surgical interventions that appear to accelerate the progression of ALSFRS29. However, this is an unlikely explanation for the intrathecal administration, which is a minor procedure and did not cause significant worsening in a controlled study27 raising questions about potential MSC off-target effects. Indeed, in the controlled study27, the authors reported that the FVC progression in points per month was not significantly different between controls (−0.8 ± 3.18, n = 25) and MSC-treated patients (−1.54 ± 3.38, n = 31) with respect to their lead-in periods, i.e. a difference of −0.74 (−2.52, 1.03) points per month in favor of controls, but note that MSC-treated patients showed a significant worsening in the follow-up period with respect to the lead-in (P = 0.023) while controls did not (P = 0.29). On the other hand, for intraparenchymal interventions, the effect of the SC treatment on ALSFRS-R could be larger if compared with a sham procedure, emphasizing the inadequacy of non-interventional controls and the need to investigate these issues in properly controlled preclinical models.
The source and class of the cells to be transplanted represent an arguable point for the implementation of cell therapy in central nervous system (CNS) disorders. Most clinical trials—and all the intrathecal ones—included in this meta-analysis were performed with autologous Bone marrow mesenchymal stromal cells (BM-MSCs). These trials presented some differences regarding cell preparation, cell dose, and administration regime and, overall, those using intrathecal repeated administration in cerebrospinal fluid vehicle appear the most promising. The rational to use MSCs in ALS is based on their capacity to produce and release neurotrophins30 and secrete molecules that can suppress the activation and function of both the innate and adaptive immune systems31. In ALS patients, increased numbers of CD4+ and CD8+ T cells and dendritic cells have been detected near dying motoneurons in the spinal cord and in brain parenchyma32. Preclinical studies have reported that human MSC transplantation attenuates neuroinflammation, improves motor performance, and extends survival in a mouse model of ALS33. In addition, it is possible that mitochondrial transfer from MSCs can improve bioenergetics of recipient cells34. However, a precise characterization of the biodistribution of MSCs and definition of the mechanism(s) of action is lacking. Since BM-MSCs were obtained from the same patient in these trials, there was no need to immunosuppress the patients. However, patient-derived SCs may express genetic and epigenetic disease-related footprints, making them unsuitable for therapeutic purposes35. Nevertheless BM-MSCs from ALS patients have been reported to maintain the cytokine secretory profile and to be more efficient in decreasing TNF alpha—but also to respond differently to induction36 (somewhat questioning NurOwn® strategy)15,25.
Initially driven to achieve neuronal cell replacement, fetal NSCs were embraced as a cell source that is renewable while being inherently non-tumorigenic and with low immunogenicity37. Nonetheless, NSCs appear more likely to differentiate into glial cells (astrocytes and oligodendrocytes) and to provide trophic support to dying motoneurons35. A toxic effect of astrocytes has been well established in ALS spinal cord, therefore, providing healthy glia into the area may have a positive impact on disease outcome. The fetal NSCs used in these studies had different origins in the spinal cord (Neuralstem Inc.)20,21 or forebrain22 which may define their restorative capacity, as regional differences in glial cells are becoming increasingly recognized38. Immunosuppression is also a factor that needs to be considered when using allogeneic cells, even if fetal NSCs appear to induce little immunogenicity39 and survival has been documented after withdrawal of immunosuppression40. In that postmortem analysis, nests of transplanted spinal NSCs were identified but did not show significant differentiation into glia and only subsets were labeled for SOX2 or NeuN (a marker of mature neurons)40. Indeed, protective mechanisms may be mediated by the NSCs or by other cell types as Blanquer et al. reported the presence of nests of MNC transplanted cells surrounding motoneurons which showed less ubiquitin inclusions and were more numerous at targeted levels (if only ~25% from control)23. With currently available datasets, these three SC types can only be compared in intraspinal administration, but the observed effects are too small to draw any conclusion.
For any therapy, but in particular for those targeting the CNS, because of the barriers and enclosures that protect it, delivery is critical. For ALS, local delivery targeting the upper motoneurons (intracortical), lower motoneurons (intraspinal), or neuromuscular junction (intramuscular) requires invasive procedures and multiple injections, each representing a risk of AEs, which, in particular, into the CNS parenchyma may have serious consequences, and therefore requires a careful risk–benefit analysis. Equally important, with local approaches the cell distribution is rather restricted to the injected segment or area (for example, with 20 intraspinal injections only about 15% (8 cm) of the spinal cord area was covered)41. This has to be taken into account when calculating the cell dose because increasing the cell number without increasing their spread could be detrimental.
On the other end, systemic (intravenous) delivery of MSCs while convenient from a logistic point of view has been found to be mostly ineffective42.
Thus, intrathecal delivery by injection into the subarachnoid space presents the most favorable profile, achieving a broader distribution and, possibly, penetration in the parenchyma through paravascular spaces, while using relatively minor invasive procedures. This route makes it feasible to repeat the administration and to perform a sham procedure allowing to evaluate separately the effect of the procedure and that of the SC product (although in this case all trials that used the intrathecal route were done with BM-MSCs so it is not possible to completely separate the effect of both variables on the outcome). A more recently proposed infusion into the subpial space41 appears promising in preclinical models, by achieving a broad distribution of infused cells, but requires surgery making more problematic to repeat the administration. Given that the effect on the clinical progression appears to be transient, an invasive procedure would not be desirable.
A compelling lesson learned from previous research is that confirmatory trials of the efficacy of new SC lines for the treatment of ALS should be initiated only after an evidence-based treatment protocol is constructed. There are distinct challenges and opportunities in performing early-phase SC therapy trials in ALS, which require special approaches and unique trial designs. The current stage of the clinical research in this field requires to identify a primary outcome measure which can show an improvement that is clinically meaningful to the patient so that the risk of the procedure is reasonably balanced by the potential clinical benefits and the value of the resulting scientific knowledge. Investigators should choose to stratify for factors most relevant to the outcome measure of the trial. Patient selection is a very important variable, but to date, there is little agreement about inclusion criteria. A more heterogeneous study population may mask efficacy of the study intervention on specific subpopulations of patients bearing genetic forms of the disease or restricted phenotypes. According to the updated guidelines for ALS clinical trial design43, responder analyses should be included, having the potential to demonstrate beneficial effects of a proposed treatment on a subset of patients with a shared unique phenotype. Moreover, in designing and implementing ALS SC clinical trials, investigators should incorporate predictive biomarkers, prognostic biomarkers, and, especially biomarkers of SC activity.
Overall, based on current evidence, we can conclude that SC therapy may have a positive, if transient, effect on ALS progression, but not on the pulmonary function, and that both the cell product and the delivery route need to be carefully reconsidered and optimized in adequately controlled preclinical models before moving forward in ALS patients. Furthermore, for a SC therapy to be successful, a third factor, the recipient, needs also to be integrated in the equation. Illustrating this point, the results of NurOwn® Phase III trial (NCT03280056) in which patients received three intrathecal injections of autologous pre-conditioned BM-MSCs or placebo were released last month. The trial did not meet the endpoint but analysis of a subgroup of early patients showed the anticipated difference between treated and placebo groups. The company has announced initiation of expanded access program (compassionate use) for some patients who completed the clinical trial and meet specific eligibility criteria (https://ir.brainstorm-cell.com/2020-11-17-BrainStorm-Announces-Topline-Results-from-NurOwn-R-Phase-3-ALS-Study). Thus, a better understanding of disease mechanisms in individual patients would greatly enhance the possibility to implement the most suitable SC strategy to modify the prognosis of ALS patients.