The results from this Phase I/IIa pilot study provide initial evidence that intramuscular injection of Auto-CD34+ cells is safe and well-tolerated in patients with moderate or high-risk CLI who are poor or non-candidates for surgical or percutaneous revascularization.
Trends toward decreased amputation in Auto-CD34+ cell-treated subjects compared with control subjects demonstrate the potential efficacy of Auto-CD34+ cell therapy in this population. One must use caution in interpreting these results, however, since a higher percentage of subjects experienced amputations during this study relative to other recent clinical studies of CLI.(8,9,10,11,4)
Several surrogate markers (ABI, TBI, leg pain, walking distance, wound healing) of limb perfusion were explored and no differences were detected between the cell-treated and control groups; however, this study was not powered to detect differences in efficacy endpoints. In addition, the high rate of amputation observed in this study resulted in missing data for several of these endpoints, making it difficult to draw any conclusions. No differences in QoL were detected among the cell-treated and control groups.
In theory, increased blood flow could be achieved by increasing the number of vessels that supply the ischemic tissue with blood. The use of pharmacological or biological therapies to induce new blood vessel growth for the treatment or prevention of pathological clinical conditions has been termed therapeutic angiogenesis.(12,13)
The mechanism of action for the majority of pharmacological therapies tested for CLI is vasodilation and promotion of angiogenesis with agents such as prostaglandins. Two randomized, double-blind, phase 3 studies of lipo-ecraprost as a parenteral therapy or as an adjunctive parenteral therapy after distal revascularization in subjects with CLI did not, however, improve major amputation or survival outcomes.(9,10)
Biological therapies, including gene therapy and stem cell therapy, have been evaluated in patients with CLI for improving perfusion in ischemic tissues.(14,15,16,17,11,18,19)
Stem cell therapy for the treatment of CLI is an emerging therapy in which unselected bone marrow mononuclear cells (BM-MNC) or BM-MNC selected to express particular cell surface markers are delivered via intramuscular or intraarterial injection. While a limited number of blinded, randomized controlled trial (RCTs) evaluating cell therapy for no-option CLI patients have been performed, results from several early phase studies show no safety signal and demonstrate favorable trends in efficacy parameters for cell-treatment versus control. In the TACT study a significant increase in ABI and TcPO2 was observed in subjects treated with BM-MNC compared with those treated with peripheral blood MNC.(20)
Interim results from RESTORE-CLI, a blinded RCT in which bone marrow aspirate was processed to generate the tissue-repair cell (TRC) population of stem and progenitor cells, demonstrated that TRC-treated subjects had increased amputation free survival and time to treatment failure compared with placebo subjects.(21)
Treatment with BM-MNCs in the PROVASA trial was associated with improved ulcer healing and reduced rest pain compared with placebo.(22)
A recent report of 12-week data from a RCT of bone marrow aspirate concentrate (BMAC) demonstrated favorable trends for BMAC versus control in major amputations and improved pain, ABI, Rutherford classification and QoL.(23)
The results of these studies are encouraging; however, the variability between studies in the efficacy endpoints that detected differences between the cell-treated and control groups highlight the challenges of choosing clinically meaningful measures of efficacy in this population.
In our study we chose to isolate and administer CD34+ cells for 2 principal reasons: 1 - because of their demonstrated pro-angiogenic potential in vivo, (5,6)
and 2- because an available, approved technology permitted the “manufacturing” of CD34+ cell preparations by standardized methods. The advantage of this approach is that selection of CD34+ cells results in a higher concentration of endothelial progenitor cells in each dose compared with unselected MNCs resulting in greater therapeutic potency in preclinical models. There are no known disadvantages of this approach other than the added step in cell processing. A theoretical disadvantage of this approach is the possibility that other cell types, which may exert pro-angiogenic or reparative functions, are removed; however the evidence from pre-clinical models does not support this concept. Similar to the studies described above, we observed favorable trends in efficacy including reduced amputation rates and improved amputation free survival in the cell-treated groups compared with the control group. The high rate of amputations observed in our study, however, limited the interpretation of other efficacy endpoints.
Taken together, the results from our pilot study and other early stage studies provide evidence for the safety and potential bioactivity of stem cell therapy for CLI. Multiple early phase studies of additional stem cell therapies are currently underway.(24)
Large randomized, placebo-controlled, double-blind studies are necessary; however the high cost of conducting trials in this patient population remains a significant challenge, particularly for earlier stage companies attempting to develop novel therapeutics. In addition, the large variability observed in amputation rates in the phase 2 and phase 3 studies of FGF1 gene therapy(8,17)
suggests that a better understanding of the no-option CLI population is necessary such that patient demographics, physiological characteristics, biomarkers or yet to be defined genetic markers can be used to better predict event rates in this population.
In conclusion, the overall positive safety profile of collecting and administering autologous CD34+ cells in this patient population as well as the potential efficacy of preventing amputations warrant larger scale studies to verify these findings, and to further refine the methods for collecting and administering Auto-CD34+ cells to patients with disabling CLI.