New Insights into Cell-Based Therapy for Heart Failure from the CHART-1 Study

Introduction

In this issue of EHJ Heart Failure, Teerlink et al. report on a follow-up study of the recently published CHART-1 study. The new analysis contains 2 important sets of results – longer term follow-up and dose response analysis. These findings provide potentially important insights in key issues in this emerging new field of cardiovascular therapeutics.

Cardiopoietic Stem Cells (C3BS-CQR-1)

Cardiopoietic cells (C3BS-CQR-1) are bone marrow mesenchymal stem cells (BM-MSCs) that have been pre-treated with a cocktail of factors: transforming growth factor-β1, fibroblast growth factor-2, insulin-like growth factor-1, Activin-A, retinoic acid, a-thrombin, bone morphogenetic protein 4, and interleukin-6. The preconditioning of MSCs with these factors guides them toward a cardiogenic oriented cell type. These cells were tested in a large, multicenter, phase I, randomized clinical trial: Cardiopoietic stem Cell therapy in heart failURE (C-CURE)¹. The results of this trial showed no treatment-associated adverse events or systemic toxicity. Moreover, superior improvements in cardiac function and physical performance measures were reported in the MSC-preconditioned group compared to standard of care alone¹. The promising results from C-CURE led to the largest phase II/III, cardiac stem cell trial to date: the CHART-1 study, which evaluated the safety and efficacy of autologous cardiopoietic cell therapy in patients with ischemic heart failure².

CHART-1 sub-analysis

The CHART-1 (The Congestive Heart Failure Cardiopoetic Regenerative Therapy) study analyzed efficacy end-points at 39 weeks post injection². While the aim was to inject up to 600 million autologous cells, the bone marrow yield from each patient was not uniform. Consequently and by design, the dosage and therefore the number of injections, varied among patients. Despite favorable trends in functional parameters, CHART-1 failed to meet its formal primary end-points³. Accordingly, it becomes increasingly important to understand the potentially correctable factors that account for the trial not meeting its primary endpoint.

As suggested by Teerlink et al. one of these factors may relate to dose and delivery of the therapeutic product. For example, perhaps myocardial damage secondary to the number of endocardial injections could have offset the beneficial effects of the therapy³. In this regard, the CHART-1 sub-analysis (Benefit of cardiopoietic mesenchymal stem cell therapy on left ventricular remodeling: Results from the Congestive Heart Failure Cardiopoietic Regenerative Therapy) sought to investigate the effectiveness of cell therapy at 52 weeks post-infusion, with an emphasis on examining efficacy after adjusting for dosage as indicated by the number of injections. Interestingly, after grouping patients into 5 groups (sham control, C3BS-CQR-1, <16 injections, 16–19 injections, and ≥20 injections), the follow-up MRI analysis revealed an improvement in left ventricular end-systolic (LVESV) and end-diastolic volumes (LVEDV) in the C3BS-CQR-1 group as compared to sham controls, and an inverse relationship between the number of endocardial injections and improvements in LVEDV and left ventricular stroke volume. Patients who received the fewest number of injections (n≤20), which correlated with a lower dose, demonstrated greater improvements in remodeling as compared to those who received >20 injections³. Similarly there was a trend toward increased troponin levels at 24 hours and at 9 months post-injection in patients receiving more doses, suggesting an adverse effect of over-dosing.

Stem Cell Dosing

The major findings suggest that fewer injections (n≤20) improves indices of remodeling and function better than a higher dosage of cells, consistent with the idea that there is an upper-limit, or perhaps a tachyphylaxis to dosing in cell-based therapy. Explanations behind this observation are: enhanced cellular destruction secondary to over-crowding during delivery, and decreased cell viability from oxygen tension deficits in the center of the injectate⁸. While the authors of this sub-analysis extrapolated that an optimal dosing range is between 15–19 injections, this study lacked a placebo control and was not designed to sufficiently assess the optimal number of injections. However, larger randomized trials designed to specifically address dosing are necessary to adequately assess such a hypothesis.

The Food and Drug Administration Center Guidance for preclinical Investigational Cellular Therapy in 2013 discussed the importance of cell dose optimization, and the establishment of a dose escalation schedule in preclinical and clinical studies.⁹ In response, a variety of clinical trials have begun to address this concern^10,2,11. In an effort to investigate stem cell dosage and efficacy, our group randomized 30 patients to receive either autologous or allogeneic MSCs and further subdivided these groups to receive one of three doses (20, 100, or 200 million cells). In this study, the POSEIDON trial, the greatest reductions in scar size, and cardiac volumes (LVESV), along with an increase in EF was observed among those who received the lowest (20 million) cell dose¹⁰. In a study testing autologous CD34+ cells in patients with refractory angina, Losordo et al. randomized 24 patients into three groups; placebo, 0.1 million and 0.5 million cells/kg. Patients who received the fewest cells (0.1 million cells/kg) experienced improvements in the frequency of anginal attacks and exercise tolerance as compared to the high dose group¹². In another dose-finding study, our group conducted a dose escalation (20, 100 or 200 million cells) study to evaluate the anti-inflammatory and restorative effects of MSCs in subjects with aging frailty. The 100 million dose group outperformed the 20 and 200 million groups by improving inflammatory biomarkers, physical components and quality of life parameters of frailty¹¹. Although these studies were not all conducted in the same disease process, they demonstrate that there may be an inverse relationship between dose and response in stem cell therapy, and there may be a maximal dose above which efficacy is reduced.

In contrast, other studies have supported that a larger dose is more efficacious; however, these findings may be confounded by other important factors that affect stem cell potency such as cell type, route of administration and timing of delivery^13,14. Studies have shown that young healthy donors are likely to harbor more robust MSCs as compared to their older counterparts^10,15. Therefore, if autologous cells are utilized, larger quantities may be required to achieve the same effects as their allogeneic counterparts^5,16. The route of cell administration is also undeniably important. While MSCs have been delivered in a variety of ways, the majority of clinical trials now utilize a transendocardial approach since it directly targets the diseased parenchyma without a majority loss in extra-cardiac tissue⁵. Many studies have demonstrated the superiority of the transendocardial approach to the intracoronary and intravenous methods, secondary to its ability to directly deliver cells to the target tissue¹⁵. Finally, when administering cells in an acute ischemic setting, the hostile hypoxic environment may necessitate larger doses to achieve the same desirable results seen in the setting of stable chronic cardiomyopathy^10,13. As such, the ideal timing of cell-based therapy may be in the subacute or chronic stages, when inflammation has subsided. Additionally, some studies may have favored the higher dose because the overall dose range was below a minimum threshold to elicit a response¹³. A summary of several dose-finding studies is shown in table 1.

Table 1.

Clinical trial evaluations of dose response in stem cell therapy

Clinical trial	Disease	ROA	Cell Type	Dose	Result
Hare et al. 10	ICM	TESI	Autologous vs Allogeneic BM-MSC	20 million cells 100 million cells 200 million cells	20 million cells; greatest reduction in scar size, cardiac volume (LVESV) and increased EF
Bartunek et al 3	ICM	TESI	Autologous Cardiopoietic BM-MSC 57–60 million cells/mL, 0.5 ml per injection	<16 injections 16–19 injections >20 injections	<20 injections; greatest improvement in LVEDV and LVESV
Golpanian et al.11	Frailty	IV	Allogeneic BM-MSC	20 million cells 100 million cells 200 million cells	100 million cells; improved physical characteristics, quality of life parameters and inflammatory biomarkers.
Losordo et al.12	Angina	TESI	Autologous CD34+	0.1 million cells/kg 0.5 million cells/kg	0.1 million cells/kg; greatest reduction of angina and improved exercise tolerance
Perin et al.14	ICM-NICM	TESI	Allogeneic MPC	25 million cells 75 million cells 150 million cells	150 million cells; reduced MACE events related to heart failure
Poglajen et al.21	ICM	TESI	Autologous CD34+	90.6±7.5 million cells	<50 million cells; no significant response.
Quyyumi et al.13	STEMI	IRA	Autologous CD34+	5 million 10 million 15 million	≥10 million cells; greatest improvement in perfusion and scar size

Abbreviations: ROA, route of administration; ICM, ischemic cardiomyopathy; NIDCM, non-ischemic cardiomyopathy; STEMI, ST-elevation myocardial infarction, IV, Intravenous, TESI, transendocardial stem cell injection, IRA, Infused Infarct-Related Coronary Artery.

Reflections of CHART-1 at 52 weeks post-injection

Another insight offered by Teerlink et al. is that longer follow-up periods may reveal accumulating benefits of cell based therapeutic strategies. The data reported in this analysis, extended the follow-up period from 39 weeks to 52 weeks. At this time-point, the anti-remodeling properties of the cardiopoeitc cells became evident. This is a fascinating finding and one that has been observed since the first report of intracardiac cell therapy for ischemic cardiomyopathy¹⁷.

Other trials investigating the effects of BM-derived cells have reported persistent and increasing improvements in LV size and function over the long-term⁴. Paradoxically, several investigations have concluded that following intramyocardial delivery of cell therapy, washout is fairly rapid (over weeks to months) resulting in a long term engraftment rate of less than 1% of the original injectate⁵. Therefore, the question becomes: what can we decipher about stem cell therapy and mechanism of action; given that cellular engraftment remains low? The evidence suggests that the paracrine effects of MSCs seem to outlast their retention rate. Once critical pro-restorative exosomes are released, their positive efforts accumulate and continue long after the cells have dissipated⁶. However, because MSCs possess inherent immunoevasive properties, new theories suggest that they do not get destroyed. Instead, MSCs migrate toward the lungs and secondary lymphoid organs such as the spleen, where they yield immunomodulatory effects and thus enhance long-term augmentation of cardiac regeneration⁷.

Conclusion

While cell-based therapy has shown promise in patients suffering from ICM, the field has yet to reach a consensus on stem cell type, route of delivery or dose¹⁸. MSCs in particular have been under evaluation in the clinical arena for more than a decade, and multiple studies have demonstrated their safety and efficacy in various disease processes⁵. Given their ease of isolation, expansion and reduced immunogenicity, it is not surprising that MSCs have become popular in regenerative medicine. Lineage directed cell therapy has emerged as a way of predetermining the fate of stem cells in an effort to increase their regenerative efficacy¹, and cardiopoietic cells are the paradigm of lineage committed mesenchymal stem cells. Despite the encouraging results in preclinical and phase I clinical trials, these cells failed to produce significant changes in cardiac structure, function and patient quality of life in a large phase II/III clinical trial². Yet smaller trials, using non-lineage directed MSCs in a similar cohort of patients, have shown significant improvements in these parameters^10,19. The difference may lie in the dosage.

While intuition tells us that more stem cells means better results, in practice the reverse may in fact be true. Dosing reaches a ceiling effect in a variety of medical treatments, with stem cell therapy being no exception. In contrast to other forms of medication, a toxic dose is yet to be described in stem cell therapy, however, at elevated levels, their restorative effects tend to wane. Postulated theories behind this observation include: effects of shear forces during injection secondary to excessive cell-cell contact, and increased viscosity of the injectate, which can induce transmural hematomas and myocyte necrosis^8,20. However, precise reasons underling this inverse dose observation are multi-factorial and remain incompletely understood.

The future of stem cell therapy for cardiac regeneration is dependent on studies to evaluate the optimal dosage. Although the initial 9 month results of CHART-1 did not meet efficacy endpoints, the sub-analysis of different dosages revealed that the low dose group, was in fact most efficacious. Therefore, further trials designed to specifically evaluate dosing are required.