Autologous mesenchymal stem cells produce concordant improvements in regional function, tissue perfusion, and fibrotic burden when administered to patients undergoing coronary artery bypass grafting: The Prospective Randomized Study of Mesenchymal Stem Cell Therapy in Patients Undergoing Cardiac Surgery (PROMETHEUS) trial

Abstract

Rationale: Although accumulating data support the efficacy of intramyocardial cell-based therapy to improve left ventricular (LV) function in patients with chronic ischemic cardiomyopathy undergoing CABG, the underlying mechanism and impact of cell injection site remain controversial. Mesenchymal stem cells (MSCs) improve LV structure and function through several effects including reducing fibrosis, neoangiogenesis, and neomyogenesis.

Objective: To test the hypothesis that the impact on cardiac structure and function after intramyocardial injections of autologous MSCs results from a concordance of prorecovery phenotypic effects.

Methods and results: Six patients were injected with autologous MSCs into akinetic/hypokinetic myocardial territories not receiving bypass graft for clinical reasons. MRI was used to measure scar, perfusion, wall thickness, and contractility at baseline, at 3, 6, and 18 months and to compare structural and functional recovery in regions that received MSC injections alone, revascularization alone, or neither. A composite score of MRI variables was used to assess concordance of antifibrotic effects, perfusion, and contraction at different regions. After 18 months, subjects receiving MSCs exhibited increased LV ejection fraction (+9.4 ± 1.7%, P=0.0002) and decreased scar mass (-47.5 ± 8.1%; P<0.0001) compared with baseline. MSC-injected segments had concordant reduction in scar size, perfusion, and contractile improvement (concordant score: 2.93 ± 0.07), whereas revascularized (0.5 ± 0.21) and nontreated segments (-0.07 ± 0.34) demonstrated nonconcordant changes (P<0.0001 versus injected segments).

Conclusions: Intramyocardial injection of autologous MSCs into akinetic yet nonrevascularized segments produces comprehensive regional functional restitution, which in turn drives improvement in global LV function. These findings, although inconclusive because of lack of placebo group, have important therapeutic and mechanistic hypothesis-generating implications.

Clinical trial registration url: http://clinicaltrials.gov/show/NCT00587990. Unique identifier: NCT00587990.

Keywords: coronary artery bypass; mesenchymal stromal cells; stem cell.

Figure 1. Scar size reduction and increase of viable tissue due to intramyocardial injection of autologous bone marrow MSCs in conjunction with CABG

(Panel A) Delayed enhancement MRI scar size images show durable reduction in scar size in patients who underwent both CABG and autologous MSCs injection, 18 months post CABG. Arrows correspond to edges of anterior wall (open arrowheads) and inferolateral wall (closed arrowheads) infarct scars, both at baseline and at 18 months post MSCs injection during CABG (Panel B) There was a significant increase of viable tissue in CABG patients who received adjunct autologous MSCs injections compared to baseline. (Panel C) Infarct size was reduced with MSC therapy during CABG compared to baseline. *p<0.05, two way ANOVA repeated measures. MSCs indicate mesenchymal stem cells; CABG, coronary artery bypass grafting. Shown are mean±SEM at each time point.

Figure 2. Increase of viable tissue determines degree of left ventricular functional restitution with autologous MSC therapy and the number of injections and of cells injected in each segment the degree segmental scar size reduction

(Panel A) Correlation between the change in viable tissue from baseline (preinjection) to 18 months post-surgery with the percentage change in EF suggests that increase of viable tissue with MSC therapy drives functional improvement in ischemic cardiomyopathy.(Panel B) The number of MSC cells injected per segment correlated with the reduction in scar tissue (r=0.6, p=0.04). (Panel C) A similar correlation was observed between the number of injections each segment received and the reduction of scar size in the respective segment in the patients that received the higher dose of MSCs (r=0.74, p=0.04). MSC indicates mesenchymal stem cells; EF ejection fraction.

Figure 3. MSCs injections lead to regional improvement

(Panel A) Infarct size was decreased significantly at 3 months that was sustained at 6 and at 18 months in the injected only segments. In the revascularized only segments, the decrease of the infarct size became significant at 18 months. (Panel B) The average upslope corrected for the left ventricular blood pool intensity is a measure of regional perfusion. After MSCs injection there was a significant increase of perfusion in the injected segments that differed significantly from the revascularized only segments.(Panel C) MSCs injections led to a significant regional increase of wall thickness compared to the revascularized only segments. (Panel D) Systolic wall thickening improved in the injected segments and in the revascularized only segments.(Panel E) Peak Ecc is a measure of contractility with lower values signifying better contractility. There was significant improvement of peak Ecc at the injected segments over time and compared to revascularized only segments. (Panel F) Peak diastolic strain rate is an index of diastolic function and a higher value signifies improved diastolic function. There was a significant improvement in diastolic function at the injected segments compared to revascularized only. □ = MSC injected only segments, ○ = revascularized only segments *p<0.05 one way ANOVA repeated measures, †p<0.05 vs. baseline, Bonferroni posttests, ‡p<0.05 two way ANOVA. Shown are mean±SEM at each time point.

Figure 4. Concordance Index Score is an indicator of simultaneous and comprehensive improvement

(Panel A) Bulls Eye map depicting the concordance of change for each variable used for the Concordance Index Score based on the average grade for each group at 18 months post treatment. A grade closer to 1 signifies a concordant improvement in the variable and a grade closer to -1 represents deterioration. The concordance index score is then derived by adding the grades for changes in scar tissue size, perfusion and the average of the grades for changes in wall thickness, wall thickening and systolic strain.The highest value (3) signifies simultaneous improvement in all 5 CMR indices and the lowest (-3) a simultaneous deterioration respectively. (Panel B) In the MSCs plus CABG group, the injected non-revascularized segments improved comprehensively and thus had a higher concordant improvement score compared to all other groups. The effect of the MSCs dissipated by a function of distance from the actual injection site (p=0.03 adjacent vs. remote revascularized and non-treated segments).□ =MSC Injected, ▲=Adjacent Revascularized, ▼=Remote Revascularized, ◇=untreated. *p<0.05 one way ANOVA repeated measures, †p<0.05 vs. baseline, Bonferroni posttests, ‡p<0.05 two way ANOVA

Figure 5. Topographic Analyses of MSCs Injected Scarred Segments

(Panel A) Topographic analysis of a late contrast enhanced image that depicts scar (red color). The total endocardial length (white double headed arrow) of the scar was measured at baseline. The center of the scar on the endocardial border was then noted and scar thickness was measured (distance from the endocardial contour to the distal border of the scar; yellow double headed arrows) at that point. The distance from the epicardial contour to the distal border of the scarred area was also measured at this point (Epicardial Cap; green double headed arrows). Then the same measurements were repeated at half the length from the lateral edges of the scar. The final values for each scar were derived by averaging the respective measurements. The same points were then identified at a slice in the same area at 18 months and the calculations were repeated. (Panel B). At 18 months the endocardial length decreased. (Panel C) Scar thickness also decreased at 18 months. (Panel D) There was also an increase in the epicardial cap thickness at 18 months. *p<0.05, paired t-test.