Long-Term Results of Intracardiac Mesenchymal Stem Cell Transplantation in Patients With Cardiomyopathy

Abstract

Background: Mesenchymal stem cells (MSCs), which have the potential to differentiate into cardiomyocytes or vascular endothelial cells, have been used clinically as therapy for cardiomyopathy. In this study, we aimed to evaluate the long-term follow-up results.

Methods and Results: We studied 8 patients with symptomatic heart failure (HF) on guideline-directed therapy (ischemic cardiomyopathy, n=3; nonischemic cardiomyopathy, n=5) who underwent intracardiac MSC transplantation using a catheter-based injection method between May 2004 and April 2006. Major adverse events and hospitalizations were investigated up to 10 years afterward. Compared with baseline, there were no significant differences in B-type natriuretic peptide (BNP) (from 211 to 173 pg/mL), left ventricular ejection fraction (LVEF) (from 24% to 26%), and peak oxygen uptake (from 16.5 to 19.2 mL/min/kg) at 2 months. During the follow-up period, no patients experienced serious adverse events such as arrhythmias. Three patients died of pneumonia in the 1st year, liver cancer in the 6th year, and HF in the 7th year. Of the remaining 5 patients, 3 patients were hospitalized for exacerbated HF, 1 of whom required heart transplantation in the 2nd year; 2 patients survived for 10 years without worsening HF.

Conclusions: The results of this exploratory study of intracardiac MSCs administration suggest further research regarding the feasibility and efficacy is warranted.

Heart failure (HF) is considered the terminal state of various heart diseases. It has a poor prognosis, even though various therapies have been recently developed. Indeed, to live longer, patients who are refractory to guideline-based treatment have no choice but heart transplantation.^1,2

Editorial p 1446

With advances in molecular and cell biology, regenerative medicine is regarded as a promising new strategy in the field of heart disease. Treatments using various types of cells with high self-proliferation potency and multipotency have been administered to patients with myocardial infarction or HF,^3,4 but concerns about serious adverse effects, such as malignant transformation and fatal arrhythmias, have been reported in previous experimental and clinical studies.^5–8 Although numerous clinical trials of cell therapy have been reported, data on the safety have been limited, and especially little is known about long-term safety.

On the basis of experimental findings that mesenchymal stem cells (MSCs) can transdifferentiate into cardiomyocytes or endothelial cells under appropriate conditions,^9,10 we conducted a clinical study of cell therapy using autologous bone marrow-derived MSCs for patients with symptomatic HF on guideline-directed therapies. Here, we report the follow-up results up to 10 years after cell therapy using MSCs.

Methods

Study Design

This study was a prospective, single-center, single-arm trial conducted at the National Cerebral and Cardiovascular Center in Suita, Japan. The institutional review board approved the protocol, which was registered with the University Hospital Medical Information Network (UMIN) clinical trial registry (number UMIN 000000656). All study patients provided written informed consent.

Study Patients

All patients were recruited between May 2004 and April 2006 at the National Cerebral and Cardiovascular Center. Inclusion and exclusion criteria for this study are described.

The inclusion criteria were: (1) left ventricular ejection fraction (LVEF) <40% because of prior myocardial infarction, dilated cardiomyopathy, or other cause; (2) symptoms of refractory HF despite optimal medical therapy; and (3) age >20 years and <80 years.

The exclusion criteria were: (1) any evidence of cancer or active infectious disease; (2) proliferative diabetic retinopathy; (3) severe renal failure or liver failure; (4) any evidence of abnormality in the peripheral blood; (5) history of percutaneous coronary intervention or coronary artery bypass grafting in the 6 months prior to study entry; (6) history of acute myocardial infarction, unstable angina, or myocarditis in the 6 months prior to study entry; (7) pregnancy or suspicion of being pregnant; (8) LV thrombus; (9) aortic valve stenosis or an artificial aortic valve; and (10) inability to tolerate invasive procedures such as catheterization and bone marrow aspiration.

Study Procedures

MSC expansion was performed according to the previously described method.^11,12 In brief, we obtained approximately 20 mL of bone marrow aspirate from the iliac crest of the patient to accumulate MSCs. The culture medium was Eagle’s minimum essential medium-alpha (Invitrogen, Carlsbad, CA, USA) containing 15% of the patient’s serum and antibiotics. To isolate MSCs, nonadherent cells were removed and adherent fibroblastic cells were expanded for approximately 3 weeks’ culture period. The definitive markers of MSCs, such as expressions of CD73, CD90, and CD105 and absence of hematopoietic markers (CD14, CD34, or CD45), were not analyzed in this study, because our animal and other clinical studies using the same method of cell cultivation demonstrated cultured cells as MSCs by the characteristics of the surface antigens.^11–17

For delivery of MSCs, we used an intramyocardial injection method involving a Myostar mapping-injection catheter (Biologics Delivery System, Cordis Corp., Miami Lakes, FL, USA). We made a transendocardial injection from the tip of the catheter directly into the myocardium using a retrograde approach with femoral artery puncture or the Brockenbrough method with femoral vein puncture. We determined the sites for injection with the NOGA 3D electromagnetic cardiac mapping system (Biologics Delivery System, Cordis Corp.), which can identify electrically active “border zone” areas with low contractility.¹⁸

Assessment of Short-Term Outcomes

The study protocol is outlined in Supplementary Figure. To assess cardiac effects, we evaluated the LVEF using myocardial scintigraphy with technetium-99 m-methoxy-isobutyl isonitrile or left ventriculography before and 2 months after the procedure. We also measured serum B-type natriuretic peptide (BNP) levels, hemodynamics (e.g., pulmonary capillary wedge pressure (PCWP) and cardiac index (CI)), peak oxygen uptake (peak V̇O₂) on cardiopulmonary exercise testing, and myocardial sympathetic activity on ¹²³I-metaiodobenzylguanidine (MIBG) scintigraphy.

We also evaluated the perioperative and short-term safety of cell transplantation, and compared the changes in BNP, LVEF, the percentage of premature ventricular contractions (PVC) on 24-hour Holter ECG, and peak V̇O₂ between the low cell number group and high cell number group of transplanted MSCs. Procedure-related complications were defined as perforation of the myocardium and induction of arrhythmia that required treatment during the procedure.

Assessment of 1-Year and Long-Term Safety

At 1 year after the procedure, computed tomography (CT) was performed to evaluate for malignant formation, and 24-hour Holter ECG was used to assess for ventricular arrhythmias. We continued to follow patients with BNP and echocardiography measurements in the outpatient clinic for 10 years. Moreover, we also compared changes in BNP, LVEF, and the New York Heart Association (NYHA) functional class between the low cell number and high cell number groups.

Statistical Analysis

Continuous variables are presented as median and interquartile range (IQR) and compared by a Mann-Whitney U-test. Categorical variables are reported as numbers and percentages, and were compared by Fisher’s exact test. To test for statistically significant differences in parameters before and after the procedure, the Wilcoxon signed-rank test was used. For all tests, P<0.05 was considered statistically significant. Data were analyzed with JMP8 software (SAS Institute, Cary, NC, USA) and R software version 3.0.2 (https://www.r-project.org/).

Results

Patient Enrollment

In total, 8 patients with a median age of 58 years (IQR, 43–60) were enrolled and treated with MSC transplantation. Baseline characteristics are summarized in Table 1 and individual profiles were presented in Table 2. The underlying cause of HF was ischemic cardiomyopathy for 3 patients and nonischemic dilated cardiomyopathy for 5 patients. The median LVEF was 24% (IQR, 19–29).

Table 1. Baseline Characteristics of the Study Patients

No. of patients	8
Age (years)	58 (43–60)
Male sex	6 (75.0)
Heart failure etiology
DCM	5 (62.5)
ICM	3 (37.5)
Cardiac function
LVEF (%)	24 (19–29)
LVEDV (mL)	246 (219–250)
Symptoms and exercise capacity
NYHA functional status
Class II	6 (75.0)
Class III	2 (25.0)
Peak V̇O2 (mL/kg/min)	16.5 (13.7–19.8)
Comorbidities
HT	3 (37.5)
DL	6 (75.0)
DM	0 (0.0)
CKD	2 (25.0)
CI	1 (12.5)
AF	5 (62.5)
Laboratory data
BNP (pg/mL)	211 (124–409)
Creatinine (mg/dL)	0.79 (0.75–1.01)
eGFR (mL/min/1.73 m2)	70 (60–87)
Hemoglobin (g/dL)	14.3 (13.4–14.8)
Sodium (mEq/L)	140 (139–142)
Medications
ACEI or ARB	7 (87.5)
β-blocker	8 (100)
Aldosterone receptor antagonist	2 (25.0)
Device therapy
ICD	3 (37.5)
CRT	1 (12.5)

Numeric values were expressed as n (%) or median (IQR). ACEI, angiotensin-converting enzyme inhibitor; AF, atrial fibrillation; ARB, angiotensin II receptor blocker; BNP, B-type natriuretic peptide; CI, prior cerebral infarction; CKD, chronic kidney disease; CRT, cardiac resynchronization therapy; DCM, dilated cardiomyopathy; DL, dyslipidemia; DM, diabetes mellitus; eGFR, estimated glomerular filtration rate; HT, hypertension; ICD, implantable cardioverter defibrillator; ICM, ischemic cardiomyopathy; IQR, interquartile range; LVEDV, left ventricular end-diastolic volume; LVEF, left ventricular ejection fraction; NYHA, New York Heart Association; V̇O₂, oxygen uptake.

Table 2. Profiles of Study Patients

Patient no.	Age (years)	Sex	Etiology	Comorbidities						Medical treatment			Device therapy
				HT	DM	DL	CKD	CI	AF	ACEI or ARB	β-blocker	Aldosterone antagonist	ICD	CRT
1	60	Female	DCM	+	−	+	−	−	−	+	+	−	−	−
2	56	Male	ICM	−	−	+	−	−	−	+	+	−	−	−
3	36	Male	DCM	−	−	−	−	+	+	+	+	+	+	−
4	70	Male	ICM	−	−	+	−	−	+	+	+	−	−	−
5	31	Male	DCM	−	−	+	−	−	+	+	+	+	−	−
6	45	Male	ICM	+	−	+	+	−	−	+	+	+	+	−
7	61	Male	DCM	+	−	+	−	−	+	−	+	+	+	−
8	60	Female	DCM	−	−	−	+	−	+	+	+	+	−	+

Abbreviations as in Table 1.

Short-Term Outcomes

The number of MSCs injected ranged from 1.2×10⁷ to 6.5×10⁷ cells. There were no complications associated with the injection procedure. Table 3 shows the injected cell number in each patient and the short-term outcomes compared with baseline data. At 2 months after MSC implantation, changes in median LVEF (from 24% to 26%), PCWP (from 12 to 12 mmHg), CI (from 2.4 to 2.5 L/min/m²), and BNP (from 211 to 173pg/mL) were not statistically significant. In addition, median peak V̇O₂ (from 16.5 to 19.2 mL/min/kg) and the heart-to-mediastinum ratio for ¹²³I-MIBG uptake (from 2.0 to 2.0) did not change.

Table 3. Numbers of Mesenchymal Stem Cells Administered and Clinical Data Before and 2 Months After Cell Transplantation

Patient no.	No. of MSCs administered (x106)	Laboratory BNP (pg/mL)		LVEF (%)		Cardiac function LVEDV (mL)		CI (mL/min/m2)		Holter ECG %PVC (%)		CPX peak V̇O2 (mL/min/kg)		MIBG Late HMR
		Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post
1	65	11	18	40	44	184	173	3.6	2.8	2.5	0.7	19.9	19.6	2.0	2.2
2	12	197	117	25	25	245	292	2.3	2.5	NA	0.04	19.7	19.2	2.1	2.0
3	54	390	371	19	21	181	182	2.3	2.2	NA	11.6	8.8	13.2	2.1	1.8
4	12	224	229	23	26	254	258	2.6	2.7	0.3	0.5	14.7	16.0	1.5	1.7
5	40	145	68	39	31	230	164	2.4	2.3	1.0	1.6	27.6	27.3	1.7	2.6
6	58	62	41	19	24	264	303	2.0	2.9	12.4	14.8	16.1	19.6	2.1	2.6
7	NA	465	485	15	25	249	308	2.1	2.5	3.2	1.6	16.8	16.9	1.7	1.9
8	23	475	677	26	26	246	226	2.5	2.4	2.4	2.0	10.8	NA	NA	NA
Median	40	211	173	24	26	246	242	2.4	2.5	1.6	2.5	16.5	19.2	2.0	2.0
(IQR)	(17.5–56)	(124–409)	(61–400)	(19–29)	(25–27)	(219–250)	(180–295)	(2.3–2.5)	(2.4–2.7)	(0.7–4.4)	(1.4–3.0)	(13.7–19.8)	(16.5–19.6)	(1.7–2.1)	(1.9–2.4)
P value		0.74		0.30		0.74		0.77		1.00		0.41		0.35

CPX, cardiopulmonary exercise testing; ECG, electrocardiogram; HMR, heart-to-mediastinum ratio; MIBG, metaiodobenzylguanidine; MSC, mesenchymal stem cell; NA, not assessed; PVC, ventricular premature contraction. Other abbreviations as in Table 1.

1-Year and Long-Term Outcomes

Table 4 shows the outcomes at 1 year. CT imaging showed no further abnormal findings. Regarding ventricular arrhythmias, 1 patient (No. 6) had a 21.8% increase in the occurrence of PVCs. An additional electrophysiological study revealed that the origin of the arrhythmia was remote from the site of cell implantation, so we concluded that the increase in PVCs in this patient was not caused by intracardiac cell transplantation.

Table 4. CT Imaging and 24-Hour Holter Results 1 Year After Mesenchymal Stem Cell Transplantation

Patient	CT imaging		Arrhythmia
	New tumor formation or new calcification	Ventricular arrhythmia requiring treatment	%PVCs in follow-up 24-hour Holter test (%)	Difference from preprocedural testing (%)
1	None observed	None	1.3	−1.2
2	None observed	None	0.3	NA
3	None observed	None	12.8	NA
4	None observed	None	2.0	+1.7
5	None observed	None	0.1	−0.9
6	None observed	None	34.2	+21.8
7	None observed	None	1.8	−1.4
8	None observed	None	NA	NA

CT, computed tomography; PVC, ventricular premature contraction.

Figure 1 shows the cardiac and fatal events in each patient during the follow-up period up to 10 years (median 8.7 years; IQR, 2.4–10 years), and the Kaplan-Meier curves on major adverse cardiac events (cardiac death and heart transplantation), all cardiac events (cardiac death, heart transplantation, and HF hospitalization), and event-free survival. Although no patient had serious adverse events such as fatal arrhythmia, 4 patients required hospitalization for exacerbation of HF. Of these, 1 patient underwent heart transplantation in the 2nd year after cell transplantation, and another patient died of HF in the 6th year. Of the remaining 4 patients without exacerbation of HF after cell transplantation, 2 died of noncardiac diseases, which were pneumonia in the 1st year and liver cancer in the 6th year, respectively. The other 2 patients remained alive without any cardiac events.

Figure 1.

Cardiac and fatal events at 1 year (Upper left panel) during long-term follow-up (Upper right panel) and Kaplan-Meier curves of major adverse cardiac events (MACE), MACE and heart failure hospitalization, and overall events (Lower panels). Arrows indicate the survival period of each patient. *Cardiac events, all of which were exacerbations of heart failure requiring hospital admission. BNP, B-type natriuretic peptide; CPX, cardiopulmonary exercise testing; CT, computed tomography; Holter, 24-hour Holter recording; LVG, left ventriculography; MSC Tx, mesenchymal stem cell transplantation; RHC, right heart catheterization; RI, radioisotope test; UCG, ultrasound cardiography.

In the 6 patients who remained alive and did not undergo heart transplantation, detailed data about BNP and echocardiography parameters before and 5 years after cell therapy are shown in Figure 2A–D. Figure 2E shows the changes in the New York Heart Association functional class during the 10-year follow-up period. From baseline to 5 years, BNP (from 210 to 152 pg/mL), LV end-systolic dimension (from 54 to 61 mm), LV end-diastolic dimension (from 66 to 67 mm), and LVEF (from 33 to 23%) did not change significantly.

Figure 2.

Changes in B-type natriuretic peptide (BNP), echocardiographic measures, and New York Heart Association (NYHA) functional class. (A) B-type natriuretic peptide. (B) Left ventricular internal dimension at end-diastole (LVDd). (C) left ventricular internal dimension at end-systole (LVDs). (D) Left ventricular ejection fraction (LVEF). (E) NYHA functional class. Dotted lines indicate changes for each patient. Continuous lines indicate changes in median values.

Comparison of the Results for the Transplanted Cell Number Groups

Table 5 presents the results for the low cell number (median; 12 million cells) and high cell number (median; 56 million cells) groups. In the comparison of changes in BNP, LVEF, the percentage of PVC on 24-hour Holter ECG, and peak V̇O₂ before and 2 months after cell transplantation, no significant differences were found between the groups. In the comparison of changes in BNP, LVEF, and NYHA class before and 5 years after transplantation, there were also no significant differences.

Table 5. Comparison of Short-Term and Long-Term Effects of Cell Therapy Between 2 Groups of Transplanted Cell Numbers

	Transplanted cell number		P value
	Low group	High group
Patient no.	2, 4, 8	1, 3, 5, 6
Cell number (×106)	12 (12, 18)	56 (51, 60)	0.04
Age (years)	60 (58, 65)	41 (35, 49)	0.15
Male sex	2	3	1.00
Etiology			0.49
DCM	1	3
ICM	2	1
Change between baseline and 2 months after cell transplantation
BNP (pg/mL)	+5 (−38, +104)	−20 (−35, −13)	0.86
LVEF (%)	0 (0, +1.5)	+3 (−0.5, +4.3)	0.59
%PVC (%)	−0.1 (−0.2, +0.4)	−0.6 (−0.6, +1.5)	0.80
Peak V̇O2 (mL/min/kg)	+0.4 (−0.1, +0.9)	+1.6 (−0.3, +3.7)	0.53
Change between baseline and 5 years after cell transplantation
BNP (pg/mL)*	+152 (+116, +189)	−35 (−41, +46)	0.40
LVEF (%)*	−13 (−16, −11)	+2.0 (+1.5, +5.5)	0.20
NYHA class			0.49
Improved	0	0
Unchanged	1	3
Worsen or Dead	2	1

Numeric values are median and IQR (25th percentile, 75th percentile). *Comparison excluding patient no. 3 (heart transplantation) and patient no. 8 (died). Abbreviations as in Table 1.

Discussion

We present the results for the periprocedural, short-term (1 year), and long-term follow-up periods (up to 10 years) of MSC transplantation in patients with symptomatic cardiomyopathy. We found no unnatural development of cancer or an obvious increase in ventricular arrhythmic events during the follow-up after cell therapy.

MSC Implantation and Duration of Follow-up in Previous Studies

MSCs have high self-proliferation and multidifferentiation capacities.^9,10 Focusing on the ability of MSCs to differentiate into cardiomyocytes and vascular endothelial cells, we conducted experimental studies in a rat myocardial infarction model and confirmed that intracardiac MSC transplantation into impaired myocardium led to an improvement in cardiac function with actual differentiation into cardiomyocytes and vascular endothelial cells.^15–17 Consequently, MSCs have been considered as a potential source for regenerative cardiology. On the other hand, because it has been pointed out that MSC transplantation could possibly cause malignant tumor formation,^7,19,20 a validation study of safety after cell therapy with MSCs was required.

Table 6 summarizes the previously reported clinical trials of MSCs administration in patients with systolic HF. Those studies have reported that cell therapy using MSCs is safe and feasible up to 6–36 months after transplantation;^21–34 however, there were few reports of follow-up in patients over a much longer period. The present report on the long-term result with regard to tumor formation and arrhythmogenicity after cell transplantation in the heart therefore, provides valuable information.

Table 6. Clinical Studies of Bone Marrow-Derived Mesenchymal Stem Cell Transplantation in Patients With Systolic Heart Failure

Study (year published)	Patient			Procedure				Follow-up and results
	Disease	LVEF (%, mean)	NYHA, n	n, MSC type	Control group	Delivery route	Cell number (×106)	Follow-up time	Ectopic tissue formation	Ventricular arrhythmia (vs. control)	Improvement of LVEF (vs. control)
POSEIDON (2012)21	ICM	29.0 (autologous) 27.1 (allogenic)	I, 4 II, 18 III, 8	15, autologous 15, allogenic	No	IM	20, 100, or 200*	13 months	None	No control (SAEs: none (0%) in allogenic group; 4 patients (27%) in autologous group)	No control (1.96% increase in overall)
C-CURE (2013)22	ICM	27.5	II or III**	21, autologous	Yes	IM	733 (mean)	24 months	None	VT/VF: 5 patients (24%) in CTx group; 8 patients (33%) in control group	Significantly improved (7.0% increase in CTx group vs. 0.2% increase in control group)
Ascheim et al (2014)23	ICM DCM	17.5	III, 3 IV, 17	20, allogenic	Yes	IM with LVAD	25*	12 months	NA	Cardiac arrest and ventricular arrhythmia: 3 patients (15%) in CTx group; 2 patients (20%) in control group	NA
PROMETHEUS (2014)24	ICM	41.1	I, 3 II, 3	6, autologous	No	IM with CABG	20 or 200*	18 months	NA	No control (NA)	No control (9.4% increase)
TAC-HFT (2014)25	ICM	35.7	I, 5 II, 12 III, 2	19, autologous	Yes	IM	200*	12 months	None	NA	Not significantly changed between groups
Perin et al (2015)26	ICM DCM	31.3	II, 31 III, 14	45, allogenic	Yes	IM	25,75, or 150*	36 months	NA	Death by cardiac arrhythmia: 2 patients (4%) in CTx group; 3 patients (20%) in control group	Not significantly changed between groups
MSC-HF (2015)27	ICM	28.2	II, 11 III, 29	40, autologous	Yes	IM	77.5 (mean)	6 months	None	VT/VF: 2 patients (5%) in CTx group; 1 patients (5%) in control group	Significantly improved (5.0% increase in CTx group vs. 1.3% decrease in control group)
MESAMI-1 (2016)28	ICM	29.4	II, 4 III, 6	10, autologous	No	IM	61.5 (mean)	24 months	None	No control (no SAEs by ventricular arrhythmia)	No control (6.3% increase)
IxCELL-DCM (2016)29	ICM	24.4	II, 2 III, 52 IV, 4	58, autologous	Yes	IM	NA	12 months	NA	NA	Not significantly changed between groups
TRIDENT (2017)30	ICM	36.1 (median)	I, 10 II, 15 III, 4 IV, 1	30, allogenic	No	IM	20 or 100*	12 months	None	No control (none experienced sustained VT)	No control (3.7% increase in high number group; 0.27% decrease in low number group)
Xiao et al (2017)31	DCM	34.1	II, III, or IV**	17, autologous	Yes	IC	490 (mean)	12 months	NA	VT: none in both CTx group and control group	Significantly improved (5.9% increase in CTx group vs. 0.6% increase in control group)
Butler et al (2017)32	DCM	31.6 (in overall)	II or III**	10, allogenic	Yes	IV	1.5/kg*	15 months	NA	VT: 1 patient (8%) in control group	Not significantly change between groups (2.3% increase in CTx group vs. 1.6% increase in control group)
CHART-1 (2017)33	ICM	27 (median)	II, 23 III, 96 IV, 1	120, autologous	Yes	IM	NA	39 weeks	NA	VT: 1 patients (0.8%) in CTx group; 4 patients (2.5%) in control group	No significant change between groups
POSEIDON-DCM (2017)34	DCM	26.5	I, 10 II, 17 III, 7	16, autologous 18, allogenic	No	IM	100*	12 months	None	No control (NA)	No control (8.0% increase in allogenic group; 5.4% increase in autologous group)
Present study	ICM DCM	24 (median)	II, 6 III, 2	8, autologous	No	IM	37.7 (mean)	10 years	None (1 patient died of liver cancer at 6 years after CTx)	No control (1 patient had 21.8% increase of PVCs on 1-year follow-up Holter ECG)	No control (increase in 3 patients and decrease in 3 patients at 5 years after CTx)

*Protocol cell number, **detailed data not available. CABG, coronary artery bypass grafting; CTx, cell therapy; IC, intracoronary injection; IM, intramyocardial injection; IV, intravenous injection; LVAD, left ventricular assist device; SAE, serious adverse event; VF, ventricular fibrillation; VT, ventricular tachycardia. Other abbreviations as in Tables 1,3.

Safety Regarding Tumor Formation

Tumor formation has been one of the most serious concerns about cell therapy regardless of cell type, target organ or method of transplantation. For cell transplantation involving the myocardium, ectopic formation (e.g., bone) that could possibly cause fatal arrhythmias or hemodynamic dysfunction is also a major concern.⁵ One study of transplantation of cultured MSCs into the myocardium in a mouse myocardial infarction model showed that cell transplantation could possibly generate malignant sarcoma at the transplant site.⁷ Several reports of experimental cell transplantation also showed the possibility of malignant tumor formation and ectopic tissue formation.^19,20 Under short-term observation (<3 years), previous clinical trials of MSC transplantation into the myocardium reported no findings of malignancy formation.^{21,22,25,27,28,30,34} In this study, we found no development of malignant tumors in the heart. Except for 1 case (Patient 7) of liver cancer developing in the 6th year, which was thought to be caused by heavy alcohol consumption and unlikely to be associated with MSC transplantation, there was also no development of malignancy in other organs through the median 8.7-year follow-up. In the clinical setting where patients are screened carefully, MSC transplantation into the myocardium would be a safe treatment in the long term.

Safety Regarding Serious Arrhythmias

The potential risk of arrhythmias after cell therapy has been another major concern.^6,8,35 Potential mechanisms of ventricular arrhythmia such as the presence of immature cardiomyocytes with spontaneous automaticity, poor cell–cell coupling to leads to a decrease in conduction velocity, and increased cardiac nerve sprouting have been suggested.⁸ A relatively high risk of ventricular arrhythmias was reported in a systematic review of cell therapy, especially with skeletal myoblasts.⁸ Although the arrhythmic risk with MSC transplantation is likely to be relatively low in the short term according to previous reports,^21–34 safety in the long term has not been adequately described. In the present study, 1 patient (No. 6) had more PVCs on 24-hour Holter ECG 1 year after MSC implantation compared with baseline. In that patient, an additional electrophysiological study revealed that the origin of the arrhythmia was far from the site of cell implantation; thus, the possibility of MSC-related arrhythmia seems unlikely. Including that case, there were no ventricular arrhythmic events that required treatment or were life-threatening in either the periprocedural period or the long term.

Efficacy to LV Function

This study did not show significant improvement in ventricular function after cell transplantation. In the comparison between 2 groups of transplanted cell numbers, we also did not find a significant difference in effect on cardiac function. However, it would be difficult to evaluate the efficacy of cell transplantation or the effect of transplanted cell number on cardiac function in the small sample size. A meta-analysis of cell therapies involving MSCs and other types of cells showed significant improvements in ventricular systolic function, though it corresponded to only 3% of LVEF. Moreover, patients’ clinical courses have also improved.³⁶ Furthermore, a former study of cell therapy for ischemic heart disease showed an association between transplanted cell number and reduction in infarct size.³⁷ Functional and clinical improvements are thought to be related to paracrine effects of cardioprotective cytokines from transplanted cells rather than differentiation to cardiomyocytes and vascular endothelial cells.³⁸ Most of the previous reports,^{25,26,29,32,33} similar to our results, did not show the degree of improvement in LV function expected, based on results of animal experiments. This discrepancy is considered to be caused by problems with engraftment, survival, differentiation efficiency, and the number of transplanted cells.^39,40 In recent years, some cell therapies with cardiac-specific stem cells have shown a more favorable improvement in LV systolic function.^41,42 Moreover, novel clinical applications, such as embryonic stem cells or induced pluripotent stem cells, have been attracting much attention.^43,44 Further advances in the methods of obtain larger numbers of cells and improving cell engraftment and survival of transplanted cells could contribute to improved treatment results.

Study Limitations

First, there was a small number of patients and no control group. Second, the efficacy of transplanted cell differentiation and survival are very important factors, but we were not able to examine them in this study because there was no pathological assessment. Third, we did not perform CT scan and Holter ECG at 10 years, which would have enabled precise assessment of long-term safety, and we also did not perform cardiac magnetic resonance imaging, which could provide detailed information about changes in cardiac function. Because of these limitations, we could not sufficiently clarify the safety and efficacy of MSCs transplantation to patients with systolic HF based on the results of the present study; however, our study has great value because it is the first to have followed patients who received MSC administration into the myocardium for such a long time (up to 10 years). Although in recent years, cell therapies using new types of cell have advanced, the concerns in the long term after cell transplantation have not been fully resolved.⁴⁵ Therefore, larger and more detailed follow-up studies are needed to demonstrate the safety and efficacy of MSC transplantation.

Conclusions

The results of this exploratory study of intracardiac MSCs administration as treatment for symptomatic HF up to 10 years warrant further research into the feasibility and efficacy of the procedure.

Source of Funding

This work was supported by a research grant for regenerative medicine (grant no. H17-009) from the Ministry of Health, Labor, and Welfare, Japan.

Conflict of Interest

The authors report no relationships that could be construed as a conflict of interest.

Supplementary Files

Please find supplementary file(s);

http://dx.doi.org/10.1253/circj.CJ-18-1179

References

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