STEM CELL THERAPIES FOR CARDIOVASCULAR
DISEASE
Myocardial infarction and congestive heart failure are the leading causes of morbidity and mortality worldwide, despite great therapeutic achievements in the treatment of cardiovascular diseases. The inability of the heart to regenerate lost cardiac muscle, coupled with a robust fibrotic repair response, contribute to adverse ventricular remodeling and decline in postinjury cardiac function. Consequently, much of the research of the last three decades has focused on reducing the atherosclerotic burden of ischemic heart disease, reperfusion, and addressing the fibrotic changes associated with heart failure. After an ischemic insult and the formation of necrotic myocardium, the process of scar formation from the recruitment of activated cardiac fibroblasts leads to reduced cardiac pump function. However, in recent years, it has been convincingly shown that the heart has the ability to regenerate cardiomyocytes, albeit at a low rate (~0.3%–1% annually). These findings and our better understanding of stem cell biology are paving the way to a new area of research, with the main goal of regenerating cardiac tissue.
Two defining features of stem cells are their ability to self-renew and to differentiate to cells of a
specific lineage under appropriate conditions. Recent observations have shed
light on the existence of cardiac stem cells and extracardiac stem cells that
are capable of leading to cardiomyocytes, smooth muscle cells, and endothelial cells.
Such therapies are still experimental but hold great promise in potentially
ushering in novel regenerative treatment strategies for heart disease.
Types of Stem Cells and Mechanisms of Benefit
Stem cells can be classified according to their level of potency. A
totipotent stem cell, such as a fertilized zygote, can lead to an entire organ-
ism. A pluripotent cell, such as an embryonic stem (ES) cell, leads to cells
from all three germ layers but is unable to generate an organism. Multipotent
stem cells, such as mesenchymal stem cells (MSCs), can lead to different types
of cells from the same cell germ layer, such as adipocytes, bone, or cartilage
cells. Skeletal muscle myoblasts, endothelial progenitor cells, and bone marrow
mononuclear cells (BN-MNCs) are other examples of multipotent stem cells that
have been studied in cardiac regeneration.
The signaling pathways that drive stem cells into cardiomyocyte fate are
areas of intense research; better understanding of these pathways can be used
as valuable therapeutic tools in enhancing cardiac regeneration.
Differentiation is the process by which stem cells can become cardiomyocytes,
whereas transdifferentiation is a process in which a somatic cell adopts
alternative cell fates (e.g., a fibroblast adopting an endothelial cell fate).
Another process by which stem cells can alter cardiac function is fusion. When
stem cells fuse with somatic cells (e.g., cardiomyocytes), the resulting cells
have characteristics of both cell types; however, the extent to which any
clinical benefit can be achieved from this process is currently unclear.
Another mechanism by which stem cells
can enhance tissue regeneration is through a paracrine mechanism. After
injection, stem cells are believed to release cytokines and/ or growth factors
that have physiological effects on other cells in the injured environment and
affect repair. For instance, injection of MSCs into the injured heart affects
the balance of Wnt signaling in the injured environment to modulate angiogenesis
and fibrosis. Identification of key stem cell secreted molecules mediating
pro-reparative effects on the injured heart may lead to development of new
therapeutic strategies in which injection of key stem cell secreted molecules,
rather than stem cells, may be sufficient to augment cardiac healing and
obviate issues such as immune response, dose titration, and availability.
ES cells have the ability to give rise to any cell type of the organism,
and under the appropriate conditions, can differentiate into cardiomyocytes.
They originate from the inner cell mass of the blastocyst during development.
Studies have shown that injection of ES cells leads to successful engraftment
of them into the surrounding cardiac tissue, making this approach appealing.
However, large-scale generation of ES cell–derived cardiomyocytes currently
remains unrealistic, because this is a field still in its infancy and filled
with several ethical and political challenges. Differentiation of bone marrow
stem cells into functioning cardiomyocytes has been more challenging.
Bone marrow contains different types of progenitor stem cells, among
which BM-MNCs and MSCs have been extensively studied. Injection of BM-MNCs into
the diseased cardiac muscle leads to improvement of cardiac function. Although
initial studies suggested that BM-MNCs transdifferentiated into myocytes, later
studies suggested an indirect effect likely related to the paracrine effects of
cells. A paracrine effect contributing to salutary effects on cardiac repair
was confirmed for MSC injection after cardiac injury. In vitro studies
suggested that these cells could also differentiate into beating
cardiomyocytes, and these findings generated a lot of excitement that led to
several preclinical studies. These studies suggested that injection of MSCs
into the injured myocardium resulted in improved cardiac function despite the
low number of mesenchymal-derived cardiomyocyte cells, which suggested a
multifactorial effect similar to BM-MNCs.
The limited supply of ES cells and associated social challenges have led
to other pathways to develop pluripotent stem cells. A type of cell that has
attracted a lot of interest recently is the resident cardiac pro- genitor cell.
The existence of these cells and their ability to differentiate into
cardiomyocytes, as well as endothelial and smooth muscle cells, has shaken the
long-standing belief that the heart is a fixed organ unable of regeneration.
Several challenges remain to fully derive the potential benefits of these
cells. First, different cell markers have been used to characterize these
cells, and it is presently unclear if there are any biological differences
between cells with different cell markers. Second, the number of these cells is
small, and their role in normal cardiac function is not clear. However,
injection of these cells into an infarcted heart leads to improvement of
cardiac function. What makes these cells particularly
attractive is their ability to differentiate into other cell types (e.g., endothelial and smooth muscle
cells) because the regenerating cardiomyocytes will need new blood vessels and
supporting cells to properly function. Also, use of these cells appears to
avoid some of the ethical challenges that can arise with the ES cells. Their
small number and the technical difficulties associated with successfully
multiplying them have led to the development of other types and techniques,
among which induced pluripotent stem cells merit special mention. These are ES
cells derived from skin fibroblasts through genetic manipulation (through the
overexpression of certain transcription factors). What makes this approach
unique and revolutionary is the ability of skin fibroblasts to be reprogrammed
into induced pluripotent stem cells that could be injected into an injured
heart. Although this field and its technology are in their infancy, it holds
great promise for future cardiovascular regeneration therapy.
Another type of stem cell that deserves special mention is the
endothelial progenitor cell, especially for vascular regeneration. Injury of
the vascular endothelium triggers a cascade of events that aims to reconstitute
the endothelium via the proliferation of the remaining endothelial cells and
differentiation of the endothelial progenitor cells. It is based on these premises
that administration of endothelial progenitor cells might indirectly improve
cardiac function by enhancing vasculature repair and angiogenesis.
Cardiospheres have also been tested in heart regeneration after
myocardial infarction. Cardiospheres are three-dimensional multicellular
structures from cardiac explant cultures in nonadhesive surfaces. It is
currently believed that injected cardiospheres modulate scar formation via
paracrine effects or secreted exosomes containing active molecules such as microRNA.
The phase I clinical trial testing cardiospheres in humans did not show any
left ventricular ejection fraction improvement; however, this trial did show a
reduction of scar mass at 6 months and an increase in viable
myocardium.
During this first wave of excitement, skeletal myoblasts have also been
studied, with promising results in early tests, but with no significant benefit
in clinical trials; therefore the use of these cells in cardiovascular
regeneration is uncertain (Table 6.1).
Approaches to
Stem Cell Administration, Technical Challenges, and Safety Concerns
The main two approaches of stem cell administration have been catheter-
based intracoronary injection and surgical-based epicardial injection; each delivery
method presents unique advantages and disadvantages. One factor to keep in mind
when selecting the appropriate approach is to have an optimal environment for
successful stem cell engraftment.
For example, an intracoronary injection approach after revascularization
is disadvantageous because the myocardial areas that need the most number of cells might have reduced
blood flow and thus would receive less stem cells. Another factor to consider
when selecting the optimal delivery method is the type of cells delivered. For
example, certain stem cells (e.g., skeletal myoblasts) are larger; therefore
they could lead to microvasculature obstruction and subsequent decreased blood
flow. The epicardial injection delivery approach solves the problem of reduced delivery
in poorly perfused areas, but cardiac perforation is a real risk, especially in
the setting of inflamed and necrotic myocardium tissue. The inflamed cardiac
microenvironment is also not optimal for engraftment of injected cells, and the
first 4 days after infarction are associated with the least amount of cell
engraftment. Focal delivery of stem cells also may not be sufficient in cases
of global myocardial dysfunction, such as in nonischemic-dilated
cardiomyopathy. Thus it is likely that the decision on what route of delivery
to choose will depend on the cardiac condition that needs to be treated and the
type of cells to be delivered.
From a safety standpoint, stem cells have been shown to be safe, but
long-term data are lacking. Beyond the technical challenges and potential
procedure-related complications, there are several specific concerns that need
to be taken into consideration when discussing stem cell–based therapy. First,
ventricular arrhythmias are a real concern with the newly differentiated and
engrafted cells. This specific adverse effect has been observed with skeletal
myoblasts, which are believed to be secondary to a lack of electromagnetic
coupling with native cardiomyocytes. The second concern is noncardiac
engraftment; if the route of administration is through the systemic
circulation, then stem cells can also populate other organs; this has been seen
in patients who have undergone stem cell injection. Localization of stem cells
to other organs can have unpredictable effects, especially tumor angiogenesis
or enhancement of malignancy.
Clinical Evidence of Stem Cell Therapy
There have been many trials involving thousands of patients and different
types of stem cells injected into patients with different cardiac pathologies
(for a more comprehensive list of selected trials, see Table
6.2). BM-MNCs have been evaluated in acute myocardial
infarction and chronic myocardial ischemia. In post–acute myocardial infarction
patients, the benefits of bone marrow cells were studied in randomized-
controlled trials such as the BOne marrOw transfer to enhance ST- elevation
infarct regeneration (BOOST) and Reinfusion of Enriched Progenitor cells And
Infarct Remodeling in Acute Myocardial Infarction (REPAIR-AMI) trials. Both studies showed improvements in left ventricular ejection fraction, and 1-year follow-up in REPAIR-AMI patients also showed a reduction in major
adverse cardiovascular events. Patients with more severely impaired left
ventricular function appeared to benefit the most according to subgroup
analysis of these studies. Bone marrow stem cell therapy for chronic myocardial
ischemia was also evaluated in several studies, including the Prospective
Randomized Trial of Direct Endomyocardial Implantation of Bone Marrow Cells for
Therapeutic Angiogenesis in Coronary Artery Disease (PROTECT- CAD) trial, as
well as in a randomized-controlled, double-blind trial with improvement in the
quality of life and cardiac perfusion. Although these findings are promising,
for most of the studies, the left ventricular ejection fraction might not be
the most accurate endpoint, and larger phase III clinical trials with
significant primary endpoints (such as mortality, recurrent myocardial
infarction, or stroke) will reveal the true impact of bone marrow stem cell
therapies and their long-term safety.
MSCs also have been tested for stem cell therapy, but clinical data are
scarce. The initial phase I clinical data used intravenous injection of these
cells, which established their safety but did not show any cardiovascular
benefit. However, the passage of these cells through the pulmonary circulation
might have hampered any potential benefit of these cells in the coronary
circulation and cardiac seeding. Subsequently, early nonrandomized studies
showed that intracoronary injection of MSCs resulted in improvement of the left
ventricular ejection fraction and symptoms. Another source of mesenchymal cells
are adipose tissue derived stem
cells. The AdiPOse-derived stem ceLLs in the treatment of patients with
ST-elevation myOcardial infarction (APOLLO trial assessed the safety of adipose
tissue–derived cells and showed a significant reduction in infarct size and
improved perfusion at 6 months in patients with acute myocardial infarction.
The AdiPose-deRived stEm Cells In the treatment of patients with nonrevaScularizable
ischEmic myocardium (PRECISE) trial evaluated the safety and benefits of these
cells in chronic myocardial ischemia, which suggested an improvement in the
functional status in patients treated with adipose tissue–derived stem cells.
After skeletal muscle injury, resident stem cells (satellite cells) contribute to skeletal muscle regeneration. Hence, there was an initial interest
in the usefulness of skeletal myoblasts in cardiovascular regeneration. Although the early nonrandomized small studies suggested a
clinical benefit, the most comprehensive trial was the randomized,
double-blind, phase II Medical Research Council Adjuvant Gastric Infusional
Chemotherapy (MAGIC) trial, which showed no benefit at 6 months. These cells
were injected around the scar tissue in patients who underwent bypass surgery.
The potential proarrhythmic and microembolization effects of myoblasts,
although not fully understood, limited the excitement for these cells as
potential source of stem cells in cardiovascular regeneration.
The recent findings that human cardiomyocyte renewal occurs naturally at
a rate of 1% in the young and 0.3% in older adults led to the quest of
identifying and characterizing these resident cardiac stem cells. The first
challenge was to fully evaluate these cells; several subtypes of cells were
described, based on the cell markers used to identify them. It is still not
clear if these are cells of different subpopulations with distinct phenotypes
or if they are part of the same subpopulation. The second challenge was to
expand them, because their number is low in the heart. Despite these
challenges, early-stage clinical studies, such as the Stem Cell Infusion in
Patients with Ischemic cardiOmyopathy (SCIPIO) and CArdiosphere-Derived
aUtologous stem CElls to reverse ventricUlar dySfunction (CADUCEUS) trials,
established their safety and feasibility, as well as showed a reduction in
infarct size. Further ongoing studies are needed to further elucidate any
clinical benefit of these cells.
FUTURE DIRECTIONS OF STEM CELL THERAPY
After an exciting decade full of both reassuring and disappointing
results, the future of stem cell therapy in cardiovascular disease is expected
to grow even more. With the discovery of inducible pluripotent cells and their
ability to differentiate into functional cardiomyocytes, a new field has been
born in stem cell therapy. However, before applying these cells in clinical
trials, certain crucial hurdles have to be resolved, especially safety concerns
regarding their tumorigenicity (which has been observed in animal models).
Furthermore, technology for upsizing to an effective number of induced
pluripotent stem cells is needed.
In addition to the previously described stem cell approaches, the concept
of pretreating the stem cells to enhance their therapeutic activity before
injection has been tested in several trials. The idea is to improve any
function that influences migration, engraftment, survival, differentiation, and
other functions that can improve efficiency. The Enhanced Angiogenic Cell
Therapy in Acute Myocardial Infarction (ENACT-AMI) trial tested the usefulness
of overexpression endothelial nitric oxide synthase in endothelial progenitor
cells, because it had previously been shown that a reduction of endothelial
nitric oxide synthase limits the repair capability of endothelial progenitor
cells in patients with hypertension and diabetes. Another trial is the
Mesenchymal Stem Cells and Myocardial Ischemia (MESAMI) II trial, which is
designed to pretreat MSCs with melatonin before injection. This approach is
based on data that suggest that the melatonin hormone improves survival and the
paracrine effects of MSCs. It is expected that many more trials will test
different pretreatment methods to enhance the effects of stem cell therapy in
cardiovascular regeneration.
Another technology that has not been applied to any clinical trials yet
is nanotechnology. The harmful microenvironment after myocardial infarction has
detrimental effects on survival and function of the newly transplanted stem
cells. Therefore there has been a growing interest in designing biomaterials
that can provide a supportive role for the transplanted cells within infarcted
myocardium. Before any human trials can be planned, several potential questions
need to be addressed, such as dosing, biodegradability, excretion of the
byproducts, and any immunological incompatibility of these bio-nanomaterials.
