Stem Cells International
Volume 2016 (2016), Article ID 3140120, 15 pages
http://dx.doi.org/10.1155/2016/3140120

Review Article

In Vivo Tracking of Cell Therapies for Cardiac Diseases with Nuclear Medicine

Mayra Lorena Moreira,¹ Priscylla da Costa Medeiros,¹ Sergio Augusto Lopes de Souza,¹ Bianca Gutfilen,¹ and Paulo Henrique Rosado-de-Castro1,2

¹Departamento de Radiologia, Faculdade de Medicina, Universidade Federal do Rio de Janeiro, 21941-913 Rio de Janeiro, RJ, Brazil
²Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, 21941-902 Rio de Janeiro, RJ, Brazil

Received 9 August 2015; Revised 18 October 2015; Accepted 20 October 2015

Academic Editor: Renke Li

Copyright © 2016 Mayra Lorena Moreira et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Even though heart diseases are amongst the main causes of mortality and morbidity in the world, existing treatments are limited in restoring cardiac lesions. Cell transplantations, originally developed for the treatment of hematologic ailments, are presently being explored in preclinical and clinical trials for cardiac diseases. Nonetheless, little is known about the possible efficacy and mechanisms for these therapies and they are the center of continuous investigation. In this scenario, noninvasive imaging techniques lead to greater comprehension of cell therapies. Radiopharmaceutical cell labeling, firstly developed to track leukocytes, has been used successfully to evaluate the migration of cell therapies for myocardial diseases. A substantial rise in the amount of reports employing this methodology has taken place in the previous years. We will review the diverse radiopharmaceuticals, imaging modalities, and results of experimental and clinical studies published until now. Also, we report on current limitations and potential advances of radiopharmaceutical labeling for cell therapies in cardiac diseases.

1. Introduction

Cardiovascular ailments are still the greatest causes of morbidity and mortality in the world, with significant financial and social consequences [1, 2]. Despite recent medical and surgical advances in the past decades, currently there are no effective therapies to allow cardiac regeneration [3]. On this scenario, experimental studies have indicated that cell therapies may target cardiac regeneration in acute and chronic myocardial diseases [3]. Although clinical studies have already been carried out, the efficacy and potential mechanisms of cell therapies for cardiac diseases are still under continuous investigation [4–6]. Possible mechanisms of action of cell therapies include the secretion of paracrine factors that reduce cardiomyocyte death, improve local microcirculation, and decrease the amount of fibrous tissue, which may improve heart function [3].

Noninvasive imaging modalities have the potential of providing better understanding of the biological process and the effectiveness of cell therapies for cardiac diseases [7]. One of the main applications of these techniques is to track the migration of cell therapies [7]. Among the different imaging techniques available, Nuclear Medicine has become one of the most employed techniques, due to its favorable characteristics, such as the availability of different radiopharmaceuticals and its high sensitivity [8]. In this paper, we will review preclinical and clinical studies that used Nuclear Medicine to evaluate cell migration and discuss important issues in this area.

2. Use of Radiopharmaceuticals for Cell Labeling

In the past decades, labeled leukocyte scintigraphy has become an important method to locate sites of infection and inflammation in the body [9, 10]. The development of this method had been a key landmark in the history of Nuclear Medicine. Conventional techniques include two-dimensional planar scintigraphies and three-dimensional single photon emission computed tomography (SPECT). Additionally, SPECT images may be acquired together with a computed tomography, resulting in hybrid SPECT/CT images [11]. This technique allows a better location of the findings of Nuclear Medicine, thus increasing the sensitivity and specificity of the method [11].

A variety of labeling methods with radionuclides has been created and used to study cell distribution in the body [12]. Currently, technetium-99m () is the most commonly utilized radionuclide in the world, due to favorable properties such as its decay by gamma emission with an energy of 140 kev and a 6-hour half-life, optimum physical characteristics for SPECT, allowing images for up to 24 hours after injection [9]. Radionuclide indium-111 (¹¹¹In) may also be used for cell labeling in SPECT, for example, through compounds ¹¹¹In-oxine and ¹¹¹In-tropolone [9].

The radionuclide fluorine-18 (¹⁸F) has a half-life of approximately 110 minutes and is the most frequently utilized in positron emission tomography (PET) and hybrid PET/CT, mainly in the radiopharmaceutical ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) [12]. PET has better spatial resolution than SPECT and allows the quantification of the standardized uptake value (SUV) [12, 13]. Zirconium-89 (⁸⁹Zr) is another promising radionuclide for cell labeling in PET that has a 78.4-hour half-life and may allow cell tracking for two to three weeks [14].

Tracking cells with SPECT and PET may be separated in two strategies: direct and indirect [15]. Direct tracking is achieved by labeling cells with a radiotracer in vitro with subsequent cell administration [7, 15]. The most widely used radionuclides for direct labeling are and ¹¹¹In to perform SPECT and ¹⁸F to perform PET [9, 16]. Indirect cell tracking may be achieved employing reporter gene/probe systems that have been the topic of exceptional reviews [8, 17]. For instance, a lentivirus may be used to deliver a reporter gene for expression of herpes simplex virus truncated thymidine kinase (TK) that catalyzes a reaction leading to the accumulation of the probe ¹⁸F-9-[4-fluoro-3-(hydroxymethyl)butyl]guanine derivatives (¹⁸F-FHBG) for PET imaging [17]. Another example of reporter gene is the Sodium Iodide Symporter (NIS), a cell surface protein expressed usually in thyroid cells, salivary glands, mammary glands, and choroid plexus, but not in organs such as the heart [18]. Cells overexpressing NIS will capture and iodine-123 (¹²³I) for SPECT, as well as iodine-124 (¹²⁴I) for PET, allowing the evaluation of viable cell homing in the heart after transplantation [18].

3. Preclinical Studies

3.1. Direct Cell Labeling

We identified 31 published articles that used direct cell labeling to track the migration and homing of cell therapies in preclinical models of heart diseases, all of them for myocardial infarction (Table 1).

Table 1: Preclinical studies that used direct radiopharmaceutical labeling for cell therapies in models of myocardial infarction.

Study reference
Radiopharmaceutical used for cell tracking
Time window of cell injection after lesion induction
Animals
Cell type
Route(s)
Number of cells injected
Radionuclide activity
Time points of analysis after cell therapy

Aicher et al., 2003 [33]
¹¹¹In-oxine
24 h
Athymic nude rats
Human EPCs
Intravenous
Intraventricular
1 × 10⁶
15 Bq/cell
1, 24, 48, and 96 hours

Barbash et al., 2003 [34]
Tc-HMPAO
2 days or 10–14 days
Athymic nude rats
Rat BM-MSC
Intravenous
Intraventricular
4 × 10⁶
Not specified
Not specified

Brenner et al., 2004 [19]
¹¹¹In-oxine
24 hours
Athymic nude rats
Human HPCs
Intraventricular
Not specified
30 Bq/cell
1, 24, 48, and 96 hours

Zhou et al., 2005 [35]
¹¹¹In-oxine
Immediately
Sprague-Dawley rats
Rats H9c2
Intramyocardial
3-4 × 10⁶
3,7 Bq/cell
2, 24, 48, 72, and 96 hours

Kraitchman et al., 2005 [30]
¹¹¹In-oxine
72 hours
Dogs
Canine BM-MSCs
Intravenous
1.6 × 10⁸
0,4 Bq/cell
Same day to 8 days

Hou et al., 2005 [97]
¹¹¹In-oxine
6 days
Pigs
Human PB-MNCs
Intracoronary
Intramyocardial
IRV
1 × 10⁷
1,85 Bq/cell
1 hour

Tran et al., 2006 [37]
¹¹¹In-oxine
3 months
Wistar rats
Rat BM-MSCs
Intramyocardial
2 × 10⁶
7,5 Bq/cell
2 days

Tran et al. 2006 [38]
¹¹¹In-oxine
1 month
Wistar rats
Rat BM-MSCs
Intramyocardial
2 × 10⁶
7,5 Bq/cell
7 days

Shen et al., 2007 [36]
¹¹¹In-oxine
Immediately
Sprague-Dawley rats
Rat embryonic cardiomyoblasts
Intramyocardial
3-4 × 10⁶
3,7 Bq/cell
30 minutes

Tran et al., 2007 [39]
¹¹¹In-oxine
4 months
Wistar
rats
Rat BM-MSCs
Intramyocardial
2 × 10⁶
7,5 Bq/cell
48 h

Qian et al., 2007 [50]
¹⁸F-FDG
7 days
Chinese
mini-pigs
Porcine BM-MNCs
Intracoronary
1.0 × 10⁹
0,185 Bq/cell
1 hour

Doyle et al., 2007 [51]
¹⁸F-FDG
48 hours
Pigs
Porcine CPCs
Intracoronary
3 × 10⁷
Not specified
1 hour

Lutz et al., 2008 [31]
¹¹¹In-oxine
24 h
C57BL/6 mice
Murine bone marrow-derived cells
Intravenous
1 × 10⁶
0,037 Bq /cell
24 and 72 hours

Tossios et al., 2008 [53]
¹¹¹In-tropolone
5 days
Pigs
Porcine BM-MNCs
Intracoronary Intramyocardial
1 × 10⁸
Not specified
Immediately; 1–24 hours

Blackwood et al. 2009 [26]
¹¹¹In-tropolone
Immediately
Dogs
Canine BM-MSCs
Intramyocardial
3.08 × 10⁷
1,7 Bq/cell
Same day and 2 weeks

Terrovitis et al., 2009 [41]
¹⁸F-FDG
Immediately
Wistar Kyoto rats
Rat CDCs
Intramyocardial
2 × 10⁶
Not specified
Not specified

Mäkelä et al., 2009 [54]
¹¹¹In-oxine
30 minutes
Pigs
Porcine BM-MNCs
Intramyocardial
Intracoronary
2 × 10⁸
0,185 Bq/cell
2 and 24 hours and 6 days

Mitchell et al., 2010 [44]
¹¹¹In-tropolone
Same day and 7 days
Dogs
Canine EPC
Epicardial
Endocardial
2.8 × 10⁷
0,1 Bq/cell
30–40 minutes
4 and 10 and up to 15 days

Forest et al., 2010 [55]
Tc
5 days
Pigs
BM-MNCs
Intravenous
Intracoronary
1 × 10⁷ (intracoronary)
1 × 10⁸ (intravenous)
Not specified
1 and 24 hours

Danoviz et al., 2010 [27]
Tc-HMPAO
24 hours
Lewis rats
Rat ADSCs
Intramyocardial
1 × 10⁶
Not specified
Not specified

Sabondjian et al., 2012 [46]
¹¹¹In-tropolone
Not specified
Dogs
Canine EPCs
Intramyocardial
Not specified
Not specified
Immediately;
4 and 10 days

Zhang et al., 2012 [25]
¹⁸F-HFB
¹⁸F-FDG
14 days
Sprague-Dawley rats
Human CPCs
Epicardial
Endocardial
2 × 10⁶
7,4 to 11,1 Bq/cell
10 min, 2 and 4 hours

Mitchell et al., 2013 [45]
¹¹¹In-tropolone
First: 4 hours or 7 days/second: 4 weeks later
Dogs
Canine EPCs
Intramyocardial
3 × 10⁷
0,1 Bq/cell
Same day and 4, 10, and 15 days

Maureira et al., 2013 [47]
¹¹¹In-oxine
4 months
Wistar rats
Rat BM-MSCs
Intramyocardial
2 × 10⁶
7,5 Bq/cell
48 hours

Lang et al., 2013 [42]
¹⁸F-FDG
5 minutes
C57BL6/N wild-type mice
Murine ESCs
Intramyocardial
3 × 10⁶
0.8 Bq/cell
25 minutes and 2 hours

Elhami et al., 2013 [49]
¹⁸F-FDG
Immediately or 7 days
Lewis rats
Rat ADSCs
Intramyocardial
Intraventricular, intravenous
4.5–6.0 × 10⁶
20 Bq/cell
4 hours

Garikipati et al., 2014 [32]
Tc-HMPAO
7 days
Sprague-Dawley rats
Rat FC-MSC
Intravenous
2 × 10⁶
37 Bq/cell
6 hours

Lang et al., 2014 [43]
¹⁸F-FDG
5 minutes
C57BL6/N wild-type mice
Murine ESCs and fibroblasts
Intramyocardial
3 × 10⁶
0,8 Bq/cell
2 hours

Kim et al., 2015 [48]
¹²⁴I-HIB
¹⁸F-FDG
Not specified
Sprague-Dawley rats
Rat ADSCs
Intramyocardial
5 × 10⁶
0,2 to 0,3 Bq/cell
1 day (¹⁸F-FDG) and 9 days (¹²⁴I-HFB)

Keith et al., 2015 [52]
¹¹¹In-oxine
1-2 months
Pigs
Human CDCs
Intracoronary
1 × 10⁷
27.7 Bq/cell
24 hours

Bansal et al., 2015 [14]
⁸⁹Zr
1 hour
Mice
Human MSCs
Intramyocardial
2 × 10⁵
0.37 Bq/cell
2, 5, and 7 days

 ¹¹¹In: indium-111; ¹⁸F: fluorine-18; ⁸⁹Zr: zirconium-89; Tc: technetium-99m; ADSCs: adipose tissue-derived stem cells; BM-MNCs: bone marrow mononuclear cells; BM-MSCs: bone marrow mesenchymal stem cells; CDCs: cardiac-derived stem/progenitor cells; CPCs: circulating progenitor cells; EPCs: endothelial progenitor cells; ESCs: embryonic stem cells; FC-MSCs: mesenchymal stem cells derived from rat fetal heart; FDG: fluorodeoxyglucose; HFB: hexadecyl-4-fluorobenzoate; HIB: hexadecyl-4-tributylstannylbenzoate; HMPAO: hexamethylpropyleneamine oxime; IRV: interstitial retrograde coronary venous; PB-MNCs: peripheral blood mononuclear cells.

3.1.1. Effect on Cell Viability, Metabolic Activity, and Migration

Although the use of ¹¹¹In radiopharmaceuticals allows cell tracking for longer periods in comparison to , it has high energy (171 and 245 keV), which leads to images of lower resolution and greater cell dose that may decrease cell viability [19–21]. ¹¹¹In can affect the viability, metabolic activity, and migration of stem cells due to internalization of Auger electrons emitted at close distances. These electrons may lead to considerable toxicity to target cells reducing cell viability [10, 19–21].

Jin et al. carried out an interesting study where they evaluated the viability of bone marrow-derived mesenchymal stem cells (BM-MSCs) labeled with ¹¹¹In [22]. Distinct samples with 5 × 10⁶ cells were labeled 0.1 to 18 MBq of ¹¹¹In-tropolone. The authors reported that cells had 100% viability when incubated with up to 0.9 MBq, which corresponded to 0.14 Bq per cell.

Brenner et al. [19] reported the impact of labeling human CD34⁺ hematopoietic progenitor cells (HPCs) with ¹¹¹In-oxine. HPCs (1 × 10⁶/mL) were incubated with 30 MBq of ¹¹¹In-oxine for 1 hour to assess cell viability at 1, 24, 48, and 96 hours. Although no significant changes were observed at 24 hours after labeling, after 48 and 96 hours the number of dead cells increased. Furthermore, cell migration was quickly reduced after 24 hours.

Suhett et al. [23] studied the binding sites for in rat bone marrow mononuclear cells (BM-MNCs). BM-MNCs were labeled with 45 MBq of O4-. After being labeled, cells were carefully disrupted and differentially centrifuged for organelle separation. Viability of the labeled cells was 93% and most of the radiation remained in the supernatant comprised of the cytosol and membrane bound ribosomes.

¹⁸F-FDG is regarded as the gold standard for the assessment of myocardial viability. ¹⁸F-FDG is a glucose analogue that enters cardiomyocytes through glucose transporters (GLUTs) such as GLUT1 and GLUT4. Within the cell, ¹⁸F-FDG suffers phosphorylation by hexokinase and converts to ¹⁸F-glucose 6-phosphate. Because it is not metabolized, it is retained within the cell. Preclinical studies made by Chan and Abraham reported that ¹⁸F-FDG caused no interference with proliferation of cardiac-derived stem/progenitor cells (CDCs) [7]. Similarly, Wolfs et al. found no significant changes to the ultrastructure and differentiation of mouse MSCs and rat multipotent adult progenitor cells [24].

Hexadecyl-4-[¹⁸F] fluorobenzoate (¹⁸F-HFB) is a lipophilic radiopharmaceutical that is absorbed through the cell membrane, allowing cell tracking by PET. Zhang et al. [25] compared the labeling of human peripheral blood-derived circulating progenitor cells (CPCs) with ¹⁸F-HFB and ¹⁸F-FDG in mice after myocardial infarction. Cells were injected close to the site of cardiac injury. The images were made in Micro-PET 10 min and 2 and 4 hours after injection. ¹³N-NH3 was used to outline the liver and the heart. Labeling with ¹⁸F-HFB showed no reduction in cell viability with 14.8–22.2 MBq of radioactivity in 2 × 10⁶; however, higher activities (185–259 MBq) resulted in significant cell death. After 24 hours, the reduction of viability in ¹⁸F-HFB-CPCs was 13.3%, whereas in controls it was 6.9%. After 5 days cell viability decreased for both groups: ¹⁸F-HFB-CPCs (10.4%) and ¹⁸F-FDG-CPCs (14.7%).

3.1.2. Radionuclide Leakage and Labeling Efficiency

Quantifying in vivo cell transplant survival may be difficult, and radionuclide leakage is an important issue that should be taken into account [26]. Radionuclide leakage may occur from viable cells and cellular debris [26]. Many authors applied different in vivo experiments to determine cell death, radiolabel leakage, and cell survival [26, 27]. Another issue to be evaluated is the normal turnover of the cells, where one may label cells and administer them in order to study the clearance characteristics from viable cells that did not die in vivo [26].

Blackwood et al. [26] quantified the survival of BM-MSCs labeled with ¹¹¹In transplanted into the canine myocardium. The authors also evaluated the clearance of lysed ¹¹¹In labeled cells. Serial SPECT images were acquired after direct epicardial injection to determine the time-dependent radiolabel clearance. The average long biologic half-life for labeled cells was 74.3 hours and for lysed cells was 19.4 hours.

The labeling efficiency of direct labels differs between different methods and needs to be taken into account [28]. For instance, it has been reported that the labeling with -tropolone was more effective and stable in comparison to -hexamethylpropyleneamine oxime (-HMPAO) [29]. In another example, Zhang et al. reported that ¹⁸F-HFB labeling showed a higher efficiency when compared with ¹⁸F-FDG [25].

3.1.3. Biodistribution after Intravenous Injection

Kraitchman et al. [30] investigated the migration of BM-MSCs labeled with ¹¹¹In-oxine, by intravenous route, 72 hours after the induction of lesion myocardial infarction in dogs. SPECT imaging was carried out up to 8 days after cell transplantation. Uptake on the same day of cell therapy was mainly restricted to the lungs in infarcted animals and control animals with low uptake in the heart. At 24 hours, uptake remained constant in the heart, decreased in the lungs, and increased in the liver and spleen.

Lutz et al. [31] studied the migration of systemically injected bone marrow-derived cells in mice after myocardial infarction. After induction of the infarction, animals received intramyocardial injections of stem cell factor (SCF) in peri-infarcted areas. Cells were labeled with ¹¹¹In-oxine and injected in the tail vein 24 hours after the infarction. Animals were sacrificed and hearts removed for analysis in a gamma counter 24 or 72 hours later. The analysis indicated that intramyocardial injections of SCF significantly increased myocardial uptake in comparison with infarcted animals that received saline injections and with sham-operated animals at both time points.

Garikipati et al. [32] investigated the efficacy of therapy with fetal cardiac mesenchymal stem cells (FC-MSCs) in rats after myocardial infarction. FC-MSCs were isolated and cultured from fetal rat hearts. Seven days after the induction of the lesion, mice were divided into FC-MSC or saline group. Cells were labeled with -HMPAO and injected into the tail vein. Multipinhole gated SPECT/CT was carried out six hours after the intravenous infusion and labeled cells were mainly present in the lungs, with focal homing in the heart.

3.1.4. Biodistribution after Intraventricular Injection

Brenner et al. [19] performed intraventricular injections of human HPCs into the left ventricular cavity of rats after myocardial infarction. SPECT was performed 1, 24, 48, and 96 hours after transplantation. Liver, kidneys, and spleen combined had 37% and lungs 17% of whole body uptake 1 h after cell transplantation. Twenty-four hours after the injection, lung uptake was no longer detected, while homing to the liver, kidneys, and spleen increased to 57%. Only 1% of the injected activity was found in the heart of transplanted animals.

Aicher et al. investigated the transplantation of ¹¹¹In-oxine labeled endothelial progenitor cells (EPCs) into rats after myocardial infarction [33]. Labeled cells were delivered in the tail vein or in the left ventricular cavity. Pinhole SPECT was performed after cell administration. Total uptake in the liver, kidneys, and spleen was 71% after 96 hours, while myocardial uptake was only 1-2% after intravenous injection and 3–5% after intraventricular cavity infusion.

Barbash et al. evaluated the effectiveness and feasibility of systemic administration of BM-MSCs in rats following myocardial infarction. Cells were labeled by incubation with -HMPAO [34]. Three injection methods were studied. The first approach was by infusion of BM-MSCs in the femoral vein. In the second strategy, BM-MSCs were infused directly into the left ventricle. In the third group, cells were injected into the right ventricle, but all animals died from pulmonary embolism. Images were acquired 4 hours after the infusion and indicated that rats with myocardial infarction had higher uptake of labeled cells in the heart than sham animals. Moreover, intravenous infusion resulted in lower myocardial homing due to pulmonary cell retention.

3.1.5. Biodistribution after Intramyocardial Injection

Zhou et al. [35] investigated the distribution of rat embryonic cardiomyoblasts (H9c2) cells after labeling with ¹¹¹In-oxine rats after myocardial infarct. Cells were intramyocardially transplanted around the infarcted region immediately after induction of the lesion and SPECT images acquired 2, 24, 48, 72, and 96 hours. The authors reported that cell uptake was detected in the injection site up to 96 hours after administration.

Shen et al. [36] used magnetic resonance imaging (MRI) and SPECT imaging to monitor H9c2 cell transplantation in rats after myocardial infarction. Myocardial infarction was induced and ¹¹¹In labeled cells were injected in regions close to the injured site. MRI was performed 5–7 days after SPECT images. Through a coregistration algorithm, it was possible to carry out the fusion of SPECT-MRI images. The authors were able to monitor the uptake of ¹¹¹In-oxine labeled cells and the perfusion in -sestamibi images.

Tran et al. [37–39] evaluated in a series of studies the migration of ¹¹¹In-oxine labeled BM-MSCs in rats one to four months after myocardial infarction in rats. Cells were injected in the infarcted areas. Cell distribution was compared with -sestamibi imaging of myocardial perfusion using a 17-segment division of the left ventricle. The authors concluded that BM-MSCs homing was heterogeneous and did not match in all occasions the infarcted regions [37–39].

Wisenberg et al. [40] evaluated dogs using both imaging of ¹¹¹Indium-tropolone labeled cells and late gadolinium enhancement cardiac MRI for up to 12 weeks after a 3-hour coronary occlusion. The animals were injected with BM-MSCs and imaged at day 0 (surgery) and after 4, 7, 10, and 14 days. SPECT imaging indicated an effective biological clearance half-life from the injection site of ~5 days, while cardiac MRI demonstrated a pattern of progressive infarct reduction over 12 weeks.

Terrovitis et al. [41] labeled rat CDCs with ¹⁸F-FDG to monitor cell therapy in rats after myocardial infarction. CDCs were injected intramyocardially. In other groups of animals, the effects of fibrin glue, bradycardia (by adenosine injection), and induction of cardiac arrest on cell homing were investigated. One hour after cell transplantation without additional measures, PET indicated that mean myocardial homing was 17.8%. Adenosine injection was able to decrease the heart rate and double cell mean cell homing to 35.4%. A comparable enhancement in cell homing was seen when the authors applied fibrin glue epicardially and mean cell homing increased to 37.5%. However, the greatest increase was seen after induction of cardiac arrest, when mean homing increased to 75.6%.

Lang et al. [42, 43] studied the distribution of ¹⁸F-FDG labeled murine embryonic stem cells (ESCs) or fibroblasts in C57BL6/N mice after myocardial infarction, five minutes after the infarct ESCs or fibroblasts were injected intramyocardially [42, 43]. Images were made in a preclinical PET. The authors reported that the percentages of uptake in the heart were 5.2–5.3% after 25 minutes, 4.8–5.0% after 1 hour, and 5.6–5.7% after 2 hours.

Danoviz et al. assessed the transplantation of adipose tissue-derived stem cells (ADSCs) with two biopolymers, fibrin and collagen, in murine model of acute myocardial infarction [27]. Cells were labeled with -HMPAO. Twenty-four hours after induction of the lesion, the animals were injected with cells suspended in 100 mL of carrier by intracoronary route. Cells were infused in the border of the lesion with fibrin, collagen, or culture medium. Radioactivity counting of the organs revealed high levels of radioactivity in the liver, kidneys, and lungs. Both biopolymers increased cellular retention, but the collagen group showed higher uptake (26.8%) when compared to fibrin and culture medium (13.7% and 4.84%, resp.).

Mitchell et al. [44, 45] and Sabondjian et al. [46] assessed the migration of EPCs in canine models of myocardial infarction up to 7 days after induction of the lesion. EPCs were labeled with ¹¹¹In-tropolone and injected by epicardial and endocardial routes. SPECT imaging was performed up to 15 days after cell transplantation. The authors reported that cell homing occurred in hypoperfused areas and that epicardial and endocardial injections led to similar uptake.

Maureira et al. [47] developed an in vivo technique with pinhole SPECT to monitor stem cell migration after myocardial infarction in rats. After coronary occlusion, autologous BM-MSCs were labeled with ¹¹¹In-oxine. An intramyocardial injection was administered in the infarcted region. Two days after the procedure, -sestamibi was injected to compare homing of ¹¹¹In labeled cells and myocardial perfusion. Left ventricle perfusion and function in all animals were monitored 2 days before cell therapy and 1–6 months after therapy using a pinhole gated SPECT. Significant improvements in cardiac perfusion were observed in injured areas and also in areas not transplanted.

Kim et al. [48] studied the homing of ADSCs after direct labeling with ¹²⁴I-hexadecyl-4-tributylstannylbenzoate (¹²⁴I-HIB) or ¹⁸F-FDG in rats after myocardial infarction. Cells were labeled with ¹²⁴I-HIB or ¹⁸F-FDG. An intramyocardial injection was performed in the infarct site. ¹²⁴I-HIB labeled cells were seen at the infarct area and monitored for up to 3 days in lesioned animals. The authors reported that labeling efficiency with ¹²⁴I-HIB was higher than with ¹⁸F-FDG, indicating it could be a good method to monitor stem cell homing.

Elhami et al. [49] investigated the migration of ¹⁸F-FDG labeled ADSCs after myocardial infarction in rats. Labeling was carried out with ¹⁸F-FDG. Immediately after the infarct induction, cell transplantation was carried out by intramyocardial, intraventricular, or intravenous route. In another group, cells were injected intramyocardially 7 days after the infarct. The authors reported that the intravenous route led to lower cell homing in the heart (1.2% of infused ADSCs) 4 hours after cell transplantation. Intraventricular injection led to an uptake of 3.5% in the heart, while intramyocardial injection led to the highest myocardial cell homing (14%). Interestingly, in the group that received an intramyocardial cell injection 7 days after the myocardial infarction, cell homing was lower (4.5%) than the group that received cells immediately after the infarct induction.

3.1.6. Biodistribution after Intracoronary Injection

Qian et al. [50] determined the distribution of BM-MNCs after myocardial infarction in Chinese mini-pigs. Cells were labeled with ¹⁸F-FDG and injected intramyocardially 7 days after the infarct. One hour after cell transplantation, 6.8% of the whole body uptake was located in the infarct site. Liver and spleen showed more than 90% of the uptake.

Doyle et al. [51] tracked CPCs in pigs after acute myocardial infarction. CPCs were labeled with ¹⁸F-FDG. One group received CPCs divided into 3 cycles after a balloon catheter was positioned and inflated in the lesioned artery. A second group received a single bolus infusion of CPCs without balloon inflation. The authors reported that one hour after cell transplantation the group that received the infusion in 3 cycles with balloon occlusion had lower uptake in the heart than the group that received a single bolus injection (8.7% versus 17.8%, resp.). The majority of activity (>60%) was concentrated in the lungs after 1 hour in both groups, and there was moderate uptake in the liver and spleen.

Keith et al. [52] investigated the impact of using intracoronary human CDC injection on cell homing in a pig model of myocardial infarction. Cells were injected with or without balloon inflation after labeling with ¹¹¹In-oxine. SPECT was carried out 24 hours after cell transplantation. The authors reported that the injection with balloon occlusion led to similar myocardial homing as the one without balloon occlusion (5.41% versus 4.87%, resp.) and concluded that the risk involved in the coronary occlusion approach would not be warranted.

Hou et al. evaluated the distribution of peripheral blood mononuclear cells (PB-MNCs), labeled with ¹¹¹In, in pigs after myocardial infarction. The lungs had 1%, 3%, and 3% of the uptake, while myocardial uptake was 2.6%, 3.2%, and 11% after intracoronary, interstitial retrograde coronary venous, or intramyocardial injections, respectively.

Tossios et al. [53] monitored the distribution of BM-MNCs following induction of myocardial infarction in pigs. After labeling with ¹¹¹In-tropolone, cells were injected by intramyocardial or by intracoronary route with or without balloon occlusion. One hour after injection, 20.7%, 4.1%, and 6.1% of the uptake were located in the heart after intramyocardial, intracoronary without balloon, and intracoronary with balloon infusions, respectively. Twenty-four hours later, myocardial uptake was 15.0%, 3.0%, and 3.3%, respectively. The lungs, liver, and spleen had 50%, 10%, and 5% of the uptake in the whole body, respectively.

Mäkelä et al. [54] evaluated the migration of BM-MNCs in a pig model of myocardial infarction. Cells were labeled with ¹¹¹In-oxine and transplanted by intramyocardial or intracoronary routes 30 minutes after induction of the lesion. SPECT was acquired 2 and 24 hours after cell transplantation and biopsies from different organs were also performed to allow gamma counting. The authors reported that the intracoronary injection led to <15% of the cardiac uptake observed after intramyocardial injection, while lung uptake after intramyocardial injection was <15% of the pulmonary uptake observed after intracoronary infusion.

Forest et al. studied a preclinical model of myocardial infarction in pigs [55]. Seven days after induction of the lesion, BM-MNCs were labeled with . Animals were divided into three groups: control group, intracoronary injection, and intravenous injection of labeled cells. Intravenous administration led to higher cell accumulation in the lungs, while intracoronary injection led to greater myocardial uptake.

3.2. Indirect Radiolabeling: Reporter Gene/Probe Systems

Reporter gene/probe imaging for SPECT and PET has been applied to evaluate the survival of transplanted cells in animal models of cardiac diseases [8]. Some of the disadvantages of using reporter genes include the possible immunogenicity of the viral reporter gene, which limits the application of the technique in humans [7]. Moreover, the stability of transfection and expression must be improved and the potential interference with stem cell function and differentiation from vector transfection or transduction must be minimized [56]. We identified 9 published articles that used indirect cell tracking to evaluate the migration and homing of cell therapies in preclinical models of heart diseases, all of them for myocardial infarction (Table 2).

Table 2: Preclinical studies that used indirect radiopharmaceutical tracking for cell therapies in models of myocardial infarction.

Study reference
Radiopharmaceutical used for cell tracking
Time window of cell injection after lesion induction
Animals
Cell type
Route(s)
Number of cells injected
Time points of analysis after cell therapy

Gyöngyösi et al., 2008 [57]
¹⁸F-FHBG
16 days
Pigs
Porcine BM-MSCs
Intravenous
1.1 × 10⁵
30 hours
7, 16, and 23 days

Terrovitis et al., 2008 [58]
¹²⁴I
Tc
Immediately
Wistar Kyoto rats
Rat CDCs
Intravenous
2 × 10⁶
1 hour

Lee et al., 2011 [59]
¹⁸F-FHBG
30 minutes
Dogs
Canine iPSCs
Intramyocardial
1 × 10⁶
8 hours

Liu et al., 2012 [60]
¹⁸F-FHBG
Immediately
SCID Beige mice
Human CDCs
Intramyocardial
1 × 10⁶
1, 7, 14, 21, and 28 days

Templin et al., 2012 [62]
¹²³I
Not specified
Pigs
Human iPSCs
Intramyocardial
1 × 10⁸
1 and 5 days and 12-15 weeks

Lan et al., 2012 [61]
¹⁸F-FHBG
Immediately
SCID Beige mice
Human CDCs
Intramyocardial
1 × 10⁶
1, 7, 14, 21, and 28 days

Yan et al., 2013 [63]
¹⁸F-FHBG
10 minutes
Nude mice
Murine BM-MSCs
Intramyocardial
1 × 10⁶
Same day, 3 and 7 days

Pei et al., 2014 [64]
¹⁸F-FHBG
Immediately
Sprague-Dawley rats
Rat BM-MSCs
Intramyocardial
5 × 10⁶
2, 3, and 7 days

Lee et al., 2015 [65]
Tc
7 days
Dogs
Canine ADSCs
Intramyocardial
1 × 10⁷
2 hours and 1, 2, 5, 7, 9, and 12 days

 ¹²³I: iodine-123; ¹²⁴I: iodine-124; ¹⁸F: fluorine-18; Tc: technetium-99m; ADSCs: adipose tissue-derived stem cells; BM-MSCs: bone marrow mesenchymal stem cells; CDCs: cardiac-derived stem/progenitor cells; FHBG: 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine derivatives; iPSCs: induced pluripotent stem cells; SCID: severe combined immunodeficiency.

3.2.1. Biodistribution after Intramyocardial Injection

Gyöngyösi et al. [57] used reporter gene imaging to monitor the migration of BM-MSCs in a pig model of myocardial infarction. Cells transfection was performed with a lentivirus for expression of TK. Sixteen days after the infarction, a group of animals received BM-MSCs by intramyocardial injection. Then, ¹⁸F-FHBG was injected 30 hours and 7 days after cell transplantation for in vivo imaging. The authors reported that there was a decrease in myocardial uptake of ¹⁸F-FHBG after 7 days in comparison with the 30-hour images, as well as mild increase in pericardial and pleural uptake.

Terrovitis et al. [58] transfected rat CDCs with a lentivirus to express the NIS gene. In vivo images were obtained after intramyocardial cell injection in mice after myocardial infarction. An injection of for SPECT imaging or ¹²⁴I for PET imaging was used to evaluate the expression of NIS gene in transplanted CDCs. The authors were able to detect the transplanted CDCs with a threshold of approximately 10⁵ cells. Cell homing was seen up to 6 days after CDC transplantation but less than 5% of cells remained in the heart, due to migration to the lungs and systemic circulation.

Lee et al. [59] investigated the homing of canine iPSCs after cell transplantation in dogs. Cells were injected intramyocardially 30 minutes after the induction of myocardial infarction. The authors injected an activity of approximately 536 MBq of ¹⁸F-FHBG and carried out PET/CT 8 hours after cell transplantation. Imaging revealed cell homing to the anterior myocardial wall.

Liu et al. [60] and Lan et al. [61] analyzed the migration of human CDCs in rats after myocardial infarction in severe combined immunodeficiency (SCID) Beige mice. A total of 1 × 10⁶ cells transfected with a TK reporter gene were injected by intramyocardial route immediately after the induction of myocardial infarction. On days 1, 7, 14, 21, and 28 after cell therapy, an activity of 7.4 MBq of ¹⁸F-FHBG was injected to allow PET imaging. A gradual decrease in the amount of surviving cells was noticed during the follow-up. Interestingly, the authors reported that early cell homing predicted ensuing functional improvement [60].

Using reporter genes, Templin et al. [62] were able to monitor human induced pluripotent stem cells (iPSCs) in pigs after myocardial infarction. Cells were labeled 90 minutes before injection with 100 MBq of ¹²³I and a volume of 250 μL was injected in three regions of the animals’ hearts. The anterior wall of the left ventricle received 50 million cells of human MSCs. The lateral and septal walls received 50 million NIS-positive [NIS(pos)] human iPSCs or 50 million NIS(pos) human iPSCs mixed with 50 million human MSCs. -tetrofosmin was intravenously injected to assess myocardial perfusion. Images were acquired for 5 minutes in SPECT/CT equipment. Images were acquired up to 15 weeks after cell transplantation through an intracoronary injection of ¹²³I. No uptake was seen outside the heart and NIS(pos) human iPSCs were detected in the site of injections, indicating successful cell homing.

Yan et al. [63] assessed the distribution of BM-MSCs in nude mice 10 minutes after induction of myocardial infarction. Cells transfected with a TK gene were injected intramyocardially after induction of the lesion. On the same day and 3 and 7 days after cell transplantation, ¹⁸F-FHBG was injected and PET was carried out. The authors described that the highest myocardial uptake occurred 3 days after cell therapy and that infarcted animals had higher homing than control animals.

Pei et al. [64] evaluated the homing of BM-MSCs in rats after myocardial infarction. Immediately after the lesion, cells were intramyocardially injected. Two, 3, and 7 days after cell transplantation, ¹⁸F-FHBG was injected to allow cell tracking. The authors reported that myocardial uptake could be seen up to 7 days following cell therapy, and homing was mostly distributed to the liver, lungs, intestines, stomach, and spleen.

Lee et al. [65] studied the distribution of ADSCs transfected with the NIS gene in dogs following myocardial infarction. NIS expressing ADSCs were intramyocardially injected 7 days after the infarct induction. O4- was injected at 2 hours and 1, 2, 5, 7, 9, and 12 days after cell transplantation. The authors reported that cell homing was identified in the apex and lateral wall of the left ventricle, reached its peak at 2 days, and was seen until 9 days after cell transplantation.

4. Clinical Trials

4.1. Direct Cell Labeling

We have found 18 published articles in English regarding 17 different trials that employed radionuclides to track cell therapies for cardiac diseases, with a total of 293 treated patients (Table 3). All studies used direct labeling methods.

Table 3: Clinical studies that used direct radiopharmaceutical labeling for cell therapies in cardiology.

Study reference
Radiopharmaceutical
Type of lesion
Cell type
Route
Number of patients
Number of cells injected
Imaging time points

Hofmann et al., 2005 [72]
¹⁸F-FDG
Acute myocardial infarction
BM-MNCs

Intracoronary
Intravenous
12
Not specified
75 min

Blocklet et al., 2006 [71]
¹⁸F-FDG
¹¹¹In-oxine
Acute myocardial infarction
PB-MNCs
Intracoronary
6
2–4 × 10⁶
1 hour (¹⁸F-FDG)
19 and 43 hours (¹¹¹In-oxine)

Kang et al., 2006 [73]
¹⁸F-FDG
Acute or nonacute myocardial infarction
PB-MNCs
Intracoronary
Intravenous
20
Not specified
2, 4, and 20 hours

Goussetis et al., 2006 [74]
Tc-HMPAO
Chronic ischemic cardiomyopathy
BM-MNCs
Intracoronary
8
0.8 × 10⁷
1 and 24 hours

Caveliers et al., 2007 [66]
¹¹¹In-oxine
Chronic ischemic cardiomyopathy
PB-MNCs
Intracoronary
8
5–35 × 10⁶
2 and 12 hours

Penicka et al., 2007 [75]
Tc-HMPAO
Acute or nonacute myocardial infarction
BM-MNCs
Intracoronary
10
24.2–57.0 × 10⁸
2 and 20 hours

Kurpisz et al., 2007 [67]
¹¹¹In-oxine
Acute myocardial infarction
BM-MNCs
Intracoronary
3
2–4 × 10⁶
24 hours

Schots et al., 2007 [68]
¹¹¹In-oxine
Nonacute myocardial infarction
PB-MNCs
Intracoronary
8
5–10 × 10⁶ 
18–50 × 10⁶
2 and 12 hours

Schächinger et al., 2008 [69]
¹¹¹In-oxine
Acute or nonacute myocardial infarction
PB-MNCs
Intracoronary
20
15 × 10⁶
1 and 24 hours

Dedobbeleer et al., 2009 [70]
¹⁸F-FDG
Nonacute myocardial infarction
PB-MNCs
Intracoronary
7
1.8 × 10⁷
1 hour

Silva et al., 2009 [76]; Moreira et al., 2011 [77]
Tc-HMPAO
Acute myocardial infarction
BM-MNCs
Intracoronary
IRCV
30
1 × 10⁷
4 and 24 hours

Musialek et al., 2011 [78]
Tc-HMPAO
Acute myocardial infarction
BM-MNCs
Intracoronary
34
0.92–7.54 × 10⁶
1 hour

Barbosa da Fonseca et al., 2011 [80]
Tc
Chronic chagasic cardiomyopathy
BM-MNCs
Intracoronary
6
4.4 × 10⁸
1, 3, and 24 hours

Kollaros et al., 2012 [81]
Tc-HMPAO
Chronic myocardial infarction
BM-MNCs
Intracoronary
13
Not specified
1 hour

Musialek et al., 2013 [79]
Tc-HMPAO
Recent myocardial infarction
BM-MNCs
Intracoronary
31
4.3 × 10⁶
1 hour

Vrtovec et al., 2013 [82]
Tc-HMPAO
Nonischemic dilated cardiomyopathy
BM-MNCs
Intracoronary transendocardial
40
1 × 10⁶
18 hours

Haddad et al., 2015 [83]
Tc-HMPAO
Nonischemic dilated cardiomyopathy
PB-MNCs
Transendocardial
37
75 × 10⁶
2 and 18 hours

 ¹¹¹In: indium-111; ¹⁸F: fluorine-18; Tc: technetium-99m; BM-MNCs: bone marrow mononuclear cells; FDG: fluorodeoxyglucose; HMPAO: hexamethylpropyleneamine oxime; IRCV: interstitial retrograde coronary venous; PB-MNCs: peripheral blood mononuclear cells.

4.1.1. Biodistribution after Intracoronary Injection

Caveliers et al. [66] conducted a cell therapy trial with eight chronic ischemic heart disease patients. They reported that infusion of CD133⁺ selected PB-MNCs labeled with ¹¹¹In-oxine is a safe and feasible procedure. They also performed -MIBI SPECT for evaluation of myocardial perfusion and compared it to cell migration. Uptake in the heart was 6.9% to 8% and 2.3% to 3.2% after 2 and 12 hours, respectively.

Kurpisz et al. [67] studied the migration of BM-MNCs in 3 patients with acute myocardial infarction. Cells were labeled with ¹¹¹In-oxine and injected by intracoronary route. Nuclear Medicine imaging was carried out 24 hours after cell transplantation. The authors reported that 2.6–11.0% of the uptake was seen in the heart, 12.3–56.7% in the liver, and 5.2–12.6% in the spleen.

Schots et al. [68] evaluated 13 patients with nonacute myocardial infarction who received CD133⁺ cells labeled with ¹¹¹In-oxine by intracoronary transplantation. Subjects had uptake of 6.9 to 8.0% in the myocardium in 2-hour images and 2.3 to 3.2% in 12-hour images.

Schächinger et al. [69] included 20 patients with ischemic myocardial disease that had myocardial viability confirmed by PET and intracoronary Doppler. The time of coronary injury to BM-MNC therapy ranged from 5 days to 17 years. After administration of ¹⁸F-FDG labeled cells, the average myocardial uptake in the first 24 hours was higher in subjects with acute myocardial infarction and gradually decreased in subjects treated in an intermediate or chronic phase. The authors concluded that the low viability of the lesioned myocardium and the reduction of coronary flow reserve were important predictors in the proangiogenic potential of progenitor cells.

Dedobbeleer et al. [70] published a study of 12 patients with nonacute myocardial infarction. Five patients were in the control group and 7 patients had CD34⁺ cells labeled with ¹⁸F-FDG. After an hour of injection, 3.2% of the radioactivity was observed in the myocardial infarction zone.

Blocklet et al. [71] evaluated the injection of PB-MNCs labeled with ¹¹¹In-oxine and ¹⁸F-FDG in 6 patients with acute myocardial infarction. The double labeling allowed monitoring of cell with high sensitivity and resolution with PET and performing late images with ¹¹¹In. Mean uptake in the myocardium after 1-hour infusion of PB-MNCs was 5.5% by PET, while in images with ¹¹¹In-oxine at 19 hours and 43 hours only 1 patient had myocardial uptake.

4.1.2. Comparison of Biodistribution of Intracoronary and Intravenous Injection

Hofmann et al. [72] carried out a cell therapy trial 5 to 10 days after a myocardial infarction in 9 patients using CD34⁺ BM-MNCs. Of the total amount of injected cells, 5% were labeled with ¹⁸F-FDG. The patients were divided into 3 protocols. In the first protocol, 3 patients received unselected BM-MNCs by intracoronary route and underwent PET imaging 55 to 75 minutes after infusion. In a second protocol, 3 patients initially received 5% of the unselected BM-MNCs by intravenous route, followed by a first PET 50 to 60 minutes after cell transplantation, and then received the remaining 95% of unselected BM-MNCs by intracoronary route, followed by a second PET 60 to 70 minutes later. In a third protocol, 3 patients received immunomagnetically enriched CD34⁺ cells by intracoronary route and underwent PET imaging 60 to 75 minutes after cell injection. In the first protocol, homing varied from 1.3% to 2.6%. In the second group, there was no detectable myocardial homing after the initial intravenous infusion, but homing increased to 1.8 to 5.3% after intracoronary injection. In the third group, in which CD34⁺ cells were injected by intracoronary route, cell homing was higher, varying from 14% to 39%.

Kang et al. [73] published a report in which 20 patients with recent or old myocardial infarctions received PB-MNCs labeled with ¹⁸F-FDG. The PB-MNCs were collected by apheresis after mobilization with granulocyte colony stimulating factor (G-CSF). Seventeen of the patients received cells by intracoronary route and 3 patients by intravenous route. The mean efficiency of cell labeling with ¹⁸F-FDG was of 72% and a total activity of 44.4 to 175 MBq was injected through a catheter after stent implantation in the infarcted artery. PET/CT images were obtained 2, 4, and 24 hours after injection. Two hours after intracoronary injection, 1.5% of the infused cells were present at the lesioned area. Delayed images up to 20 hours indicated prolonged accumulation of the cells in heart tissue. Intravenous infusion of the labeled PB-MNCs revealed high pulmonary trapping and showed no significant activity in the heart.

Goussetis et al. [74] studied 8 subjects with chronic ischemic heart disease undergoing CD133⁺ and CD133⁻CD34⁺ selected BM-MNC transplantation by intracoronary infusion. Cells were labeled with and scintigraphies acquired 1 and 24 hours after injection indicated cardiac uptake of 9.2% and 6.8%, respectively. Reevaluation with coronary angiography and echocardiography in 6 patients after 3 months of cell therapy revealed no complications.

Penicka et al. [75] included 10 patients, 5 of them with acute myocardial infarction and the other 5 with nonacute myocardial infarction. All patients received BM-MNCs labeled with -HMPAO and myocardial uptake was analyzed 2 and 20 hours after injection. There was a lack of uptake 20 hours after transplantation in subjects with acute myocardial infarction.

A randomized study of 30 subjects with acute myocardial infarction, published by Silva et al. [76] and Moreira et al. [77], compared the distribution and retention pattern of -HMPAO labeled BM-MNCs after anterograde intra-arterial or retrograde intravenous coronary routes. The early and late retention of labeled cells, evaluated in 4 and 24 hours SPECT images after injection, were higher in the group that received cells by coronary anterograde, regardless of the presence of microcirculation obstruction. Early and late retention were, respectively, 7.06% and 6.38% in the intra-arterial group and 1.4% and 0.99% in the intravenous group.

Musialek et al. [78] compared the cell transplant management techniques: perfusion technique catheter (PC) and the over-the-wire coronary occlusion technique (OTW). Thirty-four patients who suffered myocardial infarction were randomly assigned to PC or OTW infusion of autologous bone marrow CD34⁺ cells labeled with -HMPAO. One hour after infusion, the images obtained by SPECT indicated the activity of 4.86% and 5.05% in the myocardium after OTW and PC injections, respectively. The authors concluded that although the efficacy of cell delivery did not differ between infusion methods, PC infusion offered a more physiological alternative and avoided causing OTW ischemic episodes. The same group performed another study evaluating the migration of intracoronary injected -HMPAO labeled bone marrow CD34⁺ cells in subjects after myocardial infarction. The authors described that, one hour after cell transplantation, mean cardiac uptake was 5.2% [79].

Our group published a study with 6 Chagasic cardiomyopathy patients who received intracoronary injection of labeled BM-MNCs [80]. SPECT images performed 1, 3, and 24 hours after administration of the labeled cells revealed a myocardial uptake of 5.4%, 4.3%, and 2.3%, respectively. Such decrease in relative myocardial uptake could be related to leakage of from labeled cells and not to a reduction in the number of cells. We also observed that the cell distribution was heterogeneous and limited and was related with the pattern of myocardial perfusion.

Kollaros et al. [81] compared images obtained from the perfusion study with ²⁰¹Tl and images after intracoronary infusion of BM-MNCs labeled with -HMPAO. In the thirteen patients, images were complementary and revealed accurate localization of cells in the lesioned area. There was intense cell accumulation in areas without viability as evaluated by ²⁰¹Tl scintigraphy. The percentage (83.2%, ranging from 56.4 to 97.2%) of the infarcted area that had retained cells was determined by merging and ²⁰¹Tl images.

4.1.3. Comparison of Biodistribution after Intracoronary and Transendocardial Injection

Vrtovec et al. [82] included a total of 40 patients, where 20 received labeled BM-MNCs by intracoronary and another 20 by transendocardial route. The relative uptake after 18 hours after injection was 4.4% and 19.2% in intracoronary and transendocardial routes, respectively.

Haddad et al. [83] included thirty-seven patients with nonischemic dilated cardiomyopathy. On average, 75 × 10⁶ CD34⁺ PB-MNCs were labeled with -HMPAO and infused via transendocardial route. SPECT images were acquired 2 and 18 hours after infusion to assess the homing and cellular distribution as well as detect cell migration potential. Twenty-eight patients consented to further myocardial homing imaging. In those patients, the stem cells homing rate had a median value of 11.4% (range 3.8%–22.3%).

5. Alternative Approaches to Cell Tracking

Besides radionuclide labeling, different techniques may be used to study cell distribution in vivo. Fluorescence imaging (FLI) and bioluminescence imaging (BLI) have been effectively employed to track cells in preclinical studies of cell transplantation for cardiac diseases [84, 85]. Nevertheless, factors such as the limited tissue penetration of light hinder the clinical application of FLI and BLI [86]. Superparamagnetic iron oxide nanoparticles (SPIONs), originally created to detect liver tumors in patients after intravenous infusion, were adapted for preclinical exogenous cell labeling, which allowed the study of cell migration for weeks following transplantation with exceptional resolution and morphologic correspondence with MRI [87]. Early clinical studies have been conducted in studies of cell therapies for noncardiac diseases [88–92]. Nonetheless, SPION labeling has restrictions of other exogenous contrasts, for instance, the possibility of dilution with cellular division and of stem cell phagocytosis by macrophages. Moreover, there are differing data on the burden of nanoparticle cell labeling in biological properties [93–96], and exogenous SPION cell labeling has only been approved for research applications.

Due to these factors, radiopharmaceutical labeling continues to be a relevant technique for the assessment of stem cell distribution in vivo [7]. It allows more accurate definition of cell location and the combination of Nuclear Medicine with CT or MRI enables the study of diverse characteristics, for example, (1) comparison of cell migration with structural and functional results and (2) the outcome of different cell doses and injection methods on cell homing.

6. Impact of the Route of Administration

Radiopharmaceutical cell tracking has already increased understanding of cell migration in preclinical and clinical studies of cell therapies for cardiac diseases. Among other conclusions, preclinical [55] and clinical [54, 73] studies indicated that intravenous infusions of BM-MNCs and PB-MNCs lead to lower cardiac homing in comparison with intracoronary injections. On the other hand, intramyocardial injection of PB-MNCs [97] and BM-MNCs [53, 54] led to greater cardiac homing of transplanted cells in comparison to intracoronary infusion in preclinical studies. Similarly, transendocardial injection of BM-MNCs led to greater homing in comparison to intracoronary infusion in subjects with nonischemic dilated cardiomyopathy [82].

Even though there have been preclinical and clinical studies investigating the potential of MSC transplantation for cardiac diseases, to our knowledge, no clinical studies yet have tracked MSC migration with noninvasive imaging. Moreover, clinical trials of radiopharmaceutical cell tracking remain restricted to PB-MNC and BM-MNC trials.

Nevertheless, it is still unclear if more intense myocardial homing is important to improve the outcome of cell therapies for cardiac diseases. Different groups have suggested that, instead of differentiation into cardiac cells, the mechanisms of stem cell therapies may be at least partially due to interactions between injected and host cells, such as the secretion of trophic factors [98]. For example, BM-MSCs may assume distinctive phenotypes after receiving stimuli from proinflammatory cytokines or when submitted to a hypoxic milieu in vitro [98].

As previously mentioned, intravenously injected stem cells may suffer pulmonary entrapment [99]. The lungs may characterize an obstacle for cell migration [99] but might also be essential for the triggering of stem cell responses, before their homing to the heart. Lee et al. [100] reported that an increased production of the tumor necrosis factor inducible gene 6 protein (TGS-6) in BM-MSCs is entrapped in the lungs after intravenous injection in mice following acute myocardial infarction. Their report suggested that BM-MSCs were stimulated in the lungs to produce TGS-6, which controlled myocardial inflammatory response.

7. Conclusion

Methods for cell tracking with radioisotopes are feasible and efficient and different studies have used it to monitor migration in cell therapies for cardiac diseases. These techniques provide validated quantifications of cell retention in different organs and the dynamics of cell distribution in the whole body. However, additional reports are needed to increase the knowledge of the mechanisms responsible for cell migration and homing and their relationship with possible structural and functional outcomes of cell transplantation for cardiac diseases.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Authors’ Contribution

Mayra Lorena Moreira and Priscylla da Costa Medeiros contributed equally to this work and should be considered co-first authors.

Acknowledgments

This study was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ). The authors wish to thank Marcos Shimabukuro for revising and editing the language in the text.

References

C. J. L. Murray, T. Vos, R. Lozano et al., “Disability-adjusted life years (DALYs) for 291 diseases and injuries in 21 regions, 1990–2010: a systematic analysis for the Global Burden of Disease Study 2010,” The Lancet, vol. 380, no. 9859, pp. 2197–2223, 2013.

R. Lozano, M. Naghavi, K. Foreman et al., “Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010: a systematic analysis for the Global Burden of Disease Study 2010,” The Lancet, vol. 380, no. 9859, pp. 2095–2128, 2012.

V. F. M. Segers and R. T. Lee, “Stem-cell therapy for cardiac disease,” Nature, vol. 451, no. 7181, pp. 937–942, 2008.

S. A. A. Fisher, C. Doree, A. Mathur, and E. Martin-Rendon, “Meta-analysis of cell therapy trials for patients with heart failure,” Circulation Research, vol. 116, no. 8, pp. 1361–1377, 2015.

M. R. Rosen, R. J. Myerburg, D. P. Francis, G. D. Cole, and E. Marbán, “Translating stem cell research to cardiac disease therapies: pitfalls and prospects for improvement,” Journal of the American College of Cardiology, vol. 64, no. 9, pp. 922–937, 2014.

A. Behfar, R. Crespo-Diaz, A. Terzic, and B. J. Gersh, “Cell therapy for cardiac repair-lessons from clinical trials,” Nature Reviews Cardiology, vol. 11, no. 4, pp. 232–246, 2014.

A. T. Chan and M. R. Abraham, “SPECT and PET to optimize cardiac stem cell therapy,” Journal of Nuclear Cardiology, vol. 19, no. 1, pp. 118–125, 2012.

A. Ruggiero, D. L. J. Thorek, J. Guenoun, G. P. Krestin, and M. R. Bernsen, “Cell tracking in cardiac repair: what to image and how to image,” European Radiology, vol. 22, no. 1, pp. 189–204, 2012.

C. J. Palestro, C. Love, and K. K. Bhargava, “Labeled leukocyte imaging: current status and future directions,” Quarterly Journal of Nuclear Medicine and Molecular Imaging, vol. 53, no. 1, pp. 105–123, 2009.

B. Gutfilen, S. A. L. de Souza, F. P. P. Martins, L. R. Cardoso, M. C. P. Pessoa, and L. M. B. Fonseca, “Use of 99mTc-mononuclear leukocyte scintigraphy in nosocomial fever,” Acta Radiologica, vol. 47, no. 7, pp. 699–704, 2006.

G. Mariani, L. Bruselli, T. Kuwert et al., “A review on the clinical uses of SPECT/CT,” European Journal of Nuclear Medicine and Molecular Imaging, vol. 37, no. 10, pp. 1959–1985, 2010.

C. Wu, G. Ma, J. Li et al., “In vivo cell tracking via ¹⁸F-fluorodeoxyglucose labeling: a review of the preclinical and clinical applications in cell-based diagnosis and therapy,” Clinical Imaging, vol. 37, no. 1, pp. 28–36, 2013.

C. Rischpler, A. Paschali, C. Anagnostopoulos, and S. G. Nekolla, “Cardiac PET for translational imaging,” Current Cardiology Reports, vol. 17, no. 5, p. 28, 2015.

A. Bansal, M. K. Pandey, Y. E. Demirhan et al., “Novel ⁸⁹Zr cell labeling approach for PET-based cell trafficking studies,” EJNMMI Research, vol. 5, article 19, 2015.

H. Hong, Y. Yang, Y. Zhang, and W. Cai, “Non-invasive cell tracking in cancer and cancer therapy,” Current Topics in Medicinal Chemistry, vol. 10, no. 12, pp. 1237–1248, 2010.

P. H. Rosado-de-Castro, P. M. Pimentel-Coelho, B. Gutfilen et al., “Radiopharmaceutical stem cell tracking for neurological diseases,” BioMed Research International, vol. 2014, Article ID 417091, 12 pages, 2014.

S. S. Yaghoubi, D. O. Campbell, C. G. Radu, and J. Czernin, “Positron emission tomography reporter genes and reporter probes: gene and cell therapy applications,” Theranostics, vol. 2, no. 4, pp. 374–391, 2012.

A. R. Penheiter, S. J. Russell, and S. K. Carlson, “The sodium iodide symporter (NIS) as an imaging reporter for gene, viral, and cell-based therapies,” Current Gene Therapy, vol. 12, no. 1, pp. 33–47, 2012.

W. Brenner, A. Aicher, T. Eckey et al., “¹¹¹In-labeled CD34+ hematopoietic progenitor cells in a rat myocardial infarction model,” Journal of Nuclear Medicine, vol. 45, no. 3, pp. 512–518, 2004.

A. Gholamrezanezhad, S. Mirpour, J. M. Ardekani et al., “Cytotoxicity of 111In-oxine on mesenchymal stem cells: a time-dependent adverse effect,” Nuclear Medicine Communications, vol. 30, no. 3, pp. 210–216, 2009.

F. J. Gildehaus, F. Haasters, I. Drosse et al., “Impact of indium-111 oxine labelling on viability of human mesenchymal stem cells in vitro, and 3D cell-tracking using SPECT/CT in vivo,” Molecular Imaging and Biology, vol. 13, no. 6, pp. 1204–1214, 2011.

Y. Jin, H. Kong, R. Z. Stodilka et al., “Determining the minimum number of detectable cardiac-transplanted ¹¹¹In-tropolone-labelled bone-marrow-derived mesenchymal stem cells by SPECT,” Physics in Medicine and Biology, vol. 50, no. 19, pp. 4445–4455, 2005.

G. D. Suhett, S. A. de Souza, A. B. Carvalho et al., “99m-Technetium binding site in bone marrow mononuclear cells,” Stem Cell Research & Therapy, vol. 6, article 115, 2015.

E. Wolfs, T. Struys, T. Notelaers et al., “¹⁸F-FDG labeling of mesenchymal stem cells and multipotent adult progenitor cells for PET imaging: effects on ultrastructure and differentiation capacity,” Journal of Nuclear Medicine, vol. 54, no. 3, pp. 447–454, 2013.

Y. Zhang, J. N. da Silva, T. Hadizad et al., “¹⁸F-FDG cell labeling may underestimate transplanted cell homing: more accurate, efficient, and stable cell labeling with hexadecyl-4-[¹⁸F]fluorobenzoate for in vivo tracking of transplanted human progenitor cells by positron emission tomography,” Cell Transplantation, vol. 21, no. 9, pp. 1821–1835, 2012.

K. J. Blackwood, B. Lewden, R. G. Wells et al., “In vivo SPECT quantification of transplanted cell survival after engraftment using 111In-tropolone in infarcted canine myocardium,” Journal of Nuclear Medicine, vol. 50, no. 6, pp. 927–935, 2009.

M. E. Danoviz, J. S. Nakamuta, F. L. N. Marques et al., “Rat adipose tissue-derived stem cells transplantation attenuates cardiac dysfunction post infarction and biopolymers enhance cell retention,” PLoS ONE, vol. 5, no. 8, Article ID e12077, 2010.

M. M. Welling, M. Duijvestein, A. Signore, and L. van der Weerd, “In vivo biodistribution of stem cells using molecular nuclear medicine imaging,” Journal of Cellular Physiology, vol. 226, no. 6, pp. 1444–1452, 2011.

M. Welling, H. I. J. Feitsma, D. Blok et al., “A new 99mTc labelling method for leucocytes: in vitro and in vivo comparison with 99mTc-HMPAO,” Quarterly Journal of Nuclear Medicine, vol. 39, no. 2, pp. 89–98, 1995.

D. L. Kraitchman, M. Tatsumi, W. D. Gilson et al., “Dynamic imaging of allogeneic mesenchymal stem cells trafficking to myocardial infarction,” Circulation, vol. 112, no. 10, pp. 1451–1461, 2005.

M. Lutz, M. Rosenberg, F. Kiessling et al., “Local injection of stem cell factor (SCF) improves myocardial homing of systemically delivered c-kit⁺ bone marrow-derived stem cells,” Cardiovascular Research, vol. 77, no. 1, pp. 143–150, 2008.

V. N. S. Garikipati, S. Jadhav, L. Pal, P. Prakash, M. Dikshit, and S. Nityanand, “Mesenchymal stem cells from fetal heart attenuate myocardial injury after infarction: an in vivo serial pinhole gated SPECT-CT study in rats,” PLoS ONE, vol. 9, no. 6, Article ID e100982, 2014.

A. Aicher, W. Brenner, M. Zuhayra et al., “Assessment of the tissue distribution of transplanted human endothelial progenitor cells by radioactive labeling,” Circulation, vol. 107, no. 16, pp. 2134–2139, 2003.

I. M. Barbash, P. Chouraqui, J. Baron et al., “Systemic delivery of bone marrow-derived mesenchymal stem cells to the infarcted myocardium: feasibility, cell migration, and body distribution,” Circulation, vol. 108, no. 7, pp. 863–868, 2003.

R. Zhou, D. H. Thomas, H. Qiao et al., “In vivo detection of stem cells grafted in infarcted rat myocardium,” Journal of Nuclear Medicine, vol. 46, no. 5, pp. 816–822, 2005.

D. Shen, D. Liu, Z. Cao, P. D. Acton, and R. Zhou, “Coregistration of magnetic resonance and single photon emission computed tomography images for noninvasive localization of stem cells grafted in the infarcted rat myocardium,” Molecular Imaging and Biology, vol. 9, no. 1, pp. 24–31, 2007.

N. Tran, S. Poussier, P. R. Franken et al., “Feasibility of in vivo dual-energy myocardial SPECT for monitoring the distribution of transplanted cells in relation to the infarction site,” European Journal of Nuclear Medicine and Molecular Imaging, vol. 33, no. 6, pp. 709–715, 2006.

N. Tran, Y. Li, F. Maskali et al., “Short-term heart retention and distribution of intramyocardial delivered mesenchymal cells within necrotic or intact myocardium,” Cell Transplantation, vol. 15, no. 4, pp. 351–358, 2006.

N. Tran, P. R. Franken, F. Maskali et al., “Intramyocardial implantation of bone marrow-derived stem cells enhances perfusion in chronic myocardial infarction: dependency on initial perfusion depth and follow-up assessed by gated pinhole SPECT,” Journal of Nuclear Medicine, vol. 48, no. 3, pp. 405–412, 2007.

G. Wisenberg, K. Lekx, P. Zabel et al., “Cell tracking and therapy evaluation of bone marrow monocytes and stromal cells using SPECT and CMR in a canine model of myocardial infarction,” Journal of Cardiovascular Magnetic Resonance, vol. 11, article 11, 2009.

J. Terrovitis, R. Lautamäki, M. Bonios et al., “Noninvasive quantification and optimization of acute cell retention by in vivo positron emission tomography after intramyocardial cardiac-derived stem cell delivery,” Journal of the American College of Cardiology, vol. 54, no. 17, pp. 1619–1626, 2009.

C. Lang, S. Lehner, A. Todica et al., “Positron emission tomography based in-vivo imaging of early phase stem cell retention after intramyocardial delivery in the mouse model,” European Journal of Nuclear Medicine and Molecular Imaging, vol. 40, no. 11, pp. 1730–1738, 2013.

C. Lang, S. Lehner, A. Todica et al., “In-vivo comparison of the acute retention of stem cell derivatives and fibroblasts after intramyocardial transplantation in the mouse model,” European Journal of Nuclear Medicine and Molecular Imaging, vol. 41, no. 12, pp. 2325–2336, 2014.

A. J. Mitchell, E. Sabondjian, J. Sykes et al., “Comparison of initial cell retention and clearance kinetics after subendocardial or subepicardial injections of endothelial progenitor cells in a canine myocardial infarction model,” Journal of Nuclear Medicine, vol. 51, no. 3, pp. 413–417, 2010.

A. J. Mitchell, E. Sabondjian, K. J. Blackwood et al., “Comparison of the myocardial clearance of endothelial progenitor cells injected early versus late into reperfused or sustained occlusion myocardial infarction,” The International Journal of Cardiovascular Imaging, vol. 29, no. 2, pp. 497–504, 2013.

E. Sabondjian, A. J. Mitchell, G. Wisenberg et al., “Hybrid SPECT/cardiac-gated first-pass perfusion CT: locating transplanted cells relative to infarcted myocardial targets,” Contrast Media and Molecular Imaging, vol. 7, no. 1, pp. 76–84, 2012.

P. Maureira, P.-Y. Marie, Y. Liu et al., “Sustained therapeutic perfusion outside transplanted sites in chronic myocardial infarction after stem cell transplantation,” International Journal of Cardiovascular Imaging, vol. 29, no. 4, pp. 809–817, 2013.

M. H. Kim, S.-K. Woo, K. C. Lee et al., “Longitudinal monitoring adipose-derived stem cell survival by PET imaging hexadecyl-4-¹²⁴I-iodobenzoate in rat myocardial infarction model,” Biochemical and Biophysical Research Communications, vol. 456, no. 1, pp. 13–19, 2015.

E. Elhami, B. Dietz, B. Xiang et al., “Assessment of three techniques for delivering stem cells to the heart using PET and MR imaging,” EJNMMI Research, vol. 3, no. 1, article 72, 2013.

H. Qian, Y. Yang, J. Huang et al., “Intracoronary delivery of autologous bone marrow mononuclear cells radiolabeled by ¹⁸F-fluoro-deoxy-glucose: tissue distribution and impact on post-infarct swine hearts,” Journal of Cellular Biochemistry, vol. 102, no. 1, pp. 64–74, 2007.

B. Doyle, B. J. Kemp, P. Chareonthaitawee et al., “Dynamic tracking during intracoronary injection of ¹⁸F-FDG-labeled progenitor cell therapy for acute myocardial infarction,” Journal of Nuclear Medicine, vol. 48, no. 10, pp. 1708–1714, 2007.

M. C. Keith, Y. Tokita, X. Tang et al., “Effect of the stop-flow technique on cardiac retention of c-kit positive human cardiac stem cells after intracoronary infusion in a porcine model of chronic ischemic cardiomyopathy,” Basic Research in Cardiology, vol. 110, article 46, 2015.

P. Tossios, B. Krausgrill, M. Schmidt et al., “Role of balloon occlusion for mononuclear bone marrow cell deposition after intracoronary injection in pigs with reperfused myocardial infarction,” European Heart Journal, vol. 29, no. 15, pp. 1911–1921, 2008.

J. Mäkelä, V. Anttila, K. Ylitalo et al., “Acute homing of bone marrow-derived mononuclear cells in intramyocardial vs. intracoronary transplantation,” Scandinavian Cardiovascular Journal, vol. 43, no. 6, pp. 366–373, 2009.

V. F. Forest, A. M. Tirouvanziam, C. Perigaud et al., “Cell distribution after intracoronary bone marrow stem cell delivery in damaged and undamaged myocardium: implications for clinical trials,” Stem Cell Research and Therapy, vol. 1, no. 1, article 4, 2010.

S. J. Zhang and J. C. Wu, “Comparison of imaging techniques for tracking cardiac stem cell therapy,” Journal of Nuclear Medicine, vol. 48, no. 12, pp. 1916–1919, 2007.

M. Gyöngyösi, J. Blanco, T. Marian et al., “Serial noninvasive in vivo positron emission tomographic tracking of percutaneously intramyocardially injected autologous porcine mesenchymal stem cells modified for transgene reporter gene expression,” Circulation: Cardiovascular Imaging, vol. 1, no. 2, pp. 94–103, 2008.

J. Terrovitis, M. Stuber, A. Youssef et al., “Magnetic resonance imaging overestimates ferumoxide-labeled stem cell survival after transplantation in the heart,” Circulation, vol. 117, no. 12, pp. 1555–1562, 2008.

A. S. Lee, D. Xu, J. R. Plews et al., “Preclinical derivation and imaging of autologously transplanted canine induced pluripotent stem cells,” The Journal of Biological Chemistry, vol. 286, no. 37, pp. 32697–32704, 2011.

J. Liu, K. H. Narsinh, F. Lan et al., “Early stem cell engraftment predicts late cardiac functional recovery: preclinical insights from molecular imaging,” Circulation: Cardiovascular Imaging, vol. 5, no. 4, pp. 481–490, 2012.

F. Lan, J. Liu, K. H. Narsinh et al., “Safe genetic modification of cardiac stem cells using a site-specific integration technique,” Circulation, vol. 126, no. 11, supplement 1, pp. S20–S28, 2012.

C. Templin, R. Zweigerdt, K. Schwanke et al., “Transplantation and tracking of human-induced pluripotent stem cells in a pig model of myocardial infarction: assessment of cell survival, engraftment, and distribution by hybrid single photon emission computed tomography/computed tomography of sodium iodide symporter transgene expression,” Circulation, vol. 126, no. 4, pp. 430–439, 2012.

X. Yan, P. Ray, R. Paulmurugan et al., “A transgenic tri-modality reporter mouse,” PLoS ONE, vol. 8, no. 8, Article ID e73580, 2013.

Z. Pei, X. Lan, Z. Cheng et al., “Multimodality molecular imaging to monitor transplanted stem cells for the treatment of ischemic heart disease,” PLoS ONE, vol. 9, no. 3, Article ID e90543, 2014.

A. R. Lee, S. K. Woo, S. K. Kang et al., “Adenovirus-mediated expression of human sodium-iodide symporter gene permits in vivo tracking of adipose tissue-derived stem cells in a canine myocardial infarction model,” Nuclear Medicine and Biology, vol. 42, no. 7, pp. 621–629, 2015.

V. Caveliers, G. De Keulenaer, H. Everaert et al., “In vivo visualization of ¹¹¹In labeled CD133+ peripheral blood stem cells after intracoronary administration in patients with chronic ischemic heart disease,” Quarterly Journal of Nuclear Medicine and Molecular Imaging, vol. 51, no. 1, pp. 61–66, 2007.

M. Kurpisz, R. Czepczyński, B. Grygielska et al., “Bone marrow stem cell imaging after intracoronary administration,” International Journal of Cardiology, vol. 121, no. 2, pp. 194–195, 2007.

R. Schots, G. De Keulenaer, D. Schoors et al., “Evidence that intracoronary-injected CD133⁺ peripheral blood progenitor cells home to the myocardium in chronic postinfarction heart failure,” Experimental Hematology, vol. 35, no. 12, pp. 1884–1890, 2007.

V. Schächinger, A. Aicher, N. Döbert et al., “Pilot trial on determinants of progenitor cell recruitment to the infarcted human myocardium,” Circulation, vol. 118, no. 14, pp. 1425–1432, 2008.

C. Dedobbeleer, D. Blocklet, M. Toungouz et al., “Myocardial homing and coronary endothelial function after autologous blood CD34+ progenitor cells intracoronary injection in the chronic phase of myocardial infarction,” Journal of Cardiovascular Pharmacology, vol. 53, no. 6, pp. 480–485, 2009.

D. Blocklet, M. Toungouz, G. Berkenboom et al., “Myocardial homing of nonmobilized peripheral-blood CD34⁺ cells after intracoronary injection,” Stem Cells, vol. 24, no. 2, pp. 333–336, 2006.

M. Hofmann, K. C. Wollert, G. P. Meyer et al., “Monitoring of bone marrow cell homing into the infarcted human myocardium,” Circulation, vol. 111, no. 17, pp. 2198–2202, 2005.

W. J. Kang, H.-J. Kang, H.-S. Kim, J.-K. Chung, M. C. Lee, and D. S. Lee, “Tissue distribution of ¹⁸F-FDG-labeled peripheral hematopoietic stem cells after intracoronary administration in patients with myocardial infarction,” Journal of Nuclear Medicine, vol. 47, no. 8, pp. 1295–1301, 2006.

E. Goussetis, A. Manginas, M. Koutelou et al., “Intracoronary infusion of CD133⁺ and CD133⁻CD34⁺ selected autologous bone marrow progenitor cells in patients with chronic ischemic cardiomyopathy: cell isolation, adherence to the infarcted area, and body distribution,” Stem Cells, vol. 24, no. 10, pp. 2279–2283, 2006.

M. Penicka, O. Lang, P. Widimsky et al., “One-day kinetics of myocardial engraftment after intracoronary injection of bone marrow mononuclear cells in patients with acute and chronic myocardial infarction,” Heart, vol. 93, no. 7, pp. 837–841, 2007.

S. A. Silva, A. L. S. Sousa, A. F. Haddad et al., “Autologous bone-marrow mononuclear cell transplantation after acute myocardial infarction: comparison of two delivery techniques,” Cell Transplantation, vol. 18, no. 3, pp. 343–352, 2009.

R. D. C. Moreira, A. F. Haddad, S. A. Silva et al., “Intracoronary stem-cell injection after myocardial infarction: microcirculation sub-study,” Arquivos Brasileiros de Cardiologia, vol. 97, no. 5, pp. 420–426, 2011.

P. Musialek, L. Tekieli, M. Kostkiewicz et al., “Randomized transcoronary delivery of CD34⁺ cells with perfusion versus stop-flow method in patients with recent myocardial infarction: early cardiac retention of -labeled cells activity,” Journal of Nuclear Cardiology, vol. 18, no. 1, pp. 104–116, 2011.

P. Musialek, L. Tekieli, M. Kostkiewicz et al., “Infarct size determines myocardial uptake of CD34⁺ cells in the peri-infarct zone: results from a study of -extamatazime-labeled cells visualization integrated with cardiac magnetic resonance infarct imaging,” Circulation: Cardiovascular Imaging, vol. 6, no. 2, pp. 320–328, 2013.

L. M. Barbosa da Fonseca, S. S. Xavier, P. H. Rosado De Castro et al., “Biodistribution of bone marrow mononuclear cells in chronic chagasic cardiomyopathy after intracoronary injection,” International Journal of Cardiology, vol. 149, no. 3, pp. 310–314, 2011.

N. Kollaros, A. Theodorakos, A. Manginas et al., “Bone marrow stem cell adherence into old anterior myocardial infarction: a scintigraphic study using Tl-201 and Tc-99m-HMPAO,” Annals of Nuclear Medicine, vol. 26, no. 3, pp. 228–233, 2012.

B. Vrtovec, G. Poglajen, L. Lezaic et al., “Comparison of transendocardial and intracoronary CD34⁺ cell transplantation in patients with nonischemic dilated cardiomyopathy,” Circulation, vol. 128, no. 1, pp. S42–S49, 2013.

F. Haddad, M. Sever, G. Poglajen et al., “Immunologic network and response to intramyocardial CD34⁺ stem cell therapy in patients with dilated cardiomyopathy,” Journal of Cardiac Failure, vol. 21, no. 7, pp. 572–582, 2015.

S. Roura, C. Gálvez-Montón, and A. Bayes-Genis, “Bioluminescence imaging: a shining future for cardiac regeneration,” Journal of Cellular and Molecular Medicine, vol. 17, no. 6, pp. 693–703, 2013.

K. J. Ransohoff and J. C. Wu, “Advances in cardiovascular molecular imaging for tracking stem cell therapy,” Thrombosis and Haemostasis, vol. 104, no. 1, pp. 13–22, 2010.

A. S. Arbab, B. Janic, J. Haller, E. Pawelczyk, W. Liu, and J. A. Frank, “In vivo cellular imaging for translational medical research,” Current Medical Imaging Reviews, vol. 5, no. 1, pp. 19–38, 2009.

P. Hua, Y.-Y. Wang, L.-B. Liu et al., “In vivo magnetic resonance imaging tracking of transplanted superparamagnetic iron oxide-labeled bone marrow mesenchymal stem cells in rats with myocardial infarction,” Molecular Medicine Reports, vol. 11, no. 1, pp. 113–120, 2014.

F. Callera and C. M. T. P. de Melo, “Magnetic resonance tracking of magnetically labeled autologous bone marrow CD34⁺ cells transplanted into the spinal cord via lumbar puncture technique in patients with chronic spinal cord injury: CD34⁺ cells' migration into the injured site,” Stem Cells and Development, vol. 16, no. 3, pp. 461–466, 2007.

J. Zhu, L. Zhou, and F. X. Wu, “Tracking neural stem cells in patients with brain trauma,” The New England Journal of Medicine, vol. 355, no. 22, pp. 2376–2378, 2006.

I. J. M. de Vries, W. J. Lesterhuis, J. O. Barentsz et al., “Magnetic resonance tracking of dendritic cells in melanoma patients for monitoring of cellular therapy,” Nature Biotechnology, vol. 23, no. 11, pp. 1407–1413, 2005.

C. Toso, J.-P. Vallee, P. Morel et al., “Clinical magnetic resonance imaging of pancreatic islet grafts after iron nanoparticle labeling,” American Journal of Transplantation, vol. 8, no. 3, pp. 701–706, 2008.

T. J. England, M. Abaei, D. P. Auer et al., “Granulocyte-colony stimulating factor for mobilizing bone marrow stem cells in subacute stroke: the stem cell trial of recovery enhancement after stroke 2 randomized controlled trial,” Stroke, vol. 43, no. 2, pp. 405–411, 2012.

E. Farrell, P. Wielopolski, P. Pavljasevic et al., “Effects of iron oxide incorporation for long term cell tracking on MSC differentiation in vitro and in vivo,” Biochemical and Biophysical Research Communications, vol. 369, no. 4, pp. 1076–1081, 2008.

H. S. Kim, S. Y. Oh, H. J. Joo, K.-R. Son, I.-C. Song, and W. K. Moon, “The effects of clinically used MRI contrast agents on the biological properties of human mesenchymal stem cells,” NMR in Biomedicine, vol. 23, no. 5, pp. 514–522, 2010.

Jasmin, A. L. M. Torres, L. Jelicks, A. C. C. De Carvalho, D. C. Spray, and R. Mendez-Otero, “Labeling stem cells with superparamagnetic iron oxide nanoparticles: analysis of the labeling efficacy by microscopy and magnetic resonance imaging,” Methods in Molecular Biology, vol. 906, pp. 239–252, 2012.

Jasmin, A. L. M. Torres, H. M. P. Nunes et al., “Optimized labeling of bone marrow mesenchymal cells with superparamagnetic iron oxide nanoparticles and in vivo visualization by magnetic resonance imaging,” Journal of Nanobiotechnology, vol. 9, article 4, 2011.

D. Hou, E. A.-S. Youssef, T. J. Brinton et al., “Radiolabeled cell distribution after intramyocardial, intracoronary, and interstitial retrograde coronary venous delivery: implications for current clinical trials,” Circulation, vol. 112, no. 9, pp. I150–I156, 2005.

S. H. Ranganath, O. Levy, M. S. Inamdar, and J. M. Karp, “Harnessing the mesenchymal stem cell secretome for the treatment of cardiovascular disease,” Cell Stem Cell, vol. 10, no. 3, pp. 244–258, 2012.

U. M. Fischer, M. T. Harting, F. Jimenez et al., “Pulmonary passage is a major obstacle for intravenous stem cell delivery: the pulmonary first-pass effect,” Stem Cells and Development, vol. 18, no. 5, pp. 683–691, 2009.

R. H. Lee, A. A. Pulin, M. J. Seo et al., “Intravenous hMSCs improve myocardial infarction in mice because cells embolized in lung are activated to secrete the anti-inflammatory protein TSG-6,” Cell Stem Cell, vol. 5, no. 1, pp. 54–63, 2009.