Tissue Distribution of ¹⁸F-FDG-Labeled Peripheral Hematopoietic Stem Cells After Intracoronary Administration in Patients with Myocardial Infarction

Subjects

We enrolled 17 patients (14 males, 3 females; age, 58 ± 12 y) with an ST-elevated myocardial infarction who were scheduled to undergo elective percutaneous coronary intervention (PCI) and stent implantation at the infarct-related artery. Eleven patients had a history of acute chest pain (<13 d), and 5 had a history of an old myocardial infarction (>90 d) (Table 1).

To evaluate the distribution of intravenously injected stem cells, 3 additional patients with acute myocardial infarction were enrolled as the intravenous injection group (3 males, 0 females; age, 50 ± 20 y). The patients in the intravenous injection group also underwent PCI and stent implantation before stem cell injection.

Peripheral Stem Cell Preparation

After successful PCI and stent implantation, granulocyte colony-stimulating factor (G-CSF; Dong-A Pharmaceuticals) was subcutaneously injected at 10 μg/kg body weight for 4 d. Peripheral blood stem cells were obtained using a COBE spectra apheresis system (COBE BCT Inc.) with the mononuclear cell collection method recommended by the manufacturer. Stem cell collection was performed over 60–90 min with an antecubital vein access procedure. The total processing volume of blood ranged from 4,500 to 7,000 mL, and 30–45 mL of apheresis fluid were obtained.

¹⁸F-FDG Labeling

A total of 2 mL of apheresis fluid containing ethylenediaminetetraacetic acid (EDTA) as the anticoagulation agent was used for ¹⁸F-FDG labeling. Red-blood-cell hemolysis over 30 s was done with 30 mL of distilled water. After 5 min of centrifugation at 1,800 rpm, the pellet containing stem cells was obtained. A total of 175–370 MBq (5–10 mCi) of ¹⁸F-FDG in 1 mL of normal saline was mixed with the pellet. Stem cells in this ¹⁸F-FDG solution were incubated for 40 min at 37°C with gentle swirling at 5-min intervals. Labeled stem cells were resuspended in 20 mL of normal saline and then centrifuged for 5 min at 1,800 rpm. The labeling efficiency was measured by counting the radioactivity in the pellet and the supernatant. This pellet was resuspended in 1 mL of normal saline and administered immediately by intracoronary injection to the patients. Sixty minutes were required for stem cell labeling procedures.

To examine the stability of ¹⁸F-FDG binding, the pellet was incubated with plasma for 3 h. The labeling efficiency after incubation was also measured by counting the radioactivity in the pellet after centrifugation.

For the viability test, labeled stem cells suspended in plasma were stored for 3 h. Cell viability was assessed by trypan blue staining.

Stem Cell Administration and PET/CT Image Acquisition

In the coronary infusion group, ¹⁸F-FDG-labeled peripheral stem cells were administered by over-the-wire angioplasty balloon catheter through another coronary catheterization performed 4–5 d after initial PCI. Restoration of TIMI (Thrombolysis in Myocardial Infarction) grade 3 flow and preserved coronary flow reserve measured by coronary Doppler wire were confirmed before stem cell infusion.

In the intravenous injection group, ¹⁸F-FDG-labeled stem cells prepared with the same protocol were injected in bolus via a peripheral vein at 4–5 d after PCI.

After 2 h of stem cell injection, whole-body PET images were acquired with the conventional 3-dimensional protocol of ¹⁸F-FDG PET using a Gemini PET/CT camera (Philips). Low-dose CT (120 kV, 50 mA) from head to pelvis was acquired for attenuation correction and anatomic localization. Emission PET images were acquired for 2 min 30 s per each bed with a 3-dimensional acquisition mode.

To examine the delayed distribution, an additional 4-h image was acquired in 2 patients and 4- and 20-h images were acquired in another patient.

Dosimetry

Transaxial images from 2- and 4-h acquisitions of a patient were analyzed. A region of interest was drawn manually on transaxial images on the bone marrow, myocardium, liver, spleen, and bladder. The half-life and residence time were calculated for each organ from the time–activity curves of these organs. MIRDOSE3.1 was used to calculate radiation exposure (Oak Ridge Associated University, Oak Ridge, TN).

Stem Cell Labeling with ¹⁸F-FDG

Two milliliters of apheresis fluid rich in mononuclear cells were used for ¹⁸F-FDG labeling. The mononuclear cell fraction was 84.3% and the remaining cells were primarily neutrophils and their precursor cells. An initial experiment indicated that the best labeling efficiency was acquired with an EDTA-containing sample. The same samples from 1 patient showed different labeling efficiencies: 96% in EDTA, 62% in heparin, and 56% in acid–citrate–dextrose (ACD). Therefore, subsequent samples were treated with EDTA for anticoagulation.

The overall labeling efficiency was 73% ± 17% with EDTA. Finally, 136.9 ± 64.8 MBq (3.7 ± 1.75 mCi) of ¹⁸F-FDG-labeled stem cells were prepared. The injected activity of stem cells is presented in Table 1. The heparin-containing sample frequently showed cell aggregation and caused labeling failure. ACD also showed a low labeling efficiency, which was thought to be related to the glucose contained in ACD.

Stability and Viability Test

¹⁸F-FDG-labeled stem cells were incubated in plasma for 3 h and resuspended with normal saline. The radioactivity of the stem cell-containing pellet was ≥90%.

For viability assessment, microscopic examination was done on ¹⁸F-FDG-labeled stem cells after 3 h of incubation in plasma. Microscopic examination revealed that there were few trypan blue–defective stem cells (Fig. 1).

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FIGURE 1. 

Trypan blue staining of ¹⁸F-FDG-labeled stem cells. There were few trypan blue–defective cells.

Distribution of Stem Cells After Intracoronary Injection

¹⁸F-FDG-labeled stem cell PET images were acquired in 17 patients in the intracoronary injection group and in 3 in the intravenous injection group without complication during the injection period (Table 1). A total of 4.5 × 10⁸ leukocytes (8.3% ± 10.2% CD34+ cells) labeled with ¹⁸F-FDG was injected.

PET images were acquired after 2 h of stem cell injection. ¹⁸F-FDG-labeled stem cells showed physiologic distribution predominantly within the reticuloendothelial system (Figs. 2 and 3). Liver, spleen, and bone marrow showed prominent stem cell uptake. Brain and bladder activity of stem cell imaging were markedly lower than that of the pure ¹⁸F-FDG PET image (Table 2).

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FIGURE 2. 

PET/CT images of 65-y-old man with history of anterior wall infarction. After percutaneous intervention, ¹⁸F-FDG-labeled stem cells were injected via intracoronary catheter. PET/CT images were obtained 2 h after injection. Stem cell accumulation at myocardium is well visualized (arrow). Total amount of stem cells at myocardium was 2.1% of injected dose (case 1 of Table 1).

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FIGURE 3. 

PET/CT image of 30-y-old man with inferior wall infarction (case 2 of Table 1). Total amount of stem cells at myocardium (arrow) was 2.7% of injected dose.

There was no significant difference in the stem cell amount at the myocardium according to the age of infarction (1.6 ± 0.88 vs.1.48 ± 0.79; P = 0.78, in acute myocardial infarction and old myocardial infarction, respectively).

No correlation was found between the CK-MB (creatine kinase isoenzyme containing M and B subunits) level and stem cell activity at the myocardium (P = 0.372).

Stem Cell Quantification at Myocardium and Changes in Delayed Periods

Stem cell activity was found on myocardium within the territories of infarct-related arteries injected. The calculated amount on myocardium was 1.5% ± 0.8% of injected dose. It ranged from 0.2% to 3.3%. To evaluate the change of tissue distribution and homing at the myocardium, 4-h and 20-h delayed images were acquired in 2 subjects and 1 subject, respectively (Tables 3 and 4). Myocardial uptake persisted until 4-h delayed images and even remained until 20-h delayed images (Figs. 4A and 4B).

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FIGURE 4. 

Distribution of stem cells after intracoronary injection at 24 h (A, case 3 of Table 1) and at 2–20 h (B, case 4 of Table 1). Stem cell activity (arrow) remained on 4-h image, and splenic uptake was prominent. Myocardial uptake could be noted on CT-guided anatomic localization on 20-h image.

Distribution of Stem Cells After Intravenous Injection

After intravenous injection of ¹⁸F-FDG-labeled stem cells, there was no stem cell activity within the myocardium. The whole-body distribution of stem cells after intravenous injection was similar to that of intracoronary injection except for the initial high lung uptake (Table 5). The reticuloendothelial system took up most of the stem cells after intravenous injection. Initial high lung uptake on the 30-min image was cleared away and splenic uptake increased on the 2-h delayed image (Fig. 5).

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FIGURE 5. 

PET/CT images of 39-y-old male patient with anterior wall infarction (case 18 of Table 1). Stem cells were injected intravenously, and PET/CT images were acquired at 30 min (A) and at 2 h (B). Initial high lung uptake was observed at 30 min and cleared on 2-h image. No myocardial uptake was evident on both images.

Dosimetry

Similar to white blood cell images, ¹⁸F-FDG-labeled stem cell uptake occurred primarily in the reticuloendothelial system. The radiation dose to the myocardium was calculated to be 0.017 mGy/MBq (0.064 rad/mCi) (Table 6).