99mTc-S-acetylmercaptoacetyltriserine-stromal-derived factor 1α

99mTc-MAS3-SDF-1α

Chopra A.

Publication Details

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In vitro Rodents

Background

[PubMed]

The immune system is largely modulated by a variety of small, glycosylated proteins called chemokines, which vary in size from ~5 to 15 kDa. These molecules possess strong chemotactic or leukocyte activation properties and are structurally related. They are classified by the International Union of Immunological Societies and the World Health Organization into four groups designated as C, CC, CXC, and CX3C on the basis of the location of the cysteine residues in the mature protein (1). The chemokines interact with several different 7-transmembrane G-protein–coupled receptors that are designated according to the chemokine subfamily name followed by the letter “R” (for receptor) and a number based on the chronological order of its identification within a subfamily (for details see Thorpe (1)). The stromal-derived factor-1α (SDF-1α or CXC12) is a member of the CXC family of chemokines that binds to the CXCR4. SDF-1α has an important role in several immunological processes, including hematopoiesis, vasculogenesis, neuronal development, and immune cell mobilization (2-4). Because of their role in several diverse physiological processes, SDF-1α and its receptor (CXCR4) have been implicated in a variety of pathological conditions such as inflammation, cancer, and HIV infections (5-7). The United States Food and Drug Administration has approved several clinical trials to evaluate the use of SDF-1α and CXCR4 receptors in cancer and myocardial infarction (MI) treatment.

SDF-1α and its receptor have also been shown to be involved in the neointimal recruitment of smooth muscle cell (SMC) progenitors, probably as a response to SMC apoptosis, during restenosis and cardiac allograft vasculopathy (8). In addition, SDF-1α and CXCR4 are expressed on hematopoietic stem cells and endothelial progenitor cells, are necessary for cardiogenesis, and have a role in cardiac myocyte contraction in rats (9-11). The levels of the chemokine and its receptor are elevated during cardiac dysfunction and in the hearts of patients experiencing congestive heart failure (12-14). Chemokines have also been shown to mobilize stem cells after MI, probably to repair the injured myocardium by the re-expression of SDF-1α in the tissue, which could possibly help direct the homing of stem cells to the injured area (15, 16). The exact mechanism(s) of how chemokines and their receptors bring about these effects in the injured cardiac tissue are not clear. In this regard it becomes important to investigate the chemokines and their receptors noninvasively in an in vivo setting to understand the role and functioning of these molecules in the myocardium. Misra et al. labeled SDF-1α with radioactive technetium (99mTc) and used it to quantify CXCR4 expression in a rat MI model (17). The biodistribution of 99mTc-labeled SDF-1α was also studied in animals.

Synthesis

[PubMed]

99mTc-SDF-1α was synthesized using an N-hydroxysuccinimide (NHS) ester of 99mTc-S-acetylmercaptoacetyltriserine (99mTc-MAS3) as described by Misra et al. (17). 99mTc-MAS3-NHS (purity >99%) was prepared in dimethylsulfoxide (DMSO) as described elsewhere (18). Recombinant murine SDF-1α was obtained as a dry powder from commercial sources and resuspended in DMSO. The SDF-1α suspension was mixed with 99mTc-MAS3-NHS after the addition of 4 M triethylamine and stirred at room temperature for 1 h. Labeled SDF-1α was purified with gel filtration on a 60-Å Diol gel filtration column (GFC) on phosphate-buffered saline (PBS) (pH 7.4). The GFC-purified 99mTc-MAS3-SDF-1α was reported to have a specific activity of 8.0 × 107 MBq/mmol (2,166 Ci/mmol). Stability of the labeled product was studied with GFC chromatography after boiling it in PBS for 5 min, with or without dithiothreitol, and by incubating it in 100% calf serum for 4 h at 37°C (17). No dissociation or transmetallation was reported under these conditions, and >98% of 99mTc-MAS3-SDF-1α was intact as determined with GFC chromatography.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

The affinity and specificity of 99mTc-MAS3-SDF-1α for CXCR4 in PC-3 human prostate cancer cells transduced with an adenoviral vector coexpressing CXCR4 and green fluorescence protein (this was used as an indicator to confirm that the cells were transduced with the vector) was determined using a live-cell homologous competition assay as described by Misra et al. (18). Control cells were either transduced with an adenoviral vector coexpressing bone morphogenic protein-2 and green fluorescence protein or untransduced cells. A high specificity for CXCR4 was exhibited by 99mTc-MAS3-SDF-1α with an affinity of 1.0±0.1 nM, and the maximal binding was 2.6 × 105 binding sites per cell. No blocking studies were reported.

Animal Studies

Rodents

[PubMed]

The biodistribution and clearance of 99mTc-MAS3-SDF-1α was investigated in Sprague-Dawley inbred rats (the number of animals used was not reported) with or without MI as described by Misra et al. (17). The animals were administered intravenously with the radioactive compound through the retroorbital sinus, and blood was collected from the tail vein at various times from 0 to 120 min after administration. The main organs (stomach, lungs, liver, etc.) of the animals were obtained 2 h after administration of the radiochemical, washed twice with PBS, weighed, and counted for radioactivity. Heart tissue from the MI animals was divided into infarcted and noninfarcted tissue for quantitation. Control animals were also injected with 99mTc-MAS3 alone. The blood half-life of 99mTc-MAS3-SDF-1α was determined to be 25.8 ± 4.6 min compared to 5.0 ± 1.0 min for 99mTc-MAS3 alone (17). An uptake of <0.1% of the injected dose/gram tissue (% ID/g) was noted in the major organs, except the kidneys, with no uptake in the brain. The kidneys had ~0.5% ID/g, and the radioactivity was detected primarily in the urine (the exact % ID/g was not reported) with a total of 73.8 ± 6.1% ID/g in the excrement, including feces.

Using rats with an induced MI that were injected with 99mTc-MAS3-SDF-1α, the expression of CXC4 was determined to be increased in the MI tissue (17). The MI tissue showed a five-fold increase (P < 0.0001) in 99mTc-MAS3-SDF-1α binding (0.57 ± 0.04% ID/g) compared with the noninfarcted or normal heart tissue (0.11 ± 0.1% ID/g). Blocking studies, using unlabelled ligand, were not reported.

Other Non-Primate Mammals

[PubMed]

No references are currently available.

Non-Human Primates

[PubMed]

No references are currently available.

Human Studies

[PubMed]

No references are currently available.

Supplemental Information

[Disclaimers]

NIH Support

This work was supported by NIH grants R01-CA-115296, R01-HL-073458 and R01-HL-078691.

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