Macrophage mannose receptor CD206 targeting of fluoride-18 labeled mannosylated dextran: A validation study in mice

PURPOSE

Aluminum fluoride-18-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid-conjugated mannosylated dextran derivative (Al[18F]F-NOTA-D10CM) is a new tracer for PET imaging. We report here on in vitro and in vivo validation of the tracer's ability to target the macrophage mannose receptor CD206.

METHODS

First, the uptake of intravenously (i.v.) administered Al[18F]F-NOTA-D10CM was compared between wild-type (WT) and CD206-/- knockout (KO) mice. C57BL/6N mice were injected with complete Freund's adjuvant (CFA) in the left hind leg and the uptake of Al[18F]F-NOTA-D10CM after i.v. or intradermal (i.d.) injection was studied at 5 and 14 days after CFA induction of inflammation. Healthy C57BL/6N mice were studied as controls. Mice underwent PET/CT on consecutive days with [18F]FDG, i.v. Al[18F]F-NOTA-D10CM, and i.d. Al[18F]F-NOTA-D10CM. After the last imaging, Al[18F]F-NOTA-D10CM was i.v. injected for an ex vivo biodistribution study and autoradiography of inflamed tissues. Blood plasma samples were analyzed using high-performance liquid chromatography. To evaluate the specificity of Al[18F]F-NOTA-D10CM binding, an in vitro competitive displacement study was performed on inflamed tissue sections using autoradiography. CD206 expression was assessed by immunohistochemical staining.

RESULTS

Compared with WT mice, the uptake of Al[18F]F-NOTA-D10CM was significantly lower in several CD206-/- KO mice tissues, including liver (SUV 8.21 ± 2.51 vs. 1.06 ± 0.16, P < 0.001) and bone marrow (SUV 1.63 ± 0.37 vs. 0.22 ± 0.05, P < 0.0001). The uptake of i.v. injected Al[18F]F-NOTA-D10CM was significantly higher in inflamed ankle joint (SUV 0.48 ± 0.13 vs. 0.18 ± 0.05, P < 0.0001) and inflamed foot pad skin (SUV 0.41 ± 0.10 vs. 0.04 ± 0.01, P < 0.0001) than in the corresponding tissues in healthy mice. The i.d.-injected Al[18F]F-NOTA-D10CM revealed differences between CFA-induced lymph node activation and lymph nodes in healthy mice. Ex vivo γ-counting, autoradiography, and immunohistochemistry supported the results, and a decrease of ~ 80% in the binding of Al[18F]F-NOTA-D10CM in the displacement study with excess NOTA-D10CM confirmed that tracer binding was specific. At 60 min after i.v. injection, an average 96.70% of plasma radioactivity was derived from intact Al[18F]F-NOTA-D10CM, indicating good in vivo stability. The uptake of Al[18F]F-NOTA-D10CM into inflamed tissues was positively associated with the area percentage of CD206-positive staining.

CONCLUSION

The uptake of mannosylated dextran derivative Al[18F]F-NOTA-D10CM correlated with CD206 expression and the tracer appears promising for inflammation imaging.

Aluminum Fluoride-18 Labeled Mannosylated Dextran: Radiosynthesis and Initial Preclinical Positron Emission Tomography Studies

PURPOSE

In addition to being expressed on liver sinusoidal endothelial cells, mannose receptors are also found on antigen-presenting cells, including macrophages, which are mainly involved in the inflammation process. Dextran derivatives of various sizes containing cysteine and mannose moieties have previously been labeled with 99mTc and used for single-photon emission computed tomography imaging of sentinel lymph nodes. In this study, we radiolabeled 21.3-kDa D10CM with positron-emitting 18F for initial positron emission tomography (PET) studies in rats.

PROCEDURES

D10CM was conjugated with 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) chelator and radiolabeled with the aluminum fluoride-18 method. The whole-body distribution kinetics and stability of the intravenously administered tracer were studied in healthy male Sprague-Dawley rats by in vivo PET/CT imaging, ex vivo gamma counting, and high-performance liquid chromatography analysis.

RESULTS

Al[18F]F-NOTA-D10CM was obtained with a radiochemical purity of >99% and molar activity of 9.9 GBq/μmol. At 60 minutes after injection, an average of 84% of the intact tracer was found in the blood, indicating excellent in vivo stability. The highest radioactivity concentration was seen in the liver, spleen, and bone marrow, in which mannose receptors are highly expressed under physiological conditions. The uptake specificity was confirmed with in vivo blocking experiments.

CONCLUSIONS

Our results imply that Al[18F]F-NOTA-D10CM is a suitable tracer for PET imaging. Further studies in disease models with mannose receptor CD206-positive macrophages are warranted to clarify the tracer's potential for imaging of inflammation.

In Vivo Imaging of [60]Fullerene-Based Molecular Spherical Nucleic Acids by Positron Emission Tomography

18F-Labeled [60]fullerene-based molecular spherical nucleic acids (MSNAs), consisting of a human epidermal growth factor receptor 2 (HER2) mRNA antisense oligonucleotide sequence with a native phosphodiester and phosphorothioate backbone, were synthesized, site-specifically labeled with a positron emitting fluorine-18 and intravenously administrated via tail vein to HER2 expressing HCC1954 tumor-bearing mice. The biodistribution of the MSNAs was monitored in vivo by positron emission tomography/computed tomography (PET/CT) imaging. MSNA with a native phosphodiester backbone (MSNA-PO) was prone to rapid nuclease-mediated degradation, whereas the corresponding phosphorothioate analogue (MSNA-PS) with improved enzymatic stability showed an interesting biodistribution profile in vivo. One hour after the injection, majority of the radioactivity was observed in spleen and liver but also in blood with an average tumor-to-muscle ratio of 2. The prolonged radioactivity in blood circulation may open possibilities to the targeted delivery of the MSNAs.

Exploring macrophage mannose receptor expression after myocardial infarction by Al[18F]F-NOTA-DCM positron emission tomography

Introduction

Mannose receptor (CD206) is primarily expressed on the surface of alternatively activated macrophages that are involved in resolution of inflammation after myocardial injury [1]. The purpose of this study was to evaluate mannose receptor targeting positron emission tomography (PET) tracer Al[18F]F-NOTA-DCM consisting of dextran backbone with cysteine-mannose moieties for imaging of experimental acute myocardial infarction (MI) [2].

Methods

First, ALEXA-488 fluorophore-labelled DCM was used for specificity studies using flow cytometry of M1 and M2 polarized macrophages derived from human blood monocytes. Secondly, Sprague-Dawley rats were studied on day 3 and day 7 after permanent ligation of left coronary artery or after sham-operation. [18F]FDG PET (35 MBq, 10 min static scan) was performed to visualize myocardium and on the next day, 60 min dynamic PET was performed after injection of 50 MBq of Al[18F]F-NOTA-DCM. Then, rats were euthanized for biodistribution study by gamma counting followed by digital autoradiography and histology (H&E, CD206 staining) of left ventricle cryosections. In vitro Al[18F]F-NOTA-DCM blocking study was performed on left ventricle cryosection with molar excess of unlabelled DCM.

Results

Flow cytometry confirmed that ALEXA-488-DCM bound specifically to M2 macrophages. In rats, the infarcted area was clearly detected in vivo with Al[18F]F-NOTA-DCM PET and its SUV was significantly higher than that of remote area or myocardium of sham-operated rats both on day 3 (SUV 0.78±0.18 vs. 0.47±0.13 vs. 0.43±0.07, p&lt;0.005) and day 7 post-MI (SUV 0.64±0.10 vs. 0.47±0.12 vs. 0.51±0.07, p&lt;0.05). Autoradiography confirmed increased uptake in the infarcted area compared to the remote area or to the myocardium of sham-operated rats on day 3 (PSL/mm2 141.21±46.06 vs. 49.76±20.37 vs. 57.97±6.77, p&lt;0.005) and day 7 (PSL/mm2 139.22±19.44 vs. 55.38±28.83 vs. 60.83±7.63, p&lt;0.0001). In vitro blocking study indicated that the tracer binding in infarcted area was specific. The area-% of CD206-positive staining in the infarcted area was significantly higher on day 3 post-MI than on day 7 (p&lt;0.05), and higher at both time points than in remote area or myocardium of sham-operated rats (p&lt;0.0001). Area-% of CD206 staining in the MI area positively correlated with Al[18F]F-NOTA-DCM uptake and MI size (p&lt;0.05 and p&lt;0.01, respectively).

Conclusions

Al[18F]F-NOTA-DCM PET detects overexpression of mannose receptor after ischemic myocardial injury and may be a suitable biomarker for early detection of the inflammation resolution process after MI.

Funding Acknowledgement

Type of funding sources: Foundation. Main funding source(s): Jane and Aatos Erkko FoundationSigrid Juselius FoundationFInnish Foundation for Cardiovascular Research

Quantitative analysis of Gd in the protein content of the brain following single injection of gadolinium-based contrast agents (GBCAs) by size exclusion chromatography

OBJECTIVE

To investigate the role of transporter proteins in gadolinium (Gd) distribution and retention in the brain after one high-dose injection of Gd-based contrast agent (GBCA).

METHODS AND MATERIALS

30 ddY mice were randomly divided into three treatment groups to be intravenously injected with either Gadodiamide (linear GBCA), Gadobutrol (macrocyclic GBCA), or Gadoterate (macrocyclic GBCA) at a dose of 5 mmol/kg, while five mice in the control group received 250 µL saline. Five minutes (5 min) and ten days (10d) post-injection, the cerebrospinal fluid (CSF), choroid plexus (CP), and meninges and associated vasculature (MAV) were collected. The brain was then dissected to obtain the olfactory bulb, cerebral cortex, hippocampus, cerebellum, and brainstem. Proteins were extracted and separated by a size-exclusion high-performance liquid chromatography (SEC) system, and Gd concentrations were quantified by inductively coupled plasma mass spectrometry (ICP-MS).

RESULTS

5 m post-injection, the Gadodiamide group had the highest Gd concentration, while Gadoterate had the lowest Gd concentration in all parts of the brain (p < .05). Gd concentration was highest in the cerebrospinal fluid (CSF) of the Gadodiamide group (578.4 ± 135.3 nmol), while Gd concentration was highest in MAV in the Gadobutrol group (379.7 ± 75.4 nmol) at 5 min post-injection. At 10d, in spite of the significant decrease of Gd from all GBCAs ( p < 0.01), retained Gd from Gadodiamide was detected all over the brain in several molecules that varied in size. Gd from Gadobutrol detected in the olfactory bulb (8.7 ± 4.5 nmol) was significantly higher than in other parts of the brain. Although most Gd from Gadobutrol was found in molecules similar in size to Gadobutrol, it was also found in several protein molecules of molecular size larger than the contrast agents. Only a small amount of Gd from Gadoterate was found in the brain.

CONCLUSION

GBCAs may be able to pass through intact brain barriers, and the chemical structures of GBCAs may affect the penetration capability of Gd into the brain. Retained Gd in the brain tissue from Gadodiamide and Gadobutrol may be bound to some organic molecules, including proteins.

ADVANCES IN KNOWLEDGE

Intact GBCA are able to penetrate a series of brain barrier immediately after administration regardless the type of the chelate. Gd may be bound with macromolecules that may cause Gd retention in the brain.