Size and specificity of radiopharmaceuticals for sentinel lymph node detection

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.

Radiolabelled Polymeric Materials for Imaging and Treatment of Cancer: Quo Vadis?

Owing to their tunable blood circulation time and suitable plasma stability, polymer-based nanomaterials hold a great potential for designing and utilising multifunctional nanocarriers for efficient imaging and effective treatment of cancer. When tagged with appropriate radionuclides, they may allow for specific detection (diagnosis) as well as the destruction of tumours (therapy) or even customization of materials, aiming to both diagnosis and therapy (theranostic approach). This review provides an overview of recent developments of radiolabelled polymeric nanomaterials (natural and synthetic polymers) for molecular imaging of cancer, specifically, applying nuclear techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT). Different approaches to radiolabel polymers are evaluated from the methodical radiochemical point of view. This includes new bifunctional chelating agents (BFCAs) for radiometals as well as novel labelling methods. Special emphasis is given to eligible strategies employed to evade the mononuclear phagocytic system (MPS) in view of efficient targeting. The discussion encompasses promising strategies currently employed as well as emerging possibilities in radionuclide-based cancer therapy. Key issues involved in the clinical translation of radiolabelled polymers and future scopes of this intriguing research field are also discussed.

Studying the biological feasibility of [⁹⁹mTc(CO)₃]-dextran-cysteine-cysteine-mannose as a potential molecular radiopharmaceutical for sentinel node detection

OBJECTIVE

The aim of this work was to radiolabel and bioevaluate the technetium-99m labeled dextran dicysteine mannose (DCCM) [(99m)Tc(CO)₃]-DCCM for sentinel lymph node detection.

METHODS

Dextran dicysteine mannose was radiolabeled using the carbonyl method. Various parameters were studied such as in vitro stability at room temperature up to 5 h, protein binding and partition coefficient. Bioevaluation was performed in a rabbit model by developing images under a gamma camera at various time intervals. Biodistribution was performed in Wistar rat models (n = 3) by dissection and measurement of percent injected dose in various body organs, at 60 and 180 min post-injection intervals. Biodistribution was performed in two different groups of animals: in the first group, the radiolabeled compound was injected at a concentration of 200 μg/ml, thus delivering 10 μg radiolabeled compound at the site of injection; in the second group, the radiolabeled compound was injected at a concentration of 50 μg/ml, delivering 2.5 μg radiolabeled compound at the site of injection.

RESULTS

Radiolabeling efficacy was 97.5 ± 1% which remained quite stable till 5 h. Protein binding data show that 71.1 ± 5% drug exhibited binding with blood proteins. Partition coefficient results show that our radiopharmaceutical is quite hydrophilic in nature. It can be inferred from the imaging data that sentinel node can be visualized within 30 min post-injection. Rat dissection data showed that when the radiolabeled compound was injected at a concentration of 50 μg/ml, at 60 min post-injection, ~2.85% of activity was retained in the sentinel node with a significantly less accumulation, e.g., ~0.12%, in the secondary node, which resulted in very high popliteal extraction (PE) value, e.g., ~98%. At 180 min post-injection, 2.46 ± 0.29 % was found to be retained in the sentinel node and PE (99.64 ± 0.23%), thus resulting in almost complete washout from the secondary node (0.05 ± 0.01%).

CONCLUSION

The study demonstrates that radiolabeled DCCM might be a successful radiopharmaceutical for sentinel node detection.