Biological characterization of F-18-labeled rhodamine B, a potential positron emission tomography perfusion tracer

Introduction: Myocardial infarction is the leading cause of death in western countries, and positron emission tomography (PET) plays an increasing role in the diagnosis and treatment planning for this disease. However, the absence of an ¹⁸F-labeled PET myocardial perfusion tracer hampers the widespread use of PET in myocardial perfusion imaging (MPI). We recently reported a potential MPI agent based on ¹⁸F-labeled rhodamine B. The goal of this study was to more completely define the biological properties of ¹⁸F-labeled rhodamine B with respect to uptake and localization in an animal model of myocardial infarction and to evaluate the uptake ¹⁸F-labeled rhodamine B by cardiomyocytes. Methods: A total of 12 female Sprague Dawley rats with a permanent ligation of the left anterior descending artery (LAD) were studied with small-animal PET. The animals were injected with 100-150μCi of ¹⁸F-labeled rhodamine B diethylene glycol ester ([¹⁸F]RhoBDEGF) and imaged two days before ligation. The animals were imaged again two to ten days post-ligation. After the post-surgery scans, the animals were euthanized and the hearts were sectioned into 1mm slices and myocardial infarct size was determined by phosphorimaging and 2,3,5-triphenyltetrazolium chloride staining (TTC). In addition, the uptake of [¹⁸F]RhoBDEGF in isolated rat neonatal cardiomyocytes was determined by fluorescence microscopy. Results: Small-animal PET showed intense and uniform uptake of [¹⁸F]RhoBDEGF throughout the myocardium in healthy rats. After LAD ligation, well defined perfusion defects were observed in the PET images. The defect size was highly correlated with the infarct size as determined ex vivo by phosphorimaging and TTC staining. In vitro, [¹⁸F]RhoBDEGF was rapidly internalized into rat cardiomyocytes with ~40 % of the initial activity internalized within the 60min incubation time. Fluorescence microscopy clearly demonstrated localization of [¹⁸F]RhoBDEGF in the mitochondria of rat cardiomyocytes. Conclusion: Fluorine-18-labeled rhodamine B diethylene glycol ester ([¹⁸F]RhoBDEGF) provides excellent image quality and clear delineation of myocardial infarcts in a rat infarct model. In vitro studies demonstrate localization of the tracer in the mitochondria of cardiac myocytes. In combination, these results support the continued evaluation of this tracer for the PET assessment of myocardial perfusion.