Cardiac Sympathetic Positron Emission Tomography Imaging with Meta-[¹⁸F]Fluorobenzylguanidine is Sensitive to Uptake-1 in Rats

Abstract

Dysfunction of the cardiac sympathetic nervous system contributes to the development of cardiovascular diseases including ischemia, heart failure, and arrhythmias. Molecular imaging probes such as meta-[ 123 I]iodobenzylguanidine have demonstrated the utility of assessing neuronal integrity by targeting norepinephrine transporter (NET, uptake-1). However, current radiotracers can report only on innervation due to suboptimal kinetics and lack sensitivity to NET in rodents, precluding mechanistic studies in these species. The objective of this work was to characterize myocardial sympathetic neuronal uptake mechanisms and kinetics of the positron emission tomography (PET) radiotracer meta-[ 18 F]fluorobenzylguanidine ([ 18 F]mFBG) in rats. Automated synthesis using spirocyclic iodonium(III) ylide radiofluorination produces [ 18 F]mFBG in 24 ± 1% isolated radiochemical yield and 30−95 GBq/μmol molar activity. PET imaging in healthy rats delineated the left ventricle, with monoexponential washout kinetics (k mono = 0.027 ± 0.0026 min −1 , A mono = 3.08 ± 0.33 SUV). Ex vivo biodistribution studies revealed tracer retention in the myocardium, while pharmacological treatment with selective NET inhibitor desipramine, nonselective neuronal and extraneuronal uptake-2 inhibitor phenoxybenzamine, and neuronal ablation with neurotoxin 6hydroxydopamine reduced myocardial retention by 33, 76, and 36%, respectively. Clearance of [ 18 F]mFBG from the myocardium was unaffected by treatment with uptake-1 and uptake-2 inhibitors following peak myocardial activity. These results suggest that myocardial distribution of [ 18 F]mFBG in rats is dependent on both NET and extraneuronal transporters and that limited reuptake to the myocardium occurs. [ 18 F]mFBG may therefore prove useful for imaging intraneuronal dysfunction in small animals.

Recent Advances in Cardiovascular Diseases Research Using Animal Models and PET Radioisotope Tracers

“…The studies showed marker retention in ex vivo measurements and temporary disappearance after pharmacological treatment. The marker distribution in the rat heart depends on NET and extra-neuronal transporters with limited reuptake into the myocardium [145]. The group shows that the obtained images show high sensitivity and higher resolution than the comparable [ 123 I]-mIBG, indicating an excellent quantitative correlation with the size of the post-infarction zone in the rat MI model.…”