Novel ¹⁸F-Labeled PET Imaging Agent FV45 Targeting the Renin-Angiotensin System

Renin-angiotensin system (RAS) plays an important role in the regulation of blood pressure and hormonal balance. Using positron emission tomography (PET) technology, it is possible to monitor the physiological and pathological distribution of angiotensin II type 1 receptors (AT₁), which reflects the functionality of RAS. A new ¹⁸F-labeled PET tracer derived from the clinically used AT₁ antagonist valsartan showing the least possible chemical alteration from the valsartan structure has been designed and synthesized with several strategies, which can be applied for the syntheses of further derivatives. Radioligand binding study showed that the cold reference FV45 (K_i 14.6 nM) has almost equivalent binding affinity as its lead valsartan (K_i 11.8 nM) and angiotensin II (K_i 1.7 nM). Successful radiolabeling of FV45 in a one-pot radiofluorination followed by the deprotection procedure with 21.8 ± 8.5% radiochemical yield and >99% radiochemical purity (n = 5) enabled a distribution study in rats and opened a path to straightforward large-scale production. A fast and clear kidney uptake could be observed, and this renal uptake could be selectively blocked by pretreatment with AT₁-selective antagonist valsartan. Overall, as the first ¹⁸F-labeled PET tracer based on a derivation from clinically used drug valsartan with almost identical chemical structure, [¹⁸F]FV45 will be a new tool for assessing the RAS function by visualizing AT₁ receptor distributions and providing further information regarding cardiovascular system malfunction as well as possible applications in inflammation research and cancer diagnosis.