[ 18 F]Fluoro-Hydroxyphenethylguanidines: Efficient Synthesis and Comparison of Two Structural Isomers as Radiotracers of Cardiac Sympathetic Innervation.

Fluorine-18 labeled phenethylguanidines are currently under development in our laboratory as radiotracers for quantifying regional cardiac sympathetic nerve density using PET imaging techniques. In this study, we report an efficient synthesis of ¹⁸ F-hydroxyphenethylguanidines consisting of nucleophilic aromatic [ ¹⁸ F]fluorination of a protected diaryliodonium salt precursor followed by a single deprotection step to afford the desired radiolabeled compound. This approach has been shown to reliably produce 4-[ ¹⁸ F]fluoro-m-hydroxyphenethylguanidine ([ ¹⁸ F]4F-MHPG, [ ¹⁸ F]1) and its structural isomer 3-[ ¹⁸ F]fluoro-p-hydroxyphenethylguanidine ([ ¹⁸ F]3F-PHPG, [ ¹⁸ F]2) with good radiochemical yields. Preclinical evaluations of [ ¹⁸ F]2 in nonhuman primates were performed to compare its imaging properties, metabolism, and myocardial kinetics with those obtained previously with [ ¹⁸ F]1. The results of these studies have demonstrated that [ ¹⁸ F]2 exhibits imaging properties comparable to those of [ ¹⁸ F]1. Myocardial tracer kinetic analysis of each tracer provides quantitative metrics of cardiac sympathetic nerve density. Based on these findings, first-in-human PET studies with [ ¹⁸ F]1 and [ ¹⁸ F]2 are currently in progress to assess their ability to accurately measure regional cardiac sympathetic denervation in patients with heart disease, with the ultimate goal of selecting a lead compound for further clinical development.