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

Abstract

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 18F-hydroxyphenethylguanidines consisting of nucleophilic aromatic [18F]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-[18F]fluoro-m-hydroxyphenethylguanidine ([18F]4F-MHPG, [18F]1) and its structural isomer 3-[18F]fluoro-p-hydroxyphenethylguanidine ([18F]3F-PHPG, [18F]2) with good radiochemical yields. Preclinical evaluations of [18F]2 in non-human primates were performed to compare its imaging properties, metabolism, and myocardial kinetics with those obtained previously with [18F]1. The results of these studies have demonstrated that [18F]2 exhibits imaging properties comparable to those of [18F]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 [18F]1 and [18F]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.

Improved synthesis of 4‐[18F]fluoro‐m‐hydroxyphenethylguanidine using an iodonium ylide precursor

“…The synthesis of the required tetrakis ‐Boc protected spirocyclic iodonium ylide precursor 2 is shown in Scheme . Compound 1 ( N,N′,N″,N″ ‐ tetrakis ( tert ‐butoxycarbonyl)‐ N ‐3‐benzyloxy‐4‐iodophenethylguanidine), previously synthesized for our original approach to [ 18 F]4F‐MHPG, was first reacted with dimethyldioxirane (DMDO). The resulting intermediate was coupled with (1 r ,3 r ,5 r ,7 r )‐spiro[adamantan‐2,2′‐[1,3]‐dioxane]‐4′,6′‐dione (SPIAd) to give the spirocyclic iodonium ylide 2 .…”

“…For initial automated radiofluorination tests with 2 , we employed optimized reaction conditions reported by Rotstein et al, including the use of tetraethylammonium bicarbonate (TEAB; 4.0 mg) as the fluorine‐18 counter ion, anhydrous N,N ‐dimethylformamide (DMF, 0.5 mL) as the reaction solvent, 5.0 to 6.0 mg of 2 , and reaction conditions of 120°C for 10 minutes (Scheme ). The subsequent steps for simultaneous deprotection of the benzyl ether and the N,N′,N″,N″‐tetrakis‐ Boc groups using 3.0 N HBr followed by HPLC purification of [ 18 F]4F‐MHPG were identical to those used in our original method . Radiochemical yields of these pilot tests (Table , Runs 1‐3) were dramatically higher than our original approach, averaging 9.2% ± 0.7% (range 8.4%‐9.7%) at EOS (not decay corrected), more than threefold higher than the average yield of [ 18 F]4F‐MHPG achieved with our iodonium salt precursor approach …”

“…The subsequent steps for simultaneous deprotection of the benzyl ether and the N,N′,N″,N″‐tetrakis‐ Boc groups using 3.0 N HBr followed by HPLC purification of [ 18 F]4F‐MHPG were identical to those used in our original method . Radiochemical yields of these pilot tests (Table , Runs 1‐3) were dramatically higher than our original approach, averaging 9.2% ± 0.7% (range 8.4%‐9.7%) at EOS (not decay corrected), more than threefold higher than the average yield of [ 18 F]4F‐MHPG achieved with our iodonium salt precursor approach …”

“…Reagents and solvents were purchased from commercial sources and used without further purification unless otherwise noted. N,N′,N″,N″ ‐ tetrakis ( tert ‐butoxycarbonyl)‐ N ‐3‐benzyloxy‐4‐iodophenethylguanidine 1 , N,N′,N″,N″ ‐ tetrakis ( tert ‐butoxycarbonyl)‐ N ‐3‐benzyloxy‐4‐fluorophenethylguanidine 3 , and 4‐fluoro‐ m ‐hydroxyphenethylguanidine (4F‐MHPG) were previously prepared in our laboratory . Compound 1 was used to synthesize the spirocyclic iodonium ylide precursor 2 .…”

“…These radiotracers exhibit irreversible tissue kinetics through efficient intraneuronal retention in the norepinephrine storage vesicles localized in cardiac sympathetic nerve terminals. We previously described the use of iodonium salt precursors with the guanidine group completely protected ( tetrakis ‐Boc) as an effective method for automated production of [ 18 F]4F‐MHPG and [ 18 F]3F‐PHPG . For example, in production runs of [ 18 F]4F‐MHPG ( n = 15), this approach provided final product yields averaging 1.56 ± 0.68 GBq (range 0.61‐2.84 GBq) at end of synthesis (EOS) with molar activities averaging 58 ± 25 GBq/μmol (range 21‐105 GBq/μmol), which were adequate for on‐site clinical studies.…”