Ruthenium-Mediated 18F-Fluorination and Preclinical Evaluation of a New CB1 Receptor Imaging Agent [18F]FPATPP

Abstract

Cannabinoid receptor 1 (CB1R) controls various physiological and pathological conditions, including memory, motivation, and inflammation, and is thus an interesting target for positron emission tomography (PET). Herein, we report a ruthenium-mediated radiolabeling synthesis and preclinical evaluation of a new CB1R specific radiotracer, [¹⁸F]FPATPP. [¹⁸F]FPATPP was produced with 16.7 ± 5.7% decay-corrected radiochemical yield and >95 GBq/μmol molar activity. The tracer showed high stability, low defluorination, and high specific binding to CB1Rs in mouse brain.

Keywords: CB1R; FPATPP; Positron emission tomography; cannabinoid receptor; fluorine-18; radiofluorination.

Figure 1

Chemical structures of [¹¹C]1, [¹⁸F]2, [¹⁸F]3, and [¹⁸F]4.

Scheme 1. Synthesis of the Reference Standard 4

Reagents and conditions: (a) acetic acid, (b) concentrated HCl and acetic acid, (c) (R)-(+)-1-phenylethylamine and CH₂Cl₂, and (d) NaBH₃CN and acetic acid.

Figure 2

(A) Synthesis of [¹⁸F]4. (B) Results from the process optimization. SPE + temperature denotes the solid-phase extraction with subsequent reaction temperature. Azeotropic + temperature denotes the traditional drying procedure of [¹⁸F]fluoride with subsequent reaction temperature. Data are expressed as mean ± standard deviation (SD, n = 3).

Figure 3

(A) Representative coronal and transaxial [¹⁸F]4 PET/CT images summed for 90–120 min in an adult mouse brain with vehicle (left) and 2 mg/kg rimonabant pretreatment (right). (B–D) Time–activity curves for the whole brain, neocortex, and hippocampus at baseline (n = 3, 1 female) and with rimonabant pretreatment (n = 3, 1 female) and their percentage difference at 120 min. Bl., Blocking; SUV, standardized uptake value. Data are expressed as mean ± SD.

Figure 4

(A) Representative ex vivo brain autoradiography images of [¹⁸F]4 binding 120 min post injection in a nontreated mouse and a mouse pretreated with rimonabant to block the [¹⁸F]4 binding. (B) Ratios of the parietotemporal cortex (Ptc), striatum (Str), frontal cortex (Fc), hippocampus (Hippo), cortex, cerebellar gray matter (CB), and globus pallidus (Gp, n = 3, 1 female) to the thalamus (Tha) without blocking (n = 8, 1 female) and with blocking (Bl. n = 3, 1 female). ***P < 0.001, statistical analyses were performed with the Mann–Whitney U test.

Figure 5

Ex vivo biodistribution of (A) [¹⁸F]4 (30 min n = 4, 60 min n = 2, 120 min n = 8, 1 female) and (B) [¹⁸F]FMPEP-d₂ (30 min n = 3, 60 min n = 4, 120 min n = 12, 5 females). Data are presented as injected dose per gram of tissue (%ID/g) and expressed as mean ± SD. *P < 0.05, **P < 0.005, statistical analyses were performed with the Mann–Whitney U test.

Figure 6

Amount of the [¹⁸F]4 of the total ¹⁸F-radioactivity in plasma and cortex. The radiometabolites were analyzed at 30 min (n = 4), 60 min (n = 2), and 120 min (n = 5) post injection. A double exponential decay equation has been used for the fitting of the curves.