[18F]CFT synthesis and binding to monoamine transporters in rats

Abstract

Background: We present the electrophilic synthesis of [18F]2β-carbomethoxy-3β-(4-fluoro)tropane [[18F]CFT] and the pharmacological specificity and selectivity of [18F]CFT for monoamine transporters in the brain and peripheral organs of rats. The human radiation dose is extrapolated from the animal data.

Methods: [18F]CFT was synthesized by electrophilic fluorination of a stannylated precursor by using post-target-produced [18F]F2 as a fluorinating agent. The ex vivo 18F-activity biodistribution of [18F]CFT in the brain of rats was studied by autoradiography. The binding of [18F]CFT to the monoamine transporters was studied using in vivo blocking experiments with dopamine transporter [DAT], norepinephrine transporter [NET], or serotonin transporter [SERT] inhibitors. In vivo animal positron emission tomography was used as a comparative method to determine tracer kinetics. Human radiation dose was assessed using OLINDA software.

Results: The radiochemical yield of [18F]CFT from the initial [18F]F-, decay corrected to the end of bombardment, was 3.2 ± 1.0%. The specific activity [SA] was 14.5 ± 3.4 GBq/μmol, decay corrected to the end of synthesis. Radiochemical purity exceeded 99%. DAT-specific binding was found in the striatum, locus coeruleus, and pancreas. NET-specific binding was found in the locus coeruleus. SERT-specific binding was not found in any of the studied organs. Effective dose equivalent [EDE] estimated for the standard human model was 12.8 μSv/MBq. Effective dose [ED] was 9.17 μSv/MBq.

Conclusions: Post-target-produced high-SA [18F]F2 was used to incorporate18F directly into the phenyl ring of [18F]CFT. The final product had high radiochemical and chemical purities and a high SA for DAT and NET studies in vivo. In periphery, [18F]CFT showed a specific uptake in the pancreas. EDE and ED corresponded well with other18F-radioligands.

Figure 1

Scheme depicting the use of a stannylated precursor to synthesize [¹⁸F]CFT product.

Figure 2

Sample chromatogram of a semi-preparative HPLC separation of [¹⁸F]CFT from the reaction mixture.

Figure 3

Sample chromatogram of radio-HPLC analysis of formulated [¹⁸F]CFT.

Figure 4

Representative [¹⁸F]CFT autoradiographic images. Brain slices from control rats and from rats pretreated with GBR12909, fluoxetine, or nisoxetine.

Figure 5

The time courses of the brain region-to-cerebellum ratios. They are from the ex vivo study after [¹⁸F]CFT injection where n = 2 to 4/time point (a) and from the in vivo animal PET study where n = 2 (b).

Figure 6

Time course of the [¹⁸F]CFT uptake (percent injected dose per gram tissue) by selected organs. The uptake values for the liver are presented in the y-axis on the right.

Figure 7

Animal PET-CT image of [¹⁸F]CFT accumulation in the brain of rats. Data were collected for 2 to 120 min post injection.