PET Imaging of Brain 5-HT1A Receptors in Rat In Vivo with 18F-FCWAY and Improvement by Successful Inhibition of Radioligand Defluorination with Miconazole

Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, Bethesda, Maryland 20892-0135, USA.
Journal of Nuclear Medicine (Impact Factor: 6.16). 03/2006; 47(2):345-53.
Source: PubMed


18F-FCWAY (18F-trans-4-fluoro-N-(2-[4-(2-methoxyphenyl) piperazin-1-yl)ethyl]-N-(2-pyridyl)cyclohexanecarboxamide) is useful in clinical research with PET for measuring serotonin 1A (5-HT1A) receptor densities in brain regions of human subjects but has significant bone uptake of radioactivity due to defluorination. The uptake of radioactivity in skull compromises the accuracy of measurements of 5-HT1A receptor densities in adjacent areas of brain because of spillover of radioactivity through the partial-volume effect. Our aim was to demonstrate with a rat model that defluorination of 18F-FCWAY may be inhibited in vivo to improve its applicability to measuring brain regional 5-HT1A receptor densities.
PET of rat head after administration of 18F-FCWAY was used to confirm that the distribution of radioactivity measured in brain is dominated by binding to 5-HT1A receptors and to reveal the extent of defluorination of 18F-FCWAY in vivo as represented by radioactivity (18F-fluoride ion) uptake in skull. Cimetidine, diclofenac, and miconazole, known inhibitors of CYP450 2EI, were tested for the ability to inhibit defluorination of 18F-FCWAY in rat liver microsomes in vitro. The effects of miconazole treatment of rats on skull radioactivity uptake and, in turn, its spillover on brain 5-HT1A receptor imaging were assessed by PET with venous blood analysis.
PET confirmed the potential of 18F-FCWAY to act as a radioligand for 5-HT1A receptors in rat brain and also revealed extensive defluorination. In rat liver microsomes in vitro, defluorination of 18F-FCWAY was almost completely inhibited by miconazole and, to a less extent, by diclofenac. In PET experiments, treatment of rats with miconazole nitrate (60 mg/kg intravenously) over the 45-min period before administration of 18F-FCWAY almost obliterated defluorination and bone uptake of radioactivity. Also, brain radioactivity almost doubled while the ratio of radioactivity in receptor-rich ventral hippocampus to that in receptor-poor cerebellum almost tripled to 14. The plasma half-life of radioligand was also extended by miconazole treatment.
Miconazole treatment, by eliminating defluorination of 18F-FCWAY, results in effective imaging of brain 5-HT1A receptors in rat. 18F-FCWAY PET in miconazole-treated rats can serve as an effective platform for investigating 5-HT1A receptors in rodent models of neuropsychiatric conditions or drug action.

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Available from: Sami S Zoghbi, Oct 10, 2015
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    • "Since 18 F bound to an aliphatic carbon atom is often prone to defluorination, we envisioned that this might also be problematic for [ 18 F]TCO. Although tracer defluorination is known to result in high skull and other bone uptake [25] [26], it is not until 240 min pi that the bone uptake becomes prominently visible in the PET images. This makes us believe that defluorination of [ 18 F]TCO or its metabolites only becomes problematic after 120 min pi. "
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    ABSTRACT: Introduction The tetrazine-trans-cylooctene cycloaddition using radiolabeled tetrazine or radiolabeled trans-cyclooctene (TCO) has been reported to be a very fast, selective and bioorthogonal reaction that could be useful for in vivo radiolabeling of molecules. We wanted to evaluate the in vivo biodistribution profile and brain uptake of 18F-labeled TCO ([18F]TCO) to assess its potential for pre-targeted imaging in the brain. Methods We evaluated the in vivo behavior of [18F]TCO via an ex vivo biodistribution study complemented by in vivo μPET imaging at 5, 30, 60, 90, 120 and 240 min post tracer injection. An in vivo metabolite study was performed at 5 min, 30 min and 120 min post [18F]TCO injection by RP-HPLC analysis of plasma and brain extracts. Incubation with human liver microsomes was performed to further evaluate the metabolite profile of the tracer. Results μPET imaging and ex-vivo biodistribution revealed an high initial brain uptake of [18F]TCO (3.8 %ID/g at 5 min pi) followed by a washout to 3.0 %ID/g at 30 min pi. Subsequently the brain uptake increased again to 3.7 %ID/g at 120 min pi followed by a slow washout until 240 min pi (2.9 %ID/g). Autoradiography confirmed homogenous brain uptake. On the μPET images bone uptake became gradually visible after 120 min pi and was clearly visible at 240 min pi. The metabolite study revealed a fast metabolization of [18F]TCO in plasma and brain into three main polar radiometabolites. Conclusions Although [18F]TCO has previously been described to be a useful tracer for radiolabeling of tetrazine modified targeting molecules, our study indicates that its utility for in vivo chemistry and pre-targeted imaging will be limited. Although [18F]TCO clearly enters the brain, it is quickly metabolized with a non-specific accumulation of radioactivity in the brain and bone.
    Nuclear Medicine and Biology 07/2014; 41(6). DOI:10.1016/j.nucmedbio.2014.03.023 · 2.41 Impact Factor
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    • "It has been reported that [ 18 F]FCWAY is defluorinated by CYP2E1 in rats and in humans, and that this phenomenon could be blocked by pretreatment with antifungal agents (i.e. miconazole) or disulfiram (Ryu et al., 2007; Tipre et al., 2006). We thought that if [ 18 F]MeFWAY was defluorinated in vivo, CYP2E1 might be a major metabolizing enzyme as [ 18 F]MeF- WAY was derived from [ 18 F]FCWAY. "
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    ABSTRACT: Introduction: [(18) F]MeFWAY has been developed for imaging the serotonin 1A receptors in the brain. The purpose of this study were to verify the metabolic stability of [(18) F]MeFWAY, to measure the degree of defluorination of [(18) F]MeFWAY in vivo, to investigate methods of inhibition of defluorination of [(18) F]MeFWAY, and to assess the efficacy of [(18) F]MeFWAY in rat brains in vivo. Methods: MicroPET experiments in rats were conducted to confirm the distribution of radioactivity in the brain. Nondisplaceable binding potential (BP(ND) ) in the hippocampus and frontal cortex were also analyzed. Miconazole and fluconazole were tested for the ability to suppress defluorination of [(18) F]MeFWAY. We conducted a blockade and displacement experiment by treating with WAY-100635. Results: In vitro stability tests showed that MeFWAY was very stable in serum for 6 h, but PET revealed that authentic [(18) F]MeFWAY underwent significant defluorination in vivo. In vitro inhibition study against decreasing parent activity in liver microsomes, miconazole and fluconazole suppressed metabolic elimination of MeFWAY. However, in the PET study, fluconazole showed more potent inhibitory activity than miconazole. In the suppression of metabolizing enzymes using fluconazole, radioactivity in skull was dramatically decreased by 81% (compared with 69% with miconazole) and it was coupled with an increase in brain uptake. Moreover, BP(ND) in hippocampus was 5.53 and 2.66 in frontal cortex. The blockade and displacement study showed the specificity of [(18) F]MeFWAY to 5-HT(1A) receptors. Conclusion: In the rat brain, [(18) F]MeFWAY microPET showed skull uptake due to defluorination in vivo. We can effectively overcome this drawback with fluconazole.
    Synapse 12/2012; 66(12):1015-23. DOI:10.1002/syn.21607 · 2.13 Impact Factor
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    • "One reason why the FMT or FDOPA PET imaging in rats has a relatively poor signal-to-noise ratio relative to monkeys (Doudet et al., 1999; Jordan et al., 1997; Yee et al., 2001) may be more rapid defluorination in the liver and subsequent uptake in bone, as their was a strong signal in bone with the FMT PET (Fig. 2). Inhibiting defluorination is important for improving PET imaging in rats (Tipre et al., 2006), although we only attempted to block peripheral metabolism of the tracer with carbidopa. The combinatorial imaging protocol that we used may not be of great significance for humans because of the better signal-to-noise ligands, which make realignment and registration less of an issue. "
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    ABSTRACT: We attempted to monitor the nigrostriatal dopaminergic system in rats with positron emission tomography (PET) during the progression of two experimental disease states. One model was 6-hydroxydopamine (6-OHDA) lesioning and the other was direct gene transfer of the microtubule-associated protein tau to the substantia nigra using an adeno-associated virus vector (AAV9). The PET ligand was 6-[18F]fluoro-L-m-tyrosine (FMT), imaged prior to, and at two intervals after initiating dopaminergic neurodegeneration. The striatum was delineated with the aid of repeated PET imaging (FMT and sodium fluoride for bone), realignment to subsequent computed axial tomography scans, and registration to an atlas, which proved essential to tracking disease progression. The striata on the two sides of the brain were compared over time after unilateral lesioning treatments. 6-OHDA reduced uptake on the ipsilateral side relative to the untreated contralateral side at both 1 and 4 weeks after lesioning, while the AAV9 tau led to reduced uptake of the tracer in the striatum at 4 weeks, but not 1 week after treatment. The amplitude of the loss of FMT uptake in striatum at 4 weeks with either model was subtle relative to the postmortem histological analysis of the tissue, but the multi-modal imaging analysis yielded statistical effects that matched well with the histology in terms of the timing of the loss of dopaminergic markers. Live longitudinal imaging successfully tracked two distinct types of disease progression in individual rats, although the FMT is not a sensitive ligand to monitor the extent of the lesion.
    Brain research 03/2009; 1259:113-22. DOI:10.1016/j.brainres.2009.01.063 · 2.84 Impact Factor
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