Metabolism of no-carrier-added 2-[18F]fluoro-L-tyrosine in rats

Background: Several fluorine-18 labelled fluoroamino acids have been evaluated as tracers for the quantitative assessment of cerebral protein synthesis in vivo by positron emission tomography (PET). Among these, 2-[18F]fluoro-L-tyrosine (2-[18F]Tyr) has been studied in mice at a low specific activity. Its incorporation into proteins is fast and metabolism via other pathways is limited. The present in vivo study was carried out in normal awake rats using no-carrier-added 2-[18F]Tyr. Under normal physiological conditions, we have studied the incorporation into proteins and the metabolism of the tracer in different brain areas. Methods: No-carrier-added 2-[18F]Tyr was administered to awake rats equipped with chronic arterial and venous catheters. The time course of the plasma activity was studied by arterial blood sampling. The biodistribution of the activity in the main organs was studied at the end of the experiment. The distribution of radioactive species in plasma and brain regions was studied by acidic precipitation of the proteins and HPLC analysis of the supernatant. Results: The absolute uptake of radioactivity in brain regions was homogenous. In awake rats, nocarrier-added 2-[18F]Tyr exhibits a fast and almost quantitative incorporation into the proteins fractions of cerebellum and cortex. In striatum, this incorporation into proteins and the unchanged fraction of the tracer detected by HPLC could be lower than in other brain regions. Conclusion: This study confirms the potential of 2-[18F]fluoro-L-tyrosine as a tracer for the assessment of the rate of protein synthesis by positron emission tomography. The observed metabolism suggests a need for a correction for the appearance of metabolites, at least in plasma

Epimerization study on [18F]FDG produced by an alkaline hydrolysis on solid support under stringent conditions

Since 1998, routine [18F]FDG syntheses are being carried out by alkaline hydrolysis on a solid support, i.e. the labeled intermediate is trapped on a tC18 solid phase extraction cartridge, purified and finally hydrolyzed within the cartridge, at room temperature, using sodium hydroxide. The present study demonstrated that no epimerization of [18F]FDG to [18F]FDM occurs even when 12 N NaOH is used and when the hydrolysis time is extended up to 1 h. The alkaline hydrolysis on solid support appears to be a simple method leading to [18F]FDG with high purity

p-[18F]MPPF, 5-HT1A antagonist: comparison to [3H]8-OH-DPAT with autoradiography

p-MPPF 4-(2’-methoxyphenyl)-1-[2’-[N-(2’’-pyridinyl)-p-fluorobenzamido] ethyl]piperazine is the para-fluorobenzoyl analog of the highly selective 5-HT1A antagonist WAY-100635. The one step procedure used to label p-MPPF with fluorine-18 (cyclotron produced positron emitter of 110 min half-life) leads to a radiopharmaceutical compound easily prepared on a large scale. The preliminary evaluations conducted in rats and cats are good reason to consider p-[18F]MPPF as an interesting reversible radioligand to study the 5-HT1A receptor family in humans with positron emission tomography (PET). In this paper we report a careful comparison between p-[18F]MPPF and [3H]8-OH-DPAT with autoradiography and quantitative densitometry in the same animal. All experiments were conducted in Sprague Dawley male rats. For p-[18F]MMPF, the results were obtained ex-vivo after an intravenous injection of high specific activity radioligand (0.8-1.5 Ci/µmol) in vigil (no anesthesia), free moving and unstressed animals. For the purpose, permanent cannulation of the posterior vena cava were realized at least four days in advance. The [3H]8-OH-DPAT results were obtained in vitro on adjacent coronal sections to the one used for the p-[18F]MPPF autoradiography. Quantitative densitometry was employed to compare the values obtained in relevant brain structures (frontal cortex, lateral septum, hippocampus, dorsal raphe, entorhinal cortex and cerebellum). The plot of the p-[18F]MPPF values obtained for each structure against the [3H]8-OH-DPAT results displayed a significant linear correlation. These results demonstrate that from a qualitative as well as quantitative point of view, the binding of p-[18F]MPPF is totally comparable to the one of [3H]8-OH-DPAT. Supported by grants from INSERM/CGRI and FNRS Belgium

Evaluation of p-[18F]MPPF, 5-HT1A antagonist, a potential radiopharmaceutical for PET: tissue distribution, autoradiography and metabolism in rats.

peer reviewe

Tissue distribution, autoradiography, and metabolism of 4-(2'-methoxyphenyl)-1-[2' -[N-2"-pyridinyl)-p-[(18)F]fluorobenzamido]ethyl]piperazine (p-[(18)F]MPPF), a new serotonin 5-HT(1A) antagonist for positron emission tomography: An In vivo study in rats.

peer reviewedThe in vivo behavior of 4-(2'-methoxyphenyl)-1-[2'-[N-(2"-pyridinyl)-p-[(18)F]fluorobenzamido ]ethyl]-piperazine (p-[(18)F]MPPF), a new serotonin 5-HT(1A) antagonist, was studied in awake, freely moving rats. Biodistribution studies showed that the carbon-fluorine bond was stable in vivo, that this compound was able to cross the blood-brain barrier, and that a general diffusion equilibrium could account for the availability of the tracer. The great quantity of highly polar metabolites found in plasma did not contribute to the small amounts of metabolites found in hippocampus, frontal cortex, and cerebellum. Exvivo p-[(18)F]MPPF and in vitro 8-hydroxy-2-(di-n-[(3)H]propylamino)tetralin autoradiography were compared both qualitatively and quantitatively. Qualitative evaluation proved that the same brain regions were labeled and that the p-[(18)F]MPPF labeling is (a) in total agreement with the known distribution of 5-HT(1A) receptors in rats and (b) characterized by very low nonspecific binding. Quantitative comparison demonstrated that the in vivo labeling pattern obtained with p-[(18)F]MPPF cannot be explained by differences in regional blood flow, capillary density, or permeability. The 5-HT(1A) specificity of p-[(18)F]MPPF and binding reversibility were confirmed in vivo with displacement experiments. Thus, this compound can be used to evaluate parameters characterizing 5-HT(1A) binding sites in the brain