Background

[PubMed]

[¹⁸F]3-Fluoro-5-[(2-methyl-1,3-thiazol-4-yl)ethynyl]benzonitrile ([¹⁸F]F-MTEB) is a radioligand developed for positron emission tomography (PET) imaging of metabotropic glutamate receptor subtype 5 (mGlu5) in the central nervous system (CNS) (1).

Glutamate is a major excitatory neurotransmitter at CNS synapses. Many neuroanatomical CNS projection pathways contain glutamatergic neurons (2). Glutamate produces its excitatory effects by acting on cell-surface ionotropic glutamate or metabotropic glutamate (mGlu) receptors (3). The mGlu receptors are G-protein-coupled receptors, and the eight mGlu receptor subtypes are further subdivided into groups I, II, and III. The group I receptors include mGlu1 and mGlu5, and they are found mostly in postsynaptic locations. The mGlu5 receptors are found with high to moderate density in the frontal cortex, caudate, putamen, nucleus accumbens, olfactory tubercle, hippocampus, and dorsal horn of the spinal cord, whereas the density in the cerebellum is low. These receptors are coupled to phospholipase C and up- or down-regulate neuronal excitability. They have been implicated in a variety of diseases in the CNS, including anxiety, depression, schizophrenia, Parkinson’s disease, and drug addiction or withdrawal. These receptors are also involved in the modulation of various pain states. They thus are attractive targets for therapeutic drug development.

PET and single-photon emission tomography of radioligands targeting mGlu5 receptors can visualize and study the CNS mGlu5 receptors in normal and pathologic states. Some mGlu5 antagonists have been successfully labeled, but their in vivo visualization has been hampered by high lipophilicity, unfavorable brain uptake kinetics, or a high metabolism (4, 5). 2-Methyl-6-(phenylethynyl)-pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP) have been identified as potent and highly selective noncompetitive antagonists for mGlu5 receptors. Using the structure of MTEP as a template, Hamill et al. (1) and Patel et al (5). designed several MTEP analogs as PET radiotracers, and [¹⁸F]F-MTEB was one of the compounds that showed high affinity for mGlu5 receptors with moderate lipophilicity.

Synthesis

[PubMed]

Hamill et al. (1) reported the radiosynthesis of [¹⁸F]F-MTEB, from the aryl chloride via a nucleophilic aromatic substitution reaction. The precursor was obtained commercially. In the preparation, Kryptofix222 was added to the aqueous [¹⁸F]F− solution in a microwave cavity. The fluoride was dried under argon gas flow by use of microwave pulses (~45 W). The precursor, in dimethyl sulfoxide, was added to the microwave vial. The reaction mixture was then pulsed with the microwave 5 times at 15 s/pulse and a 30-s pause between pulses. After cooling for 1 min, the reaction was diluted with water and then purified by high-performance liquid chromatography. This procedure gave 259 MBq(7 mC) of [¹⁸F]F-MTEB with a specific activity (n = 2) of 49.9 ± 17.4 TBq (1,348 ± 469 Ci)/mmol, and a radiochemical purity >98%. The time of synthesis was ~ 35 min with a radiochemical yield of ~4 ± 0.9% (n = 10) at the end of synthesis (uncorrected).

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Patel et al. (5) used in vitro rat brain preparations to determine the in vitro mGlu5 binding parameters of unlabeled F-MTEB. [³H]MethoxyPyEP, a mGlu5 antagonist, was used in the equilibrium competition studies. The B_max for the rat frontal cortex was reported to be 64.7 fmol/mg of tissue (6). The K_i of F-MTEB determined from rat cortical membranes was 0.08 ± 0.02 (n = 3) nM, and the partition coefficient (log P) was 3.2 (octanol/buffer, pH 7.4). Hamill et al. (1) reported that the inhibitory concentration (IC₅₀; human mGlu5 Ca²⁺ flux assay) was 1.45 ± 0.5 nM (n = 5).

Using a rapid in vitro binding assay they had developed, Patel et al. (7) determined the in vitro regional distribution of [¹⁸F]F-MTEB in rat and rhesus monkey brain sections. Rat and rhesus brain sections were incubated with [¹⁸F]F-MTEB and assayed for 20 min, using a no-wash protocol. The percentages of specific binding (n ≥ 2) at 20 min were 69 ± 6.5 and 65 ± 11 for rat and rhesus caudate sections, respectively.

In vitro autoradiographic studies (1) of [¹⁸F]F-MTEB were carried out in rhesus monkey brain slices (from a single monkey). Good specific to nonspecific regioselective labeling with reasonable binding was found in the cortex, caudate, putamen, amygdale, hippocampus, and most thalamic nuclei. The cerebellar layers showed lower receptor density. In a saturation binding experiment, the B_max values for mGlu5 were determined to be 63 and 24 nM in the rhesus caudate and cerebellum, respectively. In vitro metabolism of [¹⁸F]F-MTEB was studied by incubation of the radiotracer with human and monkey liver microsomes. Liquid chromatography-tandem mass spectrometry showed that the radiotracer was metabolized slowly by human microsomes. Approximately 60% of the intact compound remained at the end of 30 min. In contrast, the compound was metabolized much faster by monkey liver microsomes: only 2% of the intact compound remained at the end of 30 min.

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

References

1.

Hamill TG , Krause S , Ryan C , Bonnefous C , Govek S , Seiders TJ , Cosford ND , Roppe J , Kamenecka T , Patel S , Gibson RE , Sanabria S , Riffel K , Eng W , King C , Yang X , Green MD , O'Malley SS , Hargreaves R , Burns HD . Synthesis, characterization, and first successful monkey imaging studies of metabotropic glutamate receptor subtype 5 (mGluR5) PET radiotracers. Synapse. 2005;56(4):205–216. [PubMed: 15803497]

2.

Pin JP , Duvoisin R . The metabotropic glutamate receptors: structure and functions. Neuropharmacology. 1995;34(1):1–26. [PubMed: 7623957]

3.

Slassi A , Isaac M , Edwards L , Minidis A , Wensbo D , Mattsson J , Nilsson K , Raboisson P , McLeod D , Stormann TM , Hammerland LG , Johnson E . Recent advances in non-competitive mGlu5 receptor antagonists and their potential therapeutic applications. Curr Top Med Chem. 2005;5(9):897–911. [PubMed: 16178734]

4.

Ametamey SM , Kessler LJ , Honer M , Wyss MT , Buck A , Hintermann S , Auberson YP , Gasparini F , Schubiger PA . Radiosynthesis and Preclinical Evaluation of 11C-ABP688 as a Probe for Imaging the Metabotropic Glutamate Receptor Subtype 5. J Nucl Med. 2006;47(4):698–705. [PubMed: 16595505]

5.

Patel S , Ndubizu O , Hamill T , Chaudhary A , Burns HD , Hargreaves R , Gibson RE . Screening cascade and development of potential Positron Emission Tomography radiotracers for mGluR5: in vitro and in vivo characterization. Mol Imaging Biol. 2005;7(4):314–323. [PubMed: 16080024]

6.

Patel S , Krause SM , Hamill T , Chaudhary A , Burns DH , Gibson RA . In vitro characterization of [3H]MethoxyPyEP, an mGluR5 selective radioligand. Life Sci. 2003;73(3):371–379. [PubMed: 12757844]

7.

Patel S , Hamill T , Hostetler E , Burns HD , Gibson RE . An in vitro assay for predicting successful imaging radiotracers. Mol Imaging Biol. 2003;5(2):65–71. [PubMed: 14499146]