Background

[PubMed]

Glutamate is a major excitatory neurotransmitter at neuronal synapses in the central nervous system (CNS) (1, 2). Glutamate produces excitatory effects by acting on cell-surface ionotropic glutamate or metabotropic glutamate receptors (mGluRs). The mGluRs are GTP-binding protein (G-protein)–coupled receptors that play important roles in regulating the activity of many synapses in the CNS, and many neuronal projection pathways contain mGluRs. There are eight mGluR subtypes, which are further subdivided into groups I, II, and III. The group I receptors include mGluR1 and mGluR5, and they are found predominantly in postsynaptic locations. mGluR1 is found in moderate to high density in the cerebellum, caudate, putamen, thalamus, cingulate cortex, and hippocampus, with low density in the pons. mGluR5 is usually found in moderate to high density in the frontal cortex, caudate, putamen, nucleus accumbens, olfactory tubercle, and hippocampus, with low density in the cerebellum. mGluR1 and mGluR5 are positively coupled to phospholipase C in the regulation of neuronal excitability (3). Dysfunction of mGluR1 and mGluR5 is implicated in a variety of diseases in the CNS, including anxiety, depression, schizophrenia, Parkinson’s disease, and drug addiction or withdrawal (2, 4).

Positron emission tomography (PET) radioligands targeting mGluR5 can visualize and analyze mGluR5 expression in normal physiological and pathological conditions (4-8). However, only a few mGluR1 ligands have been studied. N-(4-(6-(Isopropylamino)pyrimidin-4-yl)-1,3-thiazol-2-yl)-N-methyl-4-methylbenzamide (ITDM) was shown to be a selective mGluR1 antagonist with nanomolar affinity (K_i = 13.6 nM), with little inhibition of mGluR5 (9). Fujinaga et al. (9) labeled ITDM with ¹¹C to prepare N-(4-(6-(isopropylamino)pyrimidin-4-yl)-1,3-thiazol-2-yl)-N-methyl-4-[¹¹C]methylbenzamide ([¹¹C]ITDM) for use with in vivo PET imaging of mGluR1 distribution in rat and monkey brains.

Synthesis

[PubMed]

Fujinaga et al. (9) synthesized [¹¹C]ITDM by methylation of the arylstannane precursor with [¹¹C]MeI (prepared with [¹¹C]CO₂) using an automated synthesizer for 5 min at 80°C. Subsequent separation with high-performance liquid chromatography produced a radiochemical purity >99%. Average radiochemical yield was 19 ± 9% (n = 11) based on [¹¹C]CO₂. The specific activity of [¹¹C]ITDM was 70–170 GBq/µmol (1.89–4.59 Ci/µmol) at the end of synthesis. The total synthesis time was <31 min from the end of bombardment.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

In vitro binding affinity of ITMM was determined with rat brain homogenates using [¹⁸F]fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([¹⁸F]FITM) as the mGluR1 radioligand (9). The K_i values were 13.6 ± 3.4 nM and 5.4 ± 1.2 nM for ITDM and FITM, respectively. Both agents showed IC₅₀ values >10,000 nM for mGluR5. ITDM (LogD 1.74) was found to be slightly more lipophilic than FITM (LogD 1.45).

In vitro [¹¹C]ITDM autoradiographic imaging studies were performed on rat brain sections (9). [¹¹C]ITDM bound heterogeneously to the brain sections, with the highest accumulation of radioactivity in the mGluR1-rich cerebellum, followed by the thalamus, hippocampus, striatum, and cerebral cortex, with the lowest radioactivity in the pons. ITDM and JNJ-16259685 (1,000 nM), both mGluR1 antagonists, blocked radioactivity to homogeneity in the brain sections.

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

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