In Vivo Measurement of the Affinity and Density of Metabotropic Glutamate Receptor Subtype 1 in Rat Brain Using ^sup 18^F-FITM in Small-Animal PET

Metabotropic glutamate receptor subtype 1 (mGluR1) is a crucial molecular target in the central nervous system disorders. 4-^sup 18^F-fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide (^sup 18^F-FITM) has been recently developed as a useful PET ligand for mGluR1 imaging in our laboratory. In this study, we aimed to measure the affinity and density of mGluR1 using PET with ^sup 18^F-FITM in rat brain under the in vivo conditions. Methods: Binding potentials (BP^sub ND^) and amounts of specific binding (bound ligand concentration) at equilibrium state in brain regions were noninvasively estimated using the equilibrium analysis combined with the receptor-blocked approach (EA RBA) for kinetic analysis of ^sup 18^F-FITM PET results in place of reference tissue methods. Using BP^sub ND^ and specific binding values of rats treated with multidose ligand, we performed Scatchard analyses for in vivo measurements of mGluR1 density (maximum number of binding sites, or B^sub max^) and ligand affinity (dissociation constant, or K^sub d^) in brain regions, respectively. Results: The pretreatment of rats with unlabeled FITM (1 mg/kg) occupied an mGluR1 binding site of ^sup 18^F-FITM by more than 99% and did not affect the input function. Hence, we used the tissue time-activity curve for receptor-blocked rats as representative of the nondisplaceable (free and nonspecific binding of radioligand) compartment. The BP^sub ND^ based on EA RBA showed a high correlation with the BP^sub ND^ based on invasive Logan plot graphical analysis in the thalamus, hippocampus, striatum, and cingulate cortex. The K^sub d^ (nM) and B^sup max^ (pmol/mL) obtained by the Scatchard analyses with the multidose ligand assays were 2.1 and 36.3, respectively, for the thalamus; 2.1 and 27.5, respectively, for the hippocampus; 1.5 and 22.2, respectively, for the striatum; and 1.5 and 20.5, respectively, for the cingulate cortex with a high confidence. Conclusion: Our study is the first to our knowledge to measure the in vivo affinity (K^sub d^ and binding potential) of ^sup 18^F-FITM and mGluR1 density (B^sub max^) with a high correlation to in vitro values in rat brain regions. This measurement using PET with ^sup 18^F-FITM would be a useful index for research about mGluR1 functions in central nervous system disorders and development of new pharmaceuticals. [PUBLICATION ABSTRACT]