In vivo measurement of the affinity and density of metabotropic glutamate receptor subtype 1 in rat brain using 18F-FITM in small-animal PET

Metabotropic glutamate receptor subtype 1 (mGluR1) is a crucial molecular target in the central nervous system disorders. 4-(18)F-fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ((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 (18)F-FITM in rat brain under the in vivo conditions.

Methods: Binding potentials (BP(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 (18)F-FITM PET results in place of reference tissue methods. Using BP(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(max)) and ligand affinity (dissociation constant, or K(d)) in brain regions, respectively.

Results: The pretreatment of rats with unlabeled FITM (1 mg/kg) occupied an mGluR1 binding site of (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(ND) based on EA RBA showed a high correlation with the BP(ND) based on invasive Logan plot graphical analysis in the thalamus, hippocampus, striatum, and cingulate cortex. The K(d) (nM) and B(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(d) and binding potential) of (18)F-FITM and mGluR1 density (B(max)) with a high correlation to in vitro values in rat brain regions. This measurement using PET with (18)F-FITM would be a useful index for research about mGluR1 functions in central nervous system disorders and development of new pharmaceuticals.