Improved Visualization and Specific Binding for Metabotropic Glutamate Receptor Subtype 1 (mGluR1) Using [11C]ITMM with Ultra-High Specific Activity in Small-Animal PET

Metabotropic glutamate receptor subtype 1 (mGluR1) is a crucial target in the development of new medications to treat central nervous system (CNS) disorders. Recently, we developed N-4-6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-4-¹¹C]methoxy-N-methyl-benzamide (¹¹C]ITMM) as a useful positron emission tomography (PET) probe for mGluR1 in clinical studies. Here, we aimed to improve visualization and threshold of specific binding for mGluR1 using ¹¹C]ITMM with ultra-high specific activity (SA) of > 3,500 GBq/μmol in rat brains. A two-tissue compartment model indicated large differences between the two SAs in the constants k₃ and k₄, representing binding ability for mGluR1, while constants K ₁ and k₂ showed no differences. The total distribution volume (V_T) values of conventional and ultra-high SA were 9.1 and 11.2 in the thalamus, 7.7 and 9.7 in the striatum, and 6.4 and 8.5 mL/cm³ in the substantia nigra, respectively. The specific binding of ¹¹C]ITMM with ultra-high SA was significantly higher than the conventional SA, especially in the basal ganglia. Parametric PET images scaled with V_T of the ultra-high SA clearly identified regional differences in the rat brain. In conclusion, PET studies using ¹¹C]ITMM with ultra-high SA could sufficiently improve visualization and specific binding for mGluR1, which could help further understanding for mGluR1 functions in CNS disorders.