In vitro and In vivo Assessment of Suitable Reference Region and Kinetic Modelling for the mGluR1 Radioligand [C]ITDM in Mice.

Purpose: This study aimed at investigating binding specificity, suitability of reference region-based kinetic modelling, and pharmacokinetics of the metabotropic glutamate receptor 1 (mGluR1) radioligand [C]ITDM in mice.

Procedures: We performed in vivo blocking as well as displacement of [C]ITDM during positron emission tomography (PET) imaging using the specific mGluR1 antagonist YM-202074. Additionally, we assessed in vitro blocking of [H]ITDM at two different doses of YM-202074. As an alternative to reference region models, we validated the use of a noninvasive image-derived input function (IDIF) compared to an arterial input function measured with an invasive arteriovenous (AV) shunt using a population-based curve for radiometabolite correction and characterized the pharmacokinetic modelling of [C]ITDM in the mouse brain. Finally, we also assessed semi-quantitative approaches.

Results: In vivo blocking with YM-202074 resulted in a decreased [C]ITDM binding, ranging from - 35.8 ± 8.0 % in pons to - 65.8 ± 3.0 % in thalamus. Displacement was also markedly observed in all tested regions. In addition, in vitro [H]ITDM binding could be blocked in a dose-dependent manner. The volume of distribution (V) based on the noninvasive IDIF (V) showed excellent agreement with the V values based on the metabolite-corrected plasma input function regardless of the metabolite correction (r > 0.943, p < 0.0001). Two-tissue compartmental model (2TCM) was found to be the preferred model and showed optimal agreement with Logan plot (r > 0.960, p < 0.0001). A minimum scan duration of 80 min was required for proper parameter estimation. SUV was not reliable (r = 0.379, p = 0.0011), unlike the SUV ratio to the SUV of the input function, which showed to be a valid approach.

Conclusions: No suitable reference region could be identified for [C]ITDM as strongly supported by in vivo and in vitro evidence of specific binding in all brain regions. However, by applying appropriate kinetic models, [C]ITDM PET imaging represents a promising tool to visualize mGluR1 in the mouse brain.