In vivo quantification of benzodiazepine receptor density and affinity in rat brain using small animal PET with a multi-injection protocol

In order to follow up receptor density and affinity changes occurring in small animal models of disease, in vivo PET imaging is of great potential. Before creating a simplified non-invasive method, it is recommended to fully understand the kinetics of the radioligand-receptor interactions. In this study, we aim at fully characterizing the [18F]FMZ kinetics in healthy rats, thus identifying all the parameters of the compartmental model that describes the interactions between [18F]FMZ and the benzodiazepine (BZ) receptors in the brain. This study also investigates the feasibility of performing a multi-injection protocol in the rat with small animal PET. Methods: Five rats underwent 60-minutes PET scans after injection of (i) Tracer dose of [18F]FMZ; (ii) Pre-treatment with FMZ (150 ug/kg), then tracer dose [18F]FMZ; (iii) Tracer dose of [18F]FMZ, at T=20min: displacement with FMZ (150 ug/kg); (iv) Three-injection protocol: T=0: 0.6nmol of [18F]FMZ; T=42min: 2.7 nmol of [18F]FMZ; T=70min: 495 nmol of FMZ. Thirty-seven arterial blood samples were collected during protocol (iv), and whole blood concentration was counted. The parent compound concentration in plasma was assessed and an input function could be derived. A non-linear compartmental model describing the behaviour of [18F]FMZ and FMZ in the tissue was used for kinetics simulation. Results: The two first protocols show that uptake in brain structures is correlated to BZR density values estimated with in vitro binding and non-specific uptake is similar among most of the brain structures. Displacement studies displayed a rapid and major signal decrease in hippocampus and pons (90% and 79%, resp.). Multi-injection kinetics are shown in figure 1. The arterial input function could be accurately assessed, and the first model parameters were identified. Receptor density Bmax and affinity KdVr were consistent with values presented in a multiple-experiment study carried out with [11C]Flumazepil (Liefaard, 2005); Bmax ranged from 19.47 to 81.5 pmol/ml and KdVr from15.68 to 22.51 nM, in hippocampus and pons, respectively. This study demonstrates that the identification of the model parameters (esp. density and affinity) in the rat brain with a multi-injection protocol is feasible using small animal PET and [18F]FMZ.