Imaging of the muscarinic acetylcholine neuroreceptor in rats with the M2 selective agonist [18F]FP-TZTP

Introduction[¹⁸F]FP-TZTP is an M2 muscarinic subtype selective receptor-binding radiotracer used in vivo to image human and nonhuman primate brain following both bolus injection and infusion. In order to carry out repeated studies in rodents, the techniques developed for primates must be transferred to rodents with the same precision. This includes obtaining a metabolite-corrected input function.MethodsWe compared bolus injection with constant infusion in rats that were awake or under isoflurane anesthesia. Brain–plasma and brain–blood distribution ratios were calculated by dividing brain ¹⁸F concentrations, determined in vivo by positron emission tomography imaging with the Advanced Technology Laboratory Animal Scanner, ex vivo by direct counting in excised brain tissue or by quantitative autoradiography by the plasma or whole blood concentrations that had been corrected for metabolite contents.ResultsBlood volume constraints prevented adequate blood sampling to define a precise input function after bolus injection, thus preventing full kinetic analysis. Constant infusion, however, required fewer blood samples to define the input function, allowing calculation of distribution ratios, but complete equilibrium between plasma and tissue had not yet been reached after 120 min.ConclusionOur results showed that the blood clearance and metabolism were too rapid to obtain a reproducible input function after bolus injection. The equilibrium distribution ratios did not lead to precise biochemical parameters, but the constant infusion was more suitable in that distribution ratios between tissue and plasma were statistically more precise. Constant infusion is the better approach for studying [¹⁸F]FP-TZTP by small animal imaging.