First-in-human PET quantification study of cerebral α4ß2* nicotinic acetylcholine receptors using the novel specific radioligand (-)-[(18)F]Flubatine.

α4ß2* nicotinic receptors (α4ß2* nAChRs) could provide a biomarker in neuropsychiatric disorders (e.g., Alzheimer's and Parkinson's diseases, depressive disorders, and nicotine addiction). However, there is a lack of α4ß2* nAChR specific PET radioligands with kinetics fast enough to enable quantification of nAChR within a reasonable time frame. Following on from promising preclinical results, the aim of the present study was to evaluate for the first time in humans the novel PET radioligand (-)-[(18)F]Flubatine, formerly known as (-)-[(18)F]NCFHEB, as a tool for α4ß2* nAChR imaging and in vivo quantification. Dynamic PET emission recordings lasting 270min were acquired on an ECAT EXACT HR+ scanner in 12 healthy male non-smoking subjects (71.0±5.0years) following the intravenous injection of 353.7±9.4MBq of (-)-[(18)F]Flubatine. Individual magnetic resonance imaging (MRI) was performed for co-registration. PET frames were motion-corrected, before the kinetics in 29 brain regions were characterized using 1- and 2-tissue compartment models (1TCM, 2TCM). Given the low amounts of metabolite present in plasma, we tested arterial input functions with and without metabolite corrections. In addition, pixel-based graphical analysis (Logan plot) was used. The model's goodness of fit, with and without metabolite correction was assessed by Akaike's information criterion. Model parameters of interest were the total distribution volume VT (mL/cm(3)), and the binding potential BPND relative to the corpus callosum, which served as a reference region. The tracer proved to have high stability in vivo, with 90% of the plasma radioactivity remaining as untransformed parent compound at 90min, fast brain kinetics with rapid uptake and equilibration between free and receptor-bound tracer. Adequate fits of brain TACs were obtained with the 1TCM. VT could be reliably estimated within 90min for all regions investigated, and within 30min for low-binding regions such as the cerebral cortex. The rank order of VT by region corresponded well with the known distribution of α4ß2* receptors (VT [thalamus] 27.4±3.8, VT [putamen] 12.7±0.9, VT [frontal cortex] 10.0±0.8, and VT [corpus callosum] 6.3±0.8). The BPND, which is a parameter of α4ß2* nAChR availability, was 3.41±0.79 for the thalamus, 1.04±0.25 for the putamen and 0.61±0.23 for the frontal cortex, indicating high specific tracer binding. Use of the arterial input function without metabolite correction resulted in a 10% underestimation in VT, and was without important biasing effects on BPND. Altogether, kinetics and imaging properties of (-)-[(18)F]Flubatine appear favorable and suggest that (-)-[(18)F]Flubatine is a very suitable and clinically applicable PET tracer for in vivo imaging of α4ß2* nAChRs in neuropsychiatric disorders.

Evaluation of metabolism, plasma protein binding and other biological parameters after administration of (-)-[(18)F]Flubatine in humans.

INTRODUCTION: (-)-[(18)F]Flubatine is a PET tracer with high affinity and selectivity for the nicotinic acetylcholine α4ß2 receptor subtype. A clinical trial assessing the availability of this subtype of nAChRs was performed. From a total participant number of 21 Alzheimer's disease (AD) patients and 20 healthy controls (HCs), the following parameters were determined: plasma protein binding, metabolism and activity distribution between plasma and whole blood. METHODS: Plasma protein binding and fraction of unchanged parent compound were assessed by ultracentrifugation and HPLC, respectively. The distribution of radioactivity (parent compound+metabolites) between plasma and whole blood was determined ex vivo at different time-points after injection by gamma counting after separation of whole blood by centrifugation into the cellular and non-cellular components. In additional experiments in vitro, tracer distribution between these blood components was assessed for up to 90min. RESULTS: A fraction of 15%±2% of (-)-[(18)F]Flubatine was found to be bound to plasma proteins. Metabolic degradation of (-)-[(18)F]Flubatine was very low, resulting in almost 90% unchanged parent compound at 90min p.i. with no significant difference between AD and HC. The radioactivity distribution between plasma and whole blood changed in vivo only slightly over time from 0.82±0.03 at 3min p.i. to 0.87±0.03 at 270min p.i. indicating the contribution of only a small amount of metabolites. In vitro studies revealed that (-)-[(18)F]Flubatine was instantaneously distributed between cellular and non-cellular blood parts. DISCUSSION: (-)-[(18)F]Flubatine exhibits very favourable characteristics for a PET radiotracer such as slow metabolic degradation and moderate plasma protein binding. Equilibrium of radioactivity distribution between plasma and whole blood is reached instantaneously and remains almost constant over time allowing both convenient sample handling and facilitated fractional blood volume contribution assessment.

Fully automated radiosynthesis of both enantiomers of [18F]Flubatine under GMP conditions for human application.

A fully automatized radiosynthesis of (+)- and (-)-[(18)F]Flubatine ((+)- and (-)NCFHEB) by means of a commercially available synthesis module (TRACERlab FX FN) under GMP conditions is reported. Radiochemical yields of 30% within an overall synthesis time of 40 min were achieved in more than 70 individual syntheses. Specific activities were approximately 3000 GBq/µmol and radiochemical purity was determined to be at least 97%.

Imaging of the brain serotonin transporters (SERT) with 18F-labelled fluoromethyl-McN5652 and PET in humans.

PURPOSE: [(11)C]DASB is currently the most frequently used highly selective radiotracer for visualization and quantification of central SERT. Its use, however, is hampered by the short half-life of (11)C, the moderate cortical test-retest reliability, and the lack of quantifying endogenous serotonin. Labelling with (18)F allows in principle longer acquisition times for kinetic analysis in brain tissue and may provide higher sensitivity. The aim of our study was to firstly use the new highly SERT-selective (18)F-labelled fluoromethyl analogue of (+)-McN5652 ((+)-[(18)F]FMe-McN5652) in humans and to evaluate its potential for SERT quantification. METHODS: The PET data from five healthy volunteers (three men, two women, age 39 ± 10 years) coregistered with individual MRI scans were semiquantitatively assessed by volume-of-interest analysis using the software package PMOD. Rate constants and total distribution volumes (V (T)) were calculated using a two-tissue compartment model and arterial input function measurements were corrected for metabolite/plasma data. Standardized uptake region-to-cerebellum ratios as a measure of specific radiotracer accumulation were compared with those of a [(11)C]DASB PET dataset from 21 healthy subjects (10 men, 11 women, age 38 ± 8 years). RESULTS: The two-tissue compartment model provided adequate fits to the data. Estimates of total distribution volume (V (T)) demonstrated good identifiability based on the coefficients of variation (COV) for the volumes of interest in SERT-rich and cortical areas (COV V (T) <10%). Compared with [(11)C]DASB PET, there was a tendency to lower mean uptake values in (+)-[(18)F]FMe-McN5652 PET; however, the standard deviation was also somewhat lower. Altogether, cerebral (+)-[(18)F]FMe-McN5652 uptake corresponded well with the known SERT distribution in humans. CONCLUSION: The results showed that (+)-[(18)F]FMe-McN5652 is also suitable for in vivo quantification of SERT with PET. Because of the long half-life of (18)F, the widespread use within a satellite concept seems feasible.