First-in-human PET quantification study of cerebral α4β2* nicotinic acetylcholine receptors using the novel specific radioligand (−)-[¹⁸F]Flubatine

Highlights

• Metabolization and plasma protein binding of (−)-[¹⁸F]-Flubatine are very low.

• In cortical regions the distribution volume VT can be estimated from only 30 min PET data.

• VT values correlated well with the known brain distribution of α4β2* nAChRs in the brain.

• Kinetics is fast and can be well described by a 1-tissue compartment model.

• We examined 12 healthy subjects and no adverse effects occurred.

Abstract

α4β2* nicotinic receptors (α4β2* 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 α4β2* 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 (−)-[¹⁸F]Flubatine, formerly known as (−)-[¹⁸F]NCFHEB, as a tool for α4β2* nAChR imaging and in vivo quantification.

Dynamic PET emission recordings lasting 270 min were acquired on an ECAT EXACT HR + scanner in 12 healthy male non-smoking subjects (71.0 ± 5.0 years) following the intravenous injection of 353.7 ± 9.4 MBq of (−)-[¹⁸F]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 V_T (mL/cm³), and the binding potential BP_ND 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 90 min, fast brain kinetics with rapid uptake and equilibration between free and receptor-bound tracer. Adequate fits of brain TACs were obtained with the 1TCM. V_T could be reliably estimated within 90 min for all regions investigated, and within 30 min for low-binding regions such as the cerebral cortex.

The rank order of V_T by region corresponded well with the known distribution of α4β2* receptors (V_T [thalamus] 27.4 ± 3.8, V_T [putamen] 12.7 ± 0.9, V_T [frontal cortex] 10.0 ± 0.8, and V_T [corpus callosum] 6.3 ± 0.8). The BP_ND, which is a parameter of α4β2* 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 V_T, and was without important biasing effects on BP_ND.

Altogether, kinetics and imaging properties of (−)-[¹⁸F]Flubatine appear favorable and suggest that (−)-[¹⁸F]Flubatine is a very suitable and clinically applicable PET tracer for in vivo imaging of α4β2* nAChRs in neuropsychiatric disorders.

Introduction

Neuronal nicotinic receptors (nAChRs) are ligand-gated ion channels that are physiologically activated by acetylcholine. Each receptor is composed of five cylindrical transmembrane subunits forming the central ion channel, and can be composed of α7 subunits or some stoichiometric combination of α and β subunits (α2–10, β2–4). The various forms are widely expressed throughout the brain. In the human brain, the most abundant form is the α4β2* subtype (Dani and Bertrand, 2007). The α4β2* nAChRs play an important role in normal brain function regulating a variety of brain processes such as mood, cognition and motor control (Changeux and Edelstein, 2005). Pathological alterations of the α4β2* nAChRs are thought to contribute to several psychiatric and neurological disorders (Dani and Bertrand, 2007, Mihailescu and Drucker-Colín, 2000, Perry et al., 1995, Rinne et al., 1991).

Noninvasive imaging using positron emission tomography (PET) or single-photon emission computer tomography (SPECT) has proved essential in the quantitative assessment of in vivo pathological changes of the α4β2* nAChR availability in neuropsychiatric disorders. High-affinity α4β2* nAChR specific radioligands, such as 5-[¹²³I]IA-85380 (5-IA) for SPECT and 6-[¹⁸F]FA-85380 (6-FA) and 2-[¹⁸F]FA-85380 (2-FA) for PET, have been developed and optimized for α4β2* nAChR imaging (Chefer et al., 1998, Ding et al., 2004, Doll et al., 1999, Horti et al., 1998, Kimes et al., 2003, Mamede et al., 2004, Scheffel et al., 2000, Schildan et al., 2007). Recently, through the use of 2-FA-PET and 5-IA-SPECT, cortical and subcortical abnormalities of the α4β2* nAChR binding have been reported in a variety of neuropsychiatric disorders such as Alzheimer's disease (AD) and mild cognitive impairment (MCI) (Kendziorra et al., 2011, O'Brien et al., 2007, Okada et al., 2013, Sabri et al., 2008), Parkinson's disease (PD) (Fujita et al., 2006, Kas et al., 2009, Meyer et al., 2009), epilepsy (Picard et al., 2006), nicotine dependence (Mukhin et al., 2008, Staley et al., 2006), major depressive disease (Saricicek et al., 2012), and schizophrenia (D'Souza et al., 2012, Brašić et al., 2012).

However, the slow kinetics of the PET radioligand 2-[¹⁸F]FA-85380 and SPECT radioligand 5-[¹²³I]IA-85380 require lengthy acquisition times of up to 7 h to enable whole-brain nAChR analysis. Therefore, the use of 2-FA or 5-IA for large-scale clinical trials and routine clinical application may be limited (Horti and Villemagne, 2006, Horti et al., 2010, Sabri et al., 2008). A new generation α4β2* nAChR specific PET radioligands, such as (−)-[¹⁸F]norchloro-fluoro-homoepibatidine ((−)-[¹⁸F]NCFHEB), also known as (−)-[¹⁸F]Flubatine, [¹⁸F]AZAN and [¹⁸F]nifene, demonstrating faster kinetic properties in both preclinical and in-man investigations have been developed (Brust et al., 2008, Hillmer et al., 2011, Hockley et al., 2013, Kuwabara et al., 2012, Sabri et al., 2008, Wong et al., 2013). We recently developed (−)-[¹⁸F]Flubatine as a less toxic derivative of epibatidine (Bai et al., 1996, Brust et al., 2008, Deuther-Conrad et al., 2004, Deuther-Conrad et al., 2008, Fischer et al., 2013, Patt et al., 2003, Patt et al., 2014, Patt et al., 2013, Sabri et al., 2008, Smits et al., 2014). Preclinical studies demonstrated that (−)-[¹⁸F]Flubatine has high selectivity and specificity for α4β2* nAChRs (Brust et al., 2008, Deuther-Conrad et al., 2004, Smits et al., 2014).

The objectives of the present study were to determine the safety and tolerability of (−)-[¹⁸F]Flubatine, and to evaluate kinetic model-based approaches to quantify α4β2* nAChR binding parameters from dynamic PET and blood data in healthy volunteers.