BRIEF COMMUNICATION Age-Related Change in 5-HT6 Receptor Availability in Healthy Male Volunteers Measured with 11C-GSK215083 PET Rajiv Radhakrishnan1, Nabeel Nabulsi2, Edward Gaiser1, Jean-Dominique Gallezot2, Shannan Henry2, Beata Planeta2, Shu-fei Lin2, Jim Ropchan2, Wendol Williams1, Evan Morris2,3, Deepak Cyril D’Souza1, Yiyun Huang2, Richard E. Carson2,3, and David Matuskey1,2 1Department of Psychiatry, Yale University School of Medicine, New Haven, Connecticut; 2Department of Radiology and Biomedical Imaging, Yale University School of Medicine, New Haven, Connecticut; and 3Department of Biomedical Engineering, Yale University, New Haven, Connecticut because it presents an attractive therapeutic target for neuropsychi- Serotonin receptor 6 (5-hydroxytrypamine-6, or 5-HT6) is a potential atric disorders. therapeutic target given its distribution in brain regions that are Functionally, 5-HT6 exhibits excitatory action, but it can also important in depression, anxiety, and cognition. This study sought colocalize with g-aminobutyric acid–ergic neurons and produce an to investigate the effects of age on 5-HT6 receptor availability using inhibition of brain activity leading to complicated and discrepant 11 C-GSK215083, a PET ligand with affinity for 5-HT6 in the striatum results (2,4). Heterogeneous effects are also seen with other neu- and 5-HT in the cortex. Methods: Twenty-eight healthy male vol- 2A rotransmitters in specific brain regions, with 5-HT antagonism unteers (age range, 23–52 y) were scanned with 11C-GSK215083 6 PET. Time–activity curves in regions of interest were fitted using a resulting in increased extracellular glutamate, dopamine, and ace- multilinear analysis method. Nondisplaceable binding potential tylcholine in the frontal cortex and hippocampus (5,6), whereas (BPND) was calculated using the cerebellum as the reference region 5-HT6 agonists have been shown to produce increased extracellu- and corrected for partial-volume effects. Results: In 5-HT6–rich lar g-aminobutyric acid release in the dorsal hippocampus, stria- areas, regional 11C-GSK215083 showed a negative correlation be- tum, and amygdala (4). Thus, with a multitude of interactions not tween BP and age in the caudate (r 5 −0.41, P 5 0.03) (14% ND fully elucidated, preclinical studies on both 5-HT6 agonists and change per decade) and putamen (r 5 −0.30, P 5 0.04) (11% change 5-HT6 antagonists have shown efficacy against depression, anxiety, per decade) but not in the ventral striatum or pallidum. A negative and cognitive impairment (1). In addition, 5-HT antagonists have correlation with age was also seen in cortical regions (r 5 −0.41, P 5 6 0.03) (7% change per decade), consistent with the literature on shown promise in the treatment of obesity (7), major depressive disorder (8), age-related cognitive decline (9), and schizophrenia 5-HT2A availability. Conclusion: To our knowledge, this was the first in vivo study on humans to examine the effect of age on 5-HT6 re- (10). Currently available antidepressants and antipsychotics have ceptor availability. The study demonstrated a significant age-related also been shown to have a strong affinity for the 5-HT6 receptor BP decline in 5-HT6 availability ( ND) in the caudate and putamen. (11,12), adding speculation that these clinical effects might, at least 11 Key Words: serotonin 6; age effects; 5-HT6; PET imaging; C- in part, be mediated through this receptor. GSK215083 Age has been associated with a decline in receptor density J Nucl Med 2018; 59:1445–1450 across several 5-HT receptor subtypes (13–15). Given that some of DOI: 10.2967/jnumed.117.206516 the potential indications, such as cognitive impairment in Alz- heimer disease and age-related cognitive decline, are related to advancing age, it is important to understand the effects of age on 5-HT6 receptors and to ascertain whether they remain an important target in older age groups. This importance is further underscored by recent negative results from 2 phase III trials of the 5-HT6 The 5-hydroxytrypamine-6 (5-HT6) receptor is a 7-transmem- brane serotonin receptor subtype and one of the 14 distinct mam- antagonist idalopirdine for cognitive impairment in Alzheimer dis- malian serotonin receptors that are expressed almost exclusively ease, despite promising results in preclinical studies (16). in the central nervous system (1,2). In vitro studies demonstrated Although age has been shown to be associated with a decline in 5-HT (14), 5-HT ,5HT (15), and 5-HT receptors (13), the that 5-HT6 receptors are localized to the striatum, hippocampus, 1B 1A 2A 4 amygdala, hypothalamus, thalamus, and cerebral cortex (3). The effects of age on 5-HT6 receptor availability in humans is not localization to both basal ganglia and limbic structures suggests known. The availability of a PET ligand for the 5-HT6 receptor that this receptor may be involved in the serotonergic control of 11C-GSK215083 offers an opportunity to examine the in vivo re- motor function, mood-dependent behavior, depression, and cognition ceptor density in healthy humans and in disease states (17,18). In 5 (3). Subsequently, more attention has recently been given to 5-HT6 vitro GSK215083 exhibits high binding affinity for 5-HT6 (pKi 11 9.8) and 5-HT2A (pKi 5 9.1) receptors. Although C-GSK215083 has high affinity for 5-HT receptors in vivo, it has a nonnegligible Received Dec. 4, 2017; revision accepted Mar. 21, 2018. 6 For correspondence or reprints contact: David Matuskey, Yale University affinity (albeit 5-fold lower) for 5-HT2A receptors in cortical re- School of Medicine, 801 Howard Ave., New Haven, CT 06520. gions (17). Therefore, whereas striatal 11C-GSK215083 signal is E-mail: [email protected] Published online Apr. 6, 2018. primarily reflective of 5-HT6 availability, its binding in the cortex COPYRIGHT © 2018 by the Society of Nuclear Medicine and Molecular Imaging. is thought to reflect 5-HT2A availability (17,18). AGE EFFECTS ON 5-HT6 RECEPTOR IN MEN • Radhakrishnan et al. 1445 In this paper, we examine the effects of age on 5-HT6 receptor sample loop preloaded with a solution of 0.5 mg of desmethyl availability in healthy male volunteers using the PET ligand 11C- GSK215083 and 1 mL of 2,2,6,6,-tetramethylpiperidine in 80 mL GSK215083. of anhydrous N,N-dimethylformamide until radioactivity peaked. The reaction mixture was allowed to stand for 5 min at ambient temperature, loaded onto the semipreparative high-performance MATERIALS AND METHODS liquid chromatography column, and purified as described as above. The product fraction was collected, diluted with 50 mL of water, Participants and loaded onto a C18 Sep-Pak classic cartridge. The cartridge was Twenty-eight healthy male volunteers (mean age, 36.0 6 9.3 y; rinsed with 10 mL of 1 mM HCl and dried. The product was then range, 23–52 y) were included in the study. All underwent a compre- eluted off the SepPak with 1 mL of ethanol (USP grade), followed hensive screening assessment that included a clinical interview, com- by 3 mL of saline (USP grade). The combined solution was finally plete physical examination with medical history, routine blood tests, passed through a sterile 0.22-mm membrane filter into a sterile electrocardiogram, and urine toxicology. Individuals were excluded if dose-vial precharged with 7 mL of USP-grade saline and 20 mLof they self-reported or evaluation revealed a diagnosis of a current or 4.2% sodium bicarbonate (USP grade). Purity and molar activity were lifetime psychiatric disorder; current or past serious medical or neu- determined using a Luna C18(2) column (5 mm, 250 · 4.6 mm) eluting rologic illness (including a history of head injury with loss of con- with a mixture of 40% acetonitrile and 60% 0.1 M ammonium formate sciousness); metal in their body, which would result in MRI exclusion; at a flow rate of 1.5 mL/min. a history of substance abuse or dependence; or illicit drug use in the previous 3 mo. Image Acquisition The study was performed under protocols approved by the Yale Structural MRI was performed on a 3-T Trio system (Siemens Human Investigation Committee, the Yale University Radiation Safety Medical Solutions) with a circularly polarized head coil for purposes Committee, the Yale–New Haven Hospital Radioactive Drug Research of excluding individuals with anatomic abnormalities and anatomi- Committee, and the Yale MRI Safety Committee. The study was cally coregistering with PET scans. The dimensions and voxel size of performed in accordance with good clinical practice guidelines, reg- MR images were 256 · 256 · 176 voxels and 0.98 · 0.98 · 1.0 mm, ulatory requirements, and the code of ethics of the World Medical respectively. Association (Declaration of Helsinki). Subjects were recruited from Subjects underwent 90-min dynamic PET on an ECAT EXACT Connecticut and surrounding states by paper and online advertise- HR1 scanner (Siemens Medical Systems) in 3-dimensional mode. ments, as well as personal referrals. Written informed consent was Before each PET scan, a transmission scan was acquired for attenu- obtained from all participants at the beginning of screening after the ation correction. Up to 740 MBq (20 mCi) of radioactivity were study procedures had been fully explained. administered by an infusion pump for each scan (mean, 585.39 6 Radiosynthesis of 11C-GSK215083 on the FXC 128.34 MBq). The maximum mass dose from the radioligand was 6 Automated Module limited to minimize any self-occupancy (#10 mg) (mean, 3.03 11 11C-GSK215083 was radiolabeled with 11C-methyl triflate using an 2.32 mg). For each PET scan, C-GSK215083 was administered by FXC automated module (GE Healthcare) by modifying a previously bolus intravenous infusion over 60 s, immediately followed by acqui- described procedure (18).