Balázs Gulyás

[11C]AZ10419369: a selective 5-HT1B receptor radioligand suitable for positron emission tomography (PET). Characterization in the primate brain.

The 5-HT1B receptor has been implicated in several psychiatric disorders and is a potential pharmacological target in the treatment of depression. Here we report the synthesis of a novel PET radioligand, [11C]AZ10419369 (5-methyl-8-(4-methyl-piperazin-1-yl)-4-oxo-4H-chromene-2-carboxylic acid (4-morpholin-4-yl-phenyl)-amide), for in vivo visualization of 5-HT1B receptors in the brains of macaques and humans subjects. [11C]AZ10419369 was prepared by N-methylation of (8-(1-piperazinyl)-5-methylchrom-2-en-4-one-2-(4-morpholinophenyl) carboxamide, using carbon-11 methyl triflate. Regional brain uptake patterns of [11C]AZ10419369 were characterized by PET measurements in two macaques and a preliminary study in two human subjects. In addition, AZ10419369 was prepared in tritium labeled form for in vitro autoradiography studies in macaque brain tissue sections. The radiochemical purity of [11C]AZ10419369 was >99% and specific radioactivity was >3600 Ci/mmol. After iv injection of [11C]AZ10419369, 3-4% was in brain after 7.5 min. The regional brain distribution of radioactivity was similar in humans and macaques showing the highest uptake of radioactivity in the occipital cortex and the basal ganglia, in accord with autoradiographic studies performed using [3H]AZ10419369. Uptake was moderate in the temporal and frontal cortical regions, lower in the thalamus and lowest in the cerebellum. In macaques pre-treated with the selective 5-HT1B receptor antagonist, AR-A000002, binding was reduced in a dose-dependent manner, consistent with specific binding to 5-HT1B receptors. These data support [11C]AZ10419369 as a suitable radioligand for labeling 5-HT1B receptors in the primate brain. This radioligand may be useful in future studies evaluating drug-induced receptor occupancy and measurement of brain 5-HT1B receptor levels in patients with psychiatric disorders.

Development of a PET radioligand for the central 5-HT1B receptor: radiosynthesis and characterization in cynomolgus monkeys of eight radiolabeled compounds.

INTRODUCTIONnThe serotonin 1B (5-HT(1B)) receptor has been implicated in several psychiatric disorders and is a potential pharmacological target in the treatment of depression. The aim of this study was to develop a radioligand for positron emission tomography (PET) imaging of the 5-HT(1B) receptor in the primate brain in vivo.nnnMETHODSnEight carboxamide radioligands (1-8) from three different core structures were radiolabeled with carbon-11 employing N-methylation with [(11)C]methyl triflate on the piperazine structural moiety. In vivo PET evaluation of each radioligand was performed in cynomolgus monkeys and included analysis of radioactive metabolites measured in plasma using high-performance liquid chromatography.nnnRESULTSnIn a total of 12 radiosynthesis of the eight radioligands, the mean decay corrected yield was 11%, and the mean specific radioactivity was 299 GBq/μmol (8075 Ci/mmol) at time of administration. Of the eight tested candidates, [(11)C]6 demonstrated the most promising in vivo characteristics, showing high binding in 5-HT(1B) receptor-rich regions and low binding in the cerebellum. When inspecting data from all eight compounds, lipophilicity appeared as a physicochemical property that could be related to favorable in vivo imaging characteristics.nnnCONCLUSIONnCandidate [(11)C]6, i.e., [(11)C]AZ10419369, exhibited high binding potentials in regions known to contain 5-HT(1B) receptors and was nominated for further preclinical characterization and PET examination in human subjects.

Fluorine-18 labeling of three novel D-peptides by conjugation with N-succinimidyl-4-[18F]fluorobenzoate and preliminary examination by postmortem whole-hemisphere human brain autoradiography.

INTRODUCTIONnβ-Amyloid (Aβ) plaques and neurofibrillary tangles are the main characteristics of Alzheimers disease (AD). Positron emission tomography (PET), a high-resolution, sensitive, and noninvasive imaging technique, has been widely utilized in visualizing the localization of plaques and tangles and thereby distinguishing between AD and healthy controls. A small 12-mer D-enantiomeric peptide (amino acid sequence=QSHYRHISPAQV), denoted as D1, has high binding affinity to Aβ in vitro in the sub-micromolar range, and consequently, its radiolabeled analogues have a potential as radioligands for visualizing amyloid plaques in vivo by PET.nnnAIMnThe aims of the present work were to develop three different potent D1 derivative peptides labeled with fluorine-18 and to examine them in the AD and control postmortem human brain by autoradiography (ARG).nnnMETHODSnThree different D1 derivative peptides were radiolabeled with fluorine-18 ([(18)F]ACI-87, [(18)F]ACI-88, [(18)F]ACI-89) using the prosthetic group N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB) and purified by high performance liquid chromatography (HPLC). Preliminary ARG measurements were performed in AD and control brains.nnnRESULTSnThe three fluorine-18-labeled d-peptides were obtained in a total synthesis time of 140 min with radiochemical purity higher than 98%. The specific radioactivities of the three different D1 derivative peptides were between 9 and 113 GBq/μmol. ARG demonstrated a higher radioligand uptake in the cortical gray matter and the hippocampus in the AD brain as compared to age-matched control brain.nnnCONCLUSIONSnFluorine-18 labeling of the three novel D1 derivative peptides using [(18)F]SFB was successfully accomplished. Higher contrast between AD and control brain slices demonstrates their potential applicability for further use in vivo by PET.

Synthesis and evaluation of 2-chloro N-((S)-{(S)-1-( 11 C)methylpiperidin-2-yl} (phenyl)methyl)3-trifluoromethyl-benzamide (( 11 C)N-methyl-SSR504734) as a PET radioligand for glycine transporter 1

BackgroundDysfunction of the glycine transporter 1 (GlyT1) has been suggested to be involved in psychiatric disorders such as schizophrenia. GlyT1 inhibitors have therefore been considered to have antipsychotic therapeutic potential. Positron emission tomography (PET) imaging probes for GlyT1 are, consequently, expected to be useful for investigating the mechanism of such disease conditions and for measuring occupancy of GlyT1 inhibitors in vivo. The aim of this study was to assess the potential of 2-chloro N-[(S)-{(S)-1-[11u2009C]methylpiperidin-2-yl} (phenyl)methyl] 3-trifluoromethyl-benzamide ([11u2009C]N-methyl-SSR504734) as a PET imaging agent for GlyT1.Methods[11u2009C]N-methyl-SSR504734 was synthesized by N-[11u2009C]methylation of SSR504734 via [11u2009C]CH3OTf. In vitro brain distribution of [11u2009C]N-methyl-SSR504734 was tested in whole-hemisphere autoradiography (ARG) on human brain slices. Initial PET studies were performed using a cynomolgus monkey at baseline and after pretreatment with 0.1 to 1.5u2009mg/kg of SSR504734. Then, PET studies using rhesus monkeys were performed with arterial blood sampling at baseline and after pretreatment with 1.5 to 4.5u2009mg/kg SSR504734. Distribution volumes (VT) were calculated with a two-tissue compartment model, and GlyT1 occupancy by SSR504734 was estimated using a Lassen plot approach.Results[11u2009C]N-methyl-SSR504734 was successfully synthesized in moderate radiochemical yield and high specific radioactivity. In the ARG experiments, [11u2009C]N-methyl-SSR504734 showed specific binding in the white matter and pons. In the initial PET experiments in a cynomolgus monkey, [11u2009C]N-methyl-SSR504734 showed high brain uptake and consistent distribution with previously reported GlyT1 expression in vivo (thalamus, brainstemu2009>u2009cerebellumu2009>u2009cortical regions). However, the brain uptake increased after pretreatment with SSR504734. Further PET studies in rhesus monkeys showed a similar increase of brain uptake after pretreatment with SSR504734. However, the VT of [11u2009C]N-methyl-SSR504734 was found to decrease after pretreatment of SSR504734 in a dose-dependent manner. GlyT1 occupancy was calculated to be 45% and 73% at 1.5 and 4.5u2009mg/kg of SSR504734, respectively.Conclusions[11u2009C]N-methyl-SSR504734 is demonstrated to be a promising PET radioligand for GlyT1 in nonhuman primates. The present results warrant further PET studies in human subjects.