Comparison of 4 Radiolabeled Antagonists for Serotonin 5-HT₇ Receptor Neuroimaging: Toward the First PET Radiotracer

Abstract

Brain serotonin 7 (5-hydroxytryptamine 7, or 5-HT 7 ) is the most recently identified serotonin receptor. It is involved in mood disorders and is studied as a target for antidepressants. Because no radioligand has yet been successfully used to study this receptor by PET neuroimaging, the objective of the present study was to develop a 5-HT 7 18 F-labeled radiotracer. Methods: Four structural analogs of SB269970, a specific 5-HT 7 receptor antagonist, were synthesized. The nitro precursors of these analogs were radiolabeled by 18 F nucleophilic substitution. Analog antagonist effects were investigated by cellular functional assay. The cerebral distribution of radiolabeled molecules was studied by in vitro autoradiography in rats, and respective selectivity was determined by competition with the 5-HT 7 receptor antagonist SB269970 at different concentrations. Ex vivo small-animal PET studies in rats and in vivo PET studies in cats focused on the 1-(2-{(2R)-1-[(fluorophenyl)sulfonyl] pyrrolidin-2-yl}ethyl)-4-methylpiperidine (FP3) series. Results: Four analogs were synthesized from the SB269970 pharmacophore and divided into an FP3 ( 18 F-4FP3 and 18 F-2FP3) and an 1-(2-{(2R)-1-[(fluorophenyl)sulfonyl]pyrrolidin-2-yl}ethyl)-4-(2-methoxyphenyl) piperazine (FPMP) ( 18 F-2FPMP and 18 F-4FPMP) series. The chemical and radiochemical purities of the 4 radiolabeled molecules were greater than 98%. All presented suitable affinity for 5-HT 7 (apparent dissociation constant [K D ] between 1.6 and 14 nM), although the FPMP series showed moderate agonist activity for 5-HT 1A receptors. Lipophilicity values were predictive of good radiotracer blood-brain barrier penetration (logD from 1.4 to 3.9). In vitro competition with a 5-HT 7 antagonist, SB269970, revealed that only radioligands from the FP3 series were displaced by the 5-HT 7 -specific antagonist: subsequent in vivo study, therefore, focused on this series. Ex vivo 18 F-4FP3 and 18 F-2FP3 autoradiography was in accordance with the 5-HT 7 brain distribution, with few brain radioactive metabolites. PET scans in cats showed that pretreatment with a 5-HT 7 antagonist significantly reduced 18 F-2FP3 but not 18 F-4FP3 binding. Conclusion: The 4 PET radiotracers had suitable characteristics for 5-HT 7 receptor probing in vitro, although the FP3 series seemed to be more specific for in vivo imaging of 5-HT 7 receptors. In particular, on the basis of the in vivo results, 18 F-2FP3 appears to be the first PET radiotracer to enable in vivo imaging of 5-HT 7 receptors in animal models, possibly leading to neuroimaging studies in humans. Serot onin (5-hydroxytryptamine, or 5-HT) is a central neurotransmitter involved in many physiologic functions and in neurologic and psychiatric disorders. Pharmacologic studies identified numerous serotoninergic receptor families and subtypes, classified by structural, functional, and pharmacologic criteria into 7 distinct receptor classes (5-HT 1-7 ) (1).The 5-HT 7 subtype is the most recently cloned serotonin receptor (2). 5-HT 7 receptors are cou...

PET Tracers for Serotonin Receptors and Their Applications

“…Among these, [ 18 F]2FP3 (K i = 8.4 nM, Fig. 11) and [ 18 F]4FP3 (K i = 14 nM, Figure 11) show specific binding in vitro in brain sections of rats [175]. In vivo studies in cats show excellent brain uptake, regional distribution and specific binding for [ 18 F]2FP3 [176].…”

Evaluation of [18F]2FP3 in pigs and non‐human primates

“…The mismatch between regional distribution volumes of [ 18 F]2FP3 and the in vitro autoradiography outcomes supports that [ 18 F]2FP3 has off‐target binding. We can only speculate in the nature of the potential in vivo off‐target binding as affinity was only tested for the 5‐HT 7 , 5‐HT 1A , and 5‐HT 6 receptors and with apparent affinity for the two latter targets . Similarly, low affinity for the D 2 (790nM), D 3 (631nM), and 5‐HT 6 (3.21μM) receptors was identified of the closely related compound 12 in the publication by Lovell et al The blocking effect obtained in these studies contrasts the results obtained in cats, where the TACs strongly indicated a blocking effect after preadministration of SB‐269970 (5 mg/kg).…”

“…Similarly, low affinity for the D 2 (790nM), D 3 (631nM), and 5‐HT 6 (3.21μM) receptors was identified of the closely related compound 12 in the publication by Lovell et al The blocking effect obtained in these studies contrasts the results obtained in cats, where the TACs strongly indicated a blocking effect after preadministration of SB‐269970 (5 mg/kg). However, since the arterial input function was not determined in these experiments, the binding before and after pretreatment with SB‐269970 was not quantified …”

“…Of these, [ 11 C]Cimbi‐717 displayed the most promising results for selective 5‐HT 7 R PET imaging in pigs . A series of 18 F‐labelled compounds have been developed using SB‐269970 as lead compound because of its high affinity ( K D = 1.3nM) and high selectivity for the 5‐HT 7 R . One promising analogue of SB‐269970 is [ 18 F]2FP3 ( K D = 8.4nM, human recombinant receptors in Chinese hamster ovary [CHO] cells), which displayed promising results when evaluated in cats .…”

Radiosynthesis and In Vivo Evaluation of Novel Radioligands for PET Imaging of Cerebral 5-HT7 Receptors

“…The 5-HT 7 R antagonist (R)-3-(2-(2-(4-methylpiperidin-1-yl)ethyl)pyrrolidine-1-sulfonyl) phenol (SB-269970) has been used as a lead structure for discovering 18 F-labeled radioligands. 18 F-1-{2-[(2S)-1-(phenylsulfonyl) pyrrolidin-2-yl]ethyl}piperidin-4-yl 4-fluorobenzoate and 1-(2-{(2R)-1-[(2-18 F-fluorophenyl)sulfonyl]pyrrolidin-2-yl}ethyl)-4-methylpiperidine ( 18 F-2FP3) were evaluated ex vivo in rats and in vivo in cats (9,10). However, no input function was obtained while evaluating 18 F-2FP3.…”