11C and18F PET radioligands for the serotonin transporter (SERT)

Beyond monoamines: I. Novel targets and radiotracers for Positron emission tomography imaging in psychiatric disorders

With the emergence of positron emission tomography (PET) in the late 1970s, psychiatry had access to a tool capable of non-invasive assessment of human brain function. Early applications in psychiatry focused on identifying characteristic brain blood flow and metabolic derangements using radiotracers such as [¹⁵ O]H₂ O and [¹⁸ F]FDG. Despite the success of these techniques, it became apparent that more specific probes were needed to understand the neurochemical bases of psychiatric disorders. The first neurochemical PET imaging probes targeted sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. Based on the centrality of monoamine dysfunction in psychiatric disorders and the measured success of monoamine-enhancing drugs in treating them, the next 30 years witnessed the development of an armamentarium of PET radiopharmaceuticals and imaging methodologies for studying monoamines. Continued development of monoamine-enhancing drugs over this time however was less successful, realizing only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely paralleled drug development priorities resulting in the development of new PET imaging agents for non-monoamine targets. Part one of this review will briefly survey novel PET imaging targets with relevance to the field of psychiatry, which include the metabotropic glutamate receptor type 5 (mGluR5), purinergic P₂ X₇ receptor, type 1 cannabinoid receptor (CB₁ ), phosphodiesterase 10A (PDE10A), and describe radiotracers developed for these and other targets that have matured to human subject investigations. Current limitations of the targets and techniques will also be discussed.

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Synthesis and evaluation of 2-(2'-((dimethylamino)methyl)-4'-(2-fluoroethoxy-substituted)phenylthio)benzenamine derivatives as potential positron emission tomography imaging agents for serotonin transporters

A series of diphenylsulfide derivatives with various substitutions at the 4-position on phenyl ring A and different lengths of the 2-fluoroethoxy-substituted side-chain at the 4'-position on ring B were synthesized and evaluated as potential positron emission tomography (PET) imaging agents for serotonin transporters (SERT). These ligands exhibited high SERT binding affinities (K_i = 0.11-1.3 nM) and the 4-methyl-substituted (4-Me) compounds 7a and 8a displayed excellent selectivity for SERT versus norepinephrine transporters (NET) (392- and 700-fold, respectively). In the parallel artificial membrane permeability assay (PAMPA), these ligands demonstrated moderate to high brain penetration, and the 4-Me analogs showed higher BBB permeability than the corresponding 4-F analogs. The 2-fluoroethoxy-substituted ligands showed higher metabolic stability and lower lipophilicity than 4-F-ADAM. [¹⁸F]7a-c were readily prepared using an automatic synthesizer and exhibited significant uptake and slow washout in rat brains. At 120 min after iv injection, [¹⁸F]7a exhibited the highest uptake in the midbrain, whereas [¹⁸F]7b exhibited the highest uptake in the hypothalamus and midbrain. After treatment with citalopram, a SERT-selective ligand, the uptake of [¹⁸F]7a in the hypothalamus and striatum was significantly decreased. The potent and highly selective SERT binding and the selective and reversible accumulation in SERT-rich brain regions suggested that [¹⁸F]7a is a promising lead for the further development of novel [¹⁸F]-labeled PET imaging agents for SERT binding sites in the brain.

Fluorine-18 labeled diphenyl sulfide derivatives for imaging serotonin transporter (SERT) in the brain

OBJECTIVES

Serotonin transporters (SERT) play an important role in controlling serotonin concentration in the synaptic cleft and in managing postsynaptic signal transduction. Inhibitors of SERT binding are well known as selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine, sertraline, paroxetine, and escitalopram, that are commonly prescribed antidepressants. Positron emission tomography (PET) and single photon emission tomography (SPECT) imaging agents targeting SERT may be useful for studying its function and providing a tool for monitoring drug treatment.

METHODS

A series of novel ¹⁸F-labeled diphenyl sulfide derivatives were prepared and tested for their binding affinity. Among them, 2-((2-((dimethylamino)-methyl)-4-(2-(2-fluoroethoxy)ethoxy)phenyl)thio)aniline, 1, which showed excellent binding toward serotonin transporter (SERT) in the brain (K_i = 0.09 nM), was selected for further evaluation. An active OTs intermediate, 7, was treated with [¹⁸F]F^-/K₂₂₂ to provide [¹⁸F]1 in one step and in high radiochemical yields. This new SERT targeting agent was evaluated in rats by biodistribution studies and animal PET imaging studies.

RESULTS

The radiolabeling reaction led to the desired [¹⁸F]1. After HPLC purification no-carrier-added [¹⁸F]1 was obtained (radiochemical yield, 23-47% (n = 10,); radiochemical purity >99%; molar activity, 15-28 GBq/μmol). Biodistribution studies with [¹⁸F]1 showed good brain uptake (1.04% dose/g at 2 min post-injection), high uptake into the hypothalamus (1.55% dose/g at 30 min), and a high target-to-non-target (hypothalamus to cerebellum) ratio of 6.1 at 120 min post-injection. A PET imaging study in normal rats showed excellent uptake in the midbrain and thalamus regions known to be rich in SERT binding sites at 60 min after iv injection. Chasing experiment with escitalopram (iv, 2 mg/kg) in a rat at 60 min after iv injection caused a noticeable reduction in the regional radioactivity and the target-to-non-target ratio, suggesting binding by [¹⁸F]1 was highly specific and reversible for SERT binding sites in the brain.

CONCLUSIONS

A novel diphenyl sulfide derivative, [¹⁸F]1 for SERT imaging was successfully prepared and evaluated. Results suggest that this new chemical entity is targeting SERT binding sites in the brain, and it is a suitable candidate for future commercial development.

Small Molecule PET Tracers for Transporter Imaging

As the field of PET has expanded and an ever-increasing number and variety of compounds have been radiolabeled as potential in vivo tracers of biochemistry, transporters have become important primary targets or facilitators of radiotracer uptake and distribution. A transporter can be the primary target through the development of a specific high-affinity radioligand: examples are the multiple high-affinity radioligands for the neuronal membrane neurotransmitter or vesicular transporters, used to image nerve terminals in the brain. The goal of a radiotracer might be to study the function of a transporter through the use of a radiolabeled substrate, such as the application of 3-O-[¹¹C]methyl]glucose to measure rates of glucose transport through the blood-brain barrier. In many cases, transporters are required for radiotracer distributions, but the targeted biochemistries might be unrelated: an example is the use of 2-deoxy-2-[¹⁸F]FDG for imaging glucose metabolism, where initial passage of the radiotracer through cell membranes requires the action of specific glucose transporters. Finally, there are transporters such as p-glycoprotein that function to extrude small molecules from tissues, and can effectively work against successful uptake of radiotracers. The diversity of structures and functions of transporters, their importance in human health and disease, and their role in therapeutic drug disposition suggest that in vivo imaging of transporter location and function will continue to be a point of emphasis in PET radiopharmaceutical development. In this review, the variety of transporters and their importance for in vivo PET radiotracer development and application are discussed. Transporters have thus joined the other major protein targets such as G-protein coupled receptors, ligand-gated ion channels, enzymes, and aggregated proteins as of high interest for understanding human health and disease.

One-step preparation of [(18)F]FPBM for PET imaging of serotonin transporter (SERT) in the brain

Serotonin transporters (SERT) in the brain play an important role in normal brain function. Selective serotonin reuptake inhibitors such as fluoxetine, sertraline, paroxetine, escitalopram, etc., specifically target SERT binding in the brain. Development of SERT imaging agents may be useful for studying the function of SERT by in vivo imaging. A one-step preparation of [(18)F]FPBM, 2-(2'-(dimethylamino)methyl)-4'-(3-([(18)F]fluoropropoxy)phenylthio)benzenamine, for positron emission tomography (PET) imaging of SERT binding in the brain was achieved. An active OTs intermediate, 9, was reacted with [(18)F]F(-)/K222 to produce [(18)F]FPBM in one step and in high radiochemical yield. This labeling reaction was evaluated and optimized under different temperatures, bases, solvents, and varying amounts of precursor 9. The radiolabeling reaction led to the desired [(18)F]FPBM in one step and the crude product was purified by HPLC purification to give no-carrier-added [(18)F]FPBM (radiochemical yield, 24-33%, decay corrected; radiochemical purity >99%). PET imaging studies in normal monkeys (n=4) showed fast, pronounced uptakes in the midbrain and thalamus, regions known to be rich in SERT binding sites. A displacement experiment with escitalopram (5mg/kg iv injection at 30min after [(18)F]FPBM injection) showed a rapid and complete reversal of SERT binding, suggesting that binding by [(18)F]FPBM was highly specific and reversible. A one-step radiolabeling method coupled with HPLC purification for preparation of [(18)F]FPBM was developed. Imaging studies suggest that it is feasible to use this method to prepare [(18)F]FPBM for in vivo PET imaging of SERT binding in the brain.