Development Of Radioligands Research Articles

Radiosynthesis and evaluation of an (18)F-labeled positron emission tomography (PET) radioligand for brain histamine subtype-3 receptors based on a nonimidazole 2-aminoethylbenzofuran chemotype.

A known chemotype of H(3) receptor ligand was explored for development of a radioligand for imaging brain histamine subtype 3 (H(3)) receptors in vivo with positron emission tomography (PET), namely nonimidazole 2-aminoethylbenzofurans, represented by the compound (R)-(2-(2-(2-methylpyrrolidin-1-yl)ethyl)benzofuran-5-yl)(4-fluorophenyl)methanone (9). Compound 9 was labeled with fluorine-18 (t(1/2) = 109.7 min) in high specific activity by treating the prepared nitro analogue (12) with cyclotron-produced [(18)F]fluoride ion. [(18)F]9 was studied with PET in mouse and in monkey after intravenous injection. [(18)F]9 showed favorable properties as a candidate PET radioligand, including moderately high brain uptake with a high proportion of H(3) receptor-specific signal in the absence of radiodefluorination. The nitro compound 12 was found to have even higher H(3) receptor affinity, indicating the potential of this chemotype for the development of further promising PET radioligands.

The translocator protein ligand [18F]DPA-714 images glioma and activated microglia in vivo

PurposeIn recent years there has been an increase in the development of radioligands targeting the 18-kDa translocator protein (TSPO). TSPO expression is well documented in activated microglia and serves as a biomarker for imaging neuroinflammation. In addition, TSPO has also been reported to be overexpressed in a number of cancer cell lines and human tumours including glioma. Here we investigated the use of [18F]DPA-714, a new TSPO positron emission tomography (PET) radioligand to image glioma in vivo.MethodsWe studied the uptake of [18F]DPA-714 in three different rat strains implanted with 9L rat glioma cells: Fischer (F), Wistar (W) and Sprague Dawley (SD) rats. Dynamic [18F]DPA-714 PET imaging, kinetic modelling of PET data and in vivo displacement studies using unlabelled DPA-714 and PK11195 were performed. Validation of TSPO expression in 9L glioma cell lines and intracranial 9L gliomas were investigated using Western blotting and immunohistochemistry of brain tissue sections.ResultsAll rats showed significant [18F]DPA-714 PET accumulation at the site of 9L tumour implantation compared to the contralateral brain hemisphere with a difference in uptake among the three strains (F > W > SD). The radiotracer showed high specificity for TSPO as demonstrated by the significant reduction of [18F]DPA-714 binding in the tumour after administration of unlabelled DPA-714 or PK11195. TSPO expression was confirmed by Western blotting in 9L cells in vitro and by immunohistochemistry ex vivo.ConclusionThe TSPO radioligand [18F]DPA-714 can be used for PET imaging of intracranial 9L glioma in different rat strains. This preclinical study demonstrates the feasibility of employing [18F]DPA-714 as an alternative radiotracer to image human glioma.

Imaging the high-affinity state of the dopamine D 2 receptor in vivo: Fact or fiction?

Positron Emission Tomography (PET) has been used for more than three decades to image and quantify dopamine D 2 receptors (D 2R) in vivo with antagonist radioligands but in the recent years agonist radioligands have also been employed. In vitro competition studies have demonstrated that agonists bind to both a high and a low-affinity state of the D 2Rs, of which the high affinity state reflects receptors that are coupled to G-proteins and the low-affinity state reflects receptors uncoupled from G-proteins. In contrast, antagonists bind with uniform affinity to the total pool of receptors. Results of these studies led to the proposal that D 2Rs exist in high and low-affinity states for agonists in vivo and sparked the development and use of agonist radioligands for PET imaging with the primary purpose of measuring the proportion of receptors in the high-affinity (activating) state. Although several lines of research support the presence of high and low-affinity states of D 2Rs and their detection by in vivo imaging paradigms, a growing body of controversial data has now called this into question. These include both in vivo and ex vivo studies of anesthesia effects, rodent models with increased proportions of high-affinity state D 2Rs as well as the molecular evidence for stable receptor–G-protein complexes. In this commentary we review these data and discuss the evidence for the in vivo existence of D 2Rs configured in high and low-affinity states and whether or not the high-affinity state of the D 2R can, in fact, be imaged in vivo with agonist radioligands.

Development of radiotracers for oncology – the interface with pharmacology

There is an increasing role for positron emission tomography (PET) in oncology, particularly as a component of early phase clinical trials. As a non-invasive functional imaging modality, PET can be used to assess both pharmacokinetics and pharmacodynamics of novel therapeutics by utilizing radiolabelled compounds. These studies can provide crucial information early in the drug development process that may influence the further development of novel therapeutics. PET imaging probes can also be used as early biomarkers of clinical response and to predict clinical outcome prior to the administration of therapeutic agents. We discuss the role of PET imaging particularly as applied to phase 0 studies and discuss the regulations involved in the development and synthesis of novel radioligands. The review also discusses currently available tracers and their role in the assessment of pharmacokinetics and pharmacodynamics as applied to oncology.

5‐HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5-HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5-HT) receptors, the 5-HT transporter (SERT), and 5-HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5-HT system in humans include antagonists for the 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(4) receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging.

Molecular imaging in patients with mood disorders: a review of PET findings

Mood disorders are chronic, recurrent psychiatric disorders with high morbidity rates that cause severe disability. Researchers have used molecular imaging extensively in studies of mood disorders. In this article, we concisely and selectively review the major findings of positron emission tomography studies of patients with mood disorders. Specifically, we describe findings from cerebral blood flow, cerebral glucose/oxygen metabolism, and radioligand studies in both cross-sectional and longitudinal investigations. Patients with mood disorders have mood-correlated regional metabolism changes and molecular abnormalities in several neurotransmitter systems. Although the findings of these studies are not completely consistent and confounding factors, including drug effects and specific methodology, should be strictly controlled, these results reveal the pathophysiology of mood disorders and aid the development of novel treatment approaches for mood disorders. Future positron emission tomography research will benefit greatly from the development of better radioligands to simultaneously identify multiple neurotransmitter systems in the specific brain region and the integration of more detecting methods in specifying the neurobiological predictors of treatment response in patients with mood disorders. Understanding the molecular mechanisms in underlying mood disorders will result in aetiological diagnosis and individualization of treatment of these disorders.

N-(4-Cyanotetrahydro-2H-pyran-4-yl) and N-(1-Cyanocyclohexyl) Derivatives of 1,5-Diarylpyrazole-3-carboxamides Showing High Affinity for 18 kDa Translocator Protein and/or Cannabinoid Receptors

In order to develop improved radioligands for imaging brain CB(1) receptors with positron emission tomography (PET) based on rimonabant (5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-N-(piperidin-1-yl)-1H-pyrazole-3-carboxamide, 1), we synthesized compounds 9a-s in which the N-piperidinyl ring was replaced with a 4-(4-cyanotetrahydro-2H-pyranyl) or 1-cyanocyclohexyl ring. Such changes were expected to be almost isosteric with 1, confer greater metabolic resistance, and in the case of the 4-(4-cyanotetrahydro-2H-pyranyl) compounds, substantially reduce lipophilicity. One derivative, 1-(2-bromophenyl)-N-(1-cyanocyclohexyl)-5-(4-methoxyphenyl)-4-methylpyrazole-3-carboxamide (9n), showed high affinity (K(i) = 15.7 nM) and selectivity for binding to CB(1) receptors. The corresponding 4-(4-cyanotetrahydro-2H-pyranyl) derivative (9m) also showed quite high affinity for CB(1) receptors (K(i) = 62 nM) but was found to have even higher affinity (K(i) = 29 nM) for the structurally unrelated 18 kDa translocator protein (TSPO). Some other minor structural changes among 9a-s were also found to switch binding selectivity from CB(1) receptors to TSPO or vice versa. These unexpected findings and their implications for the development of selective ligands or PET radioligands for CB(1) receptors or TSPO are discussed in relation to current pharmacophore models of CB(1) receptor and TSPO binding sites.

Gastrin and cholecystokinin peptide‐based radiopharmaceuticals: an in vivo and in vitro comparison

The development of suitable radioligands for targeting CCK-2 receptor expressing tumors, such as medullary thyroid carcinoma, is of great clinical interest. In the search for the best CCK-2R binding peptides, we have synthesized, evaluated and compared the CCK8 peptide (Asp-Tyr-Met-Gly-Trp-Met-Asp-PheNH(2) ) and two gastrin analogs commonly referred to as MG0 (DGlu-Glu(5)-Ala-Tyr-Gly-Trp-Met-Asp-PheNH(2) ) and MG11 (DGlu(1)-Ala-Tyr-Gly-Trp-Met-Asp-PheNH(2) ). The N-terminal portion of the three peptide sequences was derivatized by introducing the DTPAGlu or DOTA chelators to allow radiolobeling with (111) In(III) and (68) Ga(III), respectively. Saturation binding and cellular internalization experiments were performed on A431 cells overexpressing CCK2R (A431-CCK2R). All compounds showed Kd values in the nM range and were internalized with similar rates in CCK2 receptor overexpressing cells. Biodistribution experiments showed higher specific uptake of both MG0-based compounds compared to conjugates containing the CCK8 and MG11 peptide sequences. The higher retention levels of MG0-based peptides were associated with markedly elevated and undesired kidney uptake compared to the other compounds. Current indications suggest that the 5 Glu N-terminal residues while improving peptide stability and receptor-mediated tumor uptake cause unacceptably high kidney retention. Although displaying lower absolute tumor uptake values, the DOTA-coupled CCK8 peptide provided the best tumor to kidney uptake ratio and appears more suitable as lead compound for improvement of radiopharmaceutical properties.

Imaging of Microglia Activation in Stroke

Activated microglia is one of the most important cellular components of poststroke neuroinflammation, which occurs early in the area of the infarct but also in remote regions with fiber tract connections to the site of the primary lesion. The development of different radioligands for the translocator protein, a mitochondrial membrane protein expressed in microglial cells when they transform from the resting to the activated state, allows to study the temporal dynamics of this cellular neuroinflammatory component in vivo in animal models and human stroke using positron emission tomography. In this article, we review the advantage and limitations of current and future methods for microglia imaging as well as new results of multimodal imaging approaches in clinical stroke, which try to combine microglia imaging with diffusion tensor imaging to investigate the clinical relevance of remote microglia activation along fiber tracts for poststroke recovery.

Editorial [Hot topic: Current Topics in the Development of Radioligands for Positron-Emission Tomography Imaging (Guest Editors: Andrew G. Horti and Robert F. Dannals)

Editorial [Hot topic: Current Topics in the Development of Radioligands for Positron-Emission Tomography Imaging (Guest Editors: Andrew G. Horti and Robert F. Dannals)

Recent Development of Radioligands for Imaging α7 Nicotinic Acetylcholine Receptors in the Brain

The α7 nicotinic acetylcholine receptors (nAChRs), ligand-gated Ca(2+) channels composed of homopentamers of α7 subunits, represent the most abundant with α4β2 nAChRs in the brain. Several lines of evidence suggest that α7 nAChRs play a role in the physiology of neuropsychiatric diseases such as schizophrenia, Alzheimer's disease, anxiety, depression, and drug addiction; hence, α7 nAChRs seem to be attractive therapeutic targets for these diseases. Several researchers have attempted to develop radioligands that can be used to selectively and quantitatively examine the distribution of α7 nAChRs in the human brain with positron emission tomography (PET) or single photon emission tomography (SPECT). Although efforts are underway, very low density of α7 nAChR and scarcity of very high affinity ligands hamper the development of α7 subtype-selective radioligands for in vivo imaging. In this article, we review the recent topics on the development of PET/SPECT ligands for in vivo imaging of α7 nAChRs in the brain.

Non-Invasive Imaging of the Type 2 Cannabinoid Receptor, Focus on Positron Emission Tomography

The type 2 cannabinoid receptor (CB(2)R) is a relatively new target for drug development, as the receptor was only discovered in 1993. The CB(2)R is mainly expressed in tissues and organs related to the immune system. However, in pathological conditions, mostly inflammatory, a strong upregulation has been observed. Because of its expression in activated microglia, the type 2 cannabinoid receptor might play an important role in pathologies with a neuroinflammatory component. Positron emission tomography provides a sensitive non-invasive imaging technique to study and quantify receptor expression. In this review, the importance of CB(2)R imaging, the current status and the future possibilities for the development of CB(2)R PET radioligands are discussed.

Applications of LC-MS in PET Radioligand Development and Metabolic Elucidation

Positron emission tomography (PET) is a very sensitive molecular imaging technique that when employed with an appropriate radioligand has the ability to quantititate physiological processes in a non-invasive manner. Since the imaging technique detects all radioactive emissions in the field of view, the presence and biological activity of radiolabeled metabolites must be determined for each radioligand in order to validate the utility of the radiotracer for measuring the desired physiological process. Thus, the identification of metabolic profiles of radiolabeled compounds is an important aspect of design, development, and validation of new radiopharmaceuticals and their applications in drug development and molecular imaging. Metabolite identification for different chemical classes of radiopharmaceuticals allows rational design to minimize the formation and accumulation of metabolites in the target tissue, either through enhanced excretion or minimized metabolism. This review will discuss methods for identifying and quantitating metabolites during the pre-clinical development of radiopharmaceuticals with special emphasis on the application of LC/MS.

The development of PET radioligands for imaging the translocator protein (18 kDa): What have we learned?

AbstractThe translocator protein (TSPO; 18 kDa), formerly known as the peripheral benzodiazepine receptor (PBR), is minimally expressed in the healthy brain. On the other hand, increased levels of TSPO have been noted in brain disorders for which an immune response is elicited. This increase in TSPO expression has been reported to coincide with the process of microglial activation making the measurement of TSPO density a useful indicator of active brain disease. To this end several new classes of TSPO positron emission tomography radioligands have been developed and evaluated. However, the incomplete pharmacological characterization of the TSPO and its ligands as well as differences in pathophysiology, pharmacology and molecular nature across species and tissue types means that caution must be exercised when comparing data obtained with various TSPO radioligands. A re‐evaluation of our interpretation of imaging data, which better correlates with our current understanding of TSPO pharmacology in disease, requires consideration. Copyright © 2010 John Wiley & Sons, Ltd.

ChemInform Abstract: Synthesis and Biological Activity of Fluoro-Substituted Pyrrolo[2,3-d]pyrimidines. The Development of Potential Positron Emission Tomography Imaging Agents for the Corticotropin-Releasing Hormone Type 1 Receptor.

A series of fluoro-substituted 4-(dialkylamino)pyrrolo[2,3-d]pyrimidines was synthesized and their binding affinity for corticotropin-releasing hormone type 1 receptor (CRHR1) was investigated. Compounds 11a and 11b possessed very high CRHR1 affinity (Ki=3.5, 0.91 nM, respectively). They are promising candidates for the development of 18F-containing nonpeptide PET radioligands for CRHR1.

Ciblage peptidique en oncologie nucléaire : intérêt de la modélisation moléculaire

Peptidic receptor targeting in nuclear oncology has been significantly improved the last 20 years. A better in-depth knowledge of peptidic receptor has permitted more extensive research and development of new radioligands. The different published studies showed the interest of radiolabelled peptide development but also the difficulties to obtain a radiopharmaceutical with pharmacologic and pharmacokinetic properties adapted to clinical applications in diagnosis and internal radiotherapy. Based on the literature, the most applied methodology is to modify the structure of the endogenous ligand iteratively and to synthesize various derivatives radiolabelled peptide tested one by one. We chose to address the issue of optimization of radiolabelled peptide analogues from a different recent method via molecular modelling and for our purpose more specifically the targeting of the peptide receptor cholecystokinin/gastrin.

Exploration of novel radioiodine‐labeling techniques for opioid peptides

A vast number of opioid peptides are derived from processing of precursors from three separate genes. The prodynorphin gene generates dynorphin A, the predominant product and the natural ligand for the kappa1 opioid receptor. However, other products of this gene have also been identified, including dynorphin B and α‐neoendorphin. Studies of kappa receptors have utilized 3H‐U69,593, a highly selective drug with poor affinity for both mu and delta opioid receptors. However, evidence suggests that 3H‐U69,593 may be labeling more than one class of kappa receptor in brain tissue. Further exploration of these binding sites requires the development of novel radioligands. In an effort to further the characterization of these potential kappa receptor subtypes, we have developed 125I‐labeled opioid peptides with high affinity and established receptor binding assays for them. Funded by grants to GWP (DA0641, DA07242, DA02615 and DA00220) and a training grant to JEP (DA07274) from the National Insitute on Drug Abuse.

Radiosynthesis of a novel potential adenosine A3 receptor ligand, 5-ethyl 2,4-diethyl-3-((2-[18F]fluoroethyl)sulfanylcarbonyl)-6-phenylpyridine-5-carboxylate ([18F]FE@SUPPY:2)

Abstract Since, to date very limited information on the distribution and function of the adenosine A3 receptor is available, the development of suitable radioligands is needed. Recently, we introduced [ 18F]FE@SUPPY (5-(2-[ 18F]fluoroethyl) 2,4-diethyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate) as the first PET-ligand for the A3R. Regarding the metabolic profile – this class of dialkylpyridines comprises two ester functions within one molecule, one carboxylic and one thiocarboxylic – one could expect carboxylesterases significantly contributing to cleavage and degradation. Therefore, our aim was the development of [ 18F]FE@SUPPY:2 (5-ethyl 2,4-diethyl-3-((2-[ 18F]fluoroethyl)sulfanylcarbonyl)-6-phenylpyridine-5-carboxylate), the functional isomer containing the label at the thiocarboxylic moiety. For satisfactory yields in high scale radiosyntheses, a reaction temperature of 75 °C has to be applied for at least 20 min using 20 mg/mL of precursor. So far, 6 complete high-scale radiosyntheses were performed. Starting from an average of 51.2±21.8 GBq (mean±SD) [ 18F]fluoride, 5.8±4.1 GBq of formulated [ 18F]FE@SUPPY:2 (12.0±5.4%, based on [ 18F]fluoride, not corrected for decay) were prepared in 75±8 min.

Fluorinated Isatin Derivatives. Part 2. New N-Substituted 5-Pyrrolidinylsulfonyl Isatins as Potential Tools for Molecular Imaging of Caspases in Apoptosis

Caspases are responsible for the execution of the cell death program and are potentially suitable targets for the specific imaging of apoptosis in vivo. A series of N-1-substituted analogues of the small molecule nonpeptide caspase inhibitor (S)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin (1), which may be useful for the development of caspase-targeted radioligands, were synthesized and their inhibition potencies were evaluated in vitro. Two of the most powerful techniques to introduce fluorine into organic compounds, viz, bromofluorination of olefins and fluorohydrin synthesis by ring-opening of epoxides, were used. Most of the target compounds are potent inhibitors of the two effector caspases-3 and -7. Furthermore, the (18)F-radiolabeled model compound (S)-1-[4-(1-[(18)F]fluoro-2-hydroxyethyl)benzyl]-5-[1-(2-methoxymethyl-pyrrolidinyl)sulfonyl]isatin ([(18)F]37), a putative tracer for the noninvasive imaging of apoptosis by positron emission tomography (PET) was synthesized by nucleophilic epoxide ring-opening of its precursor 36. The radiochemistry utilized in the (18)F-fluorination reverted to carrier-added [(18)F]Et(3)N.3HF, a new fluorine-18 source for radiolabeling.

Development of Radioligands for In Vivo Imaging of Type 1 Cannabinoid Receptors (CB1) in Human Brain

Cerebral cannabinoid receptor (CB1) and cannabinoid drugs constitute a vibrant field in modern medicine and pharmacology. However, the physiological and pharmacological roles played by the cannabinoid receptor in the central nervous system are still not fully understood. Positron-emission tomography (PET) is the most advanced technique for non-invasive research of cerebral receptors. Quantitative PET imaging of CB1 in animal and human brains has been limited by drawbacks of the available CB1 radioligands that manifested low specific binding, high non-specific binding and/or low brain uptake. The latest research revealed three CB1 PET radioligands ([(11)C]JHU75528, [(18)F]MK9470 and [(11)C]MePPEP) with improved imaging properties. These compounds are now being employed for the quantitative evaluation of CB1 in human subjects with PET. Molecular imaging of the CB1 receptor with these radioligands has now become possible and their application in healthy humans and in patients is underway. Despite the substantial progress in development of CB1 PET radioligands even the latest radioligands manifest certain disadvantages. Current research efforts on the development of CB1 radioligands with higher binding potential, greater brain uptake and more optimal brain kinetics.