Preparation and pharmaceutical evaluation for clinical application of high specific activity S-(-)[11C]CGP-12177, a radioligand for beta-adrenoreceptors

[11C]phosgene: Synthesis and application for development of PET radiotracers

Carbon-11-labeled phosgene ([¹¹C]phosgene, [¹¹C]COCl₂) is a useful labeling agent that connects two heteroatoms by inserting [¹¹C]carbonyl (¹¹C=O) function in carbamates, ureas, and carbonates, which are components of biologically important heterocyclic compounds and functional groups in drugs as a linker of fragments with in vivo stability. Development of ¹¹C-labeled PET tracers has been performed using [¹¹C]phosgene as a labeling agent. However, [¹¹C]phosgene has not been frequently used for ¹¹C-labeling because preparation of [¹¹C]phosgene required dedicated synthesis apparatus (not commercially available) and had problems in reproducibility and reliability. In our laboratory, an improved method for synthesizing [¹¹C]phosgene using a carbon tetrachloride detection tube kit in environmental air analysis and the automated synthesis system for preparing [¹¹C]phosgene have been developed in 2009. This apparatus has been used for routine synthesis of ¹¹C-labeled tracers 1-4 times/week. Using [¹¹C]phosgene we have developed and produced many PET radiotracers containing [¹¹C]urea and [¹¹C]carbamate moieties. In this review, we report the performance of our method for preparing [¹¹C]phosgene, including automated synthesis apparatus developed in house, and the application of [¹¹C]phosgene for development and production of ¹¹C-labeled PET tracers.

Molecular Imaging of the Heart

Multimodality cardiovascular imaging is routinely used to assess cardiac function, structure, and physiological parameters to facilitate the diagnosis, characterization, and phenotyping of numerous cardiovascular diseases (CVD), as well as allows for risk stratification and guidance in medical therapy decision-making. Although useful, these imaging strategies are unable to assess the underlying cellular and molecular processes that modulate pathophysiological changes. Over the last decade, there have been great advancements in imaging instrumentation and technology that have been paralleled by breakthroughs in probe development and image analysis. These advancements have been merged with discoveries in cellular/molecular cardiovascular biology to burgeon the field of cardiovascular molecular imaging. Cardiovascular molecular imaging aims to noninvasively detect and characterize underlying disease processes to facilitate early diagnosis, improve prognostication, and guide targeted therapy across the continuum of CVD. The most-widely used approaches for preclinical and clinical molecular imaging include radiotracers that allow for high-sensitivity in vivo detection and quantification of molecular processes with single photon emission computed tomography and positron emission tomography. This review will describe multimodality molecular imaging instrumentation along with established and novel molecular imaging targets and probes. We will highlight how molecular imaging has provided valuable insights in determining the underlying fundamental biology of a wide variety of CVDs, including: myocardial infarction, cardiac arrhythmias, and nonischemic and ischemic heart failure with reduced and preserved ejection fraction. In addition, the potential of molecular imaging to assist in the characterization and risk stratification of systemic diseases, such as amyloidosis and sarcoidosis will be discussed. © 2019 American Physiological Society. Compr Physiol 9:477-533, 2019.

Late-Stage Isotopic Carbon Labeling of Pharmaceutically Relevant Cyclic Ureas Directly from CO2

A robust, click-chemistry-inspired procedure for radiolabeling of cyclic ureas was developed. This protocol, suitable for all carbon isotopes (11 C, 13 C, 14 C), is based on the direct functionalization of carbon dioxide: the universal building block for carbon radiolabeling. The strategy is operationally simple and reproducible in different radiochemistry centers, exhibits remarkably wide substrate scope with short reaction times, and demonstrates superior reactivity as compared to previously reported systems. With this procedure, a variety of pharmaceuticals and an unprotected peptide were labeled with high radiochemical efficiency.

Recent Advances and Clinical Applications of PET Cardiac Autonomic Nervous System Imaging

PURPOSE OF REVIEW

The purpose of this review was to summarize current advances in positron emission tomography (PET) cardiac autonomic nervous system (ANS) imaging, with a specific focus on clinical applications of novel and established tracers.

RECENT FINDINGS

[11C]-Meta-hydroxyephedrine (HED) has provided useful information in evaluation of normal and pathological cardiovascular function. Recently, [11C]-HED PET imaging was able to predict lethal arrhythmias, sudden cardiac death (SCD), and all-cause mortality in heart failure patients with reduced ejection fraction (HFrEF). In addition, initial [11C]-HED PET imaging studies have shown the potential of this agent in elucidating the relationship between impaired cardiac sympathetic nervous system (SNS) innervation and the severity of diastolic dysfunction in HF patients with preserved ejection fraction (HFpEF) and in predicting the response to cardiac resynchronization therapy (CRT) in HFrEF patients. Longer half-life 18F-labeled presynaptic SNS tracers (e.g., [18F]-LMI1195) have been developed to facilitate clinical imaging, although no PET radiotracers that target the ANS have gained wide clinical use in the cardiovascular system. Although the use of parasympathetic nervous system radiotracers in cardiac imaging is limited, the novel tracer, [11C]-donepezil, has shown potential utility in initial studies. Many ANS radioligands have been synthesized for PET cardiac imaging, but to date, the most clinically relevant PET tracer has been [11C]-HED. Recent studies have shown the utility of [11C]-HED in relevant clinical issues, such as in the elusive clinical syndrome of HFpEF. Conversely, tracers that target cardiac PNS innervation have been used less clinically, but novel tracers show potential utility for future work. The future application of [11C]-HED and newly designed 18F-labeled tracers for targeting the ANS hold promise for the evaluation and management of a wide range of cardiovascular diseases, including the prognostication of patients with HFpEF.

(11)C[double bond, length as m-dash]O bonds made easily for positron emission tomography radiopharmaceuticals

The positron-emitting radionuclide carbon-11 ((11)C, t1/2 = 20.3 min) possesses the unique potential for radiolabeling of any biological, naturally occurring, or synthetic organic molecule for in vivo positron emission tomography (PET) imaging. Carbon-11 is most often incorporated into small molecules by methylation of alcohol, thiol, amine or carboxylic acid precursors using [(11)C]methyl iodide or [(11)C]methyl triflate (generated from [(11)C]carbon dioxide or [(11)C]methane). Consequently, small molecules that lack an easily substituted (11)C-methyl group are often considered to have non-obvious strategies for radiolabeling and require a more customized approach. [(11)C]Carbon dioxide itself, [(11)C]carbon monoxide, [(11)C]cyanide, and [(11)C]phosgene represent alternative reactants to enable (11)C-carbonylation. Methodologies developed for preparation of (11)C-carbonyl groups have had a tremendous impact on the development of novel PET tracers and provided key tools for clinical research. (11)C-Carbonyl radiopharmaceuticals based on labeled carboxylic acids, amides, carbamates and ureas now account for a substantial number of important imaging agents that have seen translation to higher species and clinical research of previously inaccessible targets, which is a testament to the creativity, utility and practicality of the underlying radiochemistry.

Molecular imaging in heart failure patients

This review focuses on molecular imaging using various radioligands for the tissue characterization of patients with heart failure. ¹²³I-labeled metaiodobenzylguanidine (MIBG), as a marker of adrenergic neuron function, plays an important role in risk stratification in heart failure and may be useful for predicting fatal arrhythmias that may require implantable cardioverter-defibrillator treatment. MIBG has also been used for monitoring treatment effects under various medications. Various positron emission tomography (PET) radioligands have been introduced for the quantitative assessment of presynaptic and postsynaptic neuronal function in vivo. ¹¹C-hydroxyephedrine, like MIBG, has potential for assessing the severity of heart failure. Our PET study using the β-receptor antagonist ¹¹C-CGP 12177 in patients with heart failure showed a reduction of β-receptor density, indicating downregulation, in most of the patients. More studies are needed to confirm the clinical utility of these molecular imaging modalities for the management of heart failure patients.

Assessment of cardiac autonomic neuronal function using PET imaging

The autonomic nervous system is the primary extrinsic control of cardiac performance, and altered autonomic activity has been recognized as an important factor in the progression of various cardiac pathologies. Molecular imaging techniques have been developed for global and regional interrogation of pre- and postsynaptic targets of the cardiac autonomic nervous system. Building on established work with the guanethidine analogue ¹²³I-metaiodobenzylguanidine (MIBG) for single-photon emission tomography (SPECT), development of radiotracers and protocols for positron emission tomography (PET) investigation of autonomic signaling has expanded. PET is limited in availability and requires specialized centers for radiosynthesis and interpretation, but the higher resolution allows for improved regional analysis and kinetic modeling provides more true quantification than is possible with SPECT. A wider array of radiolabeled catecholamines, analogues of catecholamines, and receptor ligands have been characterized and evaluated. Sympathetic neuronal PET tracers have shown promise in the identification of several cardiac pathologies. In particular, recent studies have elucidated a mechanistic role for heterogeneous sympathetic innervation in the development of lethal ventricular arrhythmias. Evaluation of cardiomyocyte adrenergic receptor expression and the parasympathetic nervous system has been slower to develop, with clinical studies beginning to emerge. This review summarizes the clinical and the experimental PET tracers currently available for autonomic imaging and discusses their application in health and cardiovascular disease, with particular emphasis on the major findings of the last decade.

Radiopharmaceutical chemistry for positron emission tomography

Molecular imaging is an emerging technology that allows the visualization of interactions between molecular probes and biological targets. Molecules that either direct or are subject to homeostatic controls in biological systems could be labeled with the appropriate radioisotopes for the quantitative measurement of selected molecular interactions during normal tissue homeostasis and again after perturbations of the normal state. In particular, positron emission tomography (PET) offers picomolar sensitivity and is a fully translational technique that requires specific probes radiolabeled with a usually short-lived positron-emitting radionuclide. PET has provided the capability of measuring biological processes at the molecular and metabolic levels in vivo by the detection of the gamma rays formed as a result of the annihilation of the positrons emitted. Despite the great wealth of information that such probes can provide, the potential of PET strongly depends on the availability of suitable PET radiotracers. However, the development of new imaging probes for PET is far from trivial and radiochemistry is a major limiting factor for the field of PET. In this review, we provided an overview of the most common chemical approaches for the synthesis of PET-labeled molecules and highlighted the most recent developments and trends. The discussed PET radionuclides include ¹¹C (t₁(/)₂=20.4min), ¹³N (t₁(/)₂=9.9min), ¹⁵O (t₁(/)₂=2min), ⁶⁸Ga (t₁(/)₂=68min), ¹⁸F (t₁(/)₂=109.8min), ⁶⁴Cu (t₁(/)₂=12.7h), and ¹²⁴I (t₁(/)₂=4.12d).

Myocardial beta-adrenergic receptor density assessed by 11C-CGP12177 PET predicts improvement of cardiac function after carvedilol treatment in patients with idiopathic dilated cardiomyopathy

We evaluated whether myocardial beta-adrenergic receptor (beta-AR) density, as determined by 11C-CGP12177 PET, could predict improvement of cardiac function by beta-blocker carvedilol treatment in patients with idiopathic dilated cardiomyopathy (IDC).

METHODS

Ten patients with IDC (left ventricular ejection fraction [LVEF]<45%) were studied. Myocardial beta-AR density was estimated using 11C-CGP12177 PET before treatment with carvedilol. Changes of LVEF in response to dobutamine infusion (DeltaLVEF-dobutamine) were also measured by echocardiography. Changes of LVEF (DeltaLVEF-carvedilol) were evaluated after 20 mo of carvedilol treatment.

RESULTS

Baseline myocardial beta-AR density significantly correlated with DeltaLVEF-carvedilol (r=-0.88, P<0.001). In contrast, DeltaLVEF-dobutamine did not correlate with DeltaLVEF-carvedilol (P=0.65). Myocardial beta-AR density was the significant multivariate independent predictor of DeltaLVEF-carvedilol (beta=-0.88, P<0.001) among univariate predictors, including functional class (r=0.76, P<0.05), plasma norepinephrine (r=0.85, P<0.01), LVEF (r=-0.64, P<0.05), and age as confounding factors. Furthermore, myocardial beta-AR density was significantly correlated with plasma norepinephrine (r=-0.79, P<0.01) and LVEF (r=0.70, P<0.05).

CONCLUSION

Myocardial beta-AR density is more tightly related to improvement of LVEF-carvedilol than is cardiac contractile reserve in patients with IDC. Patients with decreased myocardial beta-AR have higher resting adrenergic drive, as reflected by plasma norepinephrine, and may receive greater benefit from being treated by antiadrenergic drugs.

Formation of fluorine-18 labeled diaryl ureas--labeled VEGFR-2/PDGFR dual inhibitors as molecular imaging agents for angiogenesis

Urea subunits are common components of various pharmaceuticals' core structure. Since in most cases the design and development of PET biomarkers is based on approved or potential drugs, there is a growing need for a general labeling methodology of urea-containing pharmacophores. As a part of research in the field of molecular imaging of angiogenic processes, we synthesized several highly potent VEGFR-2/PDGFR dual inhibitors as potential PET biomarkers. The structure of these inhibitors is based on the N-phenyl-N'-{4-(4-quinolyloxy)phenyl}urea skeleton. A representative inhibitor was successfully labeled with fluorine-18 by a three-step process. Initially, a two-step radiosynthesis of 4-[(18)F]fluoro-aniline from 1,4-dinitrobenzene (60min, EOB decay corrected yield: 63%) was performed. At the third and final step, the 4-[(18)F]fluoro-aniline synthon reacted for 30min at room temperature with 4-(2-fluoro-4-isocyanato-phenoxy)-6,7-dimethoxy-quinoline to give complete conversion of the labeled synthon to 1-[4-(6,7-dimethoxy-quinolin-4-yloxy)-3-fluoro-phenyl]-3-(4-[(18)F]fluoro-phenyl)-urea. The desired labeled product was obtained after total radiosynthesis time of 3h including HPLC purification with 46+/-1% EOB decay corrected radiochemical yield, 99% radiochemical purity, 99% chemical purity, and a specific activity of 400+/-37GBq/mmol (n=5).

A Study on the Highly Efficient Synthesis and Pharmaceutical Evaluation of PET Radiopharmaceuticals for the Clinical Application

Positron Emission Tomography (PET) is an advanced non-invasive technology used in the field of nuclear medicine for clinical diagnosis using radiotracers labeled with short-lived positron emitting radionuclides such as (11)C (half-life: 20.4 min), (13)N, (15)O and (18)F. The present study describes an efficient rapid synthesis method for [(11)C]Phosgene ([(11)C]COCl(2)) which is an important potential precursor for preparation of PET radiopharmaceuticals. Catalytic oxidation of [(11)C]CCl(4) using Fe(2)O(3) powder mixed with Fe granules as an oxidizing agent was newly accomplished with a development of fully automated synthetic apparatus. Utilization of produced [(11)C]COCl(2) provided a substantial synthesis of [2-(11)C]thymine as a key intermediate for preparation of [2-(11)C]thymidine, a PET tracer to evaluate cellular proliferation. Direct ring closure reaction of the alkali metal salt of beta-(N-benzoyl-amino)methacrylamide with [(11)C]COCl(2) readily proceeded under mild conditions to afford [2-(11)C]thymine in fair yield reproducibly. By way of further application, a useful PET ligand for beta-adrenoreceptors, S-(-)-[(11)C]CGP-12177 (CGP) was synthesized in markedly high yield with high specific activity and radiochemical purity. CGP for intravenous injection was prepared in 25 min after EOB with a yield of 1.5+/-0.2 GBq. These results of quality control tests demonstrated that CGP preparation is suitable for routine clinical use. Thus, CGP-PET study has been newly added to clinical PET for cardiac functional investigation in Hokkaido University Hospital.