Selection, design and evaluation of new radioligands for PET studies of cardiac adrenoceptors

A Molecular Toolbox of Positron Emission Tomography Tracers for General Anesthesia Mechanism Research

With appropriate radiotracers, positron emission tomography (PET) allows direct or indirect monitoring of the spatial and temporal distribution of anesthetics, neurotransmitters, and biomarkers, making it an indispensable tool for studying the general anesthesia mechanism. In this Perspective, PET tracers that have been recruited in general anesthesia research are introduced in the following order: 1) ¹¹C/¹⁸F-labeled anesthetics, i.e., PET tracers made from inhaled and intravenous anesthetics; 2) PET tracers targeting anesthesia-related receptors, e.g., neurotransmitters and voltage-gated ion channels; and 3) PET tracers for studying anesthesia-related neurophysiological effects and neurotoxicity. The radiosynthesis, pharmacodynamics, and pharmacokinetics of the above PET tracers are mainly discussed to provide a practical molecular toolbox for radiochemists, anesthesiologists, and those who are interested in general anesthesia.

PET Radiotracers for CNS-Adrenergic Receptors: Developments and Perspectives

Epinephrine (E) and norepinephrine (NE) play diverse roles in our body’s physiology. In addition to their role in the peripheral nervous system (PNS), E/NE systems including their receptors are critical to the central nervous system (CNS) and to mental health. Various antipsychotics, antidepressants, and psychostimulants exert their influence partially through different subtypes of adrenergic receptors (ARs). Despite the potential of pharmacological applications and long history of research related to E/NE systems, research efforts to identify the roles of ARs in the human brain taking advantage of imaging have been limited by the lack of subtype specific ligands for ARs and brain penetrability issues. This review provides an overview of the development of positron emission tomography (PET) radiotracers for in vivo imaging of AR system in the brain.

Radiosynthesis and evaluation of new α1-adrenoceptor antagonists as PET radioligands for brain imaging

INTRODUCTION

Evaluation of the α1-adrenoceptors in relation to brain pathophysiology and drug treatment has been hindered by lack of α1-adrenoceptor specific radioligands with sufficient brain exposure. Our aim was to develop an α1-adrenoceptor specific PET radioligand for brain imaging.

METHODS

Two sertindole analogues 1-(4-fluorophenyl)-5-(1-methyl-1H-1,2,4-triazol-3-yl)-3-(1-[(11)C]methylpiperidin-4-yl)-1H-indole [(11)C]3 and 1-(4-fluorophenyl)-3-(1-[(11)C]methylpiperidin-4-yl)-5-(pyrimidin-5-yl)-1H-indole ([(11)C]Lu AA27122) [(11)C]4 were synthesized and evaluated as α1-adrenoceptor PET radioligands in cynomolgus monkey. Compounds 3 and 4 were selected due to their promising in vitro preclinical profile; high affinity and selectivity for the α1-adrenoceptor, favourable blood brain barrier permeability rates in Caco-2 monolayers and promising brain tissue/plasma ratio, assessed by equilibrium dialysis of free fraction in plasma and brain homogenate.

RESULTS

Compounds [(11)C]3 and [(11)C]4 were synthesized from their desmethyl piperidine precursors with high specific radioactivity (>370 GBq/μmol) using [(11)C]methyl iodide. The 1,2,4-triazole analogue [(11)C]3 exhibited poor brain uptake, but the corresponding pyrimidyl analogue [(11)C]4 exhibited high brain exposure and binding in α1-adrenoceptor rich brain regions. However, the binding could not be inhibited by pretreatment with prazosin (0.1 mg/kg and 0.3 mg/kg). The results were extended by autoradiography of [(11)C]4 binding in human brain sections and competition with antagonists from different structural families, revealing that only a minor portion of the observed binding of [(11)C]4 in brain was α1-adrenoceptor specific.

CONCLUSION

Though [(11)C]3 and [(11)C]4 proved not suitable as PET radioligands, the study provided further understanding of structural features influencing brain exposure of the chemical class of compounds related to the antipsychotic drug sertindole. It provided valuable insight in the delicacy of blood brain barrier penetration for structurally related compounds and underlines the importance for improved protocols for evaluation of brain penetration of future PET ligands.

Small molecule receptors as imaging targets

The aberrant expression and function of certain receptors in tumours and other diseased tissues make them preferable targets for molecular imaging. PET and SPECT radionuclides can be used to label specific ligands with high affinity for the target receptors. The functional information obtained from imaging these receptors can be used to better understand the systems under investigation and for diagnostic and therapeutic applications. This review discusses some of the aspects of receptor imaging with small molecule tracers by PET and SPECT and reviews some of the tracers for the receptor imaging of tumours and brain, heart and lung disorders.

Receptor imaging in the thorax with PET

This review focuses on positron emission tomography (PET)-imaging of receptors in the sympathetic and the parasympathetic systems of heart and lung and highlights the human applications of PET. For the alpha-adrenoceptor, only [11C]GB67 (N2-[6-[(4-amino-6,7-dimethoxy-2-quinazolinyl)(methyl)amino]hexyl]-N2-[11C]methyl-2-furamide hydrochloride) has been developed. Its potential for application in patients needs to be assessed. For both the beta-adrenergic and the muscarinic systems, potent PET radioligands have been prepared and evaluated in patients. It has been possible to measure receptor densities quantitatively in human heart [[11C]MQNB: [11C]methylquinuclidinyl benzilate, [11C]CGP12177: S-(3'-t-butylamino-2'-hydroxypropoxy)-benzimidazol-2-[11C]one and [11C]CGP12388: (S)-4-(3-(2'-[11C]isopropylamino)-2-hydroxypropoxy)-2H-benzimidazol-2-one] and qualitatively in lung [[11C]VC002: N-[11C]-methyl-piperidin-4-yl-2-cyclohexyl-2-hydroxy-2-phenylacetate and [11C]CGP12177]. Besides these subtype nonselective radioligands, the development of compounds that are selective for one subtype are ongoing and have not found successful application in humans yet.

New α1-adrenoceptor antagonists derived from the antipsychotic sertindole - carbon-11 labelling and pet examination of brain uptake in the cynomolgus monkey

Central alpha(1)-adrenergic receptors are potential targets for recently developed antipsychotic drugs. Two new 11C labeled potent and selective alpha(1)-adrenoceptor antagonists, 1- [2- [4-[1-(4-fluorophenyl)-5-(2-[(11)C]methyl-tetrazol-5-yl)-1H-indol-3-yl]-1-piperidinyl]ethyl]-imidazolidin-2-one ([(11)C]2) and 1- [2- [4-[1-(4-fluorophenyl)-5-(1-[(11)C]methyl-(1,2,3-triazol-4-yl)-1H-indol-3-yl]-1-piperidinyl]ethyl]-imidazolidin-2-one ([(11)C]3) were prepared and evaluated for imaging of central alpha(1)-adrenergic receptors in the cynomolgus monkey brain. For both compounds, the total brain radioactivity was only about 0.6% of the radioactivity injected i.v. There was no evident binding in regions known to contain alpha(1)-adrenoceptors. This observation suggests that the affinity of the radioligands in primates in vivo is not sufficient to provide a signal for specific binding that can be differentiated from the background. In addition, active efflux by P-glycoprotein may be responsible for the low total brain-uptake of the two radioligands. Both compounds showed a highly polarised and verapamile sensitive transport across monolayers of Caco-2 cells. The total brain-uptake of [(3)H]2 was 6 times higher in mdr1a(-/-) knock-out mice lacking the gene encoding P-glycoprotein compared to wild type mice. Pretreatment of one monkey with Cyclosporin A (15 mg/kg) resulted in 40% higher brain uptake for [(11)C]3 when compared with baseline. These observations support the view that efflux by P-glycoprotein can be of quantitative importance for the total brain-uptake of some PET radioligands.

[11C]RN5: a new agent for the in vivo imaging of myocardial alpha1-adrenoceptors

The radiolabelling with the positron-emitter Carbon-11 and the biological evaluation in rats of 3-[2-[4-(2-[11C]methoxyphenyl)piperazin-1-yl]ethyl]pyrimido[5,4-b]indole-2,4-dione ([11C]RN5), alpha1-adrenoceptor antagonist (K(i)=0.21 nM), as a putative radioligand for the non-invasive assessment of alpha1-adrenoceptors with positron emission tomography (PET) is reported. The radiosynthesis procedure consisted of O-methylation of des-methyl precursor with [11C]methyl iodide in the presence of potassium hydroxide in dimethylformamide (DMF) at 80 degrees C. [11C]RN5 was obtained in >99% radiochemical purity in 25 min with a radiochemical yield in the 20-30% range, end of synthesis (EOS) (non-decay corrected) and a specific radioactivity of 92.5+/-18.5 GBq/micromol. Pre-clinical studies in rats showed a high uptake of [11C]RN5 in heart, spleen, adrenal gland, lung and kidney but not in the brain. Inhibition studies with high doses of different adrenergic antagonists indicate that more than 70% of myocardial uptake of [11C]RN5 is due to specific binding to alpha1-adrenoceptors. Our results indicate that [11C]RN5 is suitable to be further developed as a potential radioligand for the in vivo PET imaging of myocardial alpha1-adrenoceptors in humans.

Imaging of beta-adrenoceptors in the human thorax using (S)-[(11)C]CGP12388 and positron emission tomography

We report positron emission tomography studies of beta-adrenoceptors in the human thorax with (S)-[(11)C]CGP12388 (4-(3-(2'-[(11)C]-isopropylamino)-2-hydroxypropoxy)-2H-benzimidazol-2-one). Beta-adrenoceptors have previously been quantified using (S)-[(11)C]CGP12177 (4-(3-tert-butylamino-2-hydroxypropoxy)-2H-benzimidazol-2[(11)C]-one), but (S)-[(11)C]CGP12388 is more easily prepared and therefore more suitable in a clinical setting. (S)-[(11)C]CGP12388 was administered to five healthy volunteers on two separate days (control and pindolol block study). Arterial plasma samples were used to determine clearance, metabolites, and protein binding of the radioligand. Heart, lung and spleen showed high uptake of radioactivity, which was strongly suppressed (68-77%) by pindolol. Plasma clearance of (S)-[(11)C]CGP12388 was rapid, binding to plasma proteins was low (53+/-4%), and the radioligand was slowly metabolized. (S)-[(11)C]CGP12388 produces high-quality images of the human thorax. Uptake of (S)-[(11)C]CGP12388 in heart, lung and spleen represents binding to beta-adrenoceptors. (S)-[(11)C]-CGP12388 seems useful for imaging of beta-adrenoceptors in a clinical setting.

Radioligands for PET and SPECT Imaging of the central noradrenergic system

In the central nervous system, the neurotransmitter norepinephrine is involved in normal physiology, neuropsychiatric disorders, and the effects of numerous drugs. Although alterations of the central noradrenergic system are involved in the pathophysiology and pharmacotherapy of mood disorders, the basis and nature of these changes remain unresolved. Positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging agents will be valuable for further elucidating the roles of norepinephrine in health and disease. This review discusses PET and SPECT radioligands that have been developed for the enzymes, receptors, and transporters involved in noradrenergic neurotransmission. Currently, imaging agents that exhibit specific in vivo uptake in the brain have been described for monoamine oxidase A and beta-adrenergic receptors, but have not undergone detailed evaluation or experimental application. Based on the successful development and utilization of in vivo imaging agents for elements of the central dopaminergic and serotoninergic systems, PET and SPECT radioligands are expected to serve as new tools for studying the physiology, pathophysiology, and pharmacology of the central noradrenergic system.