Myocardial adenosine A2a receptor imaging of rabbit by PET with [11C]KF17837

Non-Invasive Assessment of Locally Overexpressed Human Adenosine 2A Receptors in the Heart of Transgenic Mice

A_2A adenosine receptors (A_2A-AR) have a cardio-protective function upon ischemia and reperfusion, but on the other hand, their stimulation could lead to arrhythmias. Our aim was to investigate the potential use of the PET radiotracer [¹⁸F]FLUDA to non-invasively determine the A_2A-AR availability for diagnosis of the A_2AR status. Therefore, we compared mice with cardiomyocyte-specific overexpression of the human A_2A-AR (A_2A-AR TG) with the respective wild type (WT). We determined: (1) the functional impact of the selective A_2AR ligand FLUDA on the contractile function of atrial mouse samples, (2) the binding parameters (B_{max and} K_D) of [¹⁸F]FLUDA on mouse and human atrial tissue samples by autoradiographic studies, and (3) investigated the in vivo uptake of the radiotracer by dynamic PET imaging in A_2A-AR TG and WT. After A_2A-AR stimulation by the A_2A-AR agonist CGS 21680 in isolated atrial preparations, antagonistic effects of FLUDA were found in A_2A-AR-TG animals but not in WT. Radiolabelled [¹⁸F]FLUDA exhibited a K_D of 5.9 ± 1.6 nM and a B_max of 455 ± 78 fmol/mg protein in cardiac samples of A_2A-AR TG, whereas in WT, as well as in human atrial preparations, only low specific binding was found. Dynamic PET studies revealed a significantly higher initial uptake of [¹⁸F]FLUDA into the myocardium of A_2A-AR TG compared to WT. The hA_2A-AR-specific binding of [¹⁸F]FLUDA in vivo was verified by pre-administration of the highly affine A_2AAR-specific antagonist istradefylline. Conclusion: [¹⁸F]FLUDA is a promising PET probe for the non-invasive assessment of the A_2A-AR as a marker for pathologies linked to an increased A_2A-AR density in the heart, as shown in patients with heart failure.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

Tracers for non-invasive radionuclide imaging of immune checkpoint expression in cancer

Abstract

Immunotherapy with checkpoint inhibitors demonstrates impressive improvements in the treatment of several types of cancer. Unfortunately, not all patients respond to therapy while severe immune-related adverse effects are prevalent. Currently, patient stratification is based on immunotherapy marker expression through immunohistochemical analysis on biopsied material. However, expression can be heterogeneous within and between tumor lesions, amplifying the sampling limitations of biopsies. Analysis of immunotherapy target expression by non-invasive quantitative molecular imaging with PET or SPECT may overcome this issue. In this review, an overview of tracers that have been developed for preclinical and clinical imaging of key immunotherapy targets, such as programmed cell death-1, programmed cell death ligand-1, IDO1 and cytotoxic T lymphocyte-associated antigen-4 is presented. We discuss important aspects to consider when developing such tracers and outline the future perspectives of molecular imaging of immunotherapy markers.

Graphical abstract

Current techniques in immune checkpoint imaging and its potential for future applications

Potential Therapeutic Applications of Adenosine A2A Receptor Ligands and Opportunities for A2A Receptor Imaging

Adenosine A_2A receptors (A_2A Rs) are highly expressed in the human striatum, and at lower densities in the cerebral cortex, the hippocampus, and cells of the immune system. Antagonists of these receptors are potentially useful for the treatment of motor fluctuations, epilepsy, postischemic brain damage, or cognitive impairment, and for the control of an immune checkpoint during immunotherapy of cancer. A_2A R agonists may suppress transplant rejection and graft-versus-host disease; be used to treat inflammatory disorders such as asthma, inflammatory bowel disease, and rheumatoid arthritis; be locally applied to promote wound healing and be employed in a strategy for transient opening of the blood-brain barrier (BBB) so that therapeutic drugs and monoclonal antibodies can enter the brain. Increasing A_2A R signaling in adipose tissue is also a potential strategy to combat obesity. Several radioligands for positron emission tomography (PET) imaging of A_2A Rs have been developed in recent years. This review article presents a critical overview of the potential therapeutic applications of A_2A R ligands, the use of A_2A R imaging in drug development, and opportunities and limitations of PET imaging in future research.

Effects of adenosine monophosphate used in combination with L-arginine on female rabbit corpus cavernosum tissue

Introduction

Sexual dysfunction is significantly more prevalent in women than in men. However, to date, no satisfactory oral treatment is yet available.

Aim

The aim of this study was to study the effects of adenosine monophosphate (AMP) alone or its combination with L-Arginine on the relaxation of the female rabbit corpus cavernosum.

Methods

Cylinder strips from the corporal body of the excised clitoris from female New Zealand White rabbits were incubated in Krebs solution. Phenylephrine (PE) precontraction was achieved, then the drugs AMP and L-Arginine were administered either independently or in sequential combinations to the strips under precontracted conditions.

Main Outcome Measures

Contraction percentages were compared.

Results

When precontraction was induced by PE 8 μM or 20 μM, AMP was shown to induce relaxation up to 25% in a dose-dependent manner. The relaxation induced by L-Arginine reached 15.6% at 5.10⁻⁴ M vs. 16.5% at AMP 5.10⁻⁴ M under the same experimental conditions. Nitric oxide (NO) synthase inhibitor N-nitro-L-arginine strongly inhibited the relaxing effect provoked by AMP, suggesting that the action mechanism of this nucleotide is related to the NO pathway. The combination of L-Arginine at 5.10⁻⁴ M with AMP at different doses ranging from 5.10⁻⁴ M to 10⁻³ M significantly amplified the relaxing response up to 40.7% and 58%, respectively.

Conclusions

Our results demonstrate that AMP induces a relaxing effect on the female rabbit corpora. They also show that L-Arginine and AMP can potentiate each other and that a synergistic effect can be obtained by their combined use. Because only slight differences exist between both sexes in response to NO donors and/or nucleotide purines or in their use together, it is very likely that close biochemical mechanisms, although not to the same degree and not quite similar, are involved in the engorgement of the penis and the clitoris of New Zealand White rabbits. Stücker O, Pons C, Neuzillet Y, Laemmel E, and Lebret T. Original research-sexual medicine: Effects of adenosine monophosphate used in combination with L-Arginine on female rabbit corpus cavernosum tissue. Sex Med 2014;2:1–7.

Xanthines as adenosine receptor antagonists

The natural plant alkaloids caffeine and theophylline were the first adenosine receptor (AR) antagonists described in the literature. They exhibit micromolar affinities and are non-selective. A large number of derivatives and analogues were subsequently synthesized and evaluated as AR antagonists. Very potent antagonists have thus been developed with selectivity for each of the four AR subtypes.

Development of PET radiopharmaceuticals and their clinical applications at the Positron Medical Center

The Positron Medical Center has developed a large number of radiopharmaceuticals and 36 radiopharmaceuticals have been approved for clinical use for studying aging and geriatric diseases, especially brain functions. Positron emission tomography (PET) has been used to provide a highly advanced PET-based diagnosis. The current status of the development of radiopharmaceuticals, and representative clinical and methodological results are reviewed.

Adenosine receptor ligands and PET imaging of the CNS

Advances in radiotracer chemistry have resulted in the development of novel molecular imaging probes for adenosine receptors (ARs). With the availability of these molecules, the function of ARs in human pathophysiology as well as the safety and efficacy of approaches to the different AR targets can now be determined. Molecular imaging is a rapidly growing field of research that allows the identification of molecular targets and functional processes in vivo. It is therefore gaining increasing interest as a tool in drug development because it permits the process of evaluating promising therapeutic targets to be stratified. Further, molecular imaging has the potential to evolve into a useful diagnostic tool, particularly for neurological and psychiatric disorders. This chapter focuses on currently available AR ligands that are suitable for molecular neuroimaging and describes first applications in healthy subjects and patients using positron emission tomography (PET).

Recent developments in adenosine A2A receptor ligands

The development of potent and selective agonists and antagonists of adenosine receptors (ARs) has been a target of medicinal chemistry research for several decades, and recently the US Food and Drug Administration has approved Lexiscan, an adenosine derivative substituted at the 2 position, for use as a pharmacologic stress agent in radionuclide myocardial perfusion imaging. Currently, some other adenosine A(2A) receptor (A(2A)AR) agonists and antagonists are undergoing preclinical testing and clinical trials. While agonists are potent antiinflammatory agents also showing hypotensive effects, antagonists are being developed for the treatment of Parkinson's disease.However, since there are still major problems in this field, including side effects, low brain penetration (for the targeting of CNS diseases), short half-life, or lack of in vivo effects, the design and development of new AR ligands is a hot research topic.This review presents an update on the medicinal chemistry of A(2A)AR agonists and antagonists, and stresses the strong need for more selective ligands at the human A(2A)AR subtype, in particular in the case of agonists.

THE ADENOSINE 2A RECEPTOR AGONIST ATL-146E ATTENUATES EXPERIMENTAL POSTHEMORRHAGIC VASOSPASM

OBJECTIVE

Selective adenosine 2A receptor agonists, such as ATL-146e, are known to be potent anti-inflammatory agents devoid of systemic side effects and have been used clinically in a number of disease states. However, adenosine 2A receptor agonists have not been studied in the treatment of cerebral vasospasm after subarachnoid hemorrhage. The present study investigated the efficacy of ATL-146e in the prevention of leukocyte infiltration and attenuation of posthemorrhagic vasospasm.

METHODS

The rodent femoral artery model of vasospasm was used. Forty male Sprague-Dawley rats were randomly assigned to four different groups (vehicle, 1 ng/kg/min, 10 ng/kg/min, or 100 ng/kg/min ATL-146e administered via subcutaneous osmotic minipump). Vasospasm was evaluated at posthemorrhage Day 8 (period of peak constriction) by calculating the lumen cross-sectional area (expressed as percent change in luminal area: ratio of blood-exposed vessel to normal saline-exposed vessel) and radial wall thickness. Immunostaining with anti-CD45 monoclonal antibody to detect leukocytes was used to evaluate localized inflammation.

RESULTS

Significant vasospasm was noted in the vehicle-treated (blood-exposed) control group (78.5%, P < 0.001; expressed as a ratio of luminal area of the saline [no blood] control), but not in the ATL-146e-treated groups (lumen ratio to control: 105.0, 83.4, and 91.3% for the 1, 10, and 100 ng/kg/min groups, respectively). Additionally, infiltration of inflammatory cells was reduced significantly and radial wall thickness was decreased in the ATL-146e-treated groups compared with the vehicle-treated control group.

CONCLUSION

Selective activation of the adenosine 2A receptor with ATL-146e prevented posthemorrhagic vasospasm and reduced leukocyte infiltration in this experimental model. This agent is worthy of further investigation and lends credence to the hypothesis supporting a role for inflammation in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage.

Potential of an adenosine A2A receptor antagonist [11C]TMSX for myocardial imaging by positron emission tomography: a first human study

In previous in vivo studies with mice, rats, cats and monkeys, we have demonstrated that [7-methyl-11C]-(E)-8-(3,4,5-trimethoxystyryl)- 1,3,7-trimethylxanthine ([11C]TMSX) is a potential radioligand for mapping adenosine A2A receptors of the brain by positron emission tomography (PET). In the present study, we studied the potential of [11C]TMSX for myocardial imaging. Uptake of radioactivity by the heart was high and gradually decreased after an intravenous injection of [11C]TMSX into mice. In metabolite analysis, 54% and 76% of the radioactivity in plasma and heart, respectively, were present as the unchanged form of [11C]TMSX 60 min postinjection. The myocardial uptake was reduced by carrier-loading and by co-injection of an adenosine A2A antagonist CSC, but not by co-injection of an adenosine A1 antagonist DPCPX. Pretreatment with a high dose of a non-selective antagonist theophylline also reduced the myocardial uptake of [11C]TMSX. These findings demonstrate the specific binding of [11C]TMSX to adenosine A2A receptors in the heart. Finally we successfully performed the myocardial imaging by PET with [11C]TMSX in a normal volunteer. A graphical analysis by Logan plot supported the receptor-mediated uptake of [11C]TMSX. Peripherally [11C]TMSX was very stable in human: >90% of the radioactivity in plasma was detected as the unchanged form in a 60-min study. We concluded that [11C]TMSX PET has the potential for myocardial imaging.

Preclinical studies on [11C]TMSX for mapping adenosine A2A receptors by positron emission tomography

In previous in vivo studies with mice, rats and monkeys, we have demonstrated that [11C]TMSX ([7-methyl-11C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine) is a potential radioligand for mapping adenosine A2A receptors of the brain by positron emission tomography (PET). In the present study, we performed a preclinical study. A suitable preparation method for [11C]TMSX injection was established. The radiation absorbed-dose by [11C]TMSX in humans estimated from the tissue distribution in mice was low enough for clinical use, and the acute toxicity and mutagenicity of TMSX were not found. The striatal uptake of [11C]TMSX in mice was reduced by pretreatment with theophylline at the dose of 10 and 100 mg/kg, suggesting that the [11C]TMSX PET should be carefully performed in the patients received with theophylline. We have concluded that [11C]TMSX is suitable for mapping adenosine A2A receptors in the human brain by PET.

Comparative Pharmacological Studies on the A2 Adenosine Receptor Agonist 5'-n-Ethyl-carboxamidoadenosine and Its F19 Isotope Labelled Derivative

Adenosine receptors are expressed in various mammalian tissues where they mediate the effects of adenosine on cellular functions through a number of signalling mechanisms. 18F-NECA is the positron-emitting derivative of the A(2)-receptor agonist NECA (5'-n-ethyl-carboxamidoadenosine) and is a radioligand for PET imaging of adenosine receptors. Contractility and relaxation studies were performed on guinea pig atrial myocardium, pulmonary artery, and thoracic aorta to compare the pharmacological effects of NECA and F-NECA (a non-emitting derivative) on tissues. Furthermore, the effect of NECA and F-NECA on the potassium conductance was investigated in DDT1 MF-2 smooth muscle cells with the patch-clamp technique. Both NECA and F-NECA reduced the contractile force in atrial myocardium and evoked phasic contraction in pulmonary artery (A(1) adenosine-receptor-mediated actions) in a dose dependent manner; however, the apparent affinity was lower for F-NECA. No difference was found in relaxation induced by these compounds in 1 microM noradrenaline-precontracted aorta and pulmonary artery (in the presence of DPCPX, an A(1) adenosine receptor antagonist, tissue containing A(2B) adenosine receptors). NECA (5 microM) and F-NECA (5 microM) also decreased the peak current and accelerated activation and inactivation properties of the potassium channels, but F-NECA was less effective. These results suggest that while NECA and F-NECA are equivalent agonists of vascular A(2B) receptors, they mediate different changes of some parameters. When evaluating the data obtained by the use of radiolabelled ligands, one has to take into consideration the possible physiological effects of the ligands besides its binding properties to tissues.

Hypoglycemia activates compensatory mechanism of glucose metabolism of brain

The effect of plasma glucose concentration on the cerebral uptake of [18F]-fluorodeoxy-D-glucose (FDG) was studied in a broad concentration range in a rabbit brain model using dynamic FDG PET measurements. Hypoglycemic and hyperglycemic conditions were maintained by manipulating plasma glucose applying i.v. glucose or insulin load. FDG utilization (K) and cerebral glucose metabolic rate (CGMR) were evaluated in a plasma glucose concentration range between 0.5 mM and 26 mM from the kinetic constant k1, k2, k3 obtained by the Sokoloff model of FDG accumulation. A decreasing set of standard FDG uptake values found with increasing blood glucose concentration was explained by competition between the plasma glucose and the radiopharmacon FDG. A similar trend was observed for the forward kinetic constants k1, and k3 in the entire concentration range studied. The same decreasing tendency of k2 was of a smaller magnitude and was reverted at the lowest glucose concentrations where a pronounced decrease of this backward transport rate constant was detected. Our kinetic data indicate a modulation of the kinetics of carbohydrate metabolism by the blood glucose concentration and report on a special mechanism compensating for the low glucose supply under conditions of extremely low blood glucose level.

Influences of adenosine on the fetus and newborn

Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. In contrast to most neurotransmitters, adenosine is released by all cells and is present in all tissues. The adenosinergic system is therefore not dependent on the presence of mature synaptic structures or an intact autonomic nervous system for its release. However, similar to other signaling molecules, adenosine levels are dynamically regulated and increase with increased tissue activity, hypoxia, or stress. Local adenosine concentrations thus provide a "humoral barometer" of acute changes in cellular physiology. The receptors that transduce adenosine action include A1, A2a, A2b, and A3 adenosine receptors. These receptors differ in their affinities for adenosine and in patterns of tissues expression. During development A1 adenosine receptors (A1ARs) are especially important, and A1ARs are among the earliest receptors expressed in the embryonic brain and heart. In the developing heart, the adenosinergic system is the dominant regulator of fetal cardiac function and A1AR activation inhibits cardiac cell division leading to cardiac hypoplasia. In the forming central nervous system, A1AR activation potently inhibits the development of axons and can lead to leukomalacia. These recent data suggest that adenosine is an important modulator of mammalian development.

Evaluation of iodinated and brominated [11C]styrylxanthine derivatives as in vivo radioligands mapping adenosine A2A receptor in the central nervous system

In vivo assessment of the adenosine A2A receptors localized in the striatum by PET or SPECT offers us a new diagnostic tool for neurological disorders. In the present study, we evaluated the potential of iodinated and brominated styrylxanthine derivatives labeled with 11C as an in vivo probe. [7-Methyl-11C]-(E)-3,7-dimethyl-8-(3-iodostyryl)-1-propargylxan thine ([11C]IS-DMPX) and [7-methyl-11C]-(E)-8-(3-bromostyryl)-3,7-dimethyl-1-propargylxa nthine ([11C]BS-DMPX) were prepared by the 11C-methylation of corresponding 7-demethyl derivatives. An in vitro membrane binding study showed a high affinity (Ki values) of the two ligands for A2A receptor: 8.9 nM for IS-DMPX and 7.7 nM for BS-DMPX, and a high A2A/A1 selectivity: > 1100 for IS-DMPX and 300 for BS-DMPX. In mice, [11C]IS-DMPX and [11C]BS-DMPX were taken up slightly more in the striatum than in the reference regions such as the cortex and cerebellum. The uptake ratios of striatum to cortex and striatum to cerebellum gradually increased but were very small: 1.6-1.7 for the striatum-to-cortex ratio and 1.2 for the striatum-to-cerebellum ratio at 60 min postinjection. The uptake by these three regions was reduced by co-injection of an excess amount of carrier or an A2A antagonist KF17837, but not by an A1 antagonist KF15372. The blocking effects in the three regions were greater for [11C]BS-DMPX (32-57%) than for [11C]IS-DMPX (6-29%). Ex vivo autoradiography confirmed that the two ligands were slightly concentrated in the striatum. [11C]BS-DMPX showed more selective affinity for adenosine A2A receptors than [11C]IS-DMPX, but these results have shown that the two tracers were not suitable as in vivo ligands because of low selectivity for the striatal A2A receptors and a high nonspecific binding.

Further characterization of a CNS adenosine A2a receptor ligand [11C]KF18446 with in vitro autoradiography and in vivo tissue uptake

PET assessment of the adenosine A2a receptors localized in the striatum offers us a potential new diagnostic tool for neurological disorders. In the present study, we carried out in vitro receptor autoradiography of a newly developed PET ligand [11C]KF18446 ([7-methyl-11C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthin e) with rat brain sections. [11C]KF18446 showed a high striatum/cortex binding ratio (5.0) and low nonspecific binding (<10%), suggesting that [11C]KF18446 has characteristics comparable or slightly superior to [3H]CGS 21680 or [3H]SCH 58261, which are currently available representative A2a receptor ligands. Scatchard analysis indicated a Kd of 9.8 nM and a Bmax of 170 fmol/mm3 tissue in the striatum and a Kd of 16.4 nM and a Bmax of 33 fmol/mm3 tissue in the cortex. Seven xanthine-type and four nonxanthine-type adenosine receptor ligands with an affinity for the adenosine A2a receptors significantly reduced the in vitro binding of [11C]KF18446 to the brain section. The blocking effects were much stronger in the striatum than in the cortex, but did not necessarily parallel their affinity. On the other hand, four xanthine-type ligands and one nonxanthine-type ligand (SCH 58261) of the 11 ligands studied reduced the in vivo uptake of [11C]KF18446 in mice, but other ligands, including A1-selective and nonselective ligands and three nonxanthine-type A2a-selective antagonists did not. We conclude that [11C]KF18446 is a promising adenosine A2a receptor ligand for PET study.