Chronic therapy with a partial adenosine A1-receptor agonist improves left ventricular function and remodeling in dogs with advanced heart failure

Advances in Metabolic Remodeling and Intervention Strategies in Heart Failure

The heart is the most energy-demanding organ throughout the whole body. Perturbations or failure in energy metabolism contributes to heart failure (HF), which represents the advanced stage of various heart diseases. The poor prognosis and huge economic burden associated with HF underscore the high unmet need to explore novel therapies targeting metabolic modulators beyond conventional approaches focused on neurohormonal and hemodynamic regulators. Emerging evidence suggests that alterations in metabolic substrate reliance, metabolic pathways, metabolic by-products, and energy production collectively regulate the occurrence and progression of HF. In this review, we provide an overview of cardiac metabolic remodeling, encompassing the utilization of free fatty acids, glucose metabolism, ketone bodies, and branched-chain amino acids both in the physiological condition and heart failure. Most importantly, the latest advances in pharmacological interventions are discussed as a promising therapeutic approach to restore cardiac function, drawing insights from recent basic research, preclinical and clinical studies.

Problems and prospects for finding new pharmacological agents among adenosine receptor agonists, antagonists, or their allosteric modulators for the treatment of cardiovascular diseases

A1-adenosine receptors (A1AR) are widely distributed in the human body and mediate many different effects. They are abundantly present in the cardiovascular system, where they control angiogenesis, vascular tone, heart rate, and conduction. This makes the cardiovascular system A1AR an attractive target for the treatment of cardiovascular diseases (CVD). The review summarizes the literature data on the structure and functioning of A1AR, and analyzes their involvement in the formation of myocardial hypertrophy, ischemia-reperfusion damage, various types of heart rhythm disorders, chronic heart failure, and arterial hypertension. Special attention is paid to the role of some allosteric regulators of A1AR as potential agents for the CVD treatment.

Investigation of adenosine A1 receptor-mediated β-arrestin 2 recruitment using a split-luciferase assay

Background: Adenosine A1 receptor (A₁AR) plays a prominent role in neurological and cardiac diseases and inflammatory processes. Its endogenous ligand adenosine is known to be one of the key players in the sleep-wake cycle. Like other G protein-coupled receptors (GPCRs), stimulation of A₁AR leads to the recruitment of arrestins in addition to the activation of G proteins. So far, little is known about the role of these proteins in signal transduction and regulation of A₁AR compared to the activation of G proteins. In this work, we characterized a live cell assay for A₁AR-mediated β-arrestin 2 recruitment. We have applied this assay to a set of different compounds that interact with this receptor. Methods: Based on NanoBit^® technology, a protein complementation assay was developed in which the A₁AR is coupled to the large part of the nanoluciferase (LgBiT), whereas its small part (SmBiT) is fused to the N-terminus of β-arrestin 2. Stimulation of A₁AR results in the recruitment of β-arrestin 2 and subsequent complementation of a functional nanoluciferase. For comparison, corresponding data on the effect of receptor stimulation on intracellular cAMP levels were collected for some data sets using the GloSensor™ assay. Results: The assay gives highly reproducible results with a very good signal-to-noise ratio. Capadenoson, in contrast to adenosine, CPA, or NECA, shows only partial agonism in this assay with respect to the recruitment of β-arrestin 2, whereas it shows full agonism in the case of the inhibitory effect of A₁AR on cAMP production. By using a GRK2 inhibitor, it becomes clear that the recruitment is at least partially dependent on the phosphorylation of the receptor by this kinase. Interestingly, this was also the first time that we demonstrate the A₁AR-mediated recruitment of β-arrestin 2 by stimulation with a valerian extract. Conclusion: The presented assay is a useful tool for the quantitative study of A₁AR-mediated β-arrestin 2 recruitment. It allows data collection for stimulatory, inhibitory, and modulatory substances and is also suitable for more complex substance mixtures such as valerian extract.

Uncoupling Proteins in Striated Muscle Tissue: Known Facts and Open Questions

Significance: Uncoupling proteins (UCPs) are a family of proteins that allow proton leakage across the inner mitochondrial membrane. Although UCP1, also known as thermogenin, is well known and important for heat generation in brown adipose tissue, striated muscles express two distinct members of UCP, namely UCP2 and UCP3. Unlike UCP1, the main function of UCP2 and UCP3 does not appear to be heat production. Recent Advances: Interestingly, UCP2 is the main isoform expressed in cardiac tissues, whereas UCP3 is the dominant isoform in skeletal muscles. In the past years, researchers have started to investigate the regulation of UCP2 and UCP3 expression in striated muscles. Furthermore, concepts about the proposed functions of UCP2 and UCP3 in striated muscles are developed but are still a matter of debate. Critical Issues: Potential functions of UCP2 and UCP3 in striated muscles include a role in protection against mitochondria-dependent oxidative stress, as transporter for pyruvate, fatty acids, and protons into and out of the mitochondria, and in metabolic sensing. In this context, the different isoform expression of UCP2 and UCP3 in the skeletal and cardiac muscle may be related to different metabolic requirements of the two organs. Future Directions: The level of expression of UCP2 and UCP3 in striated muscles changes in different disease stages. This suggests that UCPs may become drug targets for therapy in the future. Antioxid. Redox Signal. 37, 324-335.

Management of Chronic Congestive Heart Failure Caused by Myxomatous Mitral Valve Disease in Dogs: A Narrative Review from 1970 to 2020

Simple Summary

Myxomatous mitral valve disease (MMVD) is the most common acquired cardiovascular disease in dogs. The progression of the disease and the increasing severity of valvular regurgitation cause a volume overload of the left heart, leading to left atrial and ventricular remodeling and congestive heart failure (CHF). The treatment of chronic CHF secondary to MMVD in dogs has not always been the same over time. In the last fifty years, the drugs utilized have considerably changed, as well as the therapeutic protocols. Some drugs have also changed their intended use. An analysis of the literature concerning the therapy of chronic heart failure in dogs affected by this widespread degenerative disease is not available; a synthesis of the published literature on this topic and a description of its current state of art are needed. To the authors’ knowledge, a review of this topic has never been published in veterinary medicine; therefore, the aim of this study is to overview the treatments of chronic CHF secondary to MMVD in dogs from 1970 to 2020 using the general framework of narrative reviews.

Abstract

The treatment of chronic congestive heart failure (CHF), secondary to myxomatous mitral valve disease (MMVD) in dogs, has considerably changed in the last fifty years. An analysis of the literature concerning the therapy of chronic CHF in dogs affected by MMVD is not available, and it is needed. Narrative reviews (NRs) are aimed at identifying and summarizing what has been previously published, avoiding duplications, and seeking new study areas that have not yet been addressed. The most accessible open-access databases, PubMed, Embase, and Google Scholar, were chosen, and the searching time frame was set in five decades, from 1970 to 2020. The 384 selected studies were classified into categories depending on the aim of the study, the population target, the pathogenesis of MMVD (natural/induced), and the resulting CHF. Over the years, the types of studies have increased considerably in veterinary medicine. In particular, there have been 43 (24.29%) clinical trials, 41 (23.16%) randomized controlled trials, 10 (5.65%) cross-over trials, 40 (22.60%) reviews, 5 (2.82%) comparative studies, 17 (9.60%) case-control studies, 2 (1.13%) cohort studies, 2 (1.13%) experimental studies, 2 (1.13%) questionnaires, 6 (3.40%) case-reports, 7 (3.95%) retrospective studies, and 2 (1.13%) guidelines. The experimental studies on dogs with an induced form of the disease were less numerous (49–27.68%) than the studies on dogs affected by spontaneous MMVD (128–72.32%). The therapy of chronic CHF in dogs has considerably changed in the last fifty years: in the last century, some of the currently prescribed drugs did not exist yet, while others had different indications.

Pharmacological Tuning of Adenosine Signal Nuances Underlying Heart Failure With Preserved Ejection Fraction

Heart failure with preserved ejection fraction (HFpEF) roughly represents half of the cardiac failure events in developed countries. The proposed 'systemic microvascular paradigm' has been used to explain HFpHF presentation heterogeneity. The lack of effective treatments with few evidence-based therapeutic recommendations makes HFpEF one of the greatest unmet clinical necessities worldwide. The endogenous levels of the purine nucleoside, adenosine, increase significantly following cardiovascular events. Adenosine exerts cardioprotective, neuromodulatory, and immunosuppressive effects by activating plasma membrane-bound P1 receptors that are widely expressed in the cardiovascular system. Its proven benefits have been demonstrated in preclinical animal tests. Here, we provide a comprehensive and up-to-date critical review about the main therapeutic advantages of tuning adenosine signalling pathways in HFpEF, without discounting their side effects and how these can be seized.

Lack of efficacy of a partial adenosine A1 receptor agonist in neuropathic pain models in mice

Previous studies suggest that adenosine A₁ receptors (A₁R) modulate the processing of pain. The aim of this study was to characterize the distribution of A₁R in nociceptive tissues and to evaluate whether targeting A₁R with the partial agonist capadenoson may reduce neuropathic pain in mice. The cellular distribution of A₁R in dorsal root ganglia (DRG) and the spinal cord was analyzed using fluorescent in situ hybridization. In behavioral experiments, neuropathic pain was induced by spared nerve injury or intraperitoneal injection of paclitaxel, and tactile hypersensitivities were determined using a dynamic plantar aesthesiometer. Whole-cell patch-clamp recordings were performed to assess electrophysiological properties of dissociated DRG neurons. We found A₁R to be expressed in populations of DRG neurons and dorsal horn neurons involved in the processing of pain. However, administration of capadenoson at established in vivo doses (0.03–1.0 mg/kg) did not alter mechanical hypersensitivity in the spared nerve injury and paclitaxel models of neuropathic pain, whereas the standard analgesic pregabalin significantly inhibited the pain behavior. Moreover, capadenoson failed to affect potassium currents in DRG neurons, in contrast to a full A₁R agonist. Despite expression of A₁R in nociceptive neurons, our data do not support the hypothesis that pharmacological intervention with partial A₁R agonists might be a valuable approach for the treatment of neuropathic pain.

Energy metabolism disorders and potential therapeutic drugs in heart failure

Heart failure (HF) is a global public health problem with high morbidity and mortality. A large number of studies have shown that HF is caused by severe energy metabolism disorders, which result in an insufficient heart energy supply. This deficiency causes cardiac pump dysfunction and systemic energy metabolism failure, which determine the development of HF and recovery of heart. Current HF therapy acts by reducing heart rate and cardiac preload and afterload, treating the HF symptomatically or delaying development of the disease. Drugs aimed at cardiac energy metabolism have not yet been developed. In this review, we outline the main characteristics of cardiac energy metabolism in healthy hearts, changes in metabolism during HF, and related pathways and targets of energy metabolism. Finally, we discuss drugs that improve cardiac function via energy metabolism to provide new research ideas for the development and application of drugs for treating HF.

Adenosine 5'-monophosphate induces hypothermia and alters gene expressions in the brain and liver of chicks

The adenosine A1 receptor is important for body temperature regulation in mammals; however, little is known about its function in avian species. In this study, we investigated the effects of the adenosine A1 receptor agonist and antagonist (adenosine 5'-monophosphate [5'-AMP] and 8 p-sulfophenyl theophylline [8-SPT], respectively) on thermoregulation in chickens. Male chicks were used in this study. After administration of 5'-AMP and 8-SPT, the rectal temperature, plasma metabolites, and gene expressions in the hypothalamus and liver were measured. The rectal temperature was reduced by peripheral administration of 5'-AMP, and the hypothermic effect of 5'-AMP was attenuated by central injection of 8-SPT in chicks. In the hypothalamus, the mRNA level of the agouti-related protein (AgRP) was increased by 5'-AMP administration, whereas it was suppressed by 8-SPT. The plasma levels of free fatty acid were elevated in 5'-AMP-treated chicks and that elevation was suppressed by the 8-SPT treatment. The gene expression of proopiomelanocortin in the hypothalamus was affected by 8-SPT. Nevertheless, the gene expressions of the thermoregulation-related genes, such as the thyrotropin-releasing hormone, were not affected by 5'-AMP and 8-SPT. Hepatic gene expressions related to lipid intake and metabolism were suppressed by 5'-AMP. However, the gene expression of the uncoupling protein was upregulated by 5'-AMP. Based on these results, birds, like mammals, will undergo adenosine A1 receptor-induced hypothermia. In conclusion, it is suggested that 5'-AMP-mediated hypothermia via the adenosine A1 receptor may affect the central melanocortin system and suppress hepatic lipid metabolism in chickens.

Pharmacological Insights Into Safety and Efficacy Determinants for the Development of Adenosine Receptor Biased Agonists in the Treatment of Heart Failure

Adenosine A₁ receptors (A₁R) are a potential target for cardiac injury treatment due to their cardioprotective/antihypertrophic actions, but drug development has been hampered by on-target side effects such as bradycardia and altered renal hemodynamics. Biased agonism has emerged as an attractive mechanism for A₁R-mediated cardioprotection that is haemodynamically safe. Here we investigate the pre-clinical pharmacology, efficacy and side-effect profile of the A₁R agonist neladenoson, shown to be safe but ineffective in phase IIb trials for the treatment of heart failure. We compare this agent with the well-characterized, pan-adenosine receptor (AR) agonist NECA, capadenoson, and the A₁R biased agonist VCP746, previously shown to be safe and cardioprotective in pre-clinical models of heart failure. We show that like VCP746, neladenoson is biased away from Ca²⁺ influx relative to NECA and the cAMP pathway at the A₁R, a profile predictive of a lack of adenosine-like side effects. Additionally, neladenoson was also biased away from the MAPK pathway at the A₁R. In contrast to VCP746, which displays more 'adenosine-like' signaling at the A_2BR, neladenoson was a highly selective A₁R agonist, with biased, weak agonism at the A_2BR. Together these results show that unwanted hemodynamic effects of A₁R agonists can be avoided by compounds biased away from Ca²⁺ influx relative to cAMP, relative to NECA. The failure of neladenoson to reach primary endpoints in clinical trials suggests that A₁R-mediated cAMP inhibition may be a poor indicator of effectiveness in chronic heart failure. This study provides additional information that can aid future screening and/or design of improved AR agonists that are safe and efficacious in treating heart failure in patients.

Biased agonism at adenosine receptors

Adenosine modulates many aspects of human physiology and pathophysiology through binding to the adenosine family of G protein-coupled receptors, which are comprised of four subtypes, the A₁R, A_2AR, A_2BR and A₃R. Modulation of adenosine receptor function by exogenous agonists, antagonists and allosteric modulators can be beneficial for a number of conditions including cardiovascular disease, Parkinson's disease, and cancer. Unfortunately, many preclinical drug candidates targeting adenosine receptors have failed in clinical trials due to limited efficacy and/or severe on-target undesired effects. To overcome the key barriers typically encountered when transitioning adenosine receptor ligands into the clinic, research efforts have focussed on exploiting the phenomenon of biased agonism. Biased agonism provides the opportunity to develop ligands that favour therapeutic signalling pathways, whilst avoiding signalling associated with on-target undesired effects. Recent studies have begun to define the structure-function relationships that underpin adenosine receptor biased agonism and establish how this phenomenon can be harnessed therapeutically. In this review we describe the recent advancements made towards achieving therapeutically relevant biased agonism at adenosine receptors.

Mitochondrial Dysfunction and Heart Disease: Critical Appraisal of an Overlooked Association

The myocardium is among the most energy-consuming tissues in the body, burning from 6 to 30 kg of ATP per day within the mitochondria, the so-called powerhouse of the cardiomyocyte. Although mitochondrial genetic disorders account for a small portion of cardiomyopathies, mitochondrial dysfunction is commonly involved in a broad spectrum of heart diseases, and it has been implicated in the development of heart failure via maladaptive circuits producing and perpetuating mitochondrial stress and energy starvation. In this bench-to-bedside review, we aimed to (i) describe the key functions of the mitochondria within the myocardium, including their role in ischemia/reperfusion injury and intracellular calcium homeostasis; (ii) examine the contribution of mitochondrial dysfunction to multiple cardiac disease phenotypes and their transition to heart failure; and (iii) discuss the rationale and current evidence for targeting mitochondrial function for the treatment of heart failure, including via sodium-glucose cotransporter 2 inhibitors.

P1 Receptor Agonists/Antagonists in Clinical Trials - Potential Drug Candidates of the Future

BACKGROUND

Adenosine mediates various physiological and pathological conditions by acting on its four P1 receptors (A1, A2A, A2B and A3 receptors). Omnipresence of P1 receptors and their activation, exert a wide range of biological activities. Thus, its modulation is implicated in various disorders like Parkinson's disease, asthma, cardiovascular disorders, cancer etc. Hence these receptors have become an interesting target for the researchers to develop potential therapeutic agents. Number of molecules were designed and developed in the past few years and evaluated for their efficacy in various disease conditions.

OBJECTIVE

The main objective is to provide an overview of new chemical entities which have crossed preclinical studies and reached clinical trials stage following their current status and future prospective.

METHODS

In this review we discuss current status of the drug candidates which have undergone clinical trials and their prospects.

RESULTS

Many chemical entities targeting various subtypes of P1 receptors are patented; twenty of them have crossed preclinical studies and reached clinical trials stage. Two of them viz adenosine and regadenoson are approved by the Food and Drug Administration.

CONCLUSION

This review is an attempt to highlight the current status, progress and probable future of P1 receptor ligands which are under clinical trials as promising novel therapeutic agents and the direction in which research should proceed with a view to come out with novel therapeutic agents.

Medicinal Chemistry and Therapeutic Potential of Agonists, Antagonists and Allosteric Modulators of A1 Adenosine Receptor: Current Status and Perspectives

Adenosine is a purine nucleoside, responsible for the regulation of a wide range of physiological and pathophysiological conditions by binding with four G-protein-coupled receptors (GPCRs), namely A1, A2A, A2B and A3 adenosine receptors (ARs). In particular, A1 AR is ubiquitously present, mediating a variety of physiological processes throughout the body, thus represents a promising drug target for the management of various pathological conditions. Agonists of A1 AR are found to be useful for the treatment of atrial arrhythmia, angina, type-2 diabetes, glaucoma, neuropathic pain, epilepsy, depression and Huntington's disease, whereas antagonists are being investigated for the treatment of diuresis, congestive heart failure, asthma, COPD, anxiety and dementia. However, treatment with full A1 AR agonists has been associated with numerous challenges like cardiovascular side effects, off-target activation as well as desensitization of A1 AR leading to tachyphylaxis. In this regard, partial agonists of A1 AR have been found to be beneficial in enhancing insulin sensitivity and subsequently reducing blood glucose level, while avoiding severe CVS side effects and tachyphylaxis. Allosteric enhancer of A1 AR is found to be potent for the treatment of neuropathic pain, culminating the side effects related to off-target tissue activation of A1 AR. This review provides an overview of the medicinal chemistry and therapeutic potential of various agonists/partial agonists, antagonists and allosteric modulators of A1 AR, with a particular emphasis on their current status and future perspectives in clinical settings.

Targeting the Mitochondria in Heart Failure: A Translational Perspective

The burden of heart failure (HF) in terms of health care expenditures, hospitalizations, and mortality is substantial and growing. The failing heart has been described as "energy-deprived" and mitochondrial dysfunction is a driving force associated with this energy supply-demand imbalance. Existing HF therapies provide symptomatic and longevity benefit by reducing cardiac workload through heart rate reduction and reduction of preload and afterload but do not address the underlying causes of abnormal myocardial energetic nor directly target mitochondrial abnormalities. Numerous studies in animal models of HF as well as myocardial tissue from explanted failed human hearts have shown that the failing heart manifests abnormalities of mitochondrial structure, dynamics, and function that lead to a marked increase in the formation of damaging reactive oxygen species and a marked reduction in on demand adenosine triphosphate synthesis. Correcting mitochondrial dysfunction therefore offers considerable potential as a new therapeutic approach to improve overall cardiac function, quality of life, and survival for patients with HF.

Microvascular Dysfunction in Heart Failure With Preserved Ejection Fraction

Heart failure with preserved ejection fraction (HFpEF) is an increasingly studied entity accounting for 50% of all diagnosed heart failure and that has claimed its own dignity being markedly different from heart failure with reduced EF in terms of etiology and natural history (Graziani et al., 2018). Recently, a growing body of evidence points the finger toward microvascular dysfunction as the major determinant of the pathological cascade that justifies clinical manifestations (Crea et al., 2017). The high burden of comorbidities such as metabolic syndrome, hypertension, atrial fibrillation, chronic kidney disease, obstructive sleep apnea, and similar, could lead to a systemic inflammatory state that impacts the physiology of the endothelium and the perivascular environment, engaging complex molecular pathways that ultimately converge to myocardial fibrosis, stiffening, and dysfunction (Paulus and Tschope, 2013). These changes could even self-perpetrate with a positive feedback where hypoxia and locally released inflammatory cytokines trigger interstitial fibrosis and hypertrophy (Ohanyan et al., 2018). Identifying microvascular dysfunction both as the cause and the maintenance mechanism of this condition has opened the field to explore specific pharmacological targets like nitric oxide (NO) pathway, sarcomeric titin, transforming growth factor beta (TGF-β) pathway, immunomodulators or adenosine receptors, trying to tackle the endothelial impairment that lies in the background of this syndrome (Graziani et al., 2018;Lam et al., 2018). Yet, many questions remain, and the new data collected still lack a translation to improved treatment strategies. To further elaborate on this tangled and exponentially growing topic, we will review the evidence favoring a microvasculature-driven etiology of this condition, its clinical correlations, the proposed diagnostic workup, and the available/hypothesized therapeutic options to address microvascular dysfunction in the failing heart.

Non-Nucleoside Agonists of the Adenosine Receptors: An Overview

Potent and selective adenosine receptor (AR) agonists are of pharmacological interest for the treatment of a wide range of diseases and conditions. Among these derivatives, nucleoside-based agonists represent the great majority of molecules developed and reported to date. However, the limited availability of compounds selective for a specific AR subtype (i.e., A2BAR) and a generally long and complex synthetic route for largely substituted nucleosides are the main drawbacks of this category of molecules. Non-nucleoside agonists represent an alternative set of compounds able to stimulate the AR function and based on simplified structures. This review provides an updated overview on the structural classes of non-nucleoside AR agonists and their biological activities, with emphasis on the main derivatives reported in the literature. A focus is also given to the synthetic routes employed to develop these derivatives and on molecular modeling studies simulating their interaction with ARs.

Safety and efficacy of the partial adenosine A1 receptor agonist neladenoson bialanate in patients with chronic heart failure with reduced ejection fraction: a phase IIb, randomized, double-blind, placebo-controlled trial

AIMS

Neladenoson bialanate is a partial adenosine A1 receptor agonist with demonstrated beneficial effects on cardiac function in animal models. We aimed to assess the dose-response effect of neladenoson bialanate on cardiac structure and function, clinical outcome, and safety in patients with heart failure (HF) with reduced ejection fraction (HFrEF).

METHODS AND RESULTS

PANTHEON was a dose-finding, phase IIb, randomized, double-blind, placebo-controlled trial conducted in 92 centres in 11 countries including 462 patients with chronic HFrEF, randomized to once daily oral dose of neladenoson bialanate (5, 10, 20, 30, and 40 mg) or placebo. The primary endpoints were change from baseline to 20 weeks in left ventricular ejection fraction (LVEF) (echocardiography) and in N-terminal pro-B-type natriuretic peptide (NT-proBNP). Mean age of the patients was 67 years, 17% were female, mean LVEF was 28%, mean NT-proBNP was 2085 ng/L. After 20 weeks of treatment, there was no dose-effect of neladenoson bialanate on changes in NT-proBNP or LVEF (primary endpoints). No effect of neladenoson bialanate was found on left ventricular volumes, high-sensitivity troponin T, or cardiovascular mortality, HF hospitalization, and urgent visits for HF (secondary endpoints). There was a dose-dependent increase in creatinine and cystatin C, and a dose-dependent decrease in estimated glomerular filtration rate and heart rate.

CONCLUSIONS

In patients with chronic HFrEF, treatment with neladenoson bialanate was not associated with dose-dependent favourable effects on cardiac structure and function, cardiac risk markers, or clinical outcome but was associated with a dose-dependent decrease in renal function.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov Identifier NCT02992288.

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

Neuroprotective potential of adenosine A1 receptor partial agonists in experimental models of cerebral ischemia

Cerebral ischemia is the second most common cause of death and a major cause of disability worldwide. Available therapies are based only on anticoagulants or recombinant tissue plasminogen activator. Extracellular adenosine increases during ischemia and acts as a neuroprotective endogenous agent mainly by activating adenosine A₁ receptors (A₁ Rs) which control calcium influx, glutamate release, membrane potential, and metabolism. Accordingly, in many experimental paradigms it has been already demonstrated that the stimulation of A₁ R with full agonists is able to reduce ischemia-related structural and functional brain damage; unfortunately, cardiovascular side effects and desensitization of A₁ R induced by these compounds have strongly limited their exploitation in stroke therapy so far. Among the newly emerging compounds, A₁ R partial agonists could be almost free of side effects and equally effective. Therefore, we decided to evaluate the neuroprotective potential of two A₁ R partial agonists, namely 2'-dCCPA and 3'-dCCPA, in in vitro and ex vivo experimental models of cerebral ischemia. Within the experimental paradigm of oxygen-glucose deprivation in vitro in human neuroblastoma (SH-SY5Y) cells both A₁ R partial agonists increased cell viability. Considering the high level of expression of A₁ Rs in the hippocampus and the susceptibility of CA1 region to hypoxia, we performed electrophysiological experiments in this subfield. The application of 7 min of oxygen-glucose deprivation constantly produces an irreversible synaptic failure in all the C57Bl/6 mice hippocampal slices evaluated; both tested compounds allowed a significant recovery of synaptic transmission. These findings demonstrate that A₁ R and its partial agonists are still of interest for cerebral ischemia therapy. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Myocardial Energetics and Heart Failure: a Review of Recent Therapeutic Trials

PURPOSE OF REVIEW

Several novel therapeutics being tested in patients with heart failure are based on myocardial energetics. This review will provide a summary of the recent trials in this area, including therapeutic options targeting various aspects of cellular and mitochondrial metabolism.

RECENT FINDINGS

Agents that improve the energetic balance in myocardial cells have the potential to improve clinical heart failure status. The most promising therapies currently under investigation in this arena include (1) elamipretide, a cardiolipin stabilizer; (2) repletion of iron deficiency with intravenous ferrous carboxymaltose; (3) coenzyme Q10; and (4) the partial adenosine receptor antagonists capadenoson and neladenosone. Myocardial energetics-based therapeutics are groundbreaking in that they utilize novel mechanisms of action to improve heart failure symptoms, without causing the adverse neurohormonal side effects associated with current guideline-based therapies. The drugs appear likely to be added to the heart failure therapy armamentarium as adjuncts to current regimens in the near future.

The adenosine A2B G protein-coupled receptor: Recent advances and therapeutic implications

The adenosine A_2B receptor (A_2BAR) is one of four adenosine receptor subtypes belonging to the Class A family of G protein-coupled receptors (GPCRs). Until recently, the A_2BAR remained poorly characterised, in part due to its relatively low affinity for the endogenous agonist adenosine and therefore presumed minor physiological significance. However, the substantial increase in extracellular adenosine concentration, the sensitisation of the receptor and the upregulation of A_2BAR expression under conditions of hypoxia and inflammation, suggest the A_2BAR as an exciting therapeutic target in a variety of pathological disease states. Here we discuss the pharmacology of the A_2BAR and outline its role in pathophysiology including ischaemia-reperfusion injury, fibrosis, inflammation and cancer.

Rationale and design of the phase 2b clinical trials to study the effects of the partial adenosine A1-receptor agonist neladenoson bialanate in patients with chronic heart failure with reduced (PANTHEON) and preserved (PANACHE) ejection fraction

Despite major advances in the treatment of chronic heart failure (HF) with reduced ejection fraction (HFrEF), morbidity and mortality associated with the condition remain high, suggesting the need for additional treatment options, particularly haemodynamically neutral treatments that do not alter blood pressure, heart rate, or renal function. HF with preserved ejection fraction (HFpEF) is also associated with high morbidity and mortality and adequate treatment options are limited; thus there is a critical unmet need for the development of novel therapies for HFpEF. Chronic HFrEF and HFpEF are both systemic disorders that affect not only the heart but several other tissues and organs including skeletal muscle, leading to exercise intolerance and dyspnoea. Partial adenosine A1-receptor agonists represent a novel potential therapy for HF regardless of underlying ejection fraction given their minimal effect on heart rate and blood pressure, and preclinical data demonstrate several possible beneficial mechanisms, including improved mitochondrial function and sarcoplasmic reticulum Ca²⁺ -ATPase (SERCA2a) activity, enhanced energy substrate utilization, reverse ventricular remodelling, and anti-ischemic, cardioprotective properties. However, data on this class of drugs in humans are scarce, and the optimal dose of the partial adenosine A1 receptor, neladenoson bialanate, has not been defined. Here we describe the design and rationale of two randomized, double-blind, placebo-controlled, parallel-group, dose-finding phase 2b trials, PANTHEON (HFrEF) and PANACHE (HFpEF), that will advance our understanding of the potential benefit and optimal dose of neladenoson bialanate and provide critical information for the planning of future phase 3 trials.

Adenosine Receptor-Mediated Cardioprotection-Current Limitations and Future Directions

Since the seminal reports of adenosine receptor-mediated cardioprotection in the early 1990s, there have been a multitude of such reports in various species and preparations. Original observations of the beneficial effects of A₁ receptor agonists have been followed up with numerous reports also implicating A_2A, A₃, and most recently A_2B, receptor agonists as cardioprotective agents. Although adenosine has been approved for clinical use in the United States for the treatment of supraventricular tachycardia and coronary artery imaging, and the selective A_2A agonist, regadenoson, for the latter, clinical use of adenosine receptor agonists for protecting the ischemic heart has not advanced beyond early trials. An examination of the literature indicates that existing experimental studies have several limitations in terms of clinical relevance, as well as lacking incorporation of recent new insights into adenosine receptor signaling. Such deficiencies include the lack of experimental studies in models that most closely mimic human cardiovascular disease. In addition, there have been very few studies in chronic models of myocardial ischemia, where limiting myocardial remodeling and heart failure, not reduction of infarct size, are the primary endpoints. Despite an increasing number of reports of the beneficial effects of adenosine receptor antagonists, not agonists, in chronic diseases, this idea has not been well-studied in experimental myocardial ischemia. There have also been few studies examining adenosine receptor subtype interactions as well as receptor heterodimerization. The purpose of this Perspective article is to discuss these deficiencies to highlight future directions of research in the field of adenosine receptor-mediated protection of ischemic myocardium.

Employing Extracellular Volume Cardiovascular Magnetic Resonance Measures of Myocardial Fibrosis to Foster Novel Therapeutics

Quantifying myocardial fibrosis (MF) with myocardial extracellular volume measures acquired during cardiovascular magnetic resonance promises to transform clinical care by advancing pathophysiologic understanding and fostering novel therapeutics. Extracellular volume quantifies MF by measuring the extracellular compartment depicted by the myocardial uptake of contrast relative to plasma. MF is a key domain of dysfunctional but viable myocardium among others (eg, microvascular dysfunction and cardiomyocyte/mitochondrial dysfunction). Although anatomically distinct, these domains may functionally interact. MF represents pathological remodeling in the heart associated with cardiac dysfunction and adverse outcomes likely mediated by interactions with the microvasculature and the cardiomyocyte. Reversal of MF improves key measures of cardiac dysfunction, so reversal of MF represents a likely mechanism for improved outcomes. Instead of characterizing the myocardium as homogenous tissue and using important yet still generic descriptors, such as thickness (hypertrophy) and function (diastolic or systolic), which lack mechanistic specificity, paradigms of cardiac disease have evolved to conceptualize myocardial disease and patient vulnerability based on the extent of disease involving its various compartments. Specifying myocardial compartmental involvement may then implicate cellular/molecular disease pathways for treatment and targeted pharmaceutical development and above all highlight the role of the cardiac-specific pathology in heart failure among myriad other changes in the heart and beyond. The cardiology community now requires phase 2 and 3 clinical trials to examine strategies for the regression/prevention of MF and eventually biomarkers to identify MF without reliance on cardiovascular magnetic resonance. It seems likely that efficacious antifibrotic therapy will improve outcomes, but definitive data are needed.

Targeting Adenosine Receptors for the Treatment of Cardiac Fibrosis

Adenosine is a ubiquitous molecule with key regulatory and cytoprotective mechanisms at times of metabolic imbalance in the body. Among a plethora of physiological actions, adenosine has an important role in attenuating ischaemia-reperfusion injury and modulating the ensuing fibrosis and tissue remodeling following myocardial damage. Adenosine exerts these actions through interaction with four adenosine G protein-coupled receptors expressed in the heart. The adenosine A_2B receptor (A_2BAR) is the most abundant adenosine receptor (AR) in cardiac fibroblasts and is largely responsible for the influence of adenosine on cardiac fibrosis. In vitro and in vivo studies demonstrate that acute A_2BAR stimulation can decrease fibrosis through the inhibition of fibroblast proliferation and reduction in collagen synthesis. However, in contrast, there is also evidence that chronic A_2BAR antagonism reduces tissue fibrosis. This review explores the opposing pro- and anti-fibrotic activity attributed to the activation of cardiac ARs and investigates the therapeutic potential of targeting ARs for the treatment of cardiac fibrosis.

Safety issues of compounds acting on adenosinergic signalling

OBJECTIVES

Much research has been performed on the field of identifying the roles of adenosine and adenosinergic signalling, but a relatively low number of marketing authorizations have been granted for adenosine receptor (AdR) ligands. In part, this could be related to their safety issues; therefore, our aim was to examine the toxicological and adverse effects data of different compounds acting on adenosinergic signalling, including different AdR ligands and compounds resembling the structure of adenosine. We also wanted to present recent pharmaceutical developments of experimental compounds that showed promising results in clinical trial setting.

KEY FINDINGS

Safety issues of compounds modulating adenosinergic signalling were investigated, and different mechanisms were presented. Structurally different classes of compounds act on AdRs, the most important being adenosine, adenosine derivatives and other non-nucleoside compounds. Many of them are either not selective enough or are targeting other targets of adenosinergic signalling such as metabolizing enzymes that regulate adenosine levels. Many other targets are also involved that are not part of adenosinergic signalling system such as GABA receptors, different channels, enzymes and others. Some synthetic AdR ligands even showed to be genotoxic.

SUMMARY

Current review presents safety data of adenosine, adenosine derivatives and other non-nucleoside compounds that modulate adenosinergic signalling. We have presented different mechanisms that participate to an adverse effect or toxic outcome. A separate section also deals with possible organ-specific toxic effects on different in-vitro and in-vivo models.

Silent disease progression in clinically stable heart failure

Heart failure with reduced ejection fraction (HFrEF) is a progressive disorder whereby cardiac structure and function continue to deteriorate, often despite the absence of clinically apparent signs and symptoms of a worsening disease state. This silent yet progressive nature of HFrEF can contribute to the increased risk of death-even in patients who are 'clinically stable', or who are asymptomatic or only mildly symptomatic-because it often goes undetected and/or undertreated. Current therapies are aimed at improving clinical symptoms, and several agents more directly target the underlying causes of disease; however, new therapies are needed that can more fully address factors responsible for underlying progressive cardiac dysfunction. In this review, mechanisms that drive HFrEF, including ongoing cardiomyocyte loss, mitochondrial abnormalities, impaired calcium cycling, elevated LV wall stress, reactive interstitial fibrosis, and cardiomyocyte hypertrophy, are discussed. Additionally, limitations of current HF therapies are reviewed, with a focus on how these therapies are designed to counteract the deleterious effects of compensatory neurohumoral activation but do not fully prevent disease progression. Finally, new investigational therapies that may improve the underlying molecular, cellular, and structural abnormalities associated with HF progression are reviewed.

Targeting mitochondrial dysfunction in the treatment of heart failure

INTRODUCTION

Heart failure (HF) has reached epidemic proportions worldwide. Despite the availability of drugs that reduce mortality and afford good symptom relief, HF continues to exact a considerable clinical and economic burden. Current HF therapies elicit benefit by reducing cardiac workload by lowering heart rate and loading conditions, thereby reducing myocardial energy demands. Areas covered: Recent recognition that the failing heart is 'energy deprived' and its primary energy source, the mitochondria, is dysfunctional, has focused attention on mitochondria as a worthy therapeutic target. In HF, mitochondrial dysfunction leads to reduced adenosine triphosphate (ATP) synthesis and excessive formation of damaging reactive oxygen species (ROS), a combination the failing heart can ill afford. Expert commentary: Correcting mitochondrial dysfunction can help forge a new therapeutic approach based on readily available energy that can meet increasing cardiac demands. This paradigm shift, once implemented successfully, is likely to elicit better overall cardiac function, better quality of life, and improved survival for patients with HF.

Risk-based evaluation of efficacy of rolofylline in patients hospitalized with acute heart failure - Post-hoc analysis of the PROTECT trial

BACKGROUND

The selective adenosine A1 receptor antagonist rolofylline showed a neutral overall result on clinical outcomes in the PROTECT trial. However, we hypothesized that response to rolofylline treatment could be influenced by underlying clinical risk.

METHODS

We performed a post-hoc analysis of the PROTECT trial - a large, double-blind, randomized, placebo-controlled trial that enrolled 2033 patients. Baseline risk of 180-day all-cause mortality was estimated using a previously published 8-item model. Evaluation of efficacy of rolofylline across subpopulations defined based on estimated risk of mortality was performed using subpopulation treatment effect pattern plot (STEPP) analysis. Findings were validated in an independent cohort of acute heart failure patients.

RESULTS

Median estimated risk of mortality was 13.0%, IQR [8.0%-23.0%] and was comparable between the rolofylline and placebo arms. In low to intermediate risk subgroups of patients, rolofylline was associated with a higher rate of 180-day all-cause mortality (11.9% in the rolofylline versus 8.4% in the placebo arms, p=0.050). In the high risk subgroup of patients, particularly those with estimated risk of mortality between 20% and 30%, 180-day all-cause mortality rate was markedly lower in the rolofylline arm (18.4% in the rolofylline versus 34.0% in the placebo arms, p=0.003). The trend towards potential harm with rolofylline treatment in the low to intermediate risk subpopulations and significant benefit in high risk patients was also observed in the validation cohort.

CONCLUSION

Our findings suggest that selective adenosine A1 receptor antagonism could be harmful in low risk acute heart failure patients, while it might significantly benefit higher risk patients.

MicroRNA and Heart Failure

Heart failure (HF) imposes significant economic and public health burdens upon modern society. It is known that disturbances in neurohormonal status play an important role in the pathogenesis of HF. Therapeutics that antagonize selected neurohormonal pathways, specifically the renin-angiotensin-aldosterone and sympathetic nervous systems, have significantly improved patient outcomes in HF. Nevertheless, mortality remains high with about 50% of HF patients dying within five years of diagnosis thus mandating ongoing efforts to improve HF management. The discovery of short noncoding microRNAs (miRNAs) and our increasing understanding of their functions, has presented potential therapeutic applications in complex diseases, including HF. Results from several genome-wide miRNA studies have identified miRNAs differentially expressed in HF cohorts suggesting their possible involvement in the pathogenesis of HF and their potential as both biomarkers and as therapeutic targets. Unravelling the functional relevance of miRNAs within pathogenic pathways is a major challenge in cardiovascular research. In this article, we provide an overview of the role of miRNAs in the cardiovascular system. We highlight several HF-related miRNAs reported from selected cohorts and review their putative roles in neurohormonal signaling.

Extracellular nucleotide regulation and signaling in cardiac fibrosis

Following myocardial infarction, purinergic nucleotides and nucleosides are released via non-specific and specific mechanisms in response to cellular activation, stress, or injury. These extracellular nucleotides are potent mediators of physiologic and pathologic responses, contributing to the inflammatory and fibrotic milieu within the injured myocardium. Via autocrine or paracrine signaling, cell-specific effects occur through differentially expressed purinergic receptors of the P2X, P2Y, and P1 families. Nucleotide activation of the ionotropic (ligand-gated) purine receptors (P2X) and several of the metabotropic (G-protein-coupled) purine receptors (P2Y) or adenosine activation of the P1 receptors can have profound effects on inflammatory cell function, fibroblast function, and cardiomyocyte function. Extracellular nucleotidases that hydrolyze released nucleotides regulate the magnitude and duration of purinergic signaling. While there are numerous studies on the role of the purinergic signaling pathway in cardiovascular disease, the extent to which the purinergic signaling pathway modulates cardiac fibrosis is incompletely understood. Here we provide an overview of the current understanding of how the purinergic signaling pathway modulates cardiac fibroblast function and myocardial fibrosis.

Partial Adenosine A1 Agonist in Heart Failure

Adenosine exerts a variety of physiological effects by binding to cell surface G-protein-coupled receptor subtypes, namely, A1, A2a, A2b, and A3. The central physiological role of adenosine is to preclude tissue injury and promote repair in response to stress. In the heart, adenosine acts as a cytoprotective modulator, linking cardiac function to metabolic demand predominantly via activation of adenosine A1 receptors (A1Rs), which leads to inhibition of adenylate cyclase activity, modulation of protein kinase C, and opening of ATP-sensitive potassium channels. Activation of myocardial adenosine A1Rs has been shown to modulate a variety of pathologies associated with ischemic cardiac injury, including arrhythmogenesis, coronary and ventricular dysfunction, apoptosis, mitochondrial dysfunction, and ventricular remodeling. Partial A1R agonists are agents that are likely to elicit favorable pharmacological responses in heart failure (HF) without giving rise to the undesirable cardiac and extra-cardiac effects observed with full A1R agonism. Preclinical data have shown that partial adenosine A1R agonists protect and improve cardiac function at doses that do not result in undesirable effects on heart rate, atrioventricular conduction, and blood pressure, suggesting that these compounds may constitute a valuable new therapy for chronic HF. Neladenoson bialanate (BAY1067197) is the first oral partial and highly selective A1R agonist that has entered clinical development for the treatment of HF. This review provides an overview of adenosine A1R-mediated signaling in the heart, summarizes the results from preclinical and clinical studies of partial A1R agonists in HF, and discusses the potential benefits of these drugs in the clinical setting.

Pentaerythritol Tetranitrate Targeting Myocardial Reactive Oxygen Species Production Improves Left Ventricular Remodeling and Function in Rats With Ischemic Heart Failure

Reduced nitric oxide bioavailability contributes to progression of cardiac dysfunction and remodeling in ischemic heart failure. Clinical use of organic nitrates as nitric oxide donors is limited by development of nitrate tolerance and reactive oxygen species formation. We investigated the effects of long-term therapy with pentaerythritol tetranitrate (PETN), an organic nitrate devoid of tolerance, in rats with congestive heart failure after extensive myocardial infarction. Seven days after coronary artery ligation, rats were randomly allocated to treatment with PETN (80 mg/kg BID) or placebo for 9 weeks. Long-term PETN therapy prevented the progressive left ventricular dilatation and improved left ventricular contractile function and relaxation in rats with congestive heart failure. Mitochondrial superoxide anion production was markedly increased in the failing left ventricular myocardium and nearly normalized by PETN treatment. Gene set enrichment analysis revealed that PETN beneficially modulated the dysregulation of mitochondrial genes involved in energy metabolism, paralleled by prevention of uncoupling protein-3, thioredoxin-2, and superoxide dismutase-2 downregulation. Moreover, PETN provided a remarkable protective effect against reactive fibrosis in chronically failing hearts. Mechanistically, induction of heme oxygenase-1 by PETN prevented mitochondrial superoxide generation, NOX4 upregulation, and ensuing formation of extracellular matrix proteins in fibroblasts from failing hearts. In summary, PETN targeting reactive oxygen species generation prevented the changes of mitochondrial antioxidant enzymes and progressive fibrotic remodeling, leading to amelioration of cardiac functional performance. Therefore, PETN might be a promising therapeutic option in the treatment of ischemic heart diseases involving oxidative stress and impairment in nitric oxide bioactivity.

GPCR signaling and cardiac function

G protein-coupled receptors (GPCRs), such as β-adrenergic and angiotensin II receptors, located in the membranes of all three major cardiac cell types, i.e. myocytes, fibroblasts and endothelial cells, play crucial roles in regulating cardiac function and morphology. Their importance in cardiac physiology and disease is reflected by the fact that, collectively, they represent the direct targets of over a third of the currently approved cardiovascular drugs used in clinical practice. Over the past few decades, advances in elucidation of their structure, function and the signaling pathways they elicit, specifically in the heart, have led to identification of an increasing number of new molecular targets for heart disease therapy. Here, we review these signaling modalities employed by GPCRs known to be expressed in the cardiac myocyte membranes and to directly modulate cardiac contractility. We also highlight drugs and drug classes that directly target these GPCRs to modulate cardiac function, as well as molecules involved in cardiac GPCR signaling that have the potential of becoming novel drug targets for modulation of cardiac function in the future.

New acyloxy nitroso compounds with improved water solubility and nitroxyl (HNO) release kinetics and inhibitors of platelet aggregation

New acyloxy nitroso compounds, 4-nitrosotetrahydro-2H-pyran-4-yl 2,2,2-trichloroacetate and 4-nitrosotetrahydro-2H-pyran-4-yl 2,2-dichloropropanoate were prepared. These compounds release HNO under neutral conditions with half-lives between 50 and 120min, identifying these HNO donors as kinetically intermediate to the much slower acetate derivative and the faster trifluoroacetic acid derivative. These compounds or HNO-derived from these compounds react with thiols, including glutathione, thiol-containing enzymes and heme-containing proteins in a similar fashion to other acyloxy nitroso compounds. HNO released from these acyloxy nitroso compounds inhibits activated platelet aggregation. These acyloxy nitroso compounds augment the range of release for this group of HNO donors and should be valuable tools in the further study of HNO biology.

Sitagliptin attenuates sympathetic innervation via modulating reactive oxygen species and interstitial adenosine in infarcted rat hearts

We investigated whether sitagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor, attenuates arrhythmias through inhibiting nerve growth factor (NGF) expression in post-infarcted normoglycemic rats, focusing on adenosine and reactive oxygen species production. DPP-4 bound adenosine deaminase has been shown to catalyse extracellular adenosine to inosine. DPP-4 inhibitors increased adenosine levels by inhibiting the complex formation. Normoglycemic male Wistar rats were subjected to coronary ligation and then randomized to either saline or sitagliptin in in vivo and ex vivo studies. Post-infarction was associated with increased oxidative stress, as measured by myocardial superoxide, nitrotyrosine and dihydroethidium fluorescent staining. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle-treated infarcted rats compared with sham. Compared with vehicle, infarcted rats treated with sitagliptin significantly increased interstitial adenosine levels and attenuated oxidative stress. Sympathetic hyperinnervation was blunted after administering sitagliptin, as assessed by immunofluorescent analysis and western blotting and real-time quantitative RT-PCR of NGF. Arrhythmic scores in the sitagliptin-treated infarcted rats were significantly lower than those in vehicle. Ex vivo studies showed a similar effect of erythro-9-(2-hydroxy-3-nonyl) adenine (an adenosine deaminase inhibitor) to sitagliptin on attenuated levels of superoxide and NGF. Furthermore, the beneficial effects of sitagliptin on superoxide anion production and NGF levels can be reversed by 8-cyclopentyl-1,3-dipropulxanthine (adenosine A₁ receptor antagonist) and exogenous hypoxanthine. Sitagliptin protects ventricular arrhythmias by attenuating sympathetic innervation via adenosine A₁ receptor and xanthine oxidase-dependent pathways, which converge through the attenuated formation of superoxide in the non-diabetic infarcted rats.