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.

Novelties in the pharmacological approaches for chronic heart failure: new drugs and cardiovascular targets

Despite recent advances in chronic heart failure (HF) management, the prognosis of HF patients is poor. This highlights the need for researching new drugs targeting, beyond neurohumoral and hemodynamic modulation approach, such as cardiomyocyte metabolism, myocardial interstitium, intracellular regulation and NO-sGC pathway. In this review we report main novelties on new possible pharmacological targets for HF therapy, mainly on new drugs acting on cardiac metabolism, GCs-cGMP pathway, mitochondrial function and intracellular calcium dysregulation.

Experimental heart failure models in small animals

Heart failure (HF) is one of the most critical health and economic burdens worldwide, and its prevalence is continuously increasing. HF is a disease that occurs due to a pathological change arising from the function or structure of the heart tissue and usually progresses. Numerous experimental HF models have been created to elucidate the pathophysiological mechanisms that cause HF. An understanding of the pathophysiology of HF is essential for the development of novel efficient therapies. During the past few decades, animal models have provided new insights into the complex pathogenesis of HF. Success in the pathophysiology and treatment of HF has been achieved by using animal models of HF. The development of new in vivo models is critical for evaluating treatments such as gene therapy, mechanical devices, and new surgical approaches. However, each animal model has advantages and limitations, and none of these models is suitable for studying all aspects of HF. Therefore, the researchers have to choose an appropriate experimental model that will fully reflect HF. Despite some limitations, these animal models provided a significant advance in the etiology and pathogenesis of HF. Also, experimental HF models have led to the development of new treatments. In this review, we discussed widely used experimental HF models that continue to provide critical information for HF patients and facilitate the development of new treatment strategies.

The Role of Purinergic Signaling in Heart Transplantation

Heart transplantation remains the optimal treatment option for patients with end-stage heart disease. Growing evidence demonstrates that purinergic signals mediated by purine nucleotides and nucleosides play vital roles in heart transplantation, especially in the era of ischemia-reperfusion injury (IRI) and allograft rejection. Purinergic signaling consists of extracellular nucleotides and nucleosides, ecto-enzymes, and cell surface receptors; it participates in the regulation of many physiological and pathological processes. During transplantation, excess adenosine triphosphate (ATP) levels are released from damaged cells, and driver detrimental inflammatory responses largely via purinergic P2 receptors. Ecto-nucleosidases sequentially dephosphorylate extracellular ATP to ADP, AMP, and finally adenosine. Adenosine exerts a cardioprotective effect by its anti-inflammatory, antiplatelet, and vasodilation properties. This review focused on the role of purinergic signaling in IRI and rejection after heart transplantation, as well as the clinical applications and prospects of purinergic signaling.

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.

New Targets in Heart Failure Drug Therapy

Despite recent advances in chronic heart failure management (either pharmacological or non-pharmacological), the prognosis of heart failure (HF) patients remains poor. This poor prognosis emphasizes the need for developing novel pathways for testing new HF drugs, beyond neurohumoral and hemodynamic modulation approaches. The development of new drugs for HF therapy must thus necessarily focus on novel approaches such as the direct effect on cardiomyocytes, coronary microcirculation, and myocardial interstitium. This review summarizes principal evidence on new possible pharmacological targets for the treatment of HF patients, mainly focusing on microcirculation, cardiomyocyte, and anti-inflammatory therapy.

2-Amino-3,5-dicarbonitrile-6-sulfanylpyridines: synthesis and multiple biological activity - a review

This review integrates the published data of the last decade (from 2010 to 2020) on the synthesis of the 2-amino-3,5-dicarbonitrile-6-sulfanylpyridine scaffold, the derivatives of which are widely used in the synthesis of biologically active compounds. Currently, no systematic accounts of synthetic routes towards this class of heterocyclic compounds can be found in the literature. The present-day trends in the catalytic synthesis of 2-amino-3,5-dicarbonitrile-6-sulfanylpyridines are considered using pseudo-four-component reaction (pseudo-4CR) by condensation of malononitrile molecules with thiols and aldehydes, and alternative three-component (3CR) condensations of malononitrile with 2-arylidenemalononitrile and S-nucleophiles.

The latest advances in the catalytic synthesis of biologically active compounds with 2-amino-3,5-dicarbonitrile-6-sulfanylpyridine scaffold via the multicomponent reactions of malononitrile have been discussed.

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 A2BR, neladenoson was a highly selective A1R agonist, with biased, weak agonism at the A2BR. 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.

Role of Adenosine and Purinergic Receptors in Myocardial Infarction: Focus on Different Signal Transduction Pathways

Myocardial infarction (MI) is a dramatic event often caused by atherosclerotic plaque erosion or rupture and subsequent thrombotic occlusion of a coronary vessel. The low supply of oxygen and nutrients in the infarcted area may result in cardiomyocytes necrosis, replacement of intact myocardium with non-contractile fibrous tissue and left ventricular (LV) function impairment if blood flow is not quickly restored. In this review, we summarized the possible correlation between adenosine system, purinergic system and Wnt/β-catenin pathway and their role in the pathogenesis of cardiac damage following MI. In this context, several pathways are involved and, in particular, the adenosine receptors system shows different interactions between its members and purinergic receptors: their modulation might be effective not only for a normal functional recovery but also for the treatment of heart diseases, thus avoiding fibrosis, reducing infarcted area and limiting scaring. Similarly, it has been shown that Wnt/β catenin pathway is activated following myocardial injury and its unbalanced activation might promote cardiac fibrosis and, consequently, LV systolic function impairment. In this regard, the therapeutic benefits of Wnt inhibitors use were highlighted, thus demonstrating that Wnt/β-catenin pathway might be considered as a therapeutic target to prevent adverse LV remodeling and heart failure following MI.

Adenosine receptors as promising targets for the management of ocular diseases

The ocular drug discovery arena has undergone a significant improvement in the last few years culminating in the FDA approvals of 8 new drugs. However, despite a large number of drugs, generics, and combination products available, it remains an urgent need to find breakthrough strategies and therapies for tackling ocular diseases. Targeting the adenosinergic system may represent an innovative strategy for discovering new ocular therapeutics. This review focused on the recent advance in the field and described the numerous nucleoside and non-nucleoside modulators of the four adenosine receptors (ARs) used as potential tools or clinical drug candidates.

"Adenosine an old player with new possibilities in kidney diseases": Preclinical evidences and clinical perspectives

Renal injury might originate from multiple factors like ischemia reperfusion (I/R), drug toxicity, cystic fibrosis, radio contrast agent etc. The four adenosine receptor subtypes have been identified and found to show diverse physiological and pathological roles in kidney diseases. The activation of A₁ adenosine receptor (A₁) protects against acute kidney injury by improving renal hemodynamic alterations, decreasing tubular necrosis and its inhibition might facilitate removal of toxin or drug metabolite in chronic kidney disease models. Furthermore, recent findings revealed that A_2A receptor subtype activation regulates macrophage phenotype in experimental models of nephritis. Interestingly the emerging role of adenosine kinase inhibitors in kidney diseases has been discussed which act by increasing adenosine availability at target sites and thereby promote A_2A receptor stimulation. In addition, the least explored adenosine receptor subtype A₃ inhibition was observed to exert anti- oxidant, immunosuppressive and anti-fibrotic effects, but more studies are required to confirm its benefits in other renal injury models. The clinical studies targeting A₁ receptor in patients with pre-existing kidney disease have yielded disappointing results, perhaps owing to the origin of unexpected neurological complications during the course of trial. Importantly, conducting well designed clinical trials and testing adenosine modulators with lesser brain penetrability could clear the way for clinical approval of these agents for patients with renal functional impairments.

Targeting of G-protein coupled receptors in sepsis

The Third International Consensus Definitions (Sepsis-3) define sepsis as life-threatening multi-organ dysfunction caused by a dysregulated host response to infection. Sepsis can progress to septic shock-an even more lethal condition associated with profound circulatory, cellular and metabolic abnormalities. Septic shock remains a leading cause of death in intensive care units and carries a mortality of almost 25%. Despite significant advances in our understanding of the pathobiology of sepsis, therapeutic interventions have not translated into tangible differences in the overall outcome for patients. Clinical trials of antagonists of various pro-inflammatory mediators in sepsis have been largely unsuccessful in the past. Given the diverse physiologic roles played by G-protein coupled receptors (GPCR), modulation of GPCR signaling for the treatment of sepsis has also been explored. Traditional pharmacologic approaches have mainly focused on ligands targeting the extracellular domains of GPCR. However, novel techniques aimed at modulating GPCR intracellularly through aptamers, pepducins and intrabodies have opened a fresh avenue of therapeutic possibilities. In this review, we summarize the diverse roles played by various subfamilies of GPCR in the pathogenesis of sepsis and identify potential targets for pharmacotherapy through these novel approaches.

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.

Focusing on Adenosine Receptors as a Potential Targeted Therapy in Human Diseases

Adenosine is involved in a range of physiological and pathological effects through membrane-bound receptors linked to G proteins. There are four subtypes of adenosine receptors, described as A1AR, A2AAR, A2BAR, and A3AR, which are the center of cAMP signal pathway-based drug development. Several types of agonists, partial agonists or antagonists, and allosteric substances have been synthesized from these receptors as new therapeutic drug candidates. Research efforts surrounding A1AR and A2AAR are perhaps the most enticing because of their concentration and affinity; however, as a consequence of distressing conditions, both A2BAR and A3AR levels might accumulate. This review focuses on the biological features of each adenosine receptor as the basis of ligand production and describes clinical studies of adenosine receptor-associated pharmaceuticals in human diseases.

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.

Targeting GPCRs Against Cardiotoxicity Induced by Anticancer Treatments

Novel anticancer medicines, including targeted therapies and immune checkpoint inhibitors, have greatly improved the management of cancers. However, both conventional and new anticancer treatments induce cardiac adverse effects, which remain a critical issue in clinic. Cardiotoxicity induced by anti-cancer treatments compromise vasospastic and thromboembolic ischemia, dysrhythmia, hypertension, myocarditis, and cardiac dysfunction that can result in heart failure. Importantly, none of the strategies to prevent cardiotoxicity from anticancer therapies is completely safe and satisfactory. Certain clinically used cardioprotective drugs can even contribute to cancer induction. Since G protein coupled receptors (GPCRs) are target of forty percent of clinically used drugs, here we discuss the newly identified cardioprotective agents that bind GPCRs of adrenalin, adenosine, melatonin, ghrelin, galanin, apelin, prokineticin and cannabidiol. We hope to provoke further drug development studies considering these GPCRs as potential targets to be translated to treatment of human heart failure induced by anticancer drugs.

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.

G-Protein-Coupled Receptors in Heart Disease

GPCRs (G-protein [guanine nucleotide-binding protein]-coupled receptors) play a central physiological role in the regulation of cardiac function in both health and disease and thus represent one of the largest class of surface receptors targeted by drugs. Several antagonists of GPCRs, such as βARs (β-adrenergic receptors) and Ang II (angiotensin II) receptors, are now considered standard of therapy for a wide range of cardiovascular disease, such as hypertension, coronary artery disease, and heart failure. Although the mechanism of action for GPCRs was thought to be largely worked out in the 80s and 90s, recent discoveries have brought to the fore new and previously unappreciated mechanisms for GPCR activation and subsequent downstream signaling. In this review, we focus on GPCRs most relevant to the cardiovascular system and discuss traditional components of GPCR signaling and highlight evolving concepts in the field, such as ligand bias, β-arrestin-mediated signaling, and conformational heterogeneity.

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.

Effect of Neladenoson Bialanate on Exercise Capacity Among Patients With Heart Failure With Preserved Ejection Fraction: A Randomized Clinical Trial

IMPORTANCE

Heart failure with preserved ejection fraction (HFpEF) lacks effective treatments. Based on preclinical studies, neladenoson bialanate, a first-in-class partial adenosine A1 receptor agonist, has the potential to improve several heart failure-related cardiac and noncardiac abnormalities but has not been evaluated to treat HFpEF.

OBJECTIVES

To determine whether neladenoson improves exercise capacity, physical activity, cardiac biomarkers, and quality of life in patients with HFpEF and to find the optimal dose.

DESIGN, SETTING, AND PARTICIPANTS

Phase 2b randomized clinical trial conducted at 76 centers in the United States, Europe, and Japan. Patients (N = 305) with New York Heart Association class II or III HFpEF with elevated natriuretic peptide levels were enrolled between May 10, 2017, and December 7, 2017 (date of final follow-up: June 20, 2018).

INTERVENTIONS

Participants were randomized (1:2:2:2:2:3) to neladenoson (n = 27 [5 mg], n = 50 [10 mg], n = 51 [20 mg], n = 50 [30 mg], and n = 51 [40 mg]) or matching placebo (n = 76) for 20 weeks of treatment.

MAIN OUTCOMES AND MEASURES

The primary end point was change in 6-minute walk test distance from baseline to 20 weeks (minimal clinically important difference, 40 m). Key safety measures included bradyarrhythmias and adverse events. To evaluate the effects of varying doses of neladenoson, a multiple comparison procedure with 5 modeling techniques (linear, Emax, 2 variations of sigmoidal Emax, and quadratic) was used to evaluate diverse dose-response profiles.

RESULTS

Among 305 patients who were randomized (mean age, 74 years; 160 [53%] women; mean 6-minute walk test distance, 321.5 m), 261 (86%) completed the trial and were included in the primary analysis. After 20 weeks of treatment, the mean absolute changes from baseline in 6-minute walk test distance were 0.2 m (95% CI, -12.1 to 12.4 m) for the placebo group; 19.4 m (95% CI, -10.8 to 49.7 m) for the 5 mg of neladenoson group; 29.4 m (95% CI, 3.0 to 55.8 m) for 10 mg of neladenoson group; 13.8 m (95% CI, -2.3 to 29.8 m) for 20 mg of neladenoson group; 16.3 m (95% CI, -1.1 to 33.6 m) for 30 mg of neladenoson group; and 13.0 m (95% CI, -5.9 to 31.9 m) for 40 mg of neladenoson group. Because none of the neladenoson groups achieved the clinically relevant 40-m increase in 6-minute walk test distance from baseline, an optimal dose of neladenoson was not identified. There was no significant dose-response relationship for the change in 6-minute walk test distance among the 5 different dose-response models (P = .05 for Emax; P = .18 for quadratic; P = .21 for sigmoidal Emax 1; P = .39 for linear; and P = .52 for sigmoidal Emax 2). Serious adverse events were similar among the neladenoson groups (61/229 [26.6%]) and the placebo group (21/76 [27.6%]).

CONCLUSIONS AND RELEVANCE

Among patients with HFpEF, there was no significant dose-response relationship detected for neladenoson with regard to the change in exercise capacity from baseline to 20 weeks. In light of these findings, novel approaches will be needed if further development of neladenoson for the treatment of patients with HFpEF is pursued.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT03098979.

Historical and Current Adenosine Receptor Agonists in Preclinical and Clinical Development

Adenosine receptors (ARs) function in the body’s response to conditions of pathology and stress associated with a functional imbalance, such as in the supply and demand of energy/oxygen/nutrients. Extracellular adenosine concentrations vary widely to raise or lower the basal activation of four subtypes of ARs. Endogenous adenosine can correct an energy imbalance during hypoxia and other stress, for example, by slowing the heart rate by A₁AR activation or increasing the blood supply to heart muscle by the A_2AAR. Moreover, exogenous AR agonists, antagonists, or allosteric modulators can be applied for therapeutic benefit, and medicinal chemists working toward that goal have reported thousands of such agents. Thus, numerous clinical trials have ensued, using promising agents to modulate adenosinergic signaling, most of which have not succeeded. Currently, short-acting, parenteral agonists, adenosine and Regadenoson, are the only AR agonists approved for human use. However, new concepts and compounds are currently being developed and applied toward preclinical and clinical evaluation, and initial results are encouraging. This review focuses on key compounds as AR agonists and positive allosteric modulators (PAMs) for disease treatment or diagnosis. AR agonists for treating inflammation, pain, cancer, non-alcoholic steatohepatitis, angina, sickle cell disease, ischemic conditions and diabetes have been under development. Multiple clinical trials with two A₃AR agonists are ongoing.

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.

Historical and Current Adenosine Receptor Agonists in Preclinical and Clinical Development

Adenosine receptors (ARs) function in the body's response to conditions of pathology and stress associated with a functional imbalance, such as in the supply and demand of energy/oxygen/nutrients. Extracellular adenosine concentrations vary widely to raise or lower the basal activation of four subtypes of ARs. Endogenous adenosine can correct an energy imbalance during hypoxia and other stress, for example, by slowing the heart rate by A₁AR activation or increasing the blood supply to heart muscle by the A_2AAR. Moreover, exogenous AR agonists, antagonists, or allosteric modulators can be applied for therapeutic benefit, and medicinal chemists working toward that goal have reported thousands of such agents. Thus, numerous clinical trials have ensued, using promising agents to modulate adenosinergic signaling, most of which have not succeeded. Currently, short-acting, parenteral agonists, adenosine and Regadenoson, are the only AR agonists approved for human use. However, new concepts and compounds are currently being developed and applied toward preclinical and clinical evaluation, and initial results are encouraging. This review focuses on key compounds as AR agonists and positive allosteric modulators (PAMs) for disease treatment or diagnosis. AR agonists for treating inflammation, pain, cancer, non-alcoholic steatohepatitis, angina, sickle cell disease, ischemic conditions and diabetes have been under development. Multiple clinical trials with two A₃AR agonists are ongoing.

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.

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

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.