Seven days treatment with the angiotensin II type 2 receptor agonist C21 in hospitalized COVID-19 patients; a placebo-controlled randomised multi-centre double-blind phase 2 trial

New insights into kidney disease after COVID-19 infection and vaccination: histopathological and clinical findings

In addition to its pulmonary effects, coronavirus disease 2019 (COVID-19) has also been found to cause acute kidney injury (AKI), which has been linked to high mortality rates. In this review, we collected data from 20 clinical studies on post-COVID-19-related AKI and 97 cases of AKI associated with COVID-19 vaccination. Acute tubular injury was by far the most common finding in the kidneys of patients with COVID-19-related AKI. Among patients hospitalized for COVID-19, 34.0% developed AKI, of which 59.0%, 19.1% and 21.9% were Stages 1, 2 and 3, respectively. Though kidney disease and other adverse effects after COVID-19 vaccination overall appear rare, case reports have accumulated suggesting that COVID-19 vaccination may be associated with a risk of subsequent kidney disease. Among the patients with post-vaccination AKI, the most common pathologic findings include crescentic glomerulonephritis (29.9%), acute tubular injury (23.7%), IgA nephropathy (18.6%), antineutrophil cytoplasmic autoantibody-associated vasculitis (17.5%), minimal change disease (17.5%) and thrombotic microangiopathy (10.3%). It is important to note that crescentic glomerulonephritis appears to be more prevalent in patients who have newly diagnosed renal involvement. The proportions of patients with AKI Stages 1, 2 and 3 after COVID-19 vaccination in case reports were 30.9%, 22.7% and 46.4%, respectively. In general, clinical cases of new-onset and recurrent nephropathy with AKI after COVID-19 vaccination have a positive prognosis. In this article, we also explore the underlying pathophysiological mechanisms of AKI associated with COVID-19 infection and its vaccination by describing key renal morphological and clinical features and prognostic findings.

Alternative Renin-Angiotensin System

The renin-angiotensin system is the most important peptide hormone system in the regulation of cardiovascular homeostasis. Its classical arm consists of the enzymes, renin, and angiotensin-converting enzyme, generating angiotensin II from angiotensinogen, which activates its AT1 receptor, thereby increasing blood pressure, retaining salt and water, and inducing cardiovascular hypertrophy and fibrosis. However, angiotensin II can also activate a second receptor, the AT2 receptor. Moreover, the removal of the C-terminal phenylalanine from angiotensin II by ACE2 (angiotensin-converting enzyme 2) yields angiotensin-(1-7), and this peptide interacts with its receptor Mas. When the aminoterminal Asp of angiotensin-(1-7) is decarboxylated, alamandine is generated, which activates the Mas-related G-protein-coupled receptor D, MrgD (Mas-related G-protein-coupled receptor type D). Since Mas, MrgD, and the AT2 receptor have opposing effects to the classical AT1 receptor, they and the enzymes and peptides activating them are called the alternative or protective arm of the renin-angiotensin system. This review will cover the historical aspects and the current standing of this recent addition to the biology of the renin-angiotensin system.

Biomarkers of early SARS-CoV-2 infection before the onset of respiratory symptoms

OBJECTIVES

Currently, limited data exist regarding the pathological changes occurring during the incubation phase of SARS-CoV-2 infection. We utilized proteomic analysis to explore changes in the circulatory host response in individuals with SARS-CoV-2 infection before the onset of symptoms.

METHODS

Participants were individuals from a randomized clinical trial of prophylaxis for COVID-19 in a workers' dormitory. Proteomic signatures of blood samples collected within 7 days before symptom onset (incubation group) were compared with those collected >21 days (non-incubation group) to derive candidate biomarkers of incubation. Candidate biomarkers were assessed by comparing levels in the incubation group with both infected individuals (positive controls) and non-infected individuals (negative controls).

RESULTS

The study included men (mean age 34.2 years and standard deviation 7.1) who were divided into three groups: an incubation group consisting of 44 men, and two control groups-positive (n = 56) and negative (n = 67) controls. Through proteomic analysis, we identified 49 proteins that, upon pathway analyses, indicated an upregulation of the renin-angiotensin and innate immune systems during the virus incubation period. Biomarker analyses revealed increased concentrations of plasma angiotensin II (mean 731 vs. 139 pg/mL), angiotensin (1-7) (302 vs. 9 pg/mL), CXCL10 (423 vs. 85 pg/mL), CXCL11 (82.7 vs. 32.1 pg/mL), interferon-gamma (0.49 vs. 0.20 pg/mL), legumain (914 vs. 743 pg/mL), galectin-9 (1443 vs. 836 pg/mL), and tumour necrosis factor (20.3 vs. 17.0 pg/mL) during virus incubation compared with non-infected controls (all p < 0.05). Plasma angiotensin (1-7) exhibited a significant increase before the onset of symptoms when compared with uninfected controls (area under the curve 0.99, sensitivity 0.97, and specificity 0.99).

DISCUSSION

Angiotensin (1-7) could play a crucial role in the progression of symptomatic COVID-19 infection, and its assessment could help identify individuals who would benefit from enhanced monitoring and early antiviral intervention.

Angiotensin II type 2 receptor agonist attenuates LPS-induced acute lung injury through modulating THP-1-derived macrophage reprogramming

Acute respiratory distress syndrome (ARDS) is a devastating respiratory disorder, characterized by overwhelming inflammation in the alveoli without effective pharmacological treatment. We aimed to investigate the effect and mechanism of angiotensin II type 2 receptor (AT2R) agonist, Compound 21 (C21), on the lipopolysaccharide (LPS)-induced acute lung injury (ALI) model. The protective effect of C21 was evaluated via enzyme-linked immunosorbent assay (ELISA), Western blot (WB), real-time PCR, and fluorescence microscopy in LPS-challenged THP1-derived macrophages. Besides, the in vivo efficacy of C21 was assessed using cell counting, ELISA, protein quantification, hematoxylin-eosin (H&E) staining, and WB in an LPS-induced ALI mouse model. The results showed that C21 significantly inhibited the secretion of pro-inflammatory cytokines (CCL-2, IL-6), overproduction of intracellular ROS, and activation of inflammatory pathways (NF-κB/NLRP3, p38/MAPK) in THP-1 cell-derived macrophages stimulated by LPS. In in vivo study, intraperitoneal injection of C21 could reduce airway leukocytes accumulation and chemokine/cytokine (keratinocyte chemoattractant (KC), IL-6) generation, as well as alleviate diffuse alveolar damage induced by LPS. Conclusively, the AT2R agonist C21 significantly inhibited LPS-stimulated excess inflammatory responses and oxidative stress in macrophages. Meanwhile, C21 could effectively alleviate acute inflammation and tissue damage in the lungs of ALI mice challenged by LPS. The results of this study bring new hope for the early treatment of ALI/ARDS.

Perspectives on Post-COVID-19 Pulmonary Fibrosis Treatment

Pulmonary fibrosis, a critical outcome of chronic inflammatory diseases, has gained prominence in the context of post-coronavirus (post-COVID-19) complications. This review delves into the multifaceted landscape of post-COVID-19 pulmonary fibrosis, elucidating the intricate molecular mechanisms underlying its pathogenesis and highlighting promising therapeutic avenues. Examining the aftermath of severe acute respiratory syndrome-2 (SARS-CoV-2) infection, the review reveals key signaling pathways implicated in the fibrotic cascade. Drawing parallels with previous coronavirus outbreaks enhances our understanding of the distinctive features of post-COVID-19 fibrosis. Antifibrotic drugs, like pirfenidone and nintedanib, take center stage; their mechanisms of action and potential applications in post-COVID-19 cases are thoroughly explored. Beyond the established treatments, this review investigates emerging therapeutic modalities, including anti-interleukin agents, immunosuppressants, and experimental compounds, like buloxybutide, saracatinib, sirolimus, and resveratrol. Emphasizing the critical importance of early intervention, this review highlights the dynamic nature of post-COVID-19 pulmonary fibrosis research. In conclusion, the synthesis of current knowledge offers a foundation for advancing our approaches to the prevention and treatment of these consequential sequelae of COVID-19.

Catch your breath: The protective role of the angiotensin AT2 receptor for the treatment of idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive interstitial lung disease whereby excessive deposition of extracellular matrix proteins (ECM) ultimately leads to respiratory failure. While there have been advances in pharmacotherapies for pulmonary fibrosis, IPF remains an incurable and irreversible disease. There remains an unmet clinical need for treatments that reverse fibrosis, or at the very least have a more tolerable side effect profile than currently available treatments. Transforming growth factor β1(TGFβ1) is considered the main driver of fibrosis in IPF. However, as our understanding of the role of the pulmonary renin-angiotensin system (PRAS) in the pathogenesis of IPF increases, it is becoming clear that targeting angiotensin receptors represents a potential novel treatment strategy for IPF - in particular, via activation of the anti-fibrotic angiotensin type 2 receptor (AT2R). This review describes the current understanding of the pathophysiology of IPF and the mediators implicated in its pathogenesis; focusing on TGFβ1, angiotensin II and related peptides in the PRAS and their contribution to fibrotic processes in the lung. Preclinical and clinical assessment of currently available AT2R agonists and the development of novel, highly selective ligands for this receptor will also be described, with a focus on compound 21, currently in clinical trials for IPF. Collectively, this review provides evidence of the potential of AT2R as a novel therapeutic target for IPF.

Coronavirus disease 2019 and acute cerebrovascular events: a comprehensive overview

Since the Corona Virus Disease 2019 (COVID-19) pandemic, there has been increasing evidence that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is associated with acute cerebrovascular events such as cerebral infarction, cerebral hemorrhage, and cerebral venous thrombosis. Although the mechanism of cerebrovascular complications among COVID-19 patients has not been adequately elucidated, the hypercoagulable state, excessive inflammation and ACE-2-associated alterations in the renin-angiotensin-aldosterone system after SARS-CoV-2 infection probably play an essential role. In this overview, we discuss the possible mechanisms underlying the SARS-CoV-2 infection leading to acute cerebrovascular events and review the characteristics of COVID-19-related acute cerebrovascular events cases and treatment options available worldwide.

Protein structure-based in-silico approaches to drug discovery: Guide to COVID-19 therapeutics

With more than 5 million fatalities and close to 300 million reported cases, COVID-19 is the first documented pandemic due to a coronavirus that continues to be a major health challenge. Despite being rapid, uncontrollable, and highly infectious in its spread, it also created incentives for technology development and redefined public health needs and research agendas to fast-track innovations to be translated. Breakthroughs in computational biology peaked during the pandemic with renewed attention to making all cutting-edge technology deliver agents to combat the disease. The demand to develop effective treatments yielded surprising collaborations from previously segregated fields of science and technology. The long-standing pharmaceutical industry's aversion to repurposing existing drugs due to a lack of exponential financial gain was overrun by the health crisis and pressures created by front-line researchers and providers. Effective vaccine development even at an unprecedented pace took more than a year to develop and commence trials. Now the emergence of variants and waning protections during the booster shots is resulting in breakthrough infections that continue to strain health care systems. As of now, every protein of SARS-CoV-2 has been structurally characterized and related host pathways have been extensively mapped out. The research community has addressed the druggability of a multitude of possible targets. This has been made possible due to existing technology for virtual computer-assisted drug development as well as new tools and technologies such as artificial intelligence to deliver new leads. Here in this article, we are discussing advances in the drug discovery field related to target-based drug discovery and exploring the implications of known target-specific agents on COVID-19 therapeutic management. The current scenario calls for more personalized medicine efforts and stratifying patient populations early on for their need for different combinations of prognosis-specific therapeutics. We intend to highlight target hotspots and their potential agents, with the ultimate goal of using rational design of new therapeutics to not only end this pandemic but also uncover a generalizable platform for use in future pandemics.

Cardiovascular Protective Effects of NP-6A4, a Drug with the FDA Designation for Pediatric Cardiomyopathy, in Female Rats with Obesity and Pre-Diabetes

BACKGROUND

Obese and pre-diabetic women have a higher risk for cardiovascular death than age-matched men with the same symptoms, and there are no effective treatments. We reported that obese and pre-diabetic female Zucker Diabetic Fatty (ZDF-F) rats recapitulate metabolic and cardiac pathology of young obese and pre-diabetic women and exhibit suppression of cardio-reparative AT2R. Here, we investigated whether NP-6A4, a new AT2R agonist with the FDA designation for pediatric cardiomyopathy, mitigate heart disease in ZDF-F rats by restoring AT2R expression.

METHODS

ZDF-F rats on a high-fat diet (to induce hyperglycemia) were treated with saline, NP-6A4 (10 mg/kg/day), or NP-6A4 + PD123319 (AT2R-specific antagonist, 5 mg/kg/day) for 4 weeks (n = 21). Cardiac functions, structure, and signaling were assessed by echocardiography, histology, immunohistochemistry, immunoblotting, and cardiac proteome analysis.

RESULTS

NP-6A4 treatment attenuated cardiac dysfunction, microvascular damage (-625%) and cardiomyocyte hypertrophy (-263%), and increased capillary density (200%) and AT2R expression (240%) (p < 0.05). NP-6A4 activated a new 8-protein autophagy network and increased autophagy marker LC3-II but suppressed autophagy receptor p62 and autophagy inhibitor Rubicon. Co-treatment with AT2R antagonist PD123319 suppressed NP-6A4's protective effects, confirming that NP-6A4 acts through AT2R. NP-6A4-AT2R-induced cardioprotection was independent of changes in body weight, hyperglycemia, hyperinsulinemia, or blood pressure.

CONCLUSIONS

Cardiac autophagy impairment underlies heart disease induced by obesity and pre-diabetes, and there are no drugs to re-activate autophagy. We propose that NP-6A4 can be an effective drug to reactivate cardiac autophagy and treat obesity- and pre-diabetes-induced heart disease, particularly for young and obese women.

Renin-Angiotensin System and Sex Differences in COVID-19: A Critical Assessment

The current epidemic of corona virus disease (COVID-19) has resulted in an immense health burden that became the third leading cause of death and potentially contributed to a decline in life expectancy in the United States. The severe acute respiratory syndrome-related coronavirus-2 binds to the surface-bound peptidase angiotensin-converting enzyme 2 (ACE2, EC 3.4.17.23) leading to tissue infection and viral replication. ACE2 is an important enzymatic component of the renin-angiotensin system (RAS) expressed in the lung and other organs. The peptidase regulates the levels of the peptide hormones Ang II and Ang-(1-7), which have distinct and opposing actions to one another, as well as other cardiovascular peptides. A potential consequence of severe acute respiratory syndrome-related coronavirus-2 infection is reduced ACE2 activity by internalization of the viral-ACE2 complex and subsequent activation of the RAS (higher ratio of Ang II:Ang-[1-7]) that may exacerbate the acute inflammatory events in COVID-19 patients and possibly contribute to the effects of long COVID-19. Moreover, COVID-19 patients present with an array of autoantibodies to various components of the RAS including the peptide Ang II, the enzyme ACE2, and the AT1 AT2 and Mas receptors. Greater disease severity is also evident in male COVID-19 patients, which may reflect underlying sex differences in the regulation of the 2 distinct functional arms of the RAS. The current review provides a critical evaluation of the evidence for an activated RAS in COVID-19 subjects and whether this system contributes to the greater severity of severe acute respiratory syndrome-related coronavirus-2 infection in males as compared with females.

Effects of the Oral Angiotensin II Type 2 Receptor Agonist C21 in Sugen-Hypoxia Induced Pulmonary Hypertension in Rats

Substantial evidence supports the involvement of the renin-angiotensin system in pulmonary hypertension (PH), and the angiotensin II type 2 receptor (AT₂R) is known to exert tissue protective actions. The effect of the selective AT₂R agonist C21 (also known as Compound 21 or buloxibutid) was evaluated in the rat Sugen-hypoxia PH model. After a single injection of Sugen 5416 and hypoxia for 21 days, C21 (2 or 20 mg/kg) or vehicle was administered perorally twice daily from Day 21 to Day 55. On Day 56, hemodynamic assessments were performed, and lung and heart tissue were prepared for quantification of cardiac and vascular remodeling and fibrosis. Treatment with C21 20 mg/kg improved cardiac output and stroke volume and decreased right ventricular hypertrophy (all p < 0.05). Treatment with C21 2 mg/kg significantly decreased vessel wall and muscular layer thickness and increased the luminal opening in vessels >100 μm (all p < 0.05). There were no significant differences between the two C21 doses on any parameter, and post hoc analyses comparing the merged C21 groups with the vehicle group showed that C21 treatment reduced vascular remodeling (reduced endothelial proliferation and thickening of the vascular wall) in vessels of all sizes; moreover, the diastolic pulmonary artery pressure and right ventricular pressure were reduced along with reduction of right ventricular hypertrophy. Sugen 5416 and hypoxia increased pulmonary collagen deposition, which was counteracted by C21 20 mg/kg. In conclusion, the effects of C21 on vascular remodeling, hemodynamic alterations, and fibrosis suggest that AT₂R agonists may have a role in Group 1 and 3 PH treatment.

Renin-Angiotensin System Modulation With Synthetic Angiotensin (1-7) and Angiotensin II Type 1 Receptor-Biased Ligand in Adults With COVID-19: Two Randomized Clinical Trials

Importance

Preclinical models suggest dysregulation of the renin-angiotensin system (RAS) caused by SARS-CoV-2 infection may increase the relative activity of angiotensin II compared with angiotensin (1-7) and may be an important contributor to COVID-19 pathophysiology.

Objective

To evaluate the efficacy and safety of RAS modulation using 2 investigational RAS agents, TXA-127 (synthetic angiotensin [1-7]) and TRV-027 (an angiotensin II type 1 receptor-biased ligand), that are hypothesized to potentiate the action of angiotensin (1-7) and mitigate the action of the angiotensin II.

Design, Setting, and Participants

Two randomized clinical trials including adults hospitalized with acute COVID-19 and new-onset hypoxemia were conducted at 35 sites in the US between July 22, 2021, and April 20, 2022; last follow-up visit: July 26, 2022.

Interventions

A 0.5-mg/kg intravenous infusion of TXA-127 once daily for 5 days or placebo. A 12-mg/h continuous intravenous infusion of TRV-027 for 5 days or placebo.

Main Outcomes and Measures

The primary outcome was oxygen-free days, an ordinal outcome that classifies a patient's status at day 28 based on mortality and duration of supplemental oxygen use; an adjusted odds ratio (OR) greater than 1.0 indicated superiority of the RAS agent vs placebo. A key secondary outcome was 28-day all-cause mortality. Safety outcomes included allergic reaction, new kidney replacement therapy, and hypotension.

Results

Both trials met prespecified early stopping criteria for a low probability of efficacy. Of 343 patients in the TXA-127 trial (226 [65.9%] aged 31-64 years, 200 [58.3%] men, 225 [65.6%] White, and 274 [79.9%] not Hispanic), 170 received TXA-127 and 173 received placebo. Of 290 patients in the TRV-027 trial (199 [68.6%] aged 31-64 years, 168 [57.9%] men, 195 [67.2%] White, and 225 [77.6%] not Hispanic), 145 received TRV-027 and 145 received placebo. Compared with placebo, both TXA-127 (unadjusted mean difference, -2.3 [95% CrI, -4.8 to 0.2]; adjusted OR, 0.88 [95% CrI, 0.59 to 1.30]) and TRV-027 (unadjusted mean difference, -2.4 [95% CrI, -5.1 to 0.3]; adjusted OR, 0.74 [95% CrI, 0.48 to 1.13]) resulted in no difference in oxygen-free days. In the TXA-127 trial, 28-day all-cause mortality occurred in 22 of 163 patients (13.5%) in the TXA-127 group vs 22 of 166 patients (13.3%) in the placebo group (adjusted OR, 0.83 [95% CrI, 0.41 to 1.66]). In the TRV-027 trial, 28-day all-cause mortality occurred in 29 of 141 patients (20.6%) in the TRV-027 group vs 18 of 140 patients (12.9%) in the placebo group (adjusted OR, 1.52 [95% CrI, 0.75 to 3.08]). The frequency of the safety outcomes was similar with either TXA-127 or TRV-027 vs placebo.

Conclusions and Relevance

In adults with severe COVID-19, RAS modulation (TXA-127 or TRV-027) did not improve oxygen-free days vs placebo. These results do not support the hypotheses that pharmacological interventions that selectively block the angiotensin II type 1 receptor or increase angiotensin (1-7) improve outcomes for patients with severe COVID-19.

Trial Registration

ClinicalTrials.gov Identifier: NCT04924660.

The angiotensin AT2-receptor agonist compound 21 is an antagonist for the thromboxane TP-receptor - Implications for preclinical studies and future clinical use

Since the AT₂-receptor (AT₂R) agonist C21 has structural similarity to the AT₁-receptor antagonists Irbesartan and Losartan, which are antagonists not only at the AT₁R, but also at thromboxane TP-receptors, we tested the hypothesis that C21 has TP-receptor antagonistic properties as well. Isolated mouse mesenteric arteries from C57BL/6 J and AT₂R-knockout mice (AT₂R^-/y) were mounted in wire myographs, contracted with either phenylephrine or the thromboxane A₂ (TXA₂) analogue U46619, and the relaxing effect of C21 (0.1 nM - 10 µM) was investigated. The effect of C21 on U46619-induced platelet aggregation was measured by an impedance aggregometer. Direct interaction of C21 with TP-receptors was determined by an β-arrestin biosensor assay. C21 caused significant, concentration-dependent relaxations in phenylephrine- and U46619-contracted mesenteric arteries from C57BL/6 J mice. The relaxing effect of C21 was absent in phenylephrine-contracted arteries from AT₂R^-/y mice, whereas it was unchanged in U46619-contracted arteries from AT₂R^-/y mice. C21 inhibited U46619-stimulated aggregation of human platelets, which was not inhibited by the AT₂R-antagonist PD123319. C21 reduced U46619-induced recruitment of β-arrestin to human thromboxane TP-receptors with a calculated K_i of 3.74 µM. We conclude that in addition to AT₂R-agonistic properties, C21 also acts as low-affinity TP-receptor antagonist, and that - depending on the constrictor - both mechanisms can be responsible for C21-induced vasorelaxation. Furthermore, by acting as a TP-receptor antagonist, C21 inhibits platelet aggregation. These findings are important for understanding potential off-target effects of C21 in the preclinical and clinical context and for the interpretation of C21-related myography data in assays with TXA₂-analogues as constrictor.

Comparison of Preprint Postings of Randomized Clinical Trials on COVID-19 and Corresponding Published Journal Articles: A Systematic Review

IMPORTANCE

Randomized clinical trials (RCTs) on COVID-19 are increasingly being posted as preprints before publication in a scientific, peer-reviewed journal.

OBJECTIVE

To assess time to journal publication for COVID-19 RCT preprints and to compare differences between pairs of preprints and corresponding journal articles.

EVIDENCE REVIEW

This systematic review used a meta-epidemiologic approach to conduct a literature search using the World Health Organization COVID-19 database and Embase to identify preprints published between January 1 and December 31, 2021. This review included RCTs with human participants and research questions regarding the treatment or prevention of COVID-19. For each preprint, a literature search was done to locate the corresponding journal article. Two independent reviewers read the full text, extracted data, and assessed risk of bias using the Cochrane Risk of Bias 2 tool. Time to publication was analyzed using a Cox proportional hazards regression model. Differences between preprint and journal article pairs in terms of outcomes, analyses, results, or conclusions were described. Statistical analysis was performed on October 17, 2022.

FINDINGS

This study included 152 preprints. As of October 1, 2022, 119 of 152 preprints (78.3%) had been published in journals. The median time to publication was 186 days (range, 17-407 days). In a multivariable model, larger sample size and low risk of bias were associated with journal publication. With a sample size of less than 200 as the reference, sample sizes of 201 to 1000 and greater than 1000 had hazard ratios (HRs) of 1.23 (95% CI, 0.80-1.91) and 2.19 (95% CI, 1.36-3.53) for publication, respectively. With high risk of bias as the reference, medium-risk articles with some concerns for bias had an HR of 1.77 (95% CI, 1.02-3.09); those with a low risk of bias had an HR of 3.01 (95% CI, 1.71-5.30). Of the 119 published preprints, there were differences in terms of outcomes, analyses, results, or conclusions in 65 studies (54.6%). The main conclusion in the preprint contradicted the conclusion in the journal article for 2 studies (1.7%).

CONCLUSIONS AND RELEVANCE

These findings suggest that there is a substantial time lag from preprint posting to journal publication. Preprints with smaller sample sizes and high risk of bias were less likely to be published. Finally, although differences in terms of outcomes, analyses, results, or conclusions were observed for preprint and journal article pairs in most studies, the main conclusion remained consistent for the majority of studies.

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Angiotensin-(1-9) in hypertension

Angiotensin-(1-9) [Ang-(1-9)] is a peptide of the non-canonical renin-angiotensin system (RAS) synthesized from angiotensin I by the monopeptidase angiotensin-converting enzyme type 2 (ACE2). Using osmotic minipumps, infusion of Ang-(1-9) consistently reduces blood pressure in several rat hypertension models. In these animals, hypertension-induced end-organ damage is also decreased. Several pieces of evidence suggest that Ang-(1-9) is the endogenous ligand that binds and activates the type-2 angiotensin II receptor (AT2R). Activation of AT2R triggers different tissue-specific signaling pathways. This phenomenon could be explained by the ability of AT2R to form different heterodimers with other G protein-coupled receptors. Because of the antihypertensive and protective effects of AT2R activation by Ang-(1-9), associated with a short half-life of RAS peptides, several synthetic AT2R agonists have been synthesized and assayed. Some of them, particularly CGP42112, C21 and novokinin, have demonstrated antihypertensive properties. Only two synthetic AT2R agonists, C21 and LP2-3, have been tested in clinical trials, but none of them like an antihypertensive. Therefore, Ang-(1-9) is a promising antihypertensive drug that reduces hypertension-induced end-organ damage. However, further research is required to translate this finding successfully to the clinic.

2-Alkyl substituted benzimidazoles as a new class of selective AT2 receptor ligands

Ligands comprising a benzimidazole rather than the imidazole ring that is common in AT₂R ligands e.g. in the AT₂R agonist C21, can provide both high affinity and receptor selectivity. In particular, compounds encompassing benzimidazoles, substituted in the 2-position with small bulky groups such as an isopropyl (K_i = 4.0 nM) or a tert-butyl (K_i = 5.3 nM) or alternatively a thiazole heterocycle (K_i = 5.1 nM) demonstrate high affinity and AT₂R selectivity. An n-butyl chain, as found in the AT₁R selective sartans, makes the ligand less receptor selective. The isobutyl group on the biaryl scaffold present in most AT₂R selective ligands reported so far was originally derived from the nonselective potent AT₁R/AT₂R ligand L-162,313. Notably, in all ligands discussed herein, the isobutyl group was substituted by an n-propyl group and ligands with high affinity to AT₂R were provided and in addition the majority of them demonstrate a favorable AT₂R/AT₁R selectivity. The introduction of fluoro atoms in various positions had no pronounced effect on the affinity data. Ligands with a thiazole or a tert-butyl group attached to the 2-position and with a terminal trifluoromethyl butoxycarbonyl sidechain exhibited a similar stability as C21 in human liver microsomes, while other ligands examined were less stable in the microsome assay.

Kidney Angiotensin in Cardiovascular Disease: Formation and Drug Targeting

The concept of local formation of angiotensin II in the kidney has changed over the last 10-15 years. Local synthesis of angiotensinogen in the proximal tubule has been proposed, combined with prorenin synthesis in the collecting duct. Binding of prorenin via the so-called (pro)renin receptor has been introduced, as well as megalin-mediated uptake of filtered plasma-derived renin-angiotensin system (RAS) components. Moreover, angiotensin metabolites other than angiotensin II [notably angiotensin-(1-7)] exist, and angiotensins exert their effects via three different receptors, of which angiotensin II type 2 and Mas receptors are considered renoprotective, possibly in a sex-specific manner, whereas angiotensin II type 1 (AT1) receptors are believed to be deleterious. Additionally, internalized angiotensin II may stimulate intracellular receptors. Angiotensin-converting enzyme 2 (ACE2) not only generates angiotensin-(1-7) but also acts as coronavirus receptor. Multiple, if not all, cardiovascular diseases involve the kidney RAS, with renal AT1 receptors often being claimed to exert a crucial role. Urinary RAS component levels, depending on filtration, reabsorption, and local release, are believed to reflect renal RAS activity. Finally, both existing drugs (RAS inhibitors, cyclooxygenase inhibitors) and novel drugs (angiotensin receptor/neprilysin inhibitors, sodium-glucose cotransporter-2 inhibitors, soluble ACE2) affect renal angiotensin formation, thereby displaying cardiovascular efficacy. Particular in the case of the latter three, an important question is to what degree they induce renoprotection (e.g., in a renal RAS-dependent manner). This review provides a unifying view, explaining not only how kidney angiotensin formation occurs and how it is affected by drugs but also why drugs are renoprotective when altering the renal RAS. SIGNIFICANCE STATEMENT: Angiotensin formation in the kidney is widely accepted but little understood, and multiple, often contrasting concepts have been put forward over the last two decades. This paper offers a unifying view, simultaneously explaining how existing and novel drugs exert renoprotection by interfering with kidney angiotensin formation.

Angiotensin II AT2 receptor ligands with phenylthiazole scaffolds

The syntheses and the AT₁R and AT₂R binding data of a series of new small molecule ligands are reported. These ligands comprise a phenylthiazole scaffold rather than the biphenyl or phenylthiophene scaffolds found in essentially all of the previously described ligands originating from the nonselective AT₁R/AT₂R ligand L-162,313 and the AT₂R selective agonist C21, the latter now in Phase II/III clinical trials. A phenylthiazole rather than the phenylthiophene scaffold that is present in the AT₂R selective agonist C21 and in the AT₂R selective antagonist C38 had a deleterious effect on the affinity to AT₂R. Nevertheless, a significant improvement could be accomplished by introduction of a small bulky alkyl group in the 2-position of the imidazole ring attached through a methylene group bridge to the phenylthiazole scaffold. Hence, a combination of a 2-tert-butyl or a 2-isopropyl group and a butoxycarbonyl furnished potent AT₂R selective ligands. Furthermore, a high affinity ligand derived from L-162,313 and exhibiting a > 35 fold selectivity for AT₁R was identified (10). The ligand 21 with the 2-tert-butyl group and ∼ 35 fold selectivity for AT₂R, demonstrated high stability in human, rat and mouse liver microsomes and a very attractive profile with regard to the inhibition of common drug-metabolizing CYP enzymes. Thus, very low levels of inhibition of CYP 3A (5%), 2D6 (12%), 2C8 (26%), 2C9 (23%) and 2B6 (24%) were observed with the 2-tert-butyl derivative comprising the methoxycarbonyl sulfonamide function, levels that are significantly lower than those obtained with C21 under the same experimental conditions.

Drugs Modulating Renin-Angiotensin System in COVID-19 Treatment

A massive worldwide vaccination campaign constitutes the main tool against the COVID-19 pandemic. However, drug treatments are also necessary. Antivirals are the most frequently considered treatments. However, strategies targeting mechanisms involved in disease aggravation may also be effective. A major role of the tissue renin-angiotensin system (RAS) in the pathophysiology and severity of COVID-19 has been suggested. The main link between RAS and COVID-19 is angiotensin-converting enzyme 2 (ACE2), a central RAS component and the primary binding site for SARS-CoV-2 that facilitates the virus entry into host cells. An initial suggestion that the susceptibility to infection and disease severity may be enhanced by angiotensin type-1 receptor blockers (ARBs) and ACE inhibitors (ACEIs) because they increase ACE2 levels, led to the consideration of discontinuing treatments in thousands of patients. More recent experimental and clinical data indicate that ACEIs and, particularly, ARBs can be beneficial for COVID-19 outcome, both by reducing inflammatory responses and by triggering mechanisms (such as ADAM17 inhibition) counteracting viral entry. Strategies directly activating RAS anti-inflammatory components such as soluble ACE2, Angiotensin 1-7 analogues, and Mas or AT2 receptor agonists may also be beneficial. However, while ACEIs and ARBs are cheap and widely used, the second type of strategies are currently under study.

The Renin Angiotensin System as a potential treatment target for Traumatic Brain Injury

Traumatic brain injury (TBI) is a major health concern and leading cause of death and disability in young adults in the UK and worldwide, however, there is a paucity of disease modifying therapies for the treatment of TBI. This review investigates the potential of the renin-angiotensin system (RAS) as a treatment pathway for traumatic brain injury (TBI) in adults. Relevant electronic databases were searched on 18 December 2019, updated 16 May 2021. All English language articles with adult human or animal populations investigating RAS drugs as an intervention for TBI or reporting genetic evidence relevant to the RAS and TBI were screened. Eighteen preclinical RCTs (n=2269) and 2 clinical cohort studies (n=771) were included. Meta-analyses of 6 preclinical randomised-controlled trials (n=99) indicated candesartan improved neurological function short-term (<7 days: standardised mean difference (SMD) 0.61, 95% confidence interval (CI) 0.19 - 1.03, I2=0%) and long-term (≥7 days: SMD 1.06, 95% CI 0.38; 1.73, I2=0%) post-TBI. Findings were similar for most secondary outcomes. There was a suggestion of benefit from other RAS-targeting drugs, although evidence was limited due to few small studies. There was insufficient evidence to enable strong assessment of these drugs on mortality post-TBI. We conclude that angiotensin-receptor blockers, especially candesartan, show positive outcomes post-TBI in preclinical studies with moderate quality of evidence (GRADE). More research into the effect of regulatory-RAS targeting drugs is needed. Clinical trials of candesartan following TBI are recommended, due to strong and consistent evidence of neuroprotection shown by these preclinical studies.