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Endothelin

Endothelin is a family of three potent 21-amino acid peptides—endothelin-1 (ET-1), endothelin-2 (ET-2), and endothelin-3 (ET-3)—that function primarily as vasoconstrictors and are produced mainly by vascular endothelial cells, as well as by vascular cells, macrophages, neurons, and other tissues. These peptides are synthesized from larger precursor proteins: preproendothelin is cleaved to big endothelin, which is then converted to the mature form by endothelin-converting enzymes (ECE-1 and ECE-2). Discovered in 1988 when ET-1 was isolated from the conditioned medium of cultured porcine aortic endothelial cells, endothelins represent the most powerful endogenous vasoconstrictors known, surpassing the potency of angiotensin II, norepinephrine, and serotonin. Their circulating is short, less than 5 minutes in human plasma, due to rapid enzymatic degradation and receptor-mediated clearance, primarily in the lungs and kidneys. Endothelins exert their biological effects through two distinct G-protein-coupled receptors: the ET<sub>A</sub> receptor, which is selective for ET-1 and ET-2 and predominantly mediates and via cells, and the ET<sub>B</sub> receptor, which binds all three isoforms with equal affinity and promotes , , and endothelin clearance through endothelial cells. Physiologically, ET-1 plays a key role in maintaining vascular tone, regulating , and modulating renal function, while also contributing to processes such as , release, and tissue remodeling. The three isoforms differ in tissue distribution and expression: ET-1 is ubiquitous and the most abundant, ET-2 is primarily found in the , intestine, and , and ET-3 is expressed in the , , and , with distinct roles in development and neural function. In disease states, dysregulated endothelin signaling is implicated in numerous cardiovascular and renal pathologies, including systemic and , , , and , where elevated ET-1 levels promote vascular , , , and . Endothelins also contribute to non-cardiovascular conditions such as , , and various cancers (e.g., ovarian, , and colorectal), where they drive tumor growth, , and via ET<sub>A</sub> receptor . Therapeutically, selective ET<sub>A</sub> or dual ET<sub>A</sub>/ET<sub>B</sub> receptor antagonists like , ambrisentan, and macitentan have been developed and approved for treating , demonstrating improved exercise capacity, , and survival by blocking endothelin-mediated and . In 2024, the dual antagonist aprocitentan was approved for resistant , expanding applications to systemic not controlled by other agents. Ongoing continues to explore further uses in endothelin-related disorders, underscoring the system's broad clinical relevance.

Discovery and Nomenclature

Historical Discovery

The initial evidence for an endothelium-derived vasoconstricting factor emerged in 1985, when researchers at the observed that conditioned medium from cultured bovine aortic endothelial cells induced potent contractions in isolated coronary artery strips, distinct from known prostanoids or other factors. This factor, later termed endothelium-derived constricting factor (EDCF), was characterized as heat-stable and resistant to inhibitors, suggesting a novel mediator. Building on these observations, Masashi Yanagisawa and colleagues at the in pursued the purification of the vasoconstrictor from porcine aortic endothelial cells, leading to its isolation and structural elucidation in 1988. The , named endothelin-1 (ET-1), was identified as a 21-amino acid sequence with two intrachain bonds, formed between residues at positions 1-15 and 3-11, conferring structural stability. This marked the first identification of a potent endothelial-derived vasoconstrictor . Early characterizations highlighted endothelin-1's exceptional potency and duration of action; it was found to be approximately 10 times more potent than angiotensin II in contracting isolated vascular and exhibited sustained effects lasting hours, far exceeding those of typical vasoconstrictors. These properties were demonstrated in bioassays using porcine coronary artery strips, where endothelin-1 induced contractions at nanomolar concentrations. The discovery of endothelin occurred amid a surge in research on endothelial-derived factors, sparked by the 1980 identification of (EDRF), later revealed as —a breakthrough recognized by the 1998 in Physiology or Medicine awarded to , Louis J. Ignarro, and . Although endothelin itself did not directly contribute to the Nobel, its identification complemented the growing understanding of endothelial regulation of vascular tone, contrasting the relaxing effects of .

Etymology and Naming Conventions

The term "endothelin" derives from "," referring to the inner cellular lining of blood vessels, prefixed with "endo-" (from , meaning "within" or "inner") and combined with the suffix "-in," a common designation for peptides and proteins, to highlight its production by vascular endothelial cells. This nomenclature was proposed in the seminal 1988 study isolating the peptide from the culture medium of porcine aortic endothelial cells, where it was sequenced as a 21-amino-acid vasoconstrictor and explicitly named endothelin to reflect its endothelial origin. Prior to this specific naming, the substance was referred to more generally in scientific literature as an endothelium-derived contracting factor (EDCF) or endothelium-derived vasoconstricting factor, based on earlier observations of vasoconstrictive activity in endothelial cell supernatants during the mid-1980s. The shift to "endothelin" marked the transition from descriptive terminology for an unidentified factor to a precise identifier for the purified , facilitating subsequent research into its family and functions. The isoforms follow a sequential numbering convention based on their order of discovery and structural similarity: endothelin-1 (ET-1), the prototype isolated from porcine and human sources; endothelin-2 (ET-2), identified shortly thereafter and primarily expressed in humans (known as vasoactive intestinal contractor or in rodents); and endothelin-3 (ET-3), cloned from and noted for its to sarafotoxins, a group of structurally related vasoconstrictive peptides originally isolated from the of the mole viper Atractaspis engaddensis. This numbering system, abbreviated as ET-1, ET-2, and ET-3, emphasizes their relatedness while distinguishing tissue-specific expressions and evolutionary origins. Endothelin receptors adhere to a subtype established in the early 1990s following their , designated as (endothelin receptor type A), which predominantly mediates via high-affinity binding to ET-1, and ETB (endothelin receptor type B), which exhibits equal affinity for all isoforms and supports diverse roles including and peptide clearance. This /ETB convention, proposed in cloning studies from 1990 onward, standardized pharmacological and physiological discussions by linking receptor subtypes to functional selectivity.

Structure and Isoforms

Molecular Structure

Endothelins are 21-amino acid peptides with a molecular weight of approximately 2.5 kDa. These peptides exhibit a compact bicyclic architecture, primarily stabilized by two intrachain disulfide bridges connecting cysteine residues at positions Cys¹-Cys¹⁵ and Cys³-Cys¹¹. These bonds create an N-terminal ring of six residues and a larger encompassing loop, which are essential for maintaining the rigid conformation required for biological function. The secondary structure includes an α-helical segment spanning residues 9–15, which positions key side chains for interactions with target receptors. Adjacent to this helix is a hydrophobic C-terminal (residues 16–21), rich in nonpolar , that enhances binding specificity and potency by inserting into hydrophobic pockets on the receptor. At the level, the overall is conserved across endothelin members, with the disulfide-stabilized providing rigidity while allowing flexibility in the tail region for receptor engagement. The tryptophan residue at position 21 (Trp²¹) is highly conserved among mammalian endothelins and across species, playing a pivotal role in receptor activation through aromatic interactions that amplify vasoconstrictive signaling. This conservation underscores the evolutionary pressure to preserve structural integrity for potent physiological effects.

Isoforms and Tissue Distribution

Endothelins comprise three primary isoforms in humans—endothelin-1 (ET-1), endothelin-2 (ET-2), and endothelin-3 (ET-3)—each encoded by distinct genes and exhibiting tissue-specific expression patterns that contribute to their diverse physiological roles. These isoforms share greater than 70% amino acid sequence homology, reflecting their common evolutionary origin and structural similarities, including a 21-amino-acid mature peptide with two disulfide bridges. The genes are located on different chromosomes: EDN1 (encoding ET-1) on chromosome 6p24.1, EDN2 (encoding ET-2) on chromosome 1p34, and EDN3 (encoding ET-3) on chromosome 20q13.2-q13.3. ET-1, the most abundant and widely studied isoform, is encoded by the EDN1 gene and is ubiquitously expressed in vascular throughout the body, with particularly high levels in the lungs and kidneys. In the lungs, ET-1 localizes primarily to vascular , airway , and cells, supporting its role in pulmonary vascular . In the kidneys, ET-1 is produced by endothelial and , contributing to renal hemodynamics. ET-1 predominates in the cardiovascular system, where it is the primary isoform released from endothelial cells. ET-2, encoded by the EDN2 gene, shows a more restricted distribution, with prominent expression in the , particularly the intestine, the kidneys, and the . It is also detected at lower levels in vascular , heart, , , , and other sites. ET-2 exhibits vasoconstrictor potency slightly lower than ET-1 due to marginally reduced affinity for the receptor, though it binds with comparable to both endothelin receptor subtypes in functional assays. ET-3, encoded by the EDN3 gene, is predominantly expressed in the , intestine, and , with high concentrations also noted in the pituitary and lungs. In the , it supports neuronal and , while in the , prepro-ET-3 mRNA is present, indicating local production. Mutations in EDN3 are associated with , a involving aganglionic due to impaired formation. Species variations exist; for instance, the counterpart to ET-3 was cloned and identified as a distinct endothelin isoform. In vascular tissues, ET-1 vastly outnumbers ET-3, establishing ET-1 as the dominant isoform in endothelial-derived endothelin production.

Biosynthesis and Regulation

Precursor Synthesis and Processing

Endothelin peptides are synthesized from precursor proteins encoded by the EDN genes, which undergo a series of proteolytic processing steps to generate the mature 21-amino-acid peptides. For endothelin-1 (ET-1), the primary isoform produced by endothelial cells, transcription of the EDN1 gene yields a prepro-endothelin-1 (preproET-1) mRNA that is translated into a 212-amino-acid precursor protein. This preproET-1 contains an N-terminal signal peptide of 17 amino acids, which directs the protein to the endoplasmic reticulum and is subsequently cleaved by signal peptidase to produce pro-endothelin-1 (proET-1). The proET-1 intermediate is then processed by furin-like proprotein convertases at paired basic residues, liberating the inactive 38-amino-acid precursor known as big ET-1. Similar precursor motifs are shared among the three endothelin isoforms (ET-1, ET-2, and ET-3), encoded by distinct EDN genes, though tissue-specific expression varies. The final maturation step involves the conversion of big ET-1 to active ET-1, catalyzed primarily by endothelin-converting (ECEs), which are zinc-dependent metalloproteases. ECE-1, a membrane-bound with four isoforms (ECE-1a through -1d), performs this cleavage at neutral (approximately 7.0) and is localized to the plasma membrane, Golgi apparatus (particularly the ECE-1b isoform in the trans-Golgi network), and endosomes. ECE-2, in contrast, is an intracellular active at acidic (approximately 5.5), residing in secretory vesicles and the trans-Golgi network, and exhibits a more restricted neuroendocrine distribution. Both enzymes cleave big ET-1 specifically between 21 and 22 (Trp<sup>21</sup>-Val<sup>22</sup>), removing a C-terminal fragment to yield the mature peptide; ECE-1 is the dominant isoform in vascular tissues, accounting for the majority of ET-1 production. This processing occurs within intracellular compartments to ensure efficient secretion of the bioactive peptide. In endothelial cells, big ET-1 is transported through the Golgi apparatus, where ECE-1 facilitates cleavage, enabling release via constitutive or regulated secretory pathways, including Weibel-Palade bodies for stimulus-induced . The membrane-bound nature of ECE-1 allows for both intracellular maturation and potential surface conversion of secreted big ET-1, contributing to localized ET-1 availability near target cells. Furin cleavage occurs earlier in the secretory pathway, typically in the trans-Golgi network or immature secretory granules, preceding ECE-mediated activation.

Factors Regulating Biosynthesis

The of endothelin-1 (ET-1), encoded by the EDN1 , is tightly controlled at the transcriptional level by various activators that bind to specific promoter s. , a mechanical force exerted by blood flow on endothelial cells, can activate ET-1 transcription through (PKC) and activator protein-1 (AP-1) pathways, particularly under cyclic strain conditions that engage the AP-1 site at -108 bp in the promoter. Hypoxia-inducible factor-1α (HIF-1α) binds to a hypoxia-responsive at -118 bp, cooperating with AP-1 and GATA-2 at adjacent sites (-108 bp and -135 bp, respectively) to enhance transcription under hypoxic conditions, as demonstrated by and electrophoretic mobility shift assays. Cytokines such as transforming growth factor-β (TGF-β) and interleukin-1β (IL-1β) further promote ET-1 expression; TGF-β acts via ALK5/Smad3 signaling and synergistic interaction with AP-1 at -191 bp, while IL-1β induces it through at -2090 bp in endothelial and renal cells. In contrast, several factors inhibit ET-1 biosynthesis transcriptionally, counterbalancing activatory signals. (NO), produced by endothelial , suppresses ET-1 by interfering with promoter activation, as evidenced by studies showing reduced ET-1 release upon NO donors or shear-induced NO elevation. (PGI2), a vasodilatory prostanoid, inhibits basal and serum-stimulated ET-1 secretion by approximately 40-50% in cultured endothelial cells, likely through cAMP-mediated repression of promoter activity. exhibits context-dependent effects, downregulating ET-1 in conditions via NO-dependent mechanisms while potentially upregulating it in oscillatory or high-magnitude scenarios, highlighting its biphasic regulation of vascular tone. Post-transcriptional regulation fine-tunes ET-1 levels through microRNAs (miRNAs). The miR-130/301 family indirectly promotes ET-1 expression by repressing (PPARγ), a transcriptional suppressor of ET-1, particularly in pulmonary arterial endothelial cells under conditions such as . Pathophysiological triggers such as angiotensin II, , and insulin enhance ET-1 production in vascular cells, amplifying under stress conditions. Angiotensin II upregulates ET-1 protein and endothelin-converting (ECE) activity in vascular via AT1 receptors, increasing release. stimulates ET-1 secretion from endothelial cells through protease-activated receptors, contributing to acute vascular responses. Insulin similarly boosts ET-1 release in endothelial cultures, linking metabolic signals to vasoconstrictor production. These triggers depend on upstream ECE processing for mature ET-1 formation but primarily act at the transcriptional and secretory levels.

Receptors and Signaling

Endothelin Receptors

Endothelin receptors are G-protein-coupled receptors (GPCRs) that mediate the biological effects of endothelin peptides, consisting of two main subtypes: endothelin receptor type A () and endothelin receptor type B (ETB). These receptors feature seven transmembrane domains typical of GPCRs and are encoded by distinct genes, with derived from the EDNRA gene on and ETB from the EDNRB gene on chromosome 13. The receptor exhibits high affinity for endothelin-1 (ET-1) and endothelin-2 (ET-2), with lower affinity for endothelin-3 (ET-3), and is predominantly expressed on vascular cells (VSMCs), where it primarily drives . It couples mainly to the protein family, activating and subsequent intracellular calcium mobilization. In contrast, the ETB receptor shows equal affinity for all three endothelin isoforms (ET-1, ET-2, and ET-3) and is expressed on endothelial cells, VSMCs, and renal tissues, with roles in and endothelin clearance depending on cellular context. Like ETA, ETB is -coupled but can engage other G proteins in a tissue-specific manner. Both receptors can undergo homo- and heterodimerization, which influences ligand binding and signaling efficiency; for instance, ETA-ETB heterodimers form in co-expressing cells and may modulate receptor function. Desensitization occurs via agonist-induced , primarily by (PKC), leading to β-arrestin recruitment and receptor internalization, thereby attenuating prolonged signaling. Tissue distribution varies between subtypes: ETA is prominently localized in the vasculature of the heart and lungs, contributing to cardiovascular tone regulation, while ETB predominates in the renal medulla and brain, including astrocytic and neuronal elements.

Intracellular Signaling Pathways

Upon binding to endothelin receptors, primarily the G protein-coupled receptors and ETB, endothelin ligands initiate diverse intracellular signaling cascades that mediate cellular responses such as , , and survival. These pathways exhibit receptor subtype selectivity, with predominantly activating vasoconstrictive signals in vascular cells (VSMCs), while ETB often promotes vasodilatory effects in endothelial cells. The primary pathway activated by both and ETB receptors involves protein coupling, which stimulates (PLC) to hydrolyze phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 then binds to receptors on the , triggering the release of Ca^{2+} stores and elevating intracellular calcium concentration ([Ca^{2+}]_i), which activates calmodulin-dependent to promote VSMC contraction. This transient [Ca^{2+}]_i increase is often followed by sustained influx through voltage-dependent and store-operated calcium channels. Endothelin also activates the (MAPK)/extracellular signal-regulated kinase (ERK) pathway via the Ras-Raf cascade, independent of c-Src but reliant on PKC and MEK1/2 intermediaries. This leads to ERK1/2 , peaking within 10 minutes of stimulation, and drives VSMC and by regulating for growth factors and components. Additional pathways include PI3K-Akt signaling, where endothelin receptor activation recruits PI3K to generate PIP3, phosphorylating and activating Akt to inhibit and support . Concurrently, the RhoA pathway is engaged through or coupling, activating Rho kinase () to induce cytoskeletal reorganization via myosin light chain inhibition and formation. In endothelial cells, ETB receptor stimulation specifically activates eNOS via βγ subunit-mediated PI3K-Akt signaling, phosphorylating eNOS at Ser-1179 to enhance (NO) production and counteract .

Physiological Functions

Cardiovascular Regulation

Endothelin-1 (ET-1) serves as a potent vasoconstrictor in the cardiovascular system, primarily acting on vascular cells to increase systemic and thereby contribute to under physiological conditions. Through activation of endothelin type A () receptors, ET-1 induces sustained of resistance vessels, with effects that are approximately 1,000 times more potent and longer-lasting than those elicited by norepinephrine, highlighting its role in maintaining vascular tone. This vasoconstrictive action is mediated by an increase in intracellular calcium levels, leading to without rapid desensitization. In the heart, ET-1 exerts direct effects on cardiomyocytes via ETA receptors, promoting positive inotropy (increased contractility) and chronotropy (elevated ), which enhance during periods of physiological demand. These responses support overall cardiovascular performance by augmenting the force and frequency of myocardial contractions. Additionally, ET-1 influences cardiac fibroblasts through ETA receptor signaling, stimulating their and into myofibroblasts, which promotes deposition and contributes to adaptive cardiac remodeling in normal . ET-1 also modulates cerebral blood flow, regulating vascular tone in the cerebrovasculature to ensure adequate to the under varying hemodynamic conditions. This involves constriction of , which helps fine-tune regional blood distribution. Furthermore, ET-1 interacts synergistically with the , exerting a sympathoexcitatory effect via receptors that amplifies norepinephrine release and enhances overall maintenance.

Renal and Pulmonary Effects

Endothelin-1 (ET-1) plays a critical role in by modulating (GFR) primarily through activation of receptors on vascular cells, leading to of afferent and and a subsequent reduction in GFR and renal blood flow. receptors provide a counterbalancing effect, promoting in the collecting duct by inhibiting sodium reabsorption via downregulation of epithelial sodium channels (ENaC) through pathways involving Src kinase, (MAPK), and production. Additionally, ETB receptors in the induce of the vasa recta, enhancing medullary blood flow and supporting overall , particularly during high salt intake. In the pulmonary vasculature, ET-1 contributes to the maintenance of basal vascular tone in pulmonary arteries via receptor-mediated contraction of cells, ensuring appropriate under normal conditions. ET-1 also supports hypoxic pulmonary vasoconstriction, a physiological response that optimizes ventilation-perfusion matching by redirecting blood flow from poorly ventilated lung regions; this involves hypoxia-induced upregulation of ET-1 expression and activation. receptors on endothelial cells aid in ET-1 clearance and promote local through and release, balancing the constrictive effects. Furthermore, ET-1 mediates through activation of receptors on airway cells, contributing to the regulation of airway tone. ET-1 influences renal fluid through interactions with hormonal systems, including antagonism of antidiuretic hormone (ADH) via ETB receptors in the collecting duct, which reduces (AQP2) expression and inhibits water reabsorption by decreasing levels and activity. Regarding aldosterone, ET-1 can stimulate its secretion to enhance sodium retention in the distal , while aldosterone in turn upregulates ET-1 production, creating a feedback loop for volume regulation; however, ETB activation may inhibit aldosterone effects to prevent excessive retention.

Pathophysiology and Clinical Relevance

Role in Cardiovascular Diseases

Endothelin-1 (ET-1) contributes to the pathogenesis of through its potent vasoconstrictive effects and promotion of vascular inflammation, , and . Plasma and vascular ET-1 levels are elevated in patients with , particularly in those with difficult-to-control disease, where enhanced prepro-ET-1 expression occurs in the of small resistance arteries. This overexpression correlates with increased vascular tone and , distinguishing pathological states from normal physiological regulation of . Blockade of receptors or dual ET receptors reduces in hypertensive models and patients; for example, the dual antagonist lowered systolic by 7-10 mmHg and diastolic pressure by approximately 6 mmHg in clinical trials of . In , ET-1 drives adverse myocardial remodeling and , especially post-myocardial , by stimulating cardiac fibroblasts to increase production and deposition, which impairs ventricular function. Plasma ET-1 concentrations are markedly elevated in heart failure patients and correlate directly with disease severity, as measured by New York Heart Association (NYHA) functional class, with levels rising progressively from NYHA class I-II to III-IV and associating with reduced left ventricular . These elevations reflect ET-1's role in exacerbating systolic and diastolic dysfunction beyond its baseline contributions to cardiovascular . ET-1 promotes by enhancing adhesion to the vascular and facilitating plaque formation through ETA receptor-mediated mechanisms on endothelial and cells. It induces expression of adhesion molecules and , leading to recruitment and differentiation into foam cells laden with oxidized lipids, which accelerates progression in hypercholesterolemic models. ET-1 immunoreactivity is particularly pronounced in macrophage-rich areas of advanced plaques, underscoring its pro-inflammatory effects in atherogenesis. Recent clinical trials, such as the PRECISION study published in 2024, have demonstrated the efficacy of dual endothelin receptor antagonists like aprocitentan in reducing in patients with resistant , further emphasizing the role of the endothelin pathway in this condition.

Involvement in Other Disorders

Endothelin-1 (ET-1) plays a significant role in the of pulmonary arterial (PAH), where it contributes to the formation of plexiform lesions characteristic of the disease. Expression of ET-1 is notably increased in the lungs of PAH patients, particularly in small resistance arteries and plexiform lesions, with levels correlating to disease severity. In idiopathic PAH, circulating and pulmonary ET-1 levels are elevated, promoting , smooth muscle cell proliferation, and that exacerbate vascular remodeling. In renal diseases, ET-1 mediates glomerular injury in primarily through activation of endothelin A () receptors on podocytes and endothelial cells, leading to , , and . Blockade of receptors has been shown to reduce and slow progression in diabetic kidney disease models and clinical trials. Additionally, deficiencies in the endothelin B (ETB) receptor are associated with salt-sensitive in animal models, such as ETB-deficient rats, where impaired renal sodium leads to elevated in response to high-salt intake. In humans, ETB receptor mutations (EDNRB gene) primarily cause , with limited directly linking them to hypertension susceptibility. Neurologically, ET-1 contributes to in conditions such as and by inducing potent cerebral . In subarachnoid hemorrhage-related , ET-1 acts as a key mediator of delayed cerebral , promoting arterial narrowing and ischemia through ETA receptor signaling on vascular . Elevated ET-1 levels have also been observed during attacks, correlating with and that may trigger headache and aura symptoms. Furthermore, endothelin-3 (ET-3) is implicated in neural crest disorders like , where homozygous mutations in the EDN3 gene disrupt and enteric development, leading to pigmentation anomalies, , and Hirschsprung . Heterozygous EDN3 variants are linked to milder forms, such as Waardenburg syndrome type 4, highlighting ET-3's role in neurocristopathy phenotypes. In , ET-1 fosters tumor progression by promoting and through autocrine and via and ETB receptors expressed on cancer cells. In , ET-1 enhances tumor cell invasion, survival, and vascular endothelial growth factor (VEGF) production, driving and peritoneal . Similarly, in , particularly hormone-refractory forms, ET-1 stimulates endothelial cell proliferation and migration, supporting while also conferring resistance to therapy and promoting . Dual /ETB antagonism has shown potential to inhibit these processes in preclinical models of both cancers. Recent advances as of 2025 underscore ET-1's involvement in VEGF inhibitor-induced hypertension, a common side effect of anti-angiogenic cancer therapies. VEGF receptor tyrosine kinase inhibitors like axitinib and lenvatinib elevate ET-1 levels, activating ETA receptors to cause endothelial dysfunction and vasoconstriction, thereby contributing to treatment-related hypertension. Endothelin receptor antagonists are emerging as adjunctive agents to mitigate this toxicity while preserving anti-tumor efficacy.

Therapeutic Interventions

Endothelin Receptor Antagonists

Endothelin receptor antagonists () are a class of drugs that block the binding of endothelin-1 (ET-1) to its receptors, primarily and ETB, thereby mitigating and associated with endothelin signaling. These agents are selective for either (predominantly mediating ) or both receptors (non-selective), with clinical applications focused on conditions involving excessive endothelin activity, such as pulmonary arterial (PAH). Non-selective ERAs target both ETA and ETB receptors. , approved by the U.S. (FDA) in 2001, is indicated for the treatment of PAH in adults and pediatric patients to improve exercise capacity and delay clinical worsening. Macitentan, approved by the FDA in 2013, is also indicated for PAH and features enhanced tissue penetration due to its physicochemical properties, allowing sustained receptor occupancy and improved efficacy in lung tissue compared to earlier agents. Selective primarily target the receptor. Ambrisentan, approved by the FDA in 2007, is indicated for PAH to improve exercise capacity and delay disease progression in adults. Sitaxsentan, an -selective initially approved for PAH, was withdrawn from the global market in 2010 due to cases of severe , including fatal liver injury. These antagonists exert their effects through at the orthosteric binding site of the endothelin receptors, preventing ET-1 from activating downstream pathways. For instance, competitively blocks ET-1 binding to and ETB receptors, thereby reducing ET-1-induced calcium rise in vascular cells, which attenuates . Aprocitentan (branded as TRYVIO), an ETA-selective ERA, was approved by the FDA in March 2024 for the treatment of hypertension in combination with other antihypertensive drugs to lower blood pressure in adults whose condition is not adequately controlled. Clinical trials demonstrated that aprocitentan (12.5 mg daily) reduces systolic blood pressure by 4-6 mmHg (placebo-corrected) in patients with resistant hypertension, with sustained effects over 40 weeks.

Emerging Therapies and Research Directions

Recent research has explored endothelin-converting enzyme (ECE) inhibitors as a strategy to reduce endothelin-1 (ET-1) production upstream of receptor activation, demonstrating preclinical efficacy in attenuating renal fibrosis and inflammation in chronic kidney disease (CKD) models by preserving renal perfusion and limiting progression to end-stage disease. Atrasentan, a selective endothelin type A (ETA) receptor antagonist, has progressed through phase 3 trials for CKD, particularly immunoglobulin A nephropathy (IgAN), with data from the ALIGN study (NCT04573478) showing a statistically significant reduction in proteinuria by 36% (95% CI: 26-45) compared to placebo when added to renin-angiotensin system inhibitors. This approval by the FDA in April 2025 marks atrasentan (branded as Vanrafia) as the first selective ETA antagonist for proteinuria reduction in primary IgAN on an accelerated basis, with ongoing assessment of kidney function preservation over 132 weeks as the confirmatory endpoint, highlighting its role in slowing CKD progression. Preclinical investigations into neutralizing antibodies and (siRNA) targeting ET-1 have shown promise in mitigating cardiac associated with , where ET-1 monoclonal antibodies abate activation and deposition in bleomycin-induced models, reducing fibrotic burden by inhibiting ETA-mediated signaling. Similarly, siRNA-mediated knockdown of ET-1 in endothelial cells has been reported to limit differentiation and synthesis in pressure-overload rodent models, preserving ventricular function through decreased inflammatory cascades. Combination regimens pairing endothelin receptor antagonists (ERAs) with phosphodiesterase-5 (PDE5) inhibitors, such as , have emerged as effective for pulmonary arterial (PAH), with dual therapy improving six-minute walk distance (6MWD) by 30-50 meters in randomized trials compared to monotherapy, while delaying clinical worsening and enhancing via synergistic and antiproliferative effects. In 2025 studies, activation of the ET-1 pathway via ETB receptor agonists has demonstrated protective effects against (AKI) by promoting and reducing tubular damage in ischemia-reperfusion models, suggesting potential renoprotective applications. Furthermore, ongoing 2025 research into ET-1 modulation in reveals that ETA/ETB antagonists enhance antitumor immune responses when combined with checkpoint inhibitors, reducing tumor-derived ET-1-mediated and in preclinical solid tumor models.

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