A phase II study of bosentan, a dual endothelin receptor antagonist, as monotherapy in patients with stage IV metastatic melanoma

Endothelin and the tumor microenvironment: a finger in every pie

The tumor microenvironment (TME) plays a central role in the development of cancer. Within this complex milieu, the endothelin (ET) system plays a key role by triggering epithelial-to-mesenchymal transition, causing degradation of the extracellular matrix and modulating hypoxia response, cell proliferation, composition, and activation. These multiple effects of the ET system on cancer progression have prompted numerous preclinical studies targeting the ET system with promising results, leading to considerable optimism for subsequent clinical trials. However, these clinical trials have not lived up to the high expectations; in fact, the clinical trials have failed to demonstrate any substantiated benefit of targeting the ET system in cancer patients. This review discusses the major and recent advances of the ET system with respect to TME and comments on past and ongoing clinical trials of the ET system.

Antibody-Drug Conjugates in the Pipeline for Treatment of Melanoma: Target and Pharmacokinetic Considerations

Melanoma is an aggressive, rapidly developing form of skin cancer that affects about 22 per 100,000 individuals. Treatment options for melanoma patients are limited and typically involve surgical excision of moles and chemotherapy. Survival has been improved in recent years through targeted small molecule inhibitors and antibody-based immunotherapies. However, the long-term side effects that arise from taking chemotherapies can negatively impact the lives of patients because they lack specificity and impact healthy cells along with the cancer cells. Antibody-drug conjugates are a promising new class of drugs for the treatment of melanoma. This review focuses on the development of antibody-drug conjugates for melanoma and discusses the existing clinical trials of antibody-drug conjugates and their use as a melanoma treatment. So far, the antibody-drug conjugates have struggled from efficacy problems, with modest effects at best, leading many to be discontinued for melanoma. At the same time, conjugates such as AMT-253, targeting melanoma cell adhesion molecule, and mecbotamab vedotin  targeting AXL receptor tyrosine kinase, are among the most exciting for melanoma treatment in the future.

Repurposing of the Drug Tezosentan for Cancer Therapy

Tezosentan is a vasodilator drug that was originally developed to treat pulmonary arterial hypertension. It acts by inhibiting endothelin (ET) receptors, which are overexpressed in many types of cancer cells. Endothelin-1 (ET1) is a substance produced by the body that causes blood vessels to narrow. Tezosentan has affinity for both ET_A and ET_B receptors. By blocking the effects of ET1, tezosentan can help to dilate blood vessels, improve the blood flow, and reduce the workload on the heart. Tezosentan has been found to have anticancer properties due to its ability to target the ET receptors, which are involved in promoting cellular processes such as proliferation, survival, neovascularization, immune cell response, and drug resistance. This review intends to demonstrate the potential of this drug in the field of oncology. Drug repurposing can be an excellent way to improve the known profiles of first-line drugs and to solve several resistance problems of these same antineoplastic drugs.

Metformin and simvastatin synergistically suppress endothelin 1-induced hypoxia and angiogenesis in multiple cancer types

Multiple cancers have been reported to be associated with angiogenesis and are sensitive to anti-angiogenic therapies. Vascular normalization, by restoring proper tumor perfusion and oxygenation, could limit tumor cell invasiveness and improve the effectiveness of anticancer treatments. However, the underlying anticancer mechanisms of antiangiogenic drugs are still unknown. Metformin (MET) and simvastatin (SVA), two metabolic-related drugs, have been shown to play important roles in modulating the hypoxic tumor microenvironment and angiogenesis. Whether the combination of MET and SVA could exert a more effective antitumor effect than individual treatments has not been examined. The antitumor effect of the synergism of SVA and MET was detected in mouse models, breast cancer patient-derived organoids, and multiple tumor cell lines compared with untreated, SVA, or MET alone. RNA sequencing revealed that the combination of MET and SVA (but not MET or SVA alone) inhibited the expression of endothelin 1 (ET-1), an important regulator of angiogenesis and the hypoxia-related pathway. We demonstrate that the MET and SVA combination showed synergistic effects on inhibiting tumor cell proliferation, promoting apoptosis, alleviating hypoxia, decreasing angiogenesis, and increasing vessel normalization compared with the use of a single agent alone. The MET and SVA combination suppressed ET-1-induced hypoxia-inducible factor 1α expression by increasing prolyl hydroxylase 2 (PHD2) expression. Furthermore, the MET and SVA combination showed a more potent anticancer effect compared with bosentan. Together, our findings suggest the potential application of the MET and SVA combination in antitumor therapy.

Endothelin-axis antagonism enhances tumor perfusion in pancreatic cancer

Delivery of therapeutic agents in pancreatic cancer (PC) is impaired due to its hypovascular and desmoplastic tumor microenvironment. The Endothelin (ET)-axis is the major regulator of vasomotor tone under physiological conditions and is highly upregulated in multiple cancers. We investigated the effect of dual endothelin receptor antagonist bosentan on perfusion and macromolecular transport in a PC cell-fibroblast co-implantation tumor model using Dynamic Contrast-Enhanced Magnetic Resonance Imaging (DCE-MRI). Following bosentan treatment, the contrast enhancement ratio and wash-in rates in tumors were two- and nine times higher, respectively, compared to the controls, whereas the time to peak was significantly shorter (7.29 ± 1.29 min v/s 22.08 ± 5.88 min; p = 0.04). Importantly, these effects were tumor selective as the magnitudes of change for these parameters were much lower in muscles. Bosentan treatment also reduced desmoplasia and improved intratumoral distribution of high molecular weight FITC-dextran. Overall, these findings support that targeting the ET-axis can serve as a potential strategy to selectively enhance tumor perfusion and improve the delivery of therapeutic agents in pancreatic tumors.

Targeting GPCRs and Their Signaling as a Therapeutic Option in Melanoma

Simple Summary

Sixteen G-protein-coupled receptors (GPCRs) have been involved in melanogenesis or melanomagenesis. Here, we review these GPCRs, their associated signaling, and therapies.

Abstract

G-protein-coupled receptors (GPCRs) serve prominent roles in melanocyte lineage physiology, with an impact at all stages of development, as well as on mature melanocyte functions. GPCR ligands are present in the skin and regulate melanocyte homeostasis, including pigmentation. The role of GPCRs in the regulation of pigmentation and, consequently, protection against external aggression, such as ultraviolet radiation, has long been established. However, evidence of new functions of GPCRs directly in melanomagenesis has been highlighted in recent years. GPCRs are coupled, through their intracellular domains, to heterotrimeric G-proteins, which induce cellular signaling through various pathways. Such signaling modulates numerous essential cellular processes that occur during melanomagenesis, including proliferation and migration. GPCR-associated signaling in melanoma can be activated by the binding of paracrine factors to their receptors or directly by activating mutations. In this review, we present melanoma-associated alterations of GPCRs and their downstream signaling and discuss the various preclinical models used to evaluate new therapeutic approaches against GPCR activity in melanoma. Recent striking advances in our understanding of the structure, function, and regulation of GPCRs will undoubtedly broaden melanoma treatment options in the future.

Inhibition of endothelin-B receptor signaling synergizes with MAPK pathway inhibitors in BRAF mutated melanoma

The clinical benefit of MAPK pathway inhibition in melanoma patients carrying BRAF mutations is temporal. After the initial response to treatment, the majority of tumors will develop resistance and patients will relapse. Here we demonstrate that the endothelin-endothelin receptor B (ETBR) signaling pathway confers resistance to MAPK pathway inhibitors in BRAF mutated melanoma. MAPK blockade, in addition to being anti-proliferative, induces a phenotypic change which is characterized by increased expression of melanocyte-specific genes including ETBR. In the presence of MAPK inhibitors, activation of ETBR by endothelin enables the sustained proliferation of melanoma cells. In mouse models of melanoma, including patient-derived xenograft models, concurrent inhibition of the MAPK pathway and ETBR signaling resulted in a more effective anti-tumor response compared to MAPK pathway inhibition alone. The combination treatment significantly reduced tumor growth and prolonged survival compared to therapies with MAPK pathway inhibitors alone. The phosphoproteomic analysis revealed that ETBR signaling did not induce resistance towards MAPK pathway inhibitors by restoring MAPK activity, but instead via multiple alternative signaling pathways downstream of the small G proteins GNAq/11. Together these data indicate that a combination of MAPK pathway inhibitors with ETBR antagonists could have a synergistically beneficial effect in melanoma patients with hyperactivated MAPK signaling pathways.

Endothelin signaling promotes melanoma tumorigenesis driven by constitutively active GNAQ

The G-protein-coupled receptor, endothelin receptor B (EDNRB), is an important regulator of melanocyte survival and proliferation. It acts by stimulating downstream heterotrimeric G proteins, such as Gα_q and Gα₁ . Constitutively active, oncogenic versions of Gα_q and Gα₁₁ drive melanomagenesis, but the role of Ednrb in the context of these mutant G proteins has not been previously examined. In this paper, we used a knock-in mouse allele at the Rosa26 locus to force oncogenic GNAQ^Q209L expression in melanocytes in combination with Ednrb gene knockout. The resulting pathological analysis revealed that every aspect of melanomagenesis driven by GNAQ^Q209L was inhibited. We conclude that even in the presence of oncogenic Gα_q , the Ednrb receptor activates normal Gα_q and Gα₁₁ proteins. This likely promotes tumorigenesis by activating phospholipase C-beta, the immediate effector of Gα_q/11 . These findings suggest that it might be possible to target upstream receptors to offset the effects of hyperactive G proteins, recognized as the cause of a growing number of human disorders.

Blockade of endothelin receptor A enhances the therapeutic efficacy of gemcitabine in pancreatic cancer cells

Pancreatic adenocarcinoma is currently one of the leading causes of cancer-related death worldwide. The high rate of mortality in pancreatic cancer patients is due to the inability to detect early-stage disease and the disease being highly refractory to therapy. Gemcitabine has been the standard chemotherapy for advanced pancreatic cancer patients for the last two decades. However, gemcitabine resistance develops within a few weeks of treatment, and the associated mechanism remains poorly understood. Therefore, a novel therapeutic strategy is needed to overcome the limited clinical efficacy of gemcitabine in pancreatic adenocarcinoma. In this study, we demonstrated that ET-1/ETAR axis gene expression was upregulated in pancreatic cancer cells after treatment with gemcitabine. Additionally, ETAR expression was significantly higher in tumor tissues than in normal tissues, and patients with high ETAR expression had a notably worse overall survival rate than those with low ETAR expression. Furthermore, our results revealed that bosentan, an ETAR antagonist, enhanced the growth-inhibiting and proapoptotic effects of gemcitabine on pancreatic cancer cells. Thus, our findings indicate that blockade of the ET-1/ETAR axis signaling pathway promotes the antiproliferative effect of gemcitabine on pancreatic cancer. Therefore, combination of ETAR blockade and gemcitabine serves as an effective therapeutic approach to achieve clinical benefits in pancreatic adenocarcinoma patients.

Endothelin Receptor Antagonists: Status Quo and Future Perspectives for Targeted Therapy

The endothelin axis, recognized for its vasoconstrictive action, plays a central role in the pathology of pulmonary arterial hypertension (PAH). Treatment with approved endothelin receptor antagonists (ERAs), such as bosentan, ambrisentan, or macitentan, slow down PAH progression and relieves symptoms. Several findings have indicated that endothelin is further involved in the pathogenesis of certain other diseases, making ERAs potentially beneficial in the treatment of various conditions. In addition to PAH, this review summarizes the use and perspectives of ERAs in cancer, renal disease, fibrotic disorders, systemic scleroderma, vasospasm, and pain management. Bosentan has proven to be effective in systemic sclerosis PAH and in decreasing the development of vasospasm-related digital ulcers. The selective ERA clazosentan has been shown to be effective in preventing cerebral vasospasm and delaying ischemic neurological deficits and new infarcts. Furthermore, in the SONAR (Study Of Diabetic Nephropathy With Atrasentan) trial, the selective ERA atrasentan reduced the risk of renal events in patients with diabetes and chronic kidney disease. These data suggest atrasentan as a new therapy in the treatment of diabetic nephropathy and possibly other renal diseases. Preclinical studies regarding heart failure, cancer, and fibrotic diseases have demonstrated promising effects, but clinical trials have not yet produced measurable results. Nevertheless, the potential benefits of ERAs may not be fully realized.

A high affinity nanobody against endothelin receptor type B: a new approach to the treatment of melanoma

The aim of the study was to produce a single-domain antibody (nanobody) specific for endothelin receptor type B (EDNRB) which has high expression in melanoma. Cultured human melanoma cells were used as antigens to immunize alpacas. After antibody generation was verified in alpaca serum, total RNA was extracted from alpaca lymphocytes and the target V_HH fragment was amplified by two-step PCR, cloned in the pCANTAB5E phagemid vector, and used to transform Escherichia coli TG1 cells to obtain a phage-display nanobody library, which was enriched by panning. The results indicated successful construction of a phage-display anti-human melanoma A375 nanobodies library with a size of 1.2 × 10⁸/ml and insertion rate of 80%. After screening, eight positive clones of anti-EDNRB nanobodies were used to infect E. coli HB2151 for production of soluble nanobodies, which were identified by ELISA. Finally, we obtained a high-affinity anti-EDNRB nanobody, which consisted of 119 amino acids (molecular weight: 12.97 kDa) with 22 amino acids in CDR3 and had good affinity in vitro. The results suggest that the nanobody may be potentially used for the treatment of human melanoma.

Endothelin: 30 Years From Discovery to Therapy

Discovered in 1987 as a potent endothelial cell-derived vasoconstrictor peptide, endothelin-1 (ET-1), the predominant member of the endothelin peptide family, is now recognized as a multifunctional peptide with cytokine-like activity contributing to almost all aspects of physiology and cell function. More than 30 000 scientific articles on endothelin were published over the past 3 decades, leading to the development and subsequent regulatory approval of a new class of therapeutics-the endothelin receptor antagonists (ERAs). This article reviews the history of the discovery of endothelin and its role in genetics, physiology, and disease. Here, we summarize the main clinical trials using ERAs and discuss the role of endothelin in cardiovascular diseases such as arterial hypertension, preecclampsia, coronary atherosclerosis, myocardial infarction in the absence of obstructive coronary artery disease (MINOCA) caused by spontaneous coronary artery dissection (SCAD), Takotsubo syndrome, and heart failure. We also discuss how endothelins contributes to diabetic kidney disease and focal segmental glomerulosclerosis, pulmonary arterial hypertension, as well as cancer, immune disorders, and allograft rejection (which all involve ET_A autoantibodies), and neurological diseases. The application of ERAs, dual endothelin receptor/angiotensin receptor antagonists (DARAs), selective ET_B agonists, novel biologics such as receptor-targeting antibodies, or immunization against ET_A receptors holds the potential to slow the progression or even reverse chronic noncommunicable diseases. Future clinical studies will show whether targeting endothelin receptors can prevent or reduce disability from disease and improve clinical outcome, quality of life, and survival in patients.

EDNRB isoform 3 confers Temozolomide resistance in A375 melanoma cells by modulating membrane potential, reactive oxygen species and mitochondrial Ca2

Background

The role of endothelin receptor type B (EDNRB) isoform 3 involved in Temozolomide (TMZ)-induced melanoma cell death has not yet been elucidated.

Methods

The subcellular localization of EDNRB isoform 3 was determined by confocal and immunoblotting assays. Silencing EDNRB isoform 3 was performed by CRISPR/Cas9. Apoptosis was assessed by annexin V/propium iodide staining and caspases 3/7/9 activity. Mitochondrial membrane potential, reactive oxygen species and mitochondrial Ca²⁺ were measured by flow cytometry. Apoptosis protein array was applied.

Results

Confocal and immunoblot analyses indicate mitochondrial localization of EDNRB isoform 3 and the first N-terminal (1–22) amino acids are sufficient for its mitochondrial targeting. EDNRB isoform 3 depleted A375 cells significantly confers chemoresistance with mitochondrial depolarization, reduced reactive oxygen species, enhanced mitochondrial Ca²⁺ uptake and decreased caspase 9 activation. Additionally, apoptosis array shows that lack of EDNRB isoform 3 has relatively lower expression of phosphorylation of p53 at S392 and a slightly higher expression of Paraoxonase 2.

Conclusion

Our findings raise the possibility of targeting EDNRB isoform 3 as a new therapeutic strategy in combination with TMZ for melanoma treatment.

Endothelin type B receptor interacts with the 78-kDa glucose-regulated protein

Endothelin (ET)-1 is involved in the vascular system, cell proliferation and apoptosis. ET receptors consist of ET type A receptor (ET_A R) and ET type B receptor (ET_B R). ET_A R and ET_B R generally exhibit opposite responses, although many exceptions exist. In the present study, we attempted to identify ET_A R- or ET_B R-specific binding proteins to understand the differences in ET_A R- and ET_B R-mediated responses after ET-1 stimulation. The 78-kDa glucose-regulated protein (GRP78) showed a stronger binding affinity towards ET_B R than towards ET_A R. Moreover, GRP78 overexpression promoted ET_B R-mediated ERK activation and GRP78 silencing suppressed ET_B R-mediated ERK activation. Furthermore, ET_B R can localize GRP78 to the cell periphery. These results suggest that the interaction of ET_B R with GRP78 affects ERK activation and GRP78 localization.

Endothelin-Receptor Antagonists beyond Pulmonary Arterial Hypertension: Cancer and Fibrosis

The endothelin axis and in particular the two endothelin receptors, ETA and ETB, are targets for therapeutic intervention in human diseases. Endothelin-receptor antagonists are in clinical use to treat pulmonary arterial hypertension and have been under clinical investigation for the treatment of several other diseases, such as systemic hypertension, cancer, vasospasm, and fibrogenic diseases. In this Perspective, we review the molecules that have been evaluated in human clinical trials for the treatment of pulmonary arterial hypertension, as well as other cardiovascular diseases, cancer, and fibrosis. We will also discuss the therapeutic consequences of receptor selectivity with regard to ETA-selective, ETB-selective, or dual ETA/ETB antagonists. We will also consider which chemical characteristics are relevant to clinical use and the properties of molecules necessary for efficacy in treating diseases against which known molecules displayed suboptimal efficacy.

Gnaq and Gna11 in the Endothelin Signaling Pathway and Melanoma

In this article, we first briefly outline the function of G protein coupled receptors in cancer, and then specifically examine the roles of the seven transmembrane G protein coupled Endothelin B receptor (Ednrb) and the G proteins, GNAQ and GNA11, in both melanocyte development and melanoma. Ednrb plays an essential role in melanocyte development. GNAQ and GNA11 are oncogenes when mutated in certain types of melanocytic lesions, being extremely frequent in uveal melanoma, which forms from melanocytes located in the eye. Previously, we reported that in mice, Schwann cell precursor derived melanocytes colonize the dermis and hair follicles, while the inter-follicular epidermis is populated by other melanocytes. A pattern has emerged whereby melanocytes whose activities are affected by gain-of-function mutations of the Endothelin 3 ligand and Gα_q/11 are the same subset that arise from Schwann cell precursors. Furthermore, the forced expression of the constitutively active human GNAQ^Q209L oncogene in mouse melanocytes only causes hyper-proliferation in the subset that arise from Schwann cell precursors. This has led us to hypothesize that in Schwann cell precursor derived melanocytes, Ednrb signals through Gα_q/11. Ednrb is promiscuous and may signal through other G protein alpha subunits in melanomas located in the inter-follicular epidermis.

Endothelin

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.

Molecular networks in melanoma invasion and metastasis

Metastatic melanoma accounts for approximately 80% of skin cancer-related deaths. Up to now there has been no effective treatment for stage IV melanoma patients due to the complexity and dissemination potential of this disease. Melanomas are heterogeneous tumors in which conventional therapies fail to improve overall survival. Targeted therapies are being developed, but the final outcome can be hampered by the incomplete knowledge of the process of melanoma progression. Even if the intracellular pathways are similar, the interaction of the cells with the surrounding environment should be taken into consideration. This article seeks to highlight some of the advances in the understanding of the molecular mechanisms underlying melanoma dissemination.

A review of the profile of endothelin axis in cancer and its management

The endothelins and their associated receptors are important controllers of vascular growth, inflammation and vascular tone. In cancer, they have roles in the control of numerous factors in cancer development and progression, including angiogenesis, stromal reaction, epithelial mesenchymal transitions, apoptosis, invasion, metastases and drug resistance. Also, we consider current information on the role of this signalling system in cancer and examine the state of the current cell, animal and clinical trials utilizing endothelin targeted drugs for cancer management. Although targeting the endothelin axis in cell lines and xenografts show some promise in retarding cellular growth, results from limited clinical trials in prostatic cancer are less encouraging and did not offer significant survival benefit. The ability to target both cancer cells and vasculature via endothelin is an important consideration that necessitates the further refining of therapeutic strategies as we continue to explore the possibilities of the endothelin axis in cancer treatment.

Endothelin 1 in cancer: biological implications and therapeutic opportunities

Activation of autocrine and paracrine signalling by endothelin 1 (ET1) binding to its receptors elicits pleiotropic effects on tumour cells and on the host microenvironment. This activation modulates cell proliferation, apoptosis, migration, epithelial-to-mesenchymal transition, chemoresistance and neovascularization, thus providing a strong rationale for targeting ET1 receptors in cancer. In this Review, we discuss the advances in our understanding of the diverse biological roles of ET1 in cancer and describe the latest preclinical and clinical progress that has been made using small-molecule antagonists of ET1 receptors that inhibit ET1-driven signalling.

Differential expression of the G-protein-coupled formyl Peptide receptor in melanoma associates with aggressive phenotype

Melanoma, due to its metastatic rate, is among the most aggressive forms of skin cancer. Human formyl peptide receptor (FPR) and its variant FPR-like 1 (FPRL1) have been associated with cell migration and invasiveness in neoplasms. We have studied the in situ expression of these receptors in a large series of melanocytic lesions and correlated the expression with clinicopathological features and prognosis. Tissue microarray blocks of 141 cases including nevi (31 cases), primary (84 cases), and metastatic melanomas (26 cases) were semiquantitatively evaluated by immunohistochemistry for the expression of FPR and FPRL1 proteins. A significant association was observed regarding diagnosis and percentage of cells showing expression of FPR (P = 0.0311) and FPRL1 (P = 0.0053). A gain of FPR immunoreactivity was observed in the lesions having ulceration (P = 0.0194) and Breslow thickness (P = 0.044). Also, high FPRL1 cytoplasmic immunoreactivity was seen in lesions without tumor regression (P = 0.04). In addition, in patients with increased cytoplasmic staining for FPR, the probability of disease-specific survival was significantly lower (log rank test, P = 0.0089). Our findings reveal that FPR and FPRL1 are overexpressed in primary melanoma and correlate with aggressive tumor characteristics, underscoring them as potential therapeutic targets.

Biologics against cancer-specific receptors - challenges to personalised medicine from early trial results

Understanding molecular mechanisms of tumourigenesis underlies new therapeutic strategies that specifically target tumours. This has led to the evolution of personalised therapy that was first used in breast cancer when hormone receptor status was determined. More recently in colorectal cancer treatment the Epidermal Growth Factor receptor and its tumourigenic role has led to its targeting by using Cetuximab and Panitumumab. Addition of these drugs to existing drug regimes (FOLFOX and FOLFIRI) showed improved respectability rates in patients with liver metastasis. Most recently the Endothelin receptor has been implicated in multiple tumourigenic processes. Interest has grown in using Endothelin A receptor antagonists as adjuvant or combination therapy as suggested by the FOLFERA and FOLFIRI trials currently on-going.

Novel targets for VEGF-independent anti-angiogenic drugs

INTRODUCTION

In the last decades, the active research in the field of tumor angiogenesis led to the development of a class of agents providing an effective inhibition of neovessels formation through the blockade of VEGF-related pathways. More recently, the identification of several non-VEGF factors such as PDGF, FGF, HGF, angiopoietins, ALK1/endoglin, endothelis and ephrins involved in tumor angiogenesis have emphasized the need to develop agents targeting multiple pro-angiogenic pathways.

AREAS COVERED

This review aimed at summarizing the role of non-VEGF molecular pathways in targeting tumor angiogenesis. Preclinical and clinical data for investigational agents against non-VEGF targets have been reviewed emphasizing the role of combined inhibition strategies.

EXPERT OPINION

Besides the successful development of drugs providing a specific VEGF blockade, novel agents targeting alternative angiogenesis-related pathways are being tested. Although it seems that the potential clinical usefulness of these novel compounds have been not yet fully investigated, sunitinib, sorafenib, pazopanib and other multikinase inhibitors have certainly displayed encouraging results. A more in-depth clarification of anti-angiogenic agents is still needed, in order to design the best clinical setting and schedule for target-based agents and possibly anticipate potential tools to overcome the emerging issue of anti-angiogenic drug resistance.

Role of the endothelin axis and its antagonists in the treatment of cancer

The endothelins (ET) are a group of proteins that act through G-protein coupled receptors. Endothelin-1 (ET-1) was initially identified as a potent vasoconstrictor and dysregulation of the ET axis contributes to pathological processes responsible for cardiovascular disease states. More recently, the ET axis, in particular ET-1 acting through the endothelin A receptor (ET(A) ), has been implicated in the development of several cancers through activation of pathways involved in cell proliferation, migration, invasion, epithelial-mesenchymal transition, osteogenesis and angiogenesis. The endothelin B receptor (ET(B) ) may counter tumour progression by promoting apoptosis and clearing ET-1; however, it has recently been implicated in the development of some tumour types including melanomas and oligodendrogliomas. Here, we review emerging preclinical and clinical data outlining the role of the ET axis in cancer, and its antagonism as an attractive and challenging approach to improve clinical cancer management. Clinical data of ET(A) antagonists in patients with prostate cancer are encouraging and provide promise for new ET(A) antagonist-based treatment strategies. Given the unexpected opportunities to affect pleiotrophic tumorigenic signals by targeting ET(A)-mediated pathways in a number of cancers, the evaluation of ET-targeted therapy in cancer warrants further investigation.

Angiogenesis and the tumor vasculature as antitumor immune modulators: the role of vascular endothelial growth factor and endothelin

Cancer immunotherapies have yielded promising results in recent years, but new approaches must be utilized if more patients are to experience the benefits of these therapies. Angiogenesis and the tumor endothelium confer unique immune privilege to a growing tumor, with significant effects on diverse immunological processes such as hematopoietic cell maturation, antigen presentation, effector T cell differentiation, cytokine production, adhesion, and T cell homing and extravasation. Here, we review the role of angiogenesis and the tumor endothelium on regulation of the antitumor immune response. We place particular emphasis on the role of vascular endothelial growth factor (VEGF) in the suppression of numerous immunological processes that control tumor progression. Further, we describe the unique crosstalk between the VEGF and endothelin systems, and how their interactions may shape the antitumor immune response. These insights establish new targets for combinatorial approaches to modify existing cancer immunotherapies.

Macitentan (ACT-064992), a tissue-targeting endothelin receptor antagonist, enhances therapeutic efficacy of paclitaxel by modulating survival pathways in orthotopic models of metastatic human ovarian cancer

Potential treatments for ovarian cancers that have become resistant to standard chemotherapies include modulators of tumor cell survival, such as endothelin receptor (ETR) antagonist. We investigated the therapeutic efficacy of the dual ETR antagonist, macitentan, on human ovarian cancer cells, SKOV3ip1 and IGROV1, growing orthotopically in nude mice. Mice with established disease were treated with vehicle (control), paclitaxel (weekly, intraperitoneal injections), macitentan (daily oral administrations), or a combination of paclitaxel and macitentan. Treatment with paclitaxel decreased tumor weight and volume of ascites. Combination therapy with macitentan and paclitaxel reduced tumor incidence and further reduced tumor weight and volume of ascites when compared with paclitaxel alone. Macitentan alone occasionally reduced tumor weight but alone had no effect on tumor incidence or ascites. Immunohistochemical analyses revealed that treatment with macitentan and macitentan plus paclitaxel inhibited the phosphorylation of ETRs and suppressed the survival pathways of tumor cells by decreasing the levels of pVEGFR2, pAkt, and pMAPK. The dose of macitentan necessary for inhibition of phosphorylation correlated with the dose required to increase antitumor efficacy of paclitaxel. Treatment with macitentan enhanced the cytotoxicity mediated by paclitaxel as measured by the degree of apoptosis in tumor cells and tumor-associated endothelial cells. Collectively, these results show that administration of macitentan in combination with paclitaxel prevents the progression of ovarian cancer in the peritoneal cavity of nude mice in part by inhibiting survival pathways of both tumor cells and tumor-associated endothelial cells.

Phase 1/2 study of atrasentan combined with pegylated liposomal doxorubicin in platinum-resistant recurrent ovarian cancer

BACKGROUND

Ovarian cancer overexpresses ET-1, and in vitro studies have shown that ET-1 confers resistance to anthracycline-containing chemotherapy. Atrasentan has been developed as an oral selective endothelin-A receptor antagonist. The objective of the study was to investigate the feasibility and toxicity of adding increasing doses of atrasentan (to a maximum of 10 mg/d) and liposomal doxorubicin in patients with progressive ovarian cancer, refractory for platinum and paclitaxel.

METHODS

Patients with platinum-resistant ovarian cancer were treated with pegylated liposomal doxorubicin (PLD) 50 mg/m(2) on day 1 (and repeated every 4 weeks) in combination with escalating doses of atrasentan once daily. The starting dose was 2.5 mg and escalated in cohorts of three patients from 5 to 10 mg.

RESULTS

Twenty-six patients (mean age = 60 years, range = 42-74 years) were treated at the three dose levels. Atrasentan could be safely administered in combination at a dose of 10 mg. All patients were evaluable for toxicity, and 19 patients, included in the phase 2 period, were evaluable for response. Adverse events included nausea, vomiting, mucositis, skin toxicity, and rhinitis. Clinical cardiac toxicity, intensively monitored, was not observed, although two patients had a decrease in cardiac ejection fraction. Three objective responses were observed and another six patients had stable disease with a median time to progression of 14 weeks and an overall survival of 13.1 months.

CONCLUSIONS

The addition of atrasentan to standard dose PLD in platinum-resistant ovarian cancer is feasible with some suggestion of prolonged survival.

Novel targeted agents for the treatment of bladder cancer: translating laboratory advances into clinical application

Bladder cancer is a common and frequently lethal cancer. Natural history studies indicate two distinct clinical and molecular entities corresponding to invasive and non-muscle invasive disease. The high frequency of recurrence of noninvasive bladder cancer and poor survival rate of invasive bladder cancer emphasizes the need for novel therapeutic approaches. These mechanisms of tumor development and promotion in bladder cancer are strongly associated with several growth factor pathways including the fibroblast, epidermal, and the vascular endothelial growth factor pathways. In this review, efforts to translate the growing body of basic science research of novel treatments into clinical applications will be explored.

Endothelin-1: a predictor of extracapsular extension in clinically localized prostate cancer?

OBJECTIVE

To assess the value of endothelin-1 (ET-1) expression in predicting extracapsular extension (ECE) in clinically localized prostate cancer (PCa).

PATIENTS AND METHODS

ET-1 expression was determined by immunohistochemistry on archival needle biopsies (NBs) from 94 patients (49 pT2 and 45 pT3a) who underwent radical prostatectomy (RP) for clinical T1-T2 PCa. Each sample was analysed independently by two pathologists blinded to the clinical data.

RESULTS

In univariate analysis, high ET-1 expression in NBs, pre-operative prostate-specific antigen (PSA) level >10 ng/ml, percentage of positive biopsy cores and NB Gleason score ≥7 were significantly associated with ECE as determined on subsequent RP. No significant association was found between clinical stage and ECE. In multivariate analysis, there was a significant association with high ET-1 expression in NBs (p = 0.006), pre-operative PSA level >10 ng/ml (p = 0.049), and NB Gleason score ≥7 (p = 0.002). These three pre-operative factors combined provided the best model for predicting ECE with 93.3% sensitivity, 49% specificity, 62.5% positive predictive value, 88.9% negative predictive value. The combination yielded a higher concordance index (0.760 vs 0.720) and offered a higher log partial likelihood than the same model without ET1 (112.8 vs 105.7, p = 0.01).

CONCLUSIONS

ET-1 expression was strongly associated with ECE and, when combined with pre-operative PSA level and Gleason score, improved the predictive accuracy of pre-operative NBs. Its assessment in patients with localized PCa might be useful when making treatment decisions. Further studies with standardisation of immunohistochemical staining and multi-institutional validation are now needed to establish the appropriate use of ET-1 staining in PCa staging and to evaluate inter-observer reproducibility.

Anti-endothelin drugs in solid tumors

IMPORTANCE OF THE FIELD

The endothelin (ET) axis, which includes the biological functions of ETs and their receptors, has played a physiological role in normal tissue, acting as a modulator of vasomotor tone, tissue differentiation and development, cell proliferation and hormone production. Interestingly, it also functions in the growth and progression of various tumors. Several researchers have identified the blockade of the ET-1 receptor as a promising therapeutic approach.

AREAS COVERED IN THIS REVIEW

The clinical investigation of an orally bioavailable ET antagonist, atrasentan, in prostate cancer, is encouraging. In this neoplasia, it has shown antitumor activity, bone metastasis control and amelioration of cancer-related pain but improvement in time to progression and overall survival has still not been demonstrated. The clinical trials of other ET antagonists are reported. Literature research was performed by Pubmed and Pharmaprojects.

WHAT THE READER WILL GAIN

A comprehensive view about the use of atrasentan in the treatment of castration-resistant prostate cancer (CRPC) is provided together with the scientific rationale based on the function of ET and its receptor in various cancer development mechanisms.

TAKE HOME MESSAGE

Atrasentan seems to be active in CRPC, although strong scientific evidence is still to be found. Interesting clinical findings regard zibotentan.

A randomized, double-blind, placebo-controlled study of high-dose bosentan in patients with stage IV metastatic melanoma receiving first-line dacarbazine chemotherapy

BACKGROUND

The endothelin system is implicated in the pathogenesis of melanoma. We evaluated the effects of bosentan - a dual endothelin receptor antagonist - in patients receiving first-line dacarbazine therapy for stage IV metastatic cutaneous melanoma in a phase 2, proof-of-concept study.

RESULTS

Eligible patients had metastatic cutaneous melanoma naïve to chemotherapy or immunotherapy, no central nervous system involvement, and serum lactate dehydrogenase <1.5 x upper limit of normal. Treatment comprised bosentan 500 mg twice daily or matching placebo, in addition to dacarbazine 1000 mg/m2 every three weeks. Eighty patients were randomized (double-blind) and 38 in each group received study treatment. Median time to tumor progression (primary endpoint) was not significantly different between the two groups (placebo, 2.8 months; bosentan, 1.6 months; bosentan/placebo hazard ratio, 1.144; 95% CI, 0.717-1.827; p = 0.5683). Incidences of most adverse events and clinically relevant increases in hepatic transaminases were similar between treatment groups although hemoglobin decrease to >8 and < or = 10 g/dL and < or = 8 g/dL was more common in the bosentan group.

CONCLUSIONS

In patients receiving dacarbazine as first-line chemotherapy for metastatic melanoma, the addition of high-dose bosentan had no effect on time to tumor progression or other efficacy parameters. There were no unexpected safety findings.

TRIAL REGISTRATION

This study is registered in ClinicalTrials.gov under the unique identifier NCT01009177.

Results of a multicenter, randomized, double-blind phase 2/3 study of lenalidomide in the treatment of pretreated relapsed or refractory metastatic malignant melanoma

BACKGROUND

The results of an international, multicenter, randomized, double-blind, controlled study assessing the efficacy and safety of lenalidomide treatment in patients with refractory stage IV metastatic malignant melanoma are reported.

METHODS

The study compared treatment with lenalidomide (25 mg/d on Days 1-21 of a 28-day cycle) to placebo in 306 patients with metastatic malignant melanoma. Treatment was continued until progression of disease or unacceptable toxicity.

RESULTS

There were no significant differences between lenalidomide and placebo in overall survival (median 5.9 months vs 7.4 months, respectively; P = .32), time to progression (median 3.0 months vs 2.1 months; P = .19), or Response Evaluation Criteria in Solid Tumors tumor response (5.3% vs 5.8%; P = .82). None of the patients given placebo discontinued treatment because of treatment-related adverse events, compared with 4.6% of those treated with lenalidomide. Treatment-related myelosuppression was observed in 2.0% of patients treated with placebo and 7.3% of patients treated with lenalidomide.

CONCLUSIONS

This study showed that treatment with lenalidomide (25 mg/d) has a manageable safety profile in patients with previously treated metastatic malignant melanoma but no benefit in tumor response, time to progression, or overall survival in these patients. Future trials for treatment of metastatic malignant melanoma with lenalidomide should focus on its use in combination therapies.

Roles of endothelin signaling in melanocyte development and melanoma

Endothelin (Edn) signaling via the G-coupled, Edn receptor type B (Ednrb) is essential for the development of melanocytes from the neural crest (NC) and has been associated with melanoma progression. Edn3 plays varying roles during melanocyte development, promoting the proliferation and self-renewal of NC-derived multi- and bi-potential precursors as well as the survival, proliferation, differentiation and migration of committed melanocyte precursors. Melanocyte differentiation is achieved via the interaction of Ednrb and Kit signaling, with Ednrb being specifically required in the final differentiation step, rather than in the initial specification of melanocytic fate. Ednrb has also been implicated in the de-differentiation of mature melanocytes, a process that takes place during the malignant transformation of these cells. Ednrb was found to be upregulated in melanoma metastases and was shown to alter tumor-host interactions leading to melanoma progression. Antagonists to this receptor were shown to inhibit melanoma cell growth and increase the apoptotic rate of these cells, and to lead to disease stabilization in melanoma patients. Thus, Edn signaling inhibition may prove useful in the treatment of certain types of melanoma.

Results of a multicenter, randomized, double-blind, dose-evaluating phase 2/3 study of lenalidomide in the treatment of metastatic malignant melanoma

BACKGROUND

There are currently no systemic treatments for stage IV melanoma, which have been proven in randomized trials to benefit overall survival (OS). Lenalidomide has efficacy against melanoma in animal models and safety in phase 1 trials. The authors reported the results of a phase 2/3 study comparing the safety and efficacy of 2 doses of lenalidomide in patients with relapsed metastatic melanoma disease refractory to previous treatment with dacarbazine, temozolomide, interleukin-2, or interferon-alpha.

METHODS

A total of 294 patients were randomized to oral lenalidomide at 5 mg or 25 mg dose. Tumor response, time to progression, and OS were evaluated. Treatment continued until disease progression or unacceptable adverse events.

RESULTS

No significant differences in response rate, OS, or time to progression were observed between lenalidomide 25 mg versus 5 mg (overall response rate: 5.5% vs 3.4%, P = .38; median OS: 6.8 months vs 7.2 months, P = .71; and median time to progression: 2.2 months vs 1.9 months, P = .24). Myelosuppression was observed in 37.0% of patients in the 25 mg group and 13.7% of patients in the 5 mg group. Treatment-related serious adverse events were seen in 39.0% of patients at the 25 mg dose and 35.4% of patients at the 5 mg dose.

CONCLUSIONS

Despite the occurrence of treatment-related serious adverse events, approximately 80% of patients continued treatment. The higher dose of lenalidomide did not improve response rate, time to progression, or OS of patients with relapsed/refractory stage IV melanoma. A parallel placebo-controlled study has been conducted to further assess the efficacy of lenalidomide in stage IV melanoma patients.

Therapeutic potential of endothelin receptor modulators: lessons from human clinical trials

The endothelin system, and in particular endothelin receptors, are targets for therapeutic intervention in human diseases. Endothelin receptor antagonists have reached clinical use for treating pulmonary arterial hypertension, and are under clinical investigation for several other diseases, such as cancer, vasospasm or fibrogenic diseases. We review the molecules that have been evaluated in the main clinical trials, from the point of view of receptor selectivity and of their chemical characteristics which were important for efficacy in pulmonary hypertension. We will also discuss future use of antagonists to endothelin receptor(s) in several human diseases and what should be the necessary properties of the future molecules for efficacy in diseases where the presently tested molecules displayed suboptimal efficacy.

Bosentan for pulmonary hypertension and other pulmonary diseases: emerging evidence

Endothelin-1 is a potent vasoconstrictor and mitogen that is primarily synthesized and released from vascular endothelial cells. Bosentan is a dual endothelin-receptor antagonist that initially received approval for treatment of WHO group I pulmonary arterial hypertension (PAH) for patients in functional classes III and IV. Analysis of a study conducted in functional class II patients (Endothelin Antagonist Trial in Mildly Symptomatic PAH Patients [EARLY] trial) suggest its efficacy for these less symptomatic patients. In addition, bosentan has demonstrated efficacy in patients with congenital heart disease and Eisenmengers syndrome with right to left shunting and in HIV-related PAH. Studies of bosentan in inoperable or residual chronic thromboembolic pulmonary hypertension suggest possible efficacy. Bosentan appears promising in patients with idiopathic pulmonary fibrosis who do not have pulmonary hypertension. Combinations of bosentan with other PAH therapies such as iloprost and sildenafil may have incremental benefit over monotherapy.

Stromal endothelin B receptor-deficiency inhibits breast cancer growth and metastasis

The endothelin (ET) axis, often deregulated in cancers, is a promising target for anticancer strategies. Whereas previous investigations have focused mostly on ET action in malignant cells, we chose a model allowing separate assessment of the effects of ETs and their receptors ET(A)R and ET(B)R in the tumor cells and the stromal compartment, which is increasingly recognized as a key player in cancer progression. In homozygous spotting lethal rats (sl/sl), a model of constitutive ET(B)R deficiency, we showed significant reduction of growth and metastasis of MAT B III rat mammary adenocarcinoma cells overexpressing ET(A)R and ET-1 but negative for ET(B)R. Lack of stromal ET(B)R expression did not influence angiogenesis. However, it was correlated with diminished infiltration by tumor-associated macrophages and with reduced production of tumor necrosis factor-alpha, both known as powerful promoters of tumor progression. These effects were almost completely abolished in transgenic sl/sl rats, wherein ET(B)R function is restored by expression of an intact ET(B)R transgene. This shows that tumor growth and metastasis are critically dependent on ET(B)R function in cells of the microenvironment and suggests that successful ETR antagonist therapy should also target the stromal component of ET signaling

Endothelin B receptor, a new target in cancer immune therapy

The endothelins and their G protein-coupled receptors A and B have been implicated in numerous diseases and have recently emerged as pivotal players in a variety of malignancies. Tumors overexpress the endothelin 1 (ET-1) ligand and the endothelin-A-receptor (ET(A)R). Their interaction induces tumor growth and metastasis by promoting tumor cell survival and proliferation, angiogenesis, and tissue remodeling. On the basis of results from xenograft models, drug development efforts have focused on antagonizing the autocrine-paracrine effects mediated by ET-1/ET(A)R. In this review, we discuss a novel role of the endothelin-B-receptor (ET(B)R) in tumorigenesis and the effect of its blockade during cancer immune therapy. We highlight key characteristics of the B receptor such as its specific overexpression in the tumor compartment; and specifically, in the tumor endothelium, where its activation by ET-1 suppresses T-cell adhesion and homing to tumors. We also review our recent findings on the effects of ET(B)R-specific blockade in increasing T-cell homing to tumors and enhancing the efficacy of otherwise ineffective immunotherapy.

Melanoma biology and the promise of zebrafish

Advantageous organismal and technical attributes of the zebrafish are being increasingly applied to study cancer biology. Along with other tumor models, zebrafish that develop melanomas have been generated. In both genetics and phenotype, zebrafish melanomas are strikingly similar to their human counterparts. For this reason, studies in the zebrafish are poised to make significant contributions to melanoma biology. In this review, we summarize important features of human melanoma and discuss how the zebrafish can be used to address many questions that remain unanswered about this devastating disease.

Endothelin receptors and pain

The endogenous endothelin (ET) peptides participate in a remarkable variety of pain-relatedprocesses. Pain that is elevated by inflammation, by skin incision, by cancer, during a Sickle Cell Disease crisis and by treatments that mimic neuropathic and inflammatory pain and are all reduced by local administration of antagonists of endothelin receptors. Many effects of endogenously released endothelin are simulated by acute, local subcutaneous administration of endothelin, which at very high concentrations causes pain and at lower concentrations sensitizes the nocifensive reactions to mechanical, thermal and chemical stimuli.

PERSPECTIVE

In this paper we review the biochemistry, second messenger pathways and hetero-receptor coupling that are activated by ET receptors, the cellular physiological responses to ET receptor activation, and the contribution to pain of such mechanisms occurring in the periphery and the CNS. Our goal is to frame the subject of endothelin and pain for a broad readership, and to present the generally accepted as well as the disputed concepts, including important unanswered questions.

The endothelin axis in cancer: the promise and the challenges of molecularly targeted therapy

The endothelin (ET) axis, which includes ET-1, ET-2, ET-3, and 2 G protein-coupled receptor subtypes, ET AR and ET BR, promotes growth and progression of a variety of tumors, such as prostatic, ovarian, renal, pulmonary, colorectal, cervical, breast, lung, bladder, endometrial carcinoma, Kaposi's sarcoma, brain tumors, and melanoma. Acting on selective receptors, ET-1 regulates mitogenesis, cell survival, angiogenesis, bone remodeling, stimulation of nociceptors, tumor-infiltrating immune cells, epithelial-to-mesenchymal transition, invasion, and metastatic dissemination. At the molecular level, endothelin receptor antagonists, besides providing ideal tools for dissecting the ET axis, have demonstrated their potential in developing novel therapeutic strategies. Emerging experimental and clinical data demonstrate that interfering with endothelin receptors provides an opportunity for the development of rational combinatorial approaches using endothelin receptor antagonists in combination with chemotherapy or molecularly targeted therapy.

Melanocytes in development, regeneration, and cancer

The genes required for stem cell specification and lineage restriction during embryogenesis also play fundamental roles in adult tissue regeneration and cancer. This "development-regeneration-cancer" axis is exemplified by the vertebrate pigmentation system. Melanocytes exhibit almost unlimited self-renewal capacity during regenerative processes such as mammalian hair recoloration and zebrafish fin regeneration. Melanoma utilizes many regulatory signals and pathways required during ontogeny and regeneration. A discussion of these interconnections highlights how studies of stem cell function in embryonic and regenerative contexts can yield insights into melanoma biology.

G-protein-coupled receptors and melanoma

G-protein-coupled receptors (GPCR) are the largest family of receptors with over 500 members. Evaluation of GPCR gene expression in primary human tumors identified over-expression of GPCR in several tumor types. Analysis of cancer samples in different disease stages also suggests that some GPCR may be involved in early tumor progression and others may play a critical role in tumor invasion and metastasis. Currently, >50% of drug targets to various human diseases are based on GPCR. In this review, the relationships between several GPCR and melanoma development and/or progression will be discussed. Finally, the possibility of using one or more of these GPCR as therapeutic targets in melanoma will be summarized.

Endothelin-1-induced pain and hyperalgesia: a review of pathophysiology, clinical manifestations and future therapeutic options

Pain in patients with metastatic cancer contributes to increased suffering in those already burdened by their advancing illness. The causes of this pain are unknown, but are likely to involve the action of tumour-associated mediators and their receptors. In recent years, several chemical mediators have increasingly come to the forefront in the pathophysiology of cancer pain. One such mediator, endothelin-1 (ET-1), is a peptide of 21 amino acids that was initially shown to be a potent vasoconstrictor. Extensive research has revealed that members of the ET family are indeed produced by several epithelial cancerous tumours, in which they act as autocrine and/or paracrine growth factors. Several preclinical and clinical studies of various malignancies have suggested that the ET axis may represent an interesting contributor to tumour progression. In addition, evidence is accumulating to suggest that ET-1 may contribute to pain states both in humans and in other animals. ET-1 both stimulates nociceptors and sensitises them to painful stimuli. Selective stimulation of ET receptors has been implicated as a cause of inflammatory, neuropathic and tumoural pain. ET-1-induced pain-related behaviour seems to be mediated either solely by one receptor type or via both endothelin-A receptors (ETAR) and endothelin-B receptors (ETBR). Whereas stimulation of ETAR on nociceptors always elicits a pain response, stimulation of ETBR may cause analgesia or elicit a pain response, depending on the conditions. The administration of ETAR antagonists in the receptive fields of these nociceptors has been shown to ameliorate pain-related behaviours in animals, as well as in some patients with advanced metastatic prostate cancer. The identification of tumour-associated mediators that might directly or indirectly cause pain in patients with metastatic disease, such as ET-1, should lead to improved, targeted analgesia for patients with advanced cancer. In this review, we will describe the current status of the role of ET-1 in different types of painful syndromes, with special emphasis on its role in the pathophysiology of cancer pain. Finally, potential new treatment options that are based on the role of the ET axis in the pathophysiology of cancer are elaborated.