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The endothelin (ET) system comprises three 21-amino acid peptides (ET-1, ET-2, and ET-3). Of these, only ET-1 appears to play an important physiological and pathophysiological role in the cardiovascular system. ET-1 acts mainly in a paracrine fashion and is an extremely potent and long-lasting vasoconstrictor. ET-1 is released principally from endothelial cells and is initially synthesized as Prepro-ET-1, a 203-amino acid peptide, which is cleaved into big ET-1, a 38-amino acid peptide. Big ET-1 is in turn cleaved into its functional form (ET-1) by endothelin converting enzyme-1 (1–3). As shown in Figure 1, the hemodynamic and proliferative effects of ET-1 are mediated through two receptor subtypes: ETA receptors located on vascular smooth muscle cells that are responsible for the vasoconstricting effects of ET-1 and ETB receptors, located on vascular endothelial cells that are responsible for the vasodilating effects of ET-1 mediated by endothelial derived relaxing factors including nitric oxide (4–6). A sub-population of ETB receptors is also found on vascular smooth muscle cells and induce vasoconstriction when activated (7). In addition to its vasoconstrictor and pressor effects, ET-1 can induce hypertrophy and hyperplasia of cardiomyocytes (8), stimulate fibroblast proliferation (9), increase extra-cellular matrix production (10), and stimulate a pro-inflammatory phenotype (11). ET-1 also alters the generation of vasomediators other than NO, including prostacyclin and platelet-activating factors (7). Circulating plasma ET-1 levels are elevated in atherosclerosis, arterial hypertension, heart failure, and pulmonary arterial hypertension (PAH) compared to the normal state (12). ET-1 plasma levels appear to correlate well with reduced pulmonary function (13) and to predict survival in patients with PAH (14).
The actions of ET-1 led to a wide-spread interest in endothelin antagonists for the treatment of various cardiovascular and other disorders that has been sustained since the initial discovery of a poorly specific endothelin receptor antagonist by Fabregat and Rozengurt in 1990 (15) and generation of compounds based on the ETA receptor antagonist BE-18257A isolated from Streptomyces misakiensis (16). These peptide inhibitors led to small molecule programs at most of the major pharmaceutical companies. We now have a wide range of compounds that are either selective for the ETA receptor, or antagonize both ETA and ETB receptors, also known as combined, or dual antagonists. The promise of therapeutic utility of endothelin antagonists has been realized with several drugs that have proven efficacy in PAH (17–19). Clinical trials continue for a range of targets such as resistant hypertension, chronic kidney disease, and prostate cancer (http://clinicaltrials.gov). One of the most significant developments during the early years of development of these compounds was the founding of a small biotech company, Actelion Pharmaceuticals Ltd. based in Allschwil, Switzerland, whose initial compounds were originally obtained from Roche Pharmaceuticals. This was important because Actelion developed the first US Food and Drug Administration-approved endothelin antagonist, bosentan (Tracleer®), a dual antagonist used clinically for the treatment of PAH (20).
While the efficacy of bosentan and other endothelin antagonists for the treatment of PAH, including selective ETA antagonists (18), is well established, efforts continue to improve efficacy and utility of this class of compounds. Indeed, there are ongoing debates of the relative merits of ETA receptor specific compared to combined receptor antagonism. On the one hand, activation of ETB receptors on endothelial cells enhances NO production (21), which would argue in favor of utilizing selective ETA receptor antagonists. The STRIDE (Sitaxsentan To Relieve Impaired Exercise) trials have demonstrated that the ETA receptor antagonist sitaxsentan, at a dose of 100 mg/day orally resulted in a significant improvements in both 6-minute-walk distance and New York Heart Association functional class as well as improvements in pulmonary vascular resistance (PVR) and cardiac index (CI) (18). However, there is also a subset of ETB receptors located on vascular smooth muscle cells that appear to cause vasoconstriction when activated. Further, at least in animal models of PAH, there can be a longitudinal change in endothelin receptor density with early up-regulation of ETA and down-regulation of ETB receptors followed by a later up-regulation of ETB receptors on vascular smooth muscle (22). Thus, the use of a combined receptor antagonist could attenuate the deleterious effects of endothelin signaling in vascular smooth muscle from both receptors. However, both ETA receptor selective and combined antagonists suffer notable limitations including elevation of liver enzymes (23), and a narrow therapeutic window to attain a significant benefit without increased incidence of adverse events in the absence of an improvement in efficacy in the case of sitaxsentan (18). Thus, the development an optimal ET receptor antagonist is ongoing.
Iglarz and colleagues from Actelion have recently published a study describing a new dual endothelin antagonist, macitentan. This antagonist was designed for tissue targeting by optimizing the chemical nature of this drug to have physicohemical properties that would improve lipophilicity (24). While these investigators are to be applauded for the novelty of their compound, there remain many unanswered questions before we can determine whether there would be any significant clinical benefit of this compound above and beyond existing ET receptor antagonists. Macitentan, also called Actelion-1 or ACT-064992, was discovered through tailored screening to have a high partition coefficient for lipid versus aqueous solutions. In theory, a compound that partitions to lipid environments will be more likely to remain in local tissue environments. This may provide therapeutic advantages such as a prolonged functional half-life in vivo, or as Iglarz and colleagues propose, more effective targeting of predominantly autocrine/paracrine systems such as the endothelin system. Another issue that will need to be investigated is whether the lipophilicity of macitentan enhances penetration of the blood-brain barrier. Although the central effects of endothelin are still poorly understood, there may be clinical benefits to targeting the brain in terms of reductions in sympathetic nerve activity (25), ameliorating deleterious effects of endothelin in stroke (26) and perhaps dementia (27, 28).
The study by Iglarz et al. clearly establishes macitentan and its major metabolite, ACT-132577, as having high affinity for both ETA and ETB receptors in a cell expression system as well as being fairly potent at inhibiting ET-1-induced increases in intracellular Ca2+ (24). Furthermore, they establish in vivo efficacy in lowering blood pressure in mineralocorticoid (DOCA)-salt hypertensive rats, improving survival in monocrotaline-induced pulmonary hypertension, and reducing proteinuria in streptozotocin-induced diabetes. These are some of the key disease models in which prior studies have shown benefit with both selective ETA and dual ETA/ETB antagonists (7). One of the hallmarks of ETB receptor blockade in vivo is elevation of plasma ET-1 levels. Since macitentan increased plasma ET-1 at a 10-fold lower dose compared to bosentan, the authors concluded that macitentan has greater in vivo potency relative to bosentan (24). Caution should be used in over-interpreting these data, however, since plasma ET-1 was measured at only one time-point following acute oral administration of the two drugs. More relevant is a functional readout, and the authors reported that acute administration of macitentan and bosentan produced similar maximal reductions in blood pressure of DOCA-salt hypertensive rats but that this was more prolonged and achieved at a 10-fold lower dose with macitentan. While the authors suggest that the greater potency of macitentan to the drug’s is due to its lipophilic nature, they will need to provide direct in vitro comparison of binding affinity of macitentan and bosentan as well as data demonstrating concentrations of the drugs achieved in the plasma. Thus one cannot exclude the possibility that the difference in potency in vivo between macitentan and bosentan may be attributable to an improved binding affinity or oral bioavailability and not due to partitioning into tissues. Importantly, these investigators will need to provide quantitative information that demonstrates actual accumulation or targeting in tissues compared to a compound with more hydrophilic properties.
One attractive feature of an endothelin antagonist that targets or accumulates in tissues is that ET-1 is a paracrine and autocrine factor. In this sense, the characteristics of macitentan to partition to a lipid environment would provide an advantage. A limitation to the Iglarz et al. study is that these investigators did not report tissue concentrations of macitentan or its major, more long-lived metabolite ACT-132577, either following acute administration or chronic treatment. It would have been particularly informative to compare tissue concentrations of these compounds and of bosentan (which is also lipophilic) in the disease models studied, especially in the relevant target organs (lungs for PAH, kidney for diabetes). The question remains whether the characteristic of macitentan to accumulate in tissues provides any improvement over a similarly potent compound in the in vivo models. Also, it also remains to be demonstrated whether a lipid targeted compound actually provides prolonged and more effective blockade of ET receptors relative to a less lipophilic compound with equal receptor binding affinity. The extracellular milieu in which the drug must first enter is clearly aqueous. Furthermore, endothelin receptors are presumed to be extracellular and are exposed to this aqueous environment. It is not clear where in the body a lipophilic endothelin antagonist would partition itself. These types of analyses will be required in order to better understand whether this is a more effective approach for endothelin blockade.
Another topic that will need to be addressed in future studies is whether prolonged tissue accumulation would represent an advantage particularly in the event of the development of unwanted side effects. Liver toxicity as determined by elevated liver aminotransferases seems to be a class effect of endothelin receptor antagonists, with the severity of this adverse effect being greater at higher doses (23). ETA receptor selective antagonists may exhibit less liver toxicity, but are frequently associated with increased incidence of peripheral edema and headache (23). Given the much lower doses of macitentan compared to bosentan (30 mg/kg vs. 300 mg/kg, respectively) required to obtain a significant effect on the PAH phenotype it is possible that this could reduce the overall liver toxicity associated with the use of endothelin receptor antagonists, although this remains to be determined. In addition, while there have been no reports of rebound effects of withdrawing either selective or dual antagonists, it remains possible that a compound that would increase plasma ET-1 at a lower dose might have a greater likelihood of ET-1-induced effects once drug treatment is withdrawn. This is a possibility that would need to be monitored in future investigations.
Macitentan represents a dual endothelin receptor antagonist by virtue of its ability to block both ETA and ETB receptors simultaneously. A fairly convincing argument has been made by the investigators at Actelion that this represents a significant therapeutic advantage because of the presence of ETB receptors on vascular smooth muscle that can contribute to the vasoconstrictor properties of ET-1 that appear to play a role in pulmonary hypertension and perhaps other vascular disorders as well. However, as activation of ETB receptors on endothelial cells is known to enhance NO generation (21), macitentan could, in theory, further attenuate NO signaling which could exacerbate a hypertensive phenotype. Moreover, ETB receptors on tubular elements within the kidney play a critical role in the control of sodium excretion, and interference with ETB receptor function promotes salt-sensitive hypertension (29, 30). Thus one could argue that ETA receptor blockade may be more advantageous than combined ETA and ETB receptor blockade. Clearly both dual and selective antagonists have proven clinical efficacy in several applications (23) and so one could argue that it does not matter whether the ETB receptor is blocked or not. Unfortunately, there are very few studies that show direct, head-to-head comparison of dual versus selective antagonists, especially in humans. What few studies have done direct comparisons do not provide sufficient answers to this question since there are data both in support of both arguments, for example Nishida et al. (31) vs. Jasmin et al. (32) in monocrotaline-induced pulmonary hypertension. Furthermore, and as discussed above, there are many circumstances where receptor expression is altered by disease making these comparisons more important. Iglarz and colleagues argue that ETB receptors mediate the deleterious effects of ET-1 in pathology, but this may only apply to specific indications. In the case of diabetic nephropathy, for example, animal studies have shown that the absence of ETB function can worsen the development of renal dysfunction and injury (33). Another unfortunate reality is the lack of direct comparison of selective vs, nonselective antagonists in clinical trials. Therefore, it will be left to academic investigators to undertake studies comparing the efficacy of dual versus selective antagonists, which may be difficult without industry support.
In conclusion, there is a clear need for more effective treatments for devastating diseases such as pulmonary hypertension, cancer, and chronic kidney disease. Targeting the endothelin system has already proven to offer benefit in these diseases, but there is still much room for improvement in this class of drugs. The approach of designing endothelin receptor antagonists to more effectively target this predominantly autocrine/paracrine system may lead to therapeutic advances. Thus, we await future studies with interest that will examine tissue accumulation and distribution, toxicity and side-effect profiles, and efficacy of more lipophilic, tissue-targeted endothelin receptor antagonists.