PD-156707: A Selective Endothelin-A Receptor Antagonist

Endothelin antagonist-induced coronary and systemic arteritis in the beagle dog

Two endothelin antagonists, ZD1611 (3-[4-[3-(3-methoxy-5-methylpyrazin-2-ylsulfamoyl)-2-pyridyl]phenyl]-2,2-dimethylpropanoic acid) and ZD2574 (2-(4-isobutylphenyl)-N-(3-methoxy-5-methylpyrazin-2-yl)pyridine-3-sulfonamide), selective for the ET(A) receptor and intended for use in pulmonary hypertension, were tested in Beagle dogs at various doses for periods of up to 4 weeks. These studies included in vivo telemetric hemodynamic assessment, full histopathological and ultrastructural pathological evaluation of coronary arteries. Both drugs produced arteritis in small- and medium-sized coronary arteries after single or multiple doses, some of which were at or below the ED50. The distribution of lesions was predominantly in extramural arteries over the atria and atrioventricular groove of the right side of the heart and consisted of epicardial hemorrhage and arteritis. Systemic arteritis was also present at a lower incidence than the coronary arteritis, was located at different sites and appeared inconsistently. Ultrastructural changes in coronary arteries suggested that damage was the result of mechanical factors. Although these patterns of vascular injury possessed features in common with those induced in dogs by high doses of vasodilating antihypertensive drugs and inotropic agents, they were atypical, as there was no left ventricular myocardial necrosis, papillary muscle damage, or subendocardial hemorrhage suggestive of ischaemia or excessive inotropism. Moreover, physiological monitoring showed no evidence of exaggerated systemic hypotension or reflex tachycardia at doses associated with vascular damage. Consequently, the changes might be the result of a localized pharmacological process such as intense, prolonged vasodilatation in unsupported arteries that are well endowed with endothelin receptors and particularly sensitive to endothelin antagonism.

Inhibition of in vivo constriction of fetal ductus arteriosus by endothelin receptor blockade in rats

The fetal ductus can be constricted by drugs, including cyclooxygenase inhibitors (indomethacin), nitric oxide synthesis antagonists [N-nitro-L-arginine monomethyl ester (L-NAME)], and glucocorticoid hormones (dexamethasone). Constriction of the fetal ductus by endothelin (ET) 1 was reported in an in vitro study. We studied the preventive effect of a dual ET receptor antagonist (bosentan) and a selective ET-A blocker (CI-1020) on pharmacologic fetal ductal constriction in rats. Near-term pregnant Wistar rats at d 21 and preterm rats at d 19 were used. The fetal ductus was constricted by four medications: orogastric administration of indomethacin (10 mg/kg) on fetal d 21, orogastric indomethacin 1 mg/kg combined with muscular injection of L-NAME (10 mg/kg) on fetal d 21, and muscular injection of L-NAME or dexamethasone (1 mg/kg) on fetal d 19. Bosentan (0.1, 1, 10, or 100 mg/kg) was injected intraperitoneally either simultaneously with indomethacin, L-NAME, or dexamethasone, or 4 h after administration of 10 mg/kg indomethacin. CI-1020 (0.01, 0.1, 1, or 10 mg/kg) was injected intraperitoneally simultaneously with indomethacin. After maternal atlas dislocation, cesarean section, fetal whole-body freezing, and cutting on the freezing microtome, measurements were made of the inner diameters of the ductus, main pulmonary artery, and ascending aorta. Bosentan blocked fetal ductal constriction by indomethacin, indomethacin plus L-NAME in the near-term rats, and constriction by L-NAME and dexamethasone in the preterm rats dose dependently. Fetal ductal constriction was nearly completely blocked by simultaneously administered 100 mg/kg of bosentan or 10 mg/kg of CI-1020. Dual ET receptor antagonist (bosentan) and selective ET-A blocker (CI-1020) prevent constriction of the fetal ductus arteriosus induced by ductus-constricting agents in rats, indicating that ET and ET-A receptors are essential in fetal ductal constriction.

Differentiating spontaneous from drug-induced vascular injury in the dog

When vascular injury is observed in dogs used in preclinical toxicology studies, careful evaluation of the lesions is warranted, especially when differentiating drug-induced vascular changes from spontaneous findings, such as idiopathic canine polyarteritis. The clinical signs as well as the nature and distribution of lesions can often be distinguishing, as is the case with vasoactive drugs, including vasodilators and/or positive inotropes (hydralazine, minoxidil, endothelin receptor antagonists, and phosphodiesterase III inhibitors). For most types of vasodilator-induced vascular injury, the lesion is often restricted to coronary arteries, whereas in idiopathic canine polyarteritis, arterial lesions not only involve coronary arteries, but also medium to small arteries of other organs. In addition, the nature of the changes in vessels yields important clues. Medial and adventitial hemorrhage is generally associated with vasodilator-induced arterial lesion, whereas hemorrhage is generally absent in idiopathic polyarteritis. Although idiopathic canine polyarteritis can generally be differentiated from vasoactive-induced vascular injury in dogs, there are increasing incidences of this type of polyarteritis in dogs receiving any 1 of a number of unrelated classes of compounds, suggestive of an exacerbation of the spontaneous disease. Therefore, in order to differentiate drug-induced injury from idiopathic canine polyarteritis, it is critical that examination of the vascular pathology be conducted with good understanding of clinical, pharmacological, and mechanistic data associated with the drug.

Studies on coronary arteriopathy in dogs following administration of CI-1020, an endothelin A receptor antagonist

A selective nonpeptide endothelin A (ETA) receptor antagonist, CI-1020, was administered to beagle dogs intravenously (i.v.) for 4 hours to 4 weeks. One animal/sex received CI-1020 at 1 mg/kg/hr intravenously for 4, 8, or 24 hours to investigate onset of arteriopathy. Control animals (1/sex) received the vehicle only. To determine reversibility of arteriopathy, 8 dogs/sex were given CI-1020 at 1 mg/kg/hr for 4 days. Two dogs/sex were sacrificed 1, 3, 8, and 29 days following cessation of infusion. Lesion development with prolonged exposure was investigated in 1 male dog. It was given CI-1020 by i.v. bolus at 120 mg/kg/day for 4 weeks and Monastral blue dye was administered i.v. to facilitate localization of vascular lesions. Coronary blood flow was determined in 4 dogs infused with CI-1020 at 0.3, 3, and 30 mg/kg for one hour at each dose. Macroscopically, hemorrhage or blue discoloration of Monastral blue was noted in the extramural coronary arteries along the coronary groove and atrium. Histologically, the earliest coronary changes were noted in animals sacrificed after 24 hours of treatment and characterized by medial hemorrhage and necrosis with a few infiltrating neutrophils. In the reversibility study, incidence and severity of arteriopathy was dependent on time of sacrifice following cessation of infusion. Acute necrotizing inflammation of arteries was present in all animals (n = 4) on day 1 postinfusion, whereas on day 8 postinfusion, lesions characterized by medial small pockets of trapped red cells, cell debris, and adventitial thickening were seen in 1 dog/sex. By day 29 postinfusion, coronary arteries were similar to controls. In the dog given daily i.v. bolus injections of CI-1020 for 4 weeks, arterial inflammatory lesions varied from acute to chronic, although most lesions were considered chronic active. Monastral blue pigments were noted in the wall of most arteries with chronic or chronic active lesions. Acute lesions were similar to those noted in day 1 postinfusion of the reversibility study. Medial smooth muscle necrosis and/or fibrosis with mixed inflammatory cell infiltrates characterized chronic or chronic active lesions. Smooth muscle proliferation and migration into the intima were also noted. There were no significant changes in coronary blood flow, coronary vascular resistance, or mean arterial blood pressure following CI-1020 infusion for 3 hours. In the 24-hour infusion study, plasma endothelin 1 (ET-1) levels were mildly elevated (1.5-4 fold) during CI-1020 infusion when compared to either pretest or control values. These results indicate that administration of endothelin antagonist (CI-1020) to dogs was associated with development of coronary arteriopathy, which was completely resolved within 29 days following cessation of treatment. With prolonged (4-week) CI-1020 treatment, arterial lesions at varying stages of development (acute, chronic active, chronic) were seen, suggesting that tolerance to treatment (up to 4 weeks) does not occur.

Patterns of cardiovascular pathology induced by diverse cardioactive drugs

In the beagle dog, exaggerated hypotension and tachycardia following administration of high doses of vasodilating antihypertensive drugs are associated with vascular injury and characteristic patterns of myocardial necrosis and haemorrhage. Cardiac and vascular inflammation and necrosis also occur in dogs in association with different functional changes including severe hypertension and the effects that follow treatment with high doses of vasoconstrictor and pressor drugs. More recently, cardioactive drugs of novel classes such as the endothelin antagonists have also been shown to produce vascular damage in the beagle dog but in the absence of ischaemic myocardial damage or significant haemodynamic alterations that typically follow administration of high doses of vasodilating antihypertensive or pressor drugs. This underlines the importance of a careful analysis of the patterns of cardiovascular pathology, their dose, temporal and spatial relationships in the context of functional changes.

Coronary arterial lesions induced by endothelin antagonists

Endothelins are potent vasoconstrictors and pressor peptides and are important mediators of cardiac, renal and endocrine functions. Increased ET-1 levels in disease states such as congestive heart failure, pulmonary hypertension, acute myocardial infarction, and renal failure suggest the endothelin system as an attractive target for pharmacotherapy. A non-peptidic, selective, competitive endothelin receptor antagonist with an affinity for the ET(A) receptor in the subnanomolar range was administered by continuous intravenous infusion to beagle dogs, rats, and Goettingen minipigs. It caused mild arteriopathy characterised by segmental degeneration in the media of mid- to large-size coronary arteries in the heart of dog, but not rat or minipig. The lesions only occurred in the atrium and ventricle. Frequency and severity of the vascular lesions was not sex or dose related. No effects were noted in blood vessels in other organs or tissue. Plasma concentrations at steady state, and overall exposure in terms of AUC((0-24h)) were higher in minipig and rat than the dog but did not cause cardiac arteriopathy. These findings concur with those caused by other endothelin anatagonists, vasodilators and positive inotropic/vasodilating drugs such as potassium channel openers, phosphodiesterase inhibitors and peripheral vasodilators, and confirm that dogs appear to be uniquely sensitive to the development of cardiac vascular lesions.

Coronary arteriopathy in monkeys following administration of CI-1020, an endothelin A receptor antagonist

A selective non-peptide endothelin A (ETA) receptor antagonist, CI-1020, was administered to cynomolgus monkeys intravenously (i.v.) for 2 or 4 wk and orally for 4 wk. Groups consisting of 3 animals of each sex received CI-1020 at 1, 5, and 10 mg/kg/hr (i.v.) or orally at 250, 500, and 750 mg/kg body weight for 4 wk. Control animals received the vehicle only. In a separate experiment, 1 male was infused with 10 mg/kg/hr for 2 wk, and Monastral blue dye was administered i.v. to facilitate localization of lesions to the vascular walls. One female was administered saline and the dye and served as a control. One female at 1 mg/kg/hr was found dead at week 2, and 1 female at 5 mg/kg/hr was euthanatized during week 4 as a result of severe thigh swelling at the catheter site. Macroscopically, extramural coronary arteries appeared thickened and nodular in the 4-wk i.v. study in the female found dead at 1 mg/kg/hr, in 1 male and 1 female at 5 mg/kg/hr, and in 2 females at 10 mg/kg/hr. Histologically, Monastral blue pigment trapped in the walls of coronary arteries with arteriopathy was observed in the male treated with CI-1020 at 10 mg/kg/hr for 2 wk. Extramural coronary arteriopathy occurred at all doses in the 4-wk i.v. study, with higher incidence occurring in females than in males (7 of 9 treated females compared with 3 of 9 treated males). In the oral study, 1 female at 500 mg/kg/day and 1 male and 2 females at 750 mg/kg/day had coronary arteriopathy. Histological changes after 2 wk of treatment were characterized by intimal thickening, fragmentation of the internal elastic lamina, necrosis and edema of the media, and mixed inflammatory-cell infiltrates in the intima, media, and adventitia. After 4 wk of i.v. administration, arteriopathy was characterized by segmental disruption of the elastic lamina and intimal and medial fibrosis with complete replacement of smooth muscle with fibrous tissue. The adventitia was thickened as a result of fibrosis and mixed or mononuclear inflammatory-cell infiltrates. CI-1020 concentrations were higher in males (1.57 to 29 micrograms/ml) than in females (0.974 to 24.4 micrograms/ml) in the i.v. study. Transient systemic exposure with high maximum plasma concentration (Cmax) (120-352 micrograms/ml) in the oral study was insufficient to provoke arterial changes of the same magnitude as those noted with continuous i.v. administration. The regeneration of the media by fibrous tissue and the disruption of the elastic lamina may weaken the arterial wall and increase the susceptibility of the artery to the development of aneurysm.