Cardiopulmonary indications for endothelin receptor antagonists: review of recent efficacy trials

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.

Attenuation of pulmonary vascular hypertension and cardiac hypertrophy with sitaxsentan sodium, an orally active ET(A) receptor antagonist

Effects of sitaxsentan (TBC11251), an orally active, highly selective antagonist of endothelin A receptors, were examined on the development and maintenance of pulmonary hypertension, pulmonary vascular remodeling, and cardiac hypertrophy in the rat. The pulmonary vasoconstrictor response to acute hypoxia (10% O(2)for 90 min) was prevented with sitaxsentan (5 mg/kg infused iv 10 min prior to the onset of hypoxia) while BQ-788 (a specific endothelin B receptor antagonist) was without effect. The same dose of sitaxsentan delivered iv 50 min after the onset of hypoxia reversed the established pulmonary vasoconstriction. In a 2-week model of hypoxia using 10% O(2), treatment with sitaxsentan (15 mg/kg per day in drinking water) attenuated pulmonary hypertension and the associated right ventricular hypertrophy, and prevented the remodeling of small pulmonary arteries (50-100 microM) without affecting systemic arterial blood pressure or heart rate. Institution of sitaxsentan treatment (15 and 30 mg/kg per day in drinking water) for 4 weeks after 2 weeks of untreated hypoxia produced a significant, dose dependent reversal of the established pulmonary hypertension, right heart hypertrophy, and pulmonary vascular remodeling despite continued hypoxic exposure. Sitaxsentan blocked increased plasma endothelin levels in the prevention protocol but did not affect the established elevated levels in the intervention study. Sitaxsentan dose dependently (10 and 50 mg/kg per day in the drinking water) attenuated right ventricular systolic pressure, right heart hypertrophy, and pulmonary vascular remodeling observed 3 weeks after a single subcutaneous injection of monocrotaline. These findings support the hypothesis that endothelin-1 plays a significant role in the development of pulmonary hypertension, pulmonary vascular remodeling, and the associated cardiac hypertrophy, and further suggest that specific endothelin-A receptor blockade may be useful in the treatment of pulmonary hypertension of diverse etiologies.

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.

Efficacy of CI-1020, an endothelin-A receptor antagonist, in hypoxic pulmonary hypertension

We previously showed that CI-1020, an endothelin (ET)-A-selective receptor antagonist, dose-dependently blocked acute hypoxic pulmonary hypertension (PH) in rats. In this study we show that CI-1020 can reverse existing PH and prevent progression of right ventricular hypertrophy (RVH) in rats exposed to chronic hypoxia. Male Sprague-Dawley rats were exposed to 20 days of hypoxia (10% O2) with CI-1020 treatment (20 or 40 mg/kg/day) starting on day 10. On day 20 of hypoxia, the rats were instrumented under anesthesia with a pulmonary artery cannula and allowed to recover to consciousness before measurement of mean pulmonary arterial pressure (MPAP). Blood samples were then collected for plasma ET-1 measurements, the rats killed, and their hearts dissected, dried, and weighed. RV/LV + septum ratio (g/g) was used as an index of RVH (RVHi). Normoxic rats and rats exposed to hypoxia for only 10 days were also evaluated as controls. Normoxic rats had MPAPs of 13 +/- 1 mm Hg, plasma ET-1 levels of 2.1 +/- 0.1 pg/ml, and an average RVHi of 0.29 +/- 0.03. Rats exposed to 10 or 20 days of hypoxia had MPAPs of 33 +/- 2 and 44 +/- 0 mm Hg, plasma ET-1 levels of 4.2 +/- 0.8 and 4.6 +/- 0.8 pg/ml, and average RVHis of 0.47 +/- 0.05 and 0.52 +/- 0.03, respectively. In comparison, rats treated with CI-1020 had MPAPs that were 37% (20 mg/kg/day) and 44% (40 mg/kg/day) lower than untreated 20-day hypoxic rats. Furthermore, rats dosed with 40 mg/kg/day of CI-1020 had MPAPs that were significantly lower (24%) than control 10-day hypoxic rats, indicating a significant reversal of PH. Along with this reversal in PH, their average RVHi was 23% lower (p < 0.05) relative to untreated 20-day hypoxic rats.