Effects of firibastat in combination with enalapril and hydrochlorothiazide on blood pressure and vasopressin release in hypertensive DOCA-salt rats

In the brain, aminopeptidase A (APA) generates angiotensin III, one of the effector peptides of the brain renin-angiotensin system (RAS), exerting tonic stimulatory control over blood pressure (BP) in hypertensive rats. Oral administration of firibastat, an APA inhibitor prodrug, in hypertensive rats, inhibits brain APA activity, blocks brain angiotensin III formation and decreases BP. In this study, we evaluated the efficacy of firibastat in combination with enalapril, an angiotensin I-converting enzyme inhibitor, and hydrochlorothiazide (HCTZ), in conscious hypertensive deoxycorticosterone acetate (DOCA)-salt rats, which display high plasma arginine-vasopressin levels, low circulating renin levels and resistance to treatment by systemic RAS blockers. We determined mean arterial BP, heart rate, plasma arginine-vasopressin levels and renin activity in DOCA-salt rats orally treated with firibastat, enalapril or HCTZ administered alone or in combination. Acute oral firibastat administration (30 mg/kg) induced a significant decrease in BP, whereas enalapril (10 mg/kg) or HCTZ (10 mg/kg) administered alone induced no significant change in BP in conscious DOCA-salt rats. The BP decrease induced by acute and nine-day chronic tritherapy [Firibastat+Enalapril+HCTZ] was significantly greater than that observed after bitherapy [Enalapril+HCTZ]. Interestingly, the chronic administration of a combination of firibastat with [Enalapril+HCTZ] reduced plasma arginine-vasopressin levels by 62% relative to those measured in DOCA-salt rats receiving bitherapy. Our data show that tritherapy with firibastat, enalapril and HCTZ improves BP control and arginine-vasopressin release in an experimental salt-dependent model of hypertension, paving the way for the development of new treatments for patients with currently difficult-to-treat or resistant hypertension.