Hans-Peter Baum

Structure-based design of aliskiren, a novel orally effective renin inhibitor

Hypertension is a major risk factor for cardiovascular diseases such as stroke, myocardial infarction, and heart failure, the leading causes of death in the Western world. Inhibitors of the renin-angiotensin system (RAS) have proven to be successful treatments for hypertension. As renin specifically catalyses the rate-limiting step of the RAS, it represents the optimal target for RAS inhibition. Several peptide-like renin inhibitors have been synthesized previously, but poor pharmacokinetic properties meant that these compounds were not clinically useful. We employed a combination of molecular modelling and crystallographic structure analysis to design renin inhibitors lacking the extended peptide-like backbone of earlier inhibitors, for improved pharmacokinetic properties. This led to the discovery of aliskiren, a highly potent and selective inhibitor of human renin in vitro, and in vivo; once-daily oral doses of aliskiren inhibit renin and lower blood pressure in sodium-depleted marmosets and hypertensive human patients. Aliskiren represents the first in a novel class of renin inhibitors with the potential for treatment of hypertension and related cardiovascular diseases.

Prolonged angiotensin II antagonism in spontaneously hypertensive rats. Hemodynamic and biochemical consequences.

The present study examines the effects of prolonged angiotensin II antagonism in spontaneously hypertensive rats by using an angiotensin II receptor antagonist (DuP 753) that is devoid of agonistic properties and selective for the subtype 1 of the angiotensin II (AT,) receptor. The antihypertensive effects of DuP 753 and its effects on circulating parameters of the reninangiotensin system were compared with those of a converting enzyme inhibitor (benazeprilat). To minimize any influence of differences in the pharmacokinetic properties of the two blockers, administration was by continuous intravenous infusion. The experiments were performed in conscious, freely moving rats with continuous 24-hour monitoring of blood pressure. DuP 753 (10 or 30 mg/kg/day) lowered mean arterial pressure to the same extent as benazeprilat (3 or 10 mg/kg/day) during a 48-hour period. The antihypertensive effect was sustained when the treatment was extended to 7 days (DuP 753,10 mg/kg/day, benazeprilat, 3 mg/kg/day). Neither of the compounds affected the baseline or diurnal rhythm of heart rate. Plasma concentrations of renin and angiotensin II were increased sevenfold and 10-fold, respectively, in the rats treated with DuP 753. In rats treated with benazeprilat, plasma renin concentration increased threefold, whereas angiotensin II was unchanged. Heart weights were significantly reduced to a similar extent by DuP 753 and benazeprilat Both compounds also induced a smaller but significant decrease in blood pressure in Wistar-Kyoto rats. Our results indicate that the antihypertensive effects of converting enzyme inhibitors in spontaneously hypertensive rats are mainly due to the blockade of the renin-angiotensin system. In this rat model, angiotensin II appears to play an important role in the maintenance of hypertension that is mediated via the AT, receptor.

Sustained Reduction in Blood Pressure During Chronic Administration of a Renin Inhibitor to Normotensive Marmosets

In this study, the hypotensive efficacy of a renin inhibitor was investigated during chronic administration. The renin inhibitor CGP 29 287 was administered by continuous intraperitoneal infusion with osmotic mini-pumps to normotensive marmosets fed a low-sodium diet. Marmosets received a dose of 0.3 (n = 5), 3.0 (n = 5), or 30 (n = 6) mg/kg/day for 14 days. Blood pressure (BP) and heart rate (HR) were measured in conscious animals by the tail-cuff method. Plasma renin activity (PRA) was reduced by 68% and 84%, respectively, by the 0.3 and 3.0 mg/kg/day doses. However, these doses had no significant effect on BP. Plasma-renin activity was reduced by 93% after 2 days of administration of the 30 mg/kg/day dose, and BP was significantly reduced (-24 ± 4 mm Hg). After 14 days, PRA had recovered to pretreatment levels, but the hypotensive response persisted (-20 ± 3 mm Hg). Despite the fall in BP, HR was not increased (292 ± 22,283 ± 16, and 267 ± 8 beats/min on days 0, 2, and 14, respectively). These findings indicate that, after 2 days of continuous administration, a dose of CGP 29 287 that induces almost complete inhibition of PRA is required to induce a significant fall in BP. Al-though there is subsequent recovery of PRA, the hypotensive response persists for 14 days. Thus, after chronic administration, the hypotensive efficacy of this renin inhibitor does not correlate with the degree of inhibition of PRA. However, these results indicate that renin inhibitors are effective hypotensive agents after chronic ad-ministration.

Comparison of Chronic Inhibition of Renin and Converting Enzyme in the Marmoset

The primate-specific renin inhibitor CGP 29287 (30 mg/kg/d, n = 5) or the converting-enzyme inhibitor CGS 14831 (30 mg/kg/d, n = 8) were administered by continuous intraperitoneal infusion via osmotic minipumps to normotensive marmosets fed a low salt diet. CGP 29287 and CGS 14831 induced a similar reduction in blood pressure after 2 days of administration (-22 +/- 6, SEM and -24 +/- 6 mmHg respectively). The hypotensive response persisted at 7 days (-20 +/- 3 and -22 +/- 8 mmHg respectively). Despite the fall in blood pressure, heart rate was not changed after either inhibitor. Blood pressure and heart rate remained stable in control marmosets that received vehicle only (0.9% saline). Plasma renin activity was inhibited after CGP 29287 (100% at day 2 and 75% at day 7) and increased after CGS 14831 (4 and 3 fold on days 2 and 7 respectively). Total plasma immunoreactive renin was increased to a similar extent after both inhibitors (approximately 5 fold on days 2 and 7). These findings show that a similar hypotensive response is induced in sodium-depleted primates after chronic inhibition of renin or converting enzyme. Thus the fall in blood pressure after chronic treatment with either inhibitor appears to be mainly due to the blockade of the renin-angiotensin system and the consequent reduction in endogenous angiotensin II formation.

Inhibition of renin-like activity in marmoset tissues by the renin inhibitor CGP 29 287.

This study investigated the contribution of tissue renin-like activity to the acute hypotensive response induced by the potent and specific inhibitor of primate renin, CGP 29 287. Furosemide-pretreated marmosets (5 mg/kg, i.m.) were killed 30 min after bolus injection of CGP 29 287 (1 mg/kg, i.v., n=4). At this time, plasma renin activity is completely inhibited and the hypotensive response is maximal. Control marmosets received vehicle (0.9% saline, 1 ml/kg, i.v.). Renin-like activity was measured by the rate of angiotensin I (ANG I) formation after incubation of tissue homogenates with sheep angiotensinogen at pH 6.0 and 7.4. Activity was detected in the brain, heart, aorta and kidney of CGP 29 287-treated and control marmosets, and there were no significant differences in the values between the two groups. Addition of CGP 29 287 (5 × 10-6mol/l) to tissue homogenates in vitro inhibited ANG I formation (> 80%). These results indicate that CGP 29 287 is an inhibitor of tissue renin-like activity in vitro, but does not inhibit activity in these tissues after acute administration in vivo. Thus, the acute hypotensive response to CGP 29 287 appears to be due to inhibition of circulating renin only.

Haemodynamic effects of acute and chronic renin inhibition in marmosets

We have developed marmoset models for the in vivo evaluation of primate-specific inhibitors of human renin. After acute intravenous administration to normotensive sodium-depleted marmosets, renin inhibitors of different structural types induced a maximum hypotensive response of a magnitude similar to that induced after angiotensin converting enzyme (ACE) inhibition. The response was prevented by pretreatment with an ACE inhibitor. A close relationship between the inhibition of plasma renin activity (PRA) and the fall in blood pressure was observed with most of the inhibitors. CGP 29287, a synthetic renin inhibitor, and R-3-36-16, a monoclonal antibody, both induced a selective increase in renal blood flow similar to that induced by an ACE inhibitor. A sustained reduction in blood pressure was observed during continuous administration of CGP 29 287 or R-3-36- 16 over 14 days, despite an increase in immunoreactive renin and an apparent recovery of PRA. A similar blood pressure fall and an increase in plasma renin was observed during continuous administration of an ACE inhibitor. The renin inhibitor CGP 29287 also lowered blood pressure after acute administration to hypertensive marmosets with normal PRA. Our studies demonstrate that renin inhibitors have similar haemodynamic effects to ACE inhibitors, and indicate that they may have a similar antihypertensive efficacy.