Studies directed toward the design of orally active renin inhibitors. 2. Development of the efficacious, bioavailable renin inhibitor (2S)-2-benzyl-3- [[(1-methylpiperazin-4-yl)sulfonyl]propionyl]-3-thiazol-4-yl-L-alanine amide of (2S,3R,4S)-2-amino-1-cyclohexyl-3,4-dihydroxy-6-methylheptane (A-72517)

Decades-old renin inhibitors are still struggling to find a niche in antihypertensive therapy. A fleeting look at the old and the promising new molecules

Hypertension is a diverse illness interlinked with cerebral, cardiovascular (CVS) and renal abnormalities. Presently, the malady is being treated by focusing on Renin- angiotensin system (RAS), voltage-gated calcium channels, peripheral vasodilators, renal and sympathetic nervous systems. Cardiovascular and renal abnormalities are associated with the overactivation of RAS, which can be constrained by angiotensin- converting enzyme inhibitors (ACEIs), angiotensin II (Ang-II) -AT1 receptor blockers (ARBs) and renin inhibitors. The latter is a new player in the old system. The renin catalyzes the conversion of angiotensinogen to Angiotensin I (Ang-I). This can be overcome by inhibiting renin, a preliminary step, eventually hinders the occurrence of the cascade of events in the RAS. Various peptidomimetics, the first-generation renin inhibitors developed six decades ago have limited drug-like properties as they suffered from poor intestinal absorption, high liver first-pass metabolism and low oral bioavailability. The development of chemically diverse molecules from peptides to nonpeptides expanded the horizon to achieving direct renin inhibition. Aliskiren, a blockbuster drug that emerged as a clinical candidate and got approved by the US FDA in 2007 was developed by molecular modeling studies. Aliskiren indicated superior to average efficacy and with minor adverse effects relative to other RAS inhibitors. However, its therapeutic use is limited by poor oral bioavailability of less than 2% that is similar to first-generation peptidic compounds. In this review, we present the development of direct renin inhibitors (DRIs) from peptidic to nonpeptidics that lead to the birth of aliskiren, its place in the treatment of cardiovascular diseases and its limitations.

Discovery of benzimidazole derivatives as orally active renin inhibitors: Optimization of 3,5-disubstituted piperidine to improve pharmacokinetic profile

We previously identified 2-tert-butyl-4-[(3-methoxypropyl)amino]-N-(2-methylpropyl)-N-[(3S,5R)-5-(morpholin-4-ylcarbonyl)piperidin-3-yl]pyrimidine-5-carboxamide 3 as a potent renin inhibitor. Since 3 showed unacceptably low bioavailability (BA) in rats, structural modification, using SBDD and focused on physicochemical properties was conducted to improve its PK profile while maintaining renin inhibitory activity. Conversion of the amino group attached at the 4-position of pyrimidine to methylene group improved PK profile and decreased renin inhibitory activity. New central cores with carbon side chains were explored to improve potency. We had designed a series of 5-membered azoles and fused heterocycles that interacted with the lipophilic S3 pocket. In the course of modification, renin inhibitory activity was enhanced by the formation of an additional hydrogen bonding with the hydroxyl group of Thr77. Consequently, a series of novel benzimidazole derivatives were discovered as potent and orally bioavailable renin inhibitors. Among those, compound 13 exhibited more than five-fold of plasma renin inhibition than aliskiren in cynomolgus monkeys at dose ratio.

N-acyldihydropyridones as synthetic intermediates. A stereoselective synthesis of acyclic amino alcohols containing multiple chiral centers

Various multisubstituted piperidines containing a phenyl group at C-2 can be opened regio- and stereoselectively with cyanogen bromide. The ring-opened products contain useful cyanamide and benzylic bromide functional groups. The benzyl bromide can be cleanly reduced, or substituted with various nucleophiles via an S(N)2 process to add additional heteroatoms stereoselectively. This methodology is useful for the stereoselective synthesis of uniquely substituted alkylamine derivatives containing multiple chiral centers and various functionality. Diastereomerically pure amino alcohols containing three to five contiguous stereocenters were prepared using this strategy.

A bioavailability score

Responding to a demonstrated need for scientists to forecast the permeability and bioavailability (F) properties of compounds before their purchase, synthesis, or advanced testing, we have developed a score that assigns the probability that a compound will have F > 10% in the rat. Neither the rule-of-five, log P, log D, nor the combination of the number of rotatable bonds and polar surface area successfully categorized compounds. Instead, different properties govern the bioavailability of compounds depending on their predominant charge at biological pH. The fraction of anions with >10% F falls from 85% if the polar surface area (PSA) is < or = 75 A(2), to 56% if 75 < PSA < 150 A(2), to 11% if PSA is > or = 150 A(2). On the other hand, whereas 55% of the neutral, zwitterionic, or cationic compounds that pass the rule-of-five have >10% F, only 17% of those that fail have > 10% F. This same categorization distinguishes compounds that are poorly permeable from those that are permeable in Caco-2 cells. Further validation is provided with human bioavailability values from the literature.

Structure–activity relationship in the oxazolidinone–quinolone hybrid series: influence of the central spacer on the antibacterial activity and the mode of action

Oxazolidinone-quinolone hybrids, which combine the pharmacophores of a quinolone and an oxazolidinone, were synthesised and shown to be active against a variety of susceptible and resistant Gram-positive and Gram-negative bacteria. The nature of the spacer greatly influences the antibacterial activity by directing the mode of action, that is quinolone- and/or oxazolidinone-like activity. The best compounds in this series have a balanced dual mode of action and overcome all types of resistance, including resistance to quinolones and linezolid, in clinically relevant Gram-positive pathogens.

IMDA/retro-Mannich approach to cis-perhydroquinoline Lycopodium alkaloids: asymmetric synthesis of (+)-luciduline

[formula: see text] The first chiral auxiliary mediated asymmetric synthesis of the naturally occurring Lycopodium alkaloid (+)-luciduline has been accomplished. Key steps include an IMDA reaction of a chiral dihydropyridine, a subsequent retro-Mannich ring opening, and a novel cationic reductive cyclization reaction.

Selective Inversion of the Proximal or Distal Hydroxyl Groups in syn,syn-3-[N-(Alkoxycarbonyl)amino] 1,2-Diols via Cyclic Sulfates

The formation of cyclic sulfates (4) from syn,syn-3-[N-(benzyloxycarbonyl)amino] 1,2-diols provides a common intermediate to access other diastereomers via two inversion procedures. Thermolysis of the cyclic sulfates in acetonitrile normally leads to inversion of the distal hydroxyl group to form a 1,3-oxazin-2-one (6). Catalytic hydrogenation of the cyclic sulfates under basic conditions (NEt(3)) results in inversion at the proximal hydroxyl group to form a 1,3-oxazolidin-2-one (5).

Diphenylpropionic acids as new AT1 selective angiotensin II antagonists

The synthesis and pharmacological evaluation of a new series of potent AT1 selective diphenylpropionic acid nonpeptide angiotensin II receptor antagonists are reported. The new compounds were evaluated for in vitro AT1 (rat liver) and AT2 (rat adrenal) binding affinity as well as for in vivo inhibition of angiotensin II-induced increase in mean arterial blood pressure in pithed rats. Unsaturation of the diphenylpropionic acids as well as substitution or replacement by alkyl groups of the pendant phenyl ring resulted in a decrease of potency. On the other hand, the presence of small alkyl groups in the alpha-position to the carboxylic acid was important for activity, with one of the resultant diastereoisomers (R*,R*) being ca. 10-fold more active than the other (R*,S*). Oral evaluation of the most active compounds in a furosemide-treated sodium-depleted rat model showed that compound 36g (UR-7198) reduced blood pressure dose dependently. This compound showed in vitro and iv potencies similar to that of the reference compound losartan but faster onset of action and somewhat greater oral activity, presumably due to its improved bioavailability.

Lead development: validation and application of high throughput screening for determination of pharmacokinetic parameters for enzyme inhibitors

An approach utilizing robotics (automation) for the rapid and reliable determination of protease inhibitor concentration in rat plasma samples is described. The bioassay protocol using an immobilized peptide substrate allows high sample throughput, compatible with parallel synthesis/SAR development strategy.

Synthetic library techniques: subjective (biased and generic) thoughts and views

Various aspects of synthetic diversity generation and screening are discussed. Controversial issues are raised and different points of view are presented. We hope the article will stimulate thinking about the utilization of library techniques and start a discussion about questions concerning their application.

Chemical approaches to improve the oral bioavailability of peptidergic molecules

This review discusses both tools and strategies that may be employed as approaches towards the pursuit of orally active compounds from peptidergic molecules. Besides providing a review of these subjects, this paper provides an example of how these were utilized in a research programme at SmithKline Beecham involving the development of orally active GPIIb/IIIa antagonists. The tools for studying oral drug absorption in-vitro include variants of the Ussing chamber which utilize either intestinal tissues or cultured epithelial cells that permit the measurement of intestinal permeability. Example absorption studies that are described are mannitol, cephalexin, the growth hormone-releasing peptide SK&F 110679 and two GPIIb/IIIa antagonist peptides SK&F 106760 and SK&F 107260. With the exception of cephalexin, these compounds cross the intestine by passive paracellular diffusion. Cephalexin, on the other hand, crosses the intestine via the oligopeptide transporter. Structure-transport studies are reviewed for this transporter. The tools for studying oral drug absorption in-vivo involve animals bearing in-dwelling intestinal or portal vein catheters. A study of the segmental absorption of SK&F 106760 is provided. The review concludes with two chemical strategies that may be taken towards the enhancement of oral bioavailability of peptidergic molecules. The first strategy involves the chemical modification of peptides which enhance intestinal permeability, specifically the modification of amide bonds. The second strategy involves the design of compounds bearing nonpeptide templates, which are more amenable to the discovery of compounds with oral activity, from peptide pharmacophore models. An example is given regarding the discovery of SB 208651, a potent orally active GPIIb/IIIa antagonist, designed from the peptides SK&F 106760 and SK&F 107260.

Renal circulation and blockade of the renin-angiotensin system. Is angiotensin-converting enzyme inhibition the last word?

The mechanism by which angiotensin-converting enzyme (ACE) inhibition influences renal perfusion and function has assumed growing importance as alternatives for blocking the system have emerged. Neither renin inhibitors nor angiotensin II (Ang II) antagonists are likely to trigger responses similar to ACE inhibitor-induced involvement of kinins, prostaglandins, or nitric oxide. Several observations suggest species variation in the contribution of these pathways to the renal response to ACE inhibition. In humans, recent investigation suggests that virtually all of the renal response is due to a fall in Ang II formation. Perhaps most persuasive is the surprising observation that the renal hemodynamic response to renin inhibitors exceeds by more than 50% the response to ACE inhibition in healthy humans. To the extent that kinins or prostaglandins contribute to the renal response to ACE inhibition, one would anticipate a smaller response to renin inhibition. Possible explanations include an unanticipated additional action of renin inhibitors, better tissue penetration of these highly lipophilic agents, or more effective blockade of Ang II formation through an action at the rate-limiting step or non-ACE-dependent Ang II generation. Substantial evidence favors the latter two possibilities. Whatever the explanation, these observations raise the intriguing possibility that the undoubted therapeutic efficacy of ACE inhibition in renal injury, documented most rigorously for type I diabetes mellitus, might be exceeded with the newer classes of agent.

Effects of SC-56525, a potent, orally active renin inhibitor, in salt-depleted and renal hypertensive dogs

SC-56525 is a nanomolar inhibitor of plasma renin activity in human, cynomolgus monkey, dog, guinea pig, Yucatan micropig, and rabbit but is less active in rat. The oral bioavailability of SC-56525 in conscious dogs at doses of 5 mg/kg IV and 30 mg/kg PO was 66.1 +/- 16.4%. Oral dosing with SC-56525 at 3, 10, and 30 mg/kg in salt-depleted dogs induced a dose-dependent reduction in mean arterial pressure and inhibition of plasma renin activity with no significant effect on heart rate. In two-kidney, one clip renal hypertensive dogs, SC-56525 given orally at 10, 30, and 60 mg/kg daily for 4 days lowered blood pressure significantly. In conscious dogs monitored in their home cages via radiotelemetry, no significant changes in heart rate occurred in response to large drops in blood pressure in both renal hypertensive and salt-depleted dogs with the renin inhibitor SC-56525. SC-56525 is a nanomolar, orally active inhibitor of renin and effectively lowers blood pressure in both salt-depleted and renal hypertensive dogs.

Discovery of inhibitors of human renin with high oral bioavailability

Knowledge of the sequence of a bioactive protein (angiotensinogen) and the availability of a natural product inhibitor lead (pepstatin) were the starting point for discovery of potent penta- and hexapeptide renin inhibitors. Study of the metabolism and disposition of these substances forced the discovery of simpler inhibitors leading to the discovery of oral activity in Terlakiren (22). Modification of physical properties led to the synthesis of aminopiperidine 30, which was identified by oral efficacy profiling. Structural modification to give enzymatic stability produced the bioavailable benzylsuccinate inhibitor 34. Its bioactive monomethylamine metabolite (35, CP-108,671) was subsequently found to have uniformly high oral bioavailability and activity in various species including primates.

Angiotensinergic versus nonangiotensinergic hemodynamic effects of converting enzyme inhibition in patients with chronic heart failure. Assessment by acute renin and converting enzyme inhibition

BACKGROUND

The contribution of nonangiotensinergic effects of converting enzyme inhibitors to their hemodynamic effects in patients with chronic heart failure is not clear. A comparison of the effects of renin and converting enzyme inhibition should help to clarify this issue.

METHODS AND RESULTS

Thirty-six patients with chronic heart failure (New York Heart Association class II or III) were randomly assigned to receive double-blind either intravenous placebo, the renin inhibitor remikiren, or the converting enzyme inhibitor enalaprilat followed by coinfusion of a second placebo infusion, the addition of remikiren to enalaprilat, or the addition of enalaprilat to remikiren, respectively. Systemic hemodynamics (Swan-Ganz and radial artery catheters) were measured before (rest and submaximal recumbent bicycle ergometry), during (rest), and at the end (rest and exercise) of each 45-minute single- or combination-infusion period. Placebo did not change hemodynamics or renin activity. Effective inhibition of the renin-angiotensin system by remikiren and enalaprilat was indicated by increases of plasma immunoreactive renin together with rapid and complete inhibition of renin activity after remikiren and an increase after enalaprilat (all P < or = .05). Remikiren and enalaprilat rapidly and to a similar extent reduced resting blood pressure through a reduction of systemic vascular resistance, and these changes were significantly correlated to baseline plasma renin activity. Both compounds also decreased pulmonary artery, pulmonary capillary wedge, and right atrial pressures to a similar extent (P < .05). During exercise, pulmonary capillary wedge and right atrial pressures were equally reduced and stroke volume index was increased with remikiren and enalaprilat (P < .05) for both). The combination of converting enzyme with renin inhibition or vice versa did not cause additional hemodynamic changes.

CONCLUSIONS

Specific renin inhibition in patients with chronic heart failure produces short-term hemodynamic effects that are almost indistinguishable from those of converting enzyme inhibition. This finding and the lack of additional effects of converting enzyme inhibition added to renin inhibition suggest that nonangiotensinergic effects of converting enzyme inhibitors do not play a significant role in their short-term hemodynamic effects in patients with chronic heart failure.

Renin inhibitor: relationship between molecular structure and oral absorption

Common problems in developing renin inhibitors are low solubility, insufficient oral absorption, and fast hepatic clearance. We focused on the molecular structure of renin inhibitors to overcome these problems. Cyclodextrins (CD) improved the low solubility of renin inhibitors, with beta-CD showing the best ability to dissolve renin inhibitors. The intestinal absorption of renin inhibitors varied with both their solubility and molecular structure. Coadministration of beta-CD improved the intestinal absorption of some renin inhibitors with low solubility as measured by transport into the mesenteric vein in the absorption experiment using the rat intestinal loop. Substitutions at both the N and C terminals was essential for absorption from the small intestine. A naphthyl group at the N-terminal further improved intestinal absorption. A carrier system appeared to be involved in the intestinal absorption of some renin inhibitors. N-methylation at the amide bond of thiazolylalanine suppressed the high hepatic clearance of one of the test compounds 18 which was well absorbed from the small intestine and it improved its oral bioavailability.

Synthesis, antiviral activity, and bioavailability studies of gamma-lactam derived HIV protease inhibitors

Incorporation of a gamma-lactam in hydroxyethylene isosteres results in modest inhibitors of HIV-1 protease. Additional structural activity studies have produced significantly more potent inhibitors with the introduction of the trisubstituted cyclopentane (see compound 20) as the optimum substituent for the C-terminus. This new amino acid amide surrogate can be readily prepared in large scale from (R)-pulegone. Optimized compounds (36) and (60) are potent antiviral agents and are well absorbed (15-20%) in a dog model after oral administration.

Renin inhibitors: C-terminal oxetanes as potent transition-state mimics

A novel transition-state mimic containing a C-terminal oxetane has been developed. Renin inhibitors incorporating this fragment exhibit enhanced potency against human plasma renin at physiological pH. The binding affinity of this new species has allowed size reductions at other sites.