Is there a future for renin inhibitors?

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.

Potential of renin inhibition in cardiovascular disease

Since renin catalyses the first and rate-limiting step of the renin-angiotensin system (RAS) cascade, interruption of the generation of angiotensin II (Ang II) by renin inhibitors at this highly specific initial step of the cascade has long been a therapeutic goal. The early development of renin inhibitors was hampered by problems with bioavailability and high costs of synthesis. However, more recently a potent non-peptidic inhibitor of renin, aliskiren, with acceptable oral bioavailability, has been synthesised. Aliskiren effectively reduces Ang II levels in normal volunteers and has been shown to lower blood pressure (BP) in patients with mild-to-moderate hypertension. Renin inhibitors would be expected to have similar, but not identical effects to those of the established RAS antagonists. Due to the lack of effective alternative enzyme pathways, blockade of Ang II production may be more effective with renin inhibition than with angiotensin-converting enzyme (ACE) inhibition. Furthermore, because renin has high specificity for only one substrate, angiotensinogen, side-effects would be expected to be less frequent. It is currently unclear whether blockade of Ang II type 1 (AT1) receptors, leaving other Ang II receptors (AT2, AT3 and AT4) unblocked, is preferable to the reduction in plasma and tissue Ang II levels achieved with either ACE or renin inhibition. Pharmacological suppression of the RAS, through ACE inhibition, or blockade of AT1, beta-adrenoceptor or mineralocorticoid receptors, has been proven to reduce morbidity and mortality in patients with hypertension, diabetes mellitus, atherosclerosis, heart failure and nephropathy. While, to date, aliskiren has only been shown to reduce BP, it appears likely that orally-active renin inhibitors could prove useful in the management of a wide range of cardiovascular pathologies.

Therapeutic potential of renin inhibitors in the management of cardiovascular disorders

The renin-angiotensin system (RAS) is well recognized for its importance in regulation of BP, electrolyte balance and vascular growth. Pharmacological suppression of the RAS, through ACE inhibition and/or angiotensin receptor blockade, is a proven effective therapeutic approach to the treatment of a range of cardiovascular diseases. Renin is the enzyme that catalyzes the first and rate-limiting step of RAS, the cleavage of angiotensinogen to angiotensin I (A-I). A-I is then further converted by ACE to the biologically active vasoconstrictor, A-II. Interruption of the generation of A-II by renin inhibitors at this highly specific initial step of the cascade would be expected to have similar but not identical effects to those of the already well established RAS antagonists. Due to the lack of effective alternative enzyme pathways, blockade of A-II production may be more effective with renin inhibition than with ACE inhibition, and because of the high specificity of renin for only one substrate, namely angiotensinogen, adverse effects would be expected to be less frequent. It is currently unclear whether blockade of angiotensin II type 1 receptors (AT(1)), leaving other A-II receptors unblocked, is preferable to the reduction in plasma and tissue A-II levels achieved with either ACE or renin inhibition. The development of early peptidic and peptidomimetic renin inhibitors was hampered by problems with oral bioavailability and high costs of synthesis. However recent work has led to the synthesis of a potent non-peptidic inhibitor of renin, aliskiren, which has acceptable oral bioavailability. This renin inhibitor has been shown to effectively reduce A-II levels in normal volunteers and to lower BP in patients with mild to moderate hypertension. It appears likely that aliskiren is the first of a new class of agents that may prove useful in the management of patients with nephropathy, heart failure and atherosclerosis in addition to hypertension.

Beta-secretase as a target for the treatment of Alzheimer's disease

Finding inhibitors of the first step of the amyloid cascade, A beta (42) generation, is a major goal of Alzheimer's disease drug development. Two target protease activities, beta-and gamma-secretase, were detected more than 10 years ago but progress in this area has been slow because the enzymes were not identified. Using an expression cloning strategy we have identified a novel membrane bound aspartic protease, BACE1, as beta-secretase. The enzyme has been characterized in detail. The X-ray crystal structure, which is critical for rational inhibitor design, has been solved and shown to be similar to that of other pepsin family members. Our recent knockout studies show that BACE1 is critical for A beta generation, but the knockout mice show an otherwise normal phenotype, raising the possibility that therapeutic BACE1 inhibition could be accomplished without major mechanism based toxicity. Target-mediated toxicity of beta-secretase inhibition cannot be ruled out, however, as long as the major substrates of this enzyme are unknown. Although various peptidic beta-secretase inhibitors have been published, the key challenge now is the generation of more drug-like compounds that could be developed for therapeutic purposes. The focus of this review is progress in the beta-secretase field from the identification of the enzyme in 1999 to the most recent publications.

Emerging Alzheimer's disease therapies: inhibition of beta-secretase

Among the approaches towards disease modifying treatment of Alzheimer's disease blocking the initial step of the amyloid cascade, Abeta42 generation, has received most attention. Abeta42 generation requires two proteases, beta- and gamma-secretase, and inhibition of these enzymes is a key focus of AD drug development. Progress in this area has been slow, because these enzymes were not identified. Using an expression cloning strategy we have identified a novel membrane bound aspartic protease, BACE1, as beta-secretase. The enzyme has been characterized in detail. The x-ray crystal structure, which is critical for rational inhibitor design, has been solved and shown to be similar to that of other pepsin family members. Our recent knockout studies show that BACE1 is critical for Abeta generation, but the knockout mice show an otherwise normal phenotype, raising the possibility that therapeutic BACE1 inhibition could be accomplished without major mechanism based toxicity. However, target-mediated toxicity of beta-secretase inhibition cannot be ruled out, as long as the major substrates of this enzyme are unknown. While various peptidic beta-secretase inhibitors have been published, the key challenge now is the generation of more drug-like compounds that could be developed for therapeutic purposes.

AT(1)-receptor blockade and the kidney: importance of non-ACE pathways in health and disease

Large-scale trials with angiotensin converting enzyme (ACE) inhibitors and angiotensin II type 1 (AT(1))-receptor blockers have clearly shown that blockade of the renin-angiotensin system reduces the deterioration in renal function associated with diabetes. AT(1)-receptor blockers represent a more rational approach to blockade of this system than ACE inhibitors, due to the presence of non-ACE pathways of angiotensin II formation. Studies in healthy volunteers maintained on a low-salt diet indicate that such pathways account for approximately 30-40% of total angiotensin II formation, and this figure increases to 60-70% in individuals maintained on a high-salt diet (resembling the situation in most human populations). Activation of the renin-angiotensin system is increased in diabetic patients, and comparison of the renal vascular responses to captopril and candesartan shows a strong correlation between the effects of ACE inhibition and AT(1)-receptor blockade, indicating that the deleterious effects of renin-angiotensin system activation in diabetes are mediated largely through angiotensin II. The presence of multiple risk factors, such as genetic predisposition, hyperglycaemia, obesity and tissue damage, places diabetic patients at high risk of disease related to activation of the renin-angiotensin system. Effective and early blockade of this system is therefore an important aspect of management.