ACE inhibitor

Captopril, the first ACE inhibitor

ACE inhibitors or angiotensin-converting enzyme inhibitors, are a group of pharmaceuticals that are used primarily in treatment of hypertension and congestive heart failure.

Clinical use

ACE inhibitors are used primarily in the treatment of hypertension, though they are also sometimes used in patients with cardiac failure, renal disease,or systemic sclerosis

Mechanism of action

Primarily angiotensin converting enzyme inhibitors reduce the activity of the renin-angiotensin-aldosterone system.

The renin-angiotensin-aldosterone system (RAAS)

Renin-angiotensin-aldosterone system

One mechanism for maintaining the blood pressure is the release of a protein called renin from cells in the kidney (specifically: the juxtaglomerular apparatus). This produces another protein called angiotensin which signals the adrenal gland to produce a hormone called aldosterone. This system is activated in response to a fall in blood pressure (hypotension) as well as markers of problems with the salt-water balance of the body, such as decreased sodium concentration in a part of the kidney known as the distal tubule, decreased blood volume and stimulation of the kidney by the sympathetic nervous system . In such a situation, the kidneys release renin which acts as an enzyme and cuts off all but the first 10 amino-acid residues of angiotensinogen (a protein made in the liver, and which circulates in the blood). These 10 residues are then known as angiotensin I. Angiotensin I is then converted to angiotensin II by angiotensin converting enzyme (ACE) which removes a further 2 residues and is found in the pulmonary circulation as well as in the endothelium of many blood vessels.^[1] The system in general aims to increase blood pressure by increasing the amount of salt and water the body retains, although angiotensin is also very good at causing the blood vessels to tighten (a potent vasoconstrictor).

Effects

ACE inhibitors block the conversion of angiotensin I to angiotensin II. ^[2] They therefore lower arteriolar resistance and increase venous capacity; increase cardiac output and cardiac index, stroke work and volume, lower renovascular resistance, and lead to increased natriuresis (excretion of sodium in the urine).

Normally, angiotensin II will have the following effects:

• vasoconstriction (narrowing of blood vessels), which may lead to increased blood pressure and hypertension

– constriction of the efferent arterioles of the kidney, leading to increased perfusion pressure in the glomeruli.

• ventricular remodeling of the heart, which may lead to ventricular hypertrophy and CHF
• stimulation of the adrenal cortex to release aldosterone, a hormone that acts on kidney tubules to retain sodium and chloride ions and excrete potassium. Sodium is a "water-holding" molecule, so water is also retained, which leads to increased blood volume, hence an increase in blood pressure.
• stimulation of the posterior pituitary to release vasopressin (also known as anti-diuretic hormone (ADH)) which also acts on the kidneys to increase water retention.
• decrease renal protein kinase C

With ACE inhibitor use, the effects of angiotensin II are prevented, leading to decreased blood pressure.

Epidemiological and clinical studies have shown that ACE inhibitors reduce the progress of diabetic nephropathy independently from their blood pressure-lowering effect.^[3] This action of ACE inhibitors is utilised in the prevention of diabetic renal failure.

ACE inhibitors have been shown to be effective for indications other than hypertension even in patients with normal blood pressure. The use of a maximum dose of ACE inhibitors in such patients (including for prevention of diabetic nephropathy, congestive heart failure, prophylaxis of cardiovascular events) is justified because it improves clinical outcomes, independent of the blood pressure lowering effect of ACE inhibitors. Such therapy, of course, requires careful and gradual titration of the dose to prevent the effects of rapidly decreasing blood pressure (dizziness, fainting, etc.).

ACE inhibitors have also been shown to cause a central enhancement of parasympathetic activity in healthy volunteers and patients with heart failure.^[4][5] This action may reduce the prevalence of malignant cardiac arrhythmias, and the reduction in sudden death reported in large clinical trials.

The ACE inhibitor enalapril has also been shown to reduce cardiac cachexia in patients with chronic heart failure.^[6] Cachexia is a poor prognostic sign in patients with chronic heart failure.^[7] ACE-inhibitors are now used to reverse frailty and muscle wasting in elderly patients without heart failure.

Adverse effects

Common adverse drug reactions include: hypotension, cough, hyperkalemia, headache, dizziness, fatigue, nausea and renal impairment.^[8] There is also some evidence to suggest that ACE inhibitors might increase inflammation-related pain^[9].

A persistent dry cough is a relatively common adverse effect believed to be associated with the increases in bradykinin levels produced by ACE inhibitors, although the role of bradykinin in producing these symptoms remains disputed by some authors.^[10] Patients who experience this cough are often switched to angiotensin II receptor antagonists.

Rash and taste disturbances, infrequent with most ACE inhibitors, are more prevalent in captopril and is attributed to its sulfhydryl moiety. This has led to decreased use of captopril in clinical setting, although it is still used in scintigraphy of the kidney.

Renal impairment is a significant adverse effect of all ACE inhibitors. The reason for this is still unknown. Some suggest that it is associated with their effect on angiotensin II-mediated homeostatic functions such as renal blood flow. Renal blood flow may be affected by angiotensin II because it vasoconstricts the efferent arterioles of the glomeruli of the kidney, thereby increasing glomerular filtration rate (GFR). Hence, by reducing angiotensin II levels, ACE inhibitors may reduce GFR, a marker of renal function. Specifically, ACE inhibitors can induce or exacerbate renal impairment in patients with renal artery stenosis. This is especially a problem if the patient is also concomitantly taking an NSAID and a diuretic. When the three drugs are taken together, there is a very high risk of developing renal failure.^[11]

ACE inhibitors may cause hyperkalemia. Suppression of angiotensin II leads to a decrease in aldosterone levels. Since aldosterone is responsible for increasing the excretion of potassium, ACE inhibitors ultimately cause retention of potassium.

A severe allergic reaction can occur that rarely can affect the bowel wall and secondarily cause abdominal pain. This "anaphylactic" reaction is very rare as well.

Some patients develop angioedema due to increased bradykinin levels. There appears to be a genetic predisposition towards this adverse effect in patients who degrade bradykinin more slowly than average.^[12]

In pregnant women, ACE inhibitors taken during the first trimester have been reported to cause major congenital malformations, stillbirths, and neonatal deaths. Commonly reported fetal abnormalities include hypotension, renal dysplasia, anuria/oliguria, oligohydramnios, intrauterine growth retardation, pulmonary hypoplasia, patent ductus arteriosus, and incomplete ossification of the skull.^[13]

Contraindications and precautions

The ACE inhibitors are contraindicated in patients with:

• Previous angioedema associated with ACE inhibitor therapy
• Renal artery stenosis (bilateral, or unilateral with a solitary functioning kidney)
• Hypersensitivity to ACE inhibitors

ACE inhibitors should be used with caution in patients with:

• Impaired renal function
• Aortic valve stenosis or cardiac outflow obstruction
• Hypovolemia or dehydration
• Hemodialysis with high flux polyacrylonitrile membranes

ACE inhibitors are ADEC Pregnancy category D, and should be avoided in women who are likely to become pregnant.^[8] In the U.S., ACE inhibitors are required to be labelled with a "black box" warning concerning the risk of birth defects when taking during the second and third trimester. It has also been found that use of ACE inhibitors in the first trimester is also associated with a risk of major congenital malformations, particularly affecting the cardiovascular and central nervous systems.^[14]

Potassium supplementation should be used with caution and under medical supervision owing to the hyperkalemic effect of ACE inhibitors.

Examples

ACE inhibitors can be divided into three groups based on their molecular structure:

Sulfhydryl-containing agents

• Captopril (trade name Capoten), the first ACE inhibitor
• Zofenopril

Dicarboxylate-containing agents

This is the largest group, including:

• Enalapril (Vasotec/Renitec)
• Ramipril (Altace/Tritace/Ramace/Ramiwin)
• Quinapril (Accupril)
• Perindopril (Coversyl/Aceon)
• Lisinopril (Lisodur/Lopril/Novatec/Prinivil/Zestril)
• Benazepril (Lotensin)

Phosphonate-containing agents

• Fosinopril (Monopril) is the only member of this group

Naturally occurring

• Casokinins and lactokinins are breakdown products of casein and whey that occur naturally after ingestion of milk products, especially cultured milk. Their role in blood pressure control is uncertain.^[15]
• The Lactotripeptides Val-Pro-Pro and Ile-Pro-Pro produced by the probiotic Lactobacillus helveticus or derived from casein have been shown to have ACE-inhibiting and antihypertensive functions.^[16][17]

Comparative information

Comparatively, all ACE inhibitors have similar antihypertensive efficacy when equivalent doses are administered. The main point-of-difference lies with captopril, the first ACE inhibitor, which has a shorter duration of action and increased incidence of certain adverse effects.

Certain agents in the ACE inhibitor class have been proven, in large clinical studies, to reduce mortality post-myocardial infarction, prevent development of heart failure, etc.. The ACE inhibitor most prominently recognized for these qualities is ramipril (Altace). Because ramipril has been shown to reduce mortality rates even among patient groups not suffering from hypertension, there are some (mostly drug-company representatives) who believe that ramipril's benefits may extend beyond those of the general abilities it holds in common with other members of the ACE inhibitor class.

ACEI equivalents

The ACE inhibitors have different strengths with different starting dosages. Dosage should be adjusted according to the clinical response. ^{[18] [19] [20]}

Angiotensin II receptor antagonists

ACE inhibitors share many common characteristics with another class of cardiovascular drugs called angiotensin II receptor antagonists, which are often used when patients are intolerant of the adverse effects produced by ACE inhibitors. ACE inhibitors do not completely prevent the formation of angiotensin II, as there are other conversion pathways, and so angiotensin II receptor antagonists may be useful because they act to prevent the action of angiotensin II at the AT₁ receptor, leaving AT₂ receptor unblocked; the latter may have consequences needing further study.

Use in combination

The combination therapy of angiotensin II receptor antagonists with ACE inhibitors may be superior to either agent alone. This combination may increase levels of bradykinin while blocking the generation of angiotensin II and its activity at the AT₁ receptor. This 'dual blockade' may be more effective than using an ACE inhibitor alone, because angiotensin II can be generated via non-ACE-dependent pathways. Preliminary studies suggest that this combination of pharmacologic agents may be advantageous in the treatment of essential hypertension, chronic heart failure, and nephropathy.^[21][22] However, more studies are needed to confirm these highly preliminary results. While statistically significant results have been obtained for its role in treating hypertension, clinical significance may be lacking.^[23]

Patients with heart failure may benefit from the combination in terms of reducing morbidity and ventricular remodeling.^[24][25]

The most compelling evidence has been found for the treatment of nephropathy: this combination therapy partially reversed the proteinuria and also exhibited a renoprotective effect in patients afflicted with diabetic nephropathy,^[21] and pediatric IgA nephropathy.^[26]

History

The first step in the development of (ACE) inhibitors was the discovery of angiotensin converting enzyme (ACE) in plasma by Leonard T. Skeggs and his colleagues in 1956. Brazilian scientist Sergio Ferreira reported in 1965 of a 'bradykinin potentiating factor (BPFs) present in the venom of bothrops jararaca, a South American pit viper.( Brit J Pharmacol & Chemother 1965). Dr SH Ferreira then proceeded to John Vanes laboratory as a Post-Doc with his already isolated BPFs. The conversion of the inactive angiotensin I to the potent angiotensin II was thought to take place in the plasma. However, in 1967, Kevin K. F. Ng and John R. Vane showed that the plasma (ACE) was too slow to account for the conversion of angiotensin I to angiotensin II in vivo. Subsequent investigation showed that rapid conversion occurs during its passage through the pulmonary circulation.^[27]

Bradykinin is rapidly inactivated in the circulating blood and it disappears completely in a single passage through the pulmonary circulation. Angiotensin I also disappears in the pulmonary circulation due to its conversion to angiotensin II. Furthermore, angiotensin II passes through the lungs without any loss. The inactivation of bradykinin and the conversion of angiotensin I to angiotensin II in the lungs was thought to be caused by the same enzyme.^[28] In 1970, Ng and Vane using bradykinin potentiating factor (BPF) provided by Sérgio Henrique Ferreira showed that the conversion of angiotensin I to angiotensin II was inhibited during its passage through the pulmonary circulation.^[29]

Bradykinin potentiating factor (BPF) is derived from the venom of the pit viper (Bothrops jararaca). It is a family of peptides and its potentiating action is linked to inhibition of bradykinin by ACE. Molecular analysis of BPF yielded a nonapeptide BPF teprotide (SQ 20,881) which showed the greatest (ACE) inhibition potency and hypotensive effect in vivo. Teprotide had limited clinical value, due to its peptide nature and lack of activity when given orally. In the early 1970s, knowledge of the structure-activity relationship required for inhibition of ACE was growing. David Cushman, Miguel Ondetti and colleagues used peptide analogues to study the structure of ACE, using carboxypeptidase A as a model. Their discoveries led to the development of captopril, the first orally-active ACE inhibitor in 1975.

Captopril was approved by the United States Food and Drug Administration in 1981. The first non-sulfhydryl-containing (ACE) inhibitor enalapril was marketed two years later. Since then, at least twelve other ACE inhibitors have been marketed.

In 1991, Japanese scientists created the first ever milk-based ACE inhibitor in the form of a fermented milk drink, using specific cultures to liberate the IPP from the dairy protein. Interestingly, Val-Pro-Pro is also liberated in this process—another milk tripeptide with a very similar chemical structure to IPP. Together, these peptides are now often referred to as lactotripeptides. Shortly after this, in 1996, the first human study confirmed the blood pressure lowering effect of IPP in fermented milk.^[30] Although twice the amount of VPP is needed to achieve the same ACE inhibiting activity as the originally discovered IPP, it is assumed that VPP also adds to the total blood pressure lowering effect.^[31][31] Since the first lactotripeptides discovery, more than 20 human clinical trials have been conducted in many different countries.^[17]

See also

• Angiotensin II receptor antagonist
• Angiotensin Receptor Blockers: Drug discovery and development

References