Discovery and development of renin inhibitors
Encyclopedia
Renin inhibitor
s are antihypertensive drugs that inhibit
the first and rate-limiting step
of the renin-angiotensin-aldosterone system (RAAS). Since the 1970s scientists have been trying to develop potent
inhibitors with acceptable oral bioavailability
. The process was difficult and took about three decades. The first and second generations faced problems like poor bioavailability and lack of potency. Finally the third generation was discovered. These compounds
were non-peptidic
renin inhibitors, had acceptable oral bioavailability and were potent enough for clinical use. The first drug in this class was aliskiren
which received a marketing approval in 2007.
in rabbits. They observed that blood pressure rose in the rabbits when extract
s of the kidneys were injected into the jugular vein
. They also discovered that this substance that was responsible for higher blood pressure was produced in the renal cortex
and they named this substance renin
. Although this experiment laid the foundation for future investigations into the RAAS pathway it had little impact on the scientific community at that time. It was not until in 1934 when Goldblatt published his work in renal ischaemia that renin came into focus again. The importance of renin in the pathogenesis
of cardiovascular disease
was however not fully understood until in the 1970s and it was not until 20 years later that the first renin inhibitors went to clinical trial
s.
Pepstatin
, which was described in 1972, was the first synthetic renin inhibitor but poor pharmacokinetic properties prevented it from entering in vivo
investigations. The first generation of renin inhibitors, such as H-142, were peptide analogues of angiotensinogen. However these inhibitors had also limited drug-like properties. It was not until 1982 that hopes of breakthrough appeared when development of the second generation renin inhibitors begins. This generation consisted of peptide-like compounds such as remikiren
, enalkiren and zanikiren. They had more drug-like rather than substrate-like properties and in 1990 they went to clinical trials. However the second generation had its limitations and never completed clinical trials.
Aliskiren is a third generation renin inhibitor, which are non-peptide-like compounds, and is the only renin inhibitor that went into phase III clinical trials. The first clinical trial of aliskiren was performed in 2000 in healthy volunteers. In 2007 aliskiren was approved by the US Food and Drug Administration and the European Medicines Agency
as a treatment for hypertension
.
of hypertension. Under normal conditions, stimulation of the RAAS occurs in response to threats that compromise blood pressure
stability, such as hypotension
, blood loss and excessive loss of sodium and water. Blood pressure is a result of total peripheral resistance
and cardiac output
. Research has shown that renin is in high concentration
when vasoconstriction is the main reason for high blood pressure. On the other hand, renin is in low concentration when cardiac output is the main reason.
The highly selective
aspartic protease renin is secreted
from the juxtaglomerular apparatus
as well as locally in other tissues
, such as in the brain, heart and blood vessels. Renin is a circulating enzyme
that acts on a circulating peptide
, angiotensinogen. Renin cleaves the peptide at the Leu10–Val11 bond and this reaction is the rate-determining step
of the RAAS. This leads to the product angiotensin I (Ang I) which is a decapeptide. Ang I is broken down
by the angiotensin-converting enzyme
(ACE) to the active octapeptide angiotensin II (Ang II) which is the principal effector of the RAAS. Ang II is a powerful vasoconstrictor and leads to the release of catecholamine
s from the adrenal medulla
and prejunctional nerve endings. Ang II also leads to the secretion of aldosterone
which is the main salt-retaining hormone. This stimulates the kidneys to retain salt. Ang II also provides a negative feedback
to the system by its inhibiton on renin release. Ang II interacts with at least two classes of Ang II receptors
, AT1
and AT2
. This mechanism, which runs from renin through Ang II and to aldosterone as well as the negative feedback that Ang II has on renin secretion, is known as RAAS. The short term effects are increased blood pressure and long term effects can be end organ damage
.
of renin and inhibit the binding of renin to angiotensinogen which is the rate determining step of the RAAS cascade. By inhibiting the RAAS at the beginning renin inhibitors prevent the formation of Ang I and Ang II. It is also thought that renin inhibitors prevent Ang-(1-7), Ang-(1-9) and Ang-(1-5) formation. Renin is profoundly selective for its only naturally occurring substrate which is angiotensinogen. This means that renin inhibitors that have a high specificity for human renin, theoretically should have very few side effect
s, if any. This has been shown for aliskiren which has a placebo-like side-effect profile.
Ang II also functions within the RAAS as a negative feedback to suppress further release of renin. This reduction in Ang II levels suppress the feedback loop. That leads to further increased plasma renin concentrations (PRC) and plasma renin activity (PRA). This can be problematic for ACE inhibitor
and angiotensin II receptor antagonist
therapy since increased PRA could partially overcome the pharmacologic inhibition of the RAAS cascade. Because renin inhibitors directly effect renin activity, decrease of PRA despite the increased PRC (from loss of the negative feedback) may be clinically advantageous.
was the first synthetic
renin inhibitor. It is of microbial
origin and is an N-acyl-pentapeptide, more accurately: isovaleryl-L-valyl-L-valyl-statyl-L-alanyl-statine. Pepstatin was found to be a potent competitive inhibitor of most aspartic proteases but a weak inhibitor of renin. Originally it was thought to be effective in the treatment of duodenal ulcer and went through clinical trials but had no success. Statine
, an amino acid
, is thought to be responsible for the inhibitory activity of pepstatin. That is because it mimics the tetrahedral transition state
of the peptide catalysis
. Because of hydrophobic properties of statine, pepstatin has very low solubility
in physiological media. Since it had low potency and poor solubility it did not enter in vivo studies.
compound was the first renin inhibitor that proved effective when taken orally. It was tested on marmoset
s and was only active at high doses. Development of new drugs in the second generation continued to improve pharmacokinetic properties. Remikiren, enalkiren and zankiren were then discovered. These were peptidomimetic inhibitors with improved structure that made them more specific, potent and stable. Unfortunately the clinical development was terminated because the drugs had poor oral bioavailability (it was poorly absorbed and rapidly metabolized) and lowering blood pressure activity still remained low including short half-life
.
and molecular modeling techniques. Now a solution has been found to the problem that impeded the development of the renin inhibitors of the previous generations. It was known that non-peptide mimetics would solve the problems of poor pharmacokinetic properties and low specificity. This led to the design of small molecules, nonpeptide inhibitors, which were very potent and specific of human renin.
conformation. Between the two lobes, deep within the enzyme, resides the active site
and its catalytic activity is due to two aspartic acid
residues
(Asp32 and Asp 215, one from each lobe in the renin molecule). A flexible flap made from amino acids formed in a β-hairpin
closes the active site by covering the cleft.
The renin molecule contains both hydrophobic and hydrophilic amino acids. The hydrophilic tend to be on the outside of the molecule while the hydrophobic tend to be more on the inside and form the active site, a large hydrophobic cavity that can accommodate a ligand
with at least seven residues. The principal connection between a ligand and the enzyme is by hydrogen bond
ing. The residues are named after their place in the ligand, the residues closest to the cleavage site are named P1 and P1′ and they bind into the S1 and S1′ pockets respectively. There are four S pockets, and three S′ pockets, see table 1. The pockets alternate on either side of the backbone in the ligand. This alternation affects the orientation of the pockets making the S3 and S1 pockets arrange together and it makes the S2 pocket close to both S4 and S1′ pockets. Evidence suggest that the closely arranged S1 and S3 pockets merge to form a spacious superpocket.
Ligands that fill the superpocket have greater potency than those who do not, occupying increases potency
200 fold. These ligands can be structurally diverse and form van der Waals
bonds to the surface of the superpocket.
From the S3 pocket stretches a binding site that is distinct for renin, the S3sp subpocket.
The S3sp subpocket can accommodate both hydrophobic and polar
residues, the pocket can accommodate three water molecules but has also lipophillic nature. The S3sp subpocket is not conformationally flexible so the residues occupying the pocket must have certain characteristics. They can not be sterically demanding and must have reasonably high number of rotatable bonds and be able to connect with hydrogen bonds.
The S2 pocket is large, bipartite
and hydrophobic but can accommodate both hydrophobic and polar ligands. This diversity of possible polarity offers the P2 residue opportunity of variation in its connection to the enzyme.
The S3-S1 and the S3sp subpocket have been the main target of drug design but recent discoveries have indicated that there are other sites of interest. Interactions to the pockets on the S′ site have been proven to be critical for affinity
, especially the S1′ and S2′, and in vitro test have indicated that interaction with the flap region could be important to affinity.
Interaction with both aspartic acids in the active site results in a higher affinity. Higher affinity also results by occupying more active site pockets. However some pockets contribute more to the affinity than others, see table 1. A hydrophobic interaction with the S3sp subpocket, S1 and S3 contribute to higher potency and affinity.
By having a large and aromatic residue in P3 increases inhibitory activity. Occupation of the S3sp subpocket can increase potency by 50 fold and results in tight binding.
Example of binding to the renin inhibitor:
Aliskiren is a peptide like renin inhibitor and unlike most it is rather hydrophilic. It blocks the catalytic function of the enzyme by occupying the S3 to S2′ pockets, except the S2 pocket. Aliskiren also binds to the S3sp subpocket and because that pocket is distinct for renin aliskiren doesn't inhibit other aspartic proteases such as cathepsin D and pepsin
.
The side chain of aliskiren that binds the S3sp subpocket is ideal and leads to its quality as an inhibitor of human renin.
The hydroxyl group in aliskiren forms a hydrogen bond with both oxygen atoms of the Asp
32. The amine
group forms a hydrogen bond with the carboxylic acid
group of Gly
217 and the oxygen atom of the Asp32. The methoxy
group on the aromatic ring fills the S3 pocket and may possibly form a hydrogen bond with a secondary amine group of Tyr
14. The amide group forms a hydrogen bond with a secondary amine group of Ser
76. The S1 and S1′ pockets are occupied by the two propyl
groups in positions P1 and P1′. The terminal amide in position P2′ anchors the amide tail in the active site by forming a hydrogen bond with Arg
74 in the S2′ pocket.
and cardiovascular disease
.
Aliskiren has been in development as a monotherapy and in combination with other high blood pressure medicines. Aliskiren in combination with hydrochlorothiazide
was approved by the FDA in 2008 under the tradename Tekturna HCT.
In 2007 the companies Actelion/Merck and Speedel announced that they have the next generation of renin inhibitors in clinical research. The lead compound from Actelion/Merck has entered phase II trials. One compound from Speedel, SPP635, has completed phase IIa. The results showed that it was safe and well tolerated over four weeks period and it reduced the blood pressure by 9,8 mmHg – 17,9 mmHg. In 2008 SPP635 was continuing phase II development for hypertension in diabetic
patients. More renin inhibitors from Speedel are in clinical trials. Two of them, SPP1148 and SPP676, have entered phase I. Other are in preclinical phases
, the compound SPP1234 and compounds from the SPP800 series.
The next generation of renin inhibitors have shown potential improvements over previous generations were bioavailability has increased up to 30% in humans, and they have better tissue distribution
.
Renin inhibitor
Renin inhibitor, or inhibitors of renin, are a new group of pharmaceuticals that are used primarily in treatment of hypertension.They act on the juxtaglomerular cells of kidney, which produce renin in response to decreased blood flow....
s are antihypertensive drugs that inhibit
Enzyme inhibitor
An enzyme inhibitor is a molecule that binds to enzymes and decreases their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides...
the first and rate-limiting step
Rate-determining step
The rate-determining step is a chemistry term for the slowest step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in. In...
of the renin-angiotensin-aldosterone system (RAAS). Since the 1970s scientists have been trying to develop potent
Potency (pharmacology)
In the field of pharmacology, potency is a measure of drug activity expressed in terms of the amount required to produce an effect of given intensity. A highly potent drug evokes a larger response at low concentrations, while a drug of lower potency evokes a small response at low concentrations...
inhibitors with acceptable oral bioavailability
Bioavailability
In pharmacology, bioavailability is a subcategory of absorption and is used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered...
. The process was difficult and took about three decades. The first and second generations faced problems like poor bioavailability and lack of potency. Finally the third generation was discovered. These compounds
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
were non-peptidic
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
renin inhibitors, had acceptable oral bioavailability and were potent enough for clinical use. The first drug in this class was aliskiren
Aliskiren
Aliskiren is the first in a class of drugs called direct renin inhibitor. Its current licensed indication is essential hypertension....
which received a marketing approval in 2007.
History
In 1896 the Finnish physiologist Robert Tigerstedt and the Swedish physician Per Bergman did an experiment on kidneys and the circulationCirculatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
in rabbits. They observed that blood pressure rose in the rabbits when extract
Extract
An extract is a substance made by extracting a part of a raw material, often by using a solvent such as ethanol or water. Extracts may be sold as tinctures or in powder form....
s of the kidneys were injected into the jugular vein
Jugular vein
The jugular veins are veins that bring deoxygenated blood from the head back to the heart via the superior vena cava.-Internal and external:There are two sets of jugular veins: external and internal....
. They also discovered that this substance that was responsible for higher blood pressure was produced in the renal cortex
Renal cortex
The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla. In the adult, it forms a continuous smooth outer zone with a number of projections that extend down between the pyramids. It contains the renal corpuscles and the renal tubules except for parts of...
and they named this substance renin
Renin
Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...
. Although this experiment laid the foundation for future investigations into the RAAS pathway it had little impact on the scientific community at that time. It was not until in 1934 when Goldblatt published his work in renal ischaemia that renin came into focus again. The importance of renin in the pathogenesis
Pathogenesis
The pathogenesis of a disease is the mechanism by which the disease is caused. The term can also be used to describe the origin and development of the disease and whether it is acute, chronic or recurrent...
of cardiovascular disease
Cardiovascular disease
Heart disease or cardiovascular disease are the class of diseases that involve the heart or blood vessels . While the term technically refers to any disease that affects the cardiovascular system , it is usually used to refer to those related to atherosclerosis...
was however not fully understood until in the 1970s and it was not until 20 years later that the first renin inhibitors went to clinical trial
Clinical trial
Clinical trials are a set of procedures in medical research and drug development that are conducted to allow safety and efficacy data to be collected for health interventions...
s.
Pepstatin
Pepstatin
Pepstatin is a potent inhibitor of aspartyl proteases. It is a hexa-peptide containing the unusual amino acid statine , having the sequence Isovaleryl-Val-Val-Sta-Ala-Sta...
, which was described in 1972, was the first synthetic renin inhibitor but poor pharmacokinetic properties prevented it from entering in vivo
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
investigations. The first generation of renin inhibitors, such as H-142, were peptide analogues of angiotensinogen. However these inhibitors had also limited drug-like properties. It was not until 1982 that hopes of breakthrough appeared when development of the second generation renin inhibitors begins. This generation consisted of peptide-like compounds such as remikiren
Remikiren
Remikiren is a renin inhibitor under development for the treatment of hypertension . It was first developed by Hoffmann–La Roche in 1996....
, enalkiren and zanikiren. They had more drug-like rather than substrate-like properties and in 1990 they went to clinical trials. However the second generation had its limitations and never completed clinical trials.
Aliskiren is a third generation renin inhibitor, which are non-peptide-like compounds, and is the only renin inhibitor that went into phase III clinical trials. The first clinical trial of aliskiren was performed in 2000 in healthy volunteers. In 2007 aliskiren was approved by the US Food and Drug Administration and the European Medicines Agency
European Medicines Agency
The European Medicines Agency is a European agency for the evaluation of medicinal products. From 1995 to 2004, the European Medicines Agency was known as European Agency for the Evaluation of Medicinal Products.Roughly parallel to the U.S...
as a treatment for hypertension
Hypertension
Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...
.
The renin-angiotensin-aldosterone system
The renin-angiotensin-aldosterone system (RAAS) plays a key role in the pathologyPathology
Pathology is the precise study and diagnosis of disease. The word pathology is from Ancient Greek , pathos, "feeling, suffering"; and , -logia, "the study of". Pathologization, to pathologize, refers to the process of defining a condition or behavior as pathological, e.g. pathological gambling....
of hypertension. Under normal conditions, stimulation of the RAAS occurs in response to threats that compromise blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
stability, such as hypotension
Hypotension
In physiology and medicine, hypotension is abnormally low blood pressure, especially in the arteries of the systemic circulation. It is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the...
, blood loss and excessive loss of sodium and water. Blood pressure is a result of total peripheral resistance
Total peripheral resistance
Vasculature throughout the entire body can be thought of as two separate circuits - one is the systemic circulation, while the other is the pulmonary circulation. Total peripheral resistance is the sum of the resistance of all peripheral vasculature in the systemic circulation...
and cardiac output
Cardiac output
Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...
. Research has shown that renin is in high concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
when vasoconstriction is the main reason for high blood pressure. On the other hand, renin is in low concentration when cardiac output is the main reason.
The highly selective
Functional Selectivity
Functional selectivity is the ligand-dependent selectivity for certain signal transduction pathways in one and the same receptor. This can be present when a receptor has several possible signal transduction pathways...
aspartic protease renin is secreted
Secretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
from the juxtaglomerular apparatus
Juxtaglomerular apparatus
The juxtaglomerular apparatus is a microscopic structure in the kidney, which regulates the function of each nephron. The juxtaglomerular apparatus is named for its proximity to the glomerulus: it is found between the vascular pole of the renal corpuscle and the returning distal convoluted tubule...
as well as locally in other tissues
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
, such as in the brain, heart and blood vessels. Renin is a circulating enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
that acts on a circulating peptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
, angiotensinogen. Renin cleaves the peptide at the Leu10–Val11 bond and this reaction is the rate-determining step
Rate-determining step
The rate-determining step is a chemistry term for the slowest step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in. In...
of the RAAS. This leads to the product angiotensin I (Ang I) which is a decapeptide. Ang I is broken down
Chemical decomposition
Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...
by the angiotensin-converting enzyme
Angiotensin-converting enzyme
Angiotensin I-converting enzyme , an exopeptidase, is a circulating enzyme that participates in the body's renin-angiotensin system , which mediates extracellular volume , and arterial vasoconstriction...
(ACE) to the active octapeptide angiotensin II (Ang II) which is the principal effector of the RAAS. Ang II is a powerful vasoconstrictor and leads to the release of catecholamine
Catecholamine
Catecholamines are molecules that have a catechol nucleus consisting of benzene with two hydroxyl side groups and a side-chain amine. They include dopamine, as well as the "fight-or-flight" hormones adrenaline and noradrenaline released by the adrenal medulla of the adrenal glands in response to...
s from the adrenal medulla
Adrenal medulla
The adrenal medulla is part of the adrenal gland. It is located at the center of the gland, being surrounded by the adrenal cortex. It is the innermost part of the adrenal gland, consisting of cells that secrete epinephrine , norepinephrine , and a small amount of dopamine in response to...
and prejunctional nerve endings. Ang II also leads to the secretion of aldosterone
Aldosterone
Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...
which is the main salt-retaining hormone. This stimulates the kidneys to retain salt. Ang II also provides a negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
to the system by its inhibiton on renin release. Ang II interacts with at least two classes of Ang II receptors
Angiotensin receptor
The angiotensin receptors are a class of G protein-coupled receptors with angiotensin II as their ligands. They are important in the renin-angiotensin system: they are responsible for the signal transduction of the vasoconstricting stimulus of the main effector hormone, angiotensin...
, AT1
Angiotensin II receptor type 1
Angiotensin II receptor, type 1 or AT1 receptor is an angiotensin receptor. It has vasopressor effects and regulates aldosterone secretion. It is an important effector controlling blood pressure and volume in the cardiovascular system...
and AT2
Angiotensin II receptor type 2
Angiotensin II receptor type 2, also known as the AT2 receptor is a protein that in humans is encoded by the AGTR2 gene.- Function :...
. This mechanism, which runs from renin through Ang II and to aldosterone as well as the negative feedback that Ang II has on renin secretion, is known as RAAS. The short term effects are increased blood pressure and long term effects can be end organ damage
End organ damage
End organ or target organ damage usually refers to damage occurring in major organs fed by the circulatory system which can sustain damage due to uncontrolled hypertension, hypotension, or hypovolemia....
.
Mechanism of action
Renin inhibitors bind to the active siteActive site
In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...
of renin and inhibit the binding of renin to angiotensinogen which is the rate determining step of the RAAS cascade. By inhibiting the RAAS at the beginning renin inhibitors prevent the formation of Ang I and Ang II. It is also thought that renin inhibitors prevent Ang-(1-7), Ang-(1-9) and Ang-(1-5) formation. Renin is profoundly selective for its only naturally occurring substrate which is angiotensinogen. This means that renin inhibitors that have a high specificity for human renin, theoretically should have very few side effect
Side effect
In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequences of the use of a drug.Occasionally, drugs are...
s, if any. This has been shown for aliskiren which has a placebo-like side-effect profile.
Ang II also functions within the RAAS as a negative feedback to suppress further release of renin. This reduction in Ang II levels suppress the feedback loop. That leads to further increased plasma renin concentrations (PRC) and plasma renin activity (PRA). This can be problematic for ACE inhibitor
ACE inhibitor
ACE inhibitors or angiotensin-converting enzyme inhibitors are a group of drugs used primarily for the treatment of hypertension and congestive heart failure...
and angiotensin II receptor antagonist
Angiotensin II receptor antagonist
Angiotensin II receptor antagonists, also known as angiotensin receptor blockers , AT1-receptor antagonists or sartans, are a group of pharmaceuticals which modulate the renin-angiotensin-aldosterone system...
therapy since increased PRA could partially overcome the pharmacologic inhibition of the RAAS cascade. Because renin inhibitors directly effect renin activity, decrease of PRA despite the increased PRC (from loss of the negative feedback) may be clinically advantageous.
Pepstatin – the first renin inhibitor
PepstatinPepstatin
Pepstatin is a potent inhibitor of aspartyl proteases. It is a hexa-peptide containing the unusual amino acid statine , having the sequence Isovaleryl-Val-Val-Sta-Ala-Sta...
was the first synthetic
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
renin inhibitor. It is of microbial
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
origin and is an N-acyl-pentapeptide, more accurately: isovaleryl-L-valyl-L-valyl-statyl-L-alanyl-statine. Pepstatin was found to be a potent competitive inhibitor of most aspartic proteases but a weak inhibitor of renin. Originally it was thought to be effective in the treatment of duodenal ulcer and went through clinical trials but had no success. Statine
Statine
Statine is a gamma amino acid that occurs twice in the sequence of pepstatin, a protease inhibitor that is active against pepsin and other acid proteases. It is thought to be responsible for the inhibitory activity of pepstatin because it mimics the tetrahedral transition state of peptide catalysis....
, an amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
, is thought to be responsible for the inhibitory activity of pepstatin. That is because it mimics the tetrahedral transition state
Transition state
The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...
of the peptide catalysis
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
. Because of hydrophobic properties of statine, pepstatin has very low solubility
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
in physiological media. Since it had low potency and poor solubility it did not enter in vivo studies.
First generation – peptide analogues
This generation consists of two groups of compounds, either peptide analogues of the prosegment of renin or peptide analogues of the amino-terminal part of the substrate angiotensinogen. The drugs in the latter group seemed to be effective in inhibiting renin activity and lowering blood pressure in both animals and humans. Unfortunately they had to be given parenterally because of poor bioavailability. They also turned out to have a short duration of action, low potency and their ability to lower blood pressure was inadequate. None of these drugs completed clinical investigations.Second generation – peptide mimetics
Compounds in this generation were more potent, more stable and had longer duration of action. One of these, CGP2928, a peptidomimeticPeptidomimetic
A peptidomimetic is a small protein-like chain designed to mimic a peptide. They typically arise either from modification of an existing peptide, or by designing similar systems that mimic peptides, such as peptoids and β-peptides...
compound was the first renin inhibitor that proved effective when taken orally. It was tested on marmoset
Marmoset
Marmosets are the 22 New World monkey species of the genera Callithrix, Cebuella, Callibella, and Mico. All four genera are part of the biological family Callitrichidae. The term marmoset is also used in reference to the Goeldi's Monkey, Callimico goeldii, which is closely related.Most marmosets...
s and was only active at high doses. Development of new drugs in the second generation continued to improve pharmacokinetic properties. Remikiren, enalkiren and zankiren were then discovered. These were peptidomimetic inhibitors with improved structure that made them more specific, potent and stable. Unfortunately the clinical development was terminated because the drugs had poor oral bioavailability (it was poorly absorbed and rapidly metabolized) and lowering blood pressure activity still remained low including short half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
.
Third generation – non-peptide mimetics
Aliskiren is an orally active non-peptide renin inhibitor. It was the first drug in its class on the market. It is used to treat hypertension as monotherapy or in combination with other antihypertensive agents. The key to the discovery of aliskiren was crystallographyCrystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...
and molecular modeling techniques. Now a solution has been found to the problem that impeded the development of the renin inhibitors of the previous generations. It was known that non-peptide mimetics would solve the problems of poor pharmacokinetic properties and low specificity. This led to the design of small molecules, nonpeptide inhibitors, which were very potent and specific of human renin.
Binding and structure activity relationship of renin inhibitors
The renin molecule is a monospecific enzyme that belongs to the aspartic protease family. Its structure is complex and consists of two homologous lobes that fold mainly in a β-sheetBeta sheet
The β sheet is the second form of regular secondary structure in proteins, only somewhat less common than the alpha helix. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet...
conformation. Between the two lobes, deep within the enzyme, resides the active site
Active site
In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...
and its catalytic activity is due to two aspartic acid
Aspartic acid
Aspartic acid is an α-amino acid with the chemical formula HOOCCHCH2COOH. The carboxylate anion, salt, or ester of aspartic acid is known as aspartate. The L-isomer of aspartate is one of the 20 proteinogenic amino acids, i.e., the building blocks of proteins...
residues
Residue (chemistry)
In chemistry, residue is the material remaining after a distillation or an evaporation, or to a portion of a larger molecule, such as a methyl group. It may also refer to the undesired byproducts of a reaction....
(Asp32 and Asp 215, one from each lobe in the renin molecule). A flexible flap made from amino acids formed in a β-hairpin
Beta hairpin
The beta hairpin structural motif is the simplest protein motif involving two beta strands that look like a hairpin. The motif consists of two strands that are adjacent in primary structure oriented in an antiparallel arrangement and linked by a short loop of two to five amino acids...
closes the active site by covering the cleft.
The renin molecule contains both hydrophobic and hydrophilic amino acids. The hydrophilic tend to be on the outside of the molecule while the hydrophobic tend to be more on the inside and form the active site, a large hydrophobic cavity that can accommodate a ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
with at least seven residues. The principal connection between a ligand and the enzyme is by hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
ing. The residues are named after their place in the ligand, the residues closest to the cleavage site are named P1 and P1′ and they bind into the S1 and S1′ pockets respectively. There are four S pockets, and three S′ pockets, see table 1. The pockets alternate on either side of the backbone in the ligand. This alternation affects the orientation of the pockets making the S3 and S1 pockets arrange together and it makes the S2 pocket close to both S4 and S1′ pockets. Evidence suggest that the closely arranged S1 and S3 pockets merge to form a spacious superpocket.
Ligands that fill the superpocket have greater potency than those who do not, occupying increases potency
Potency
Potency may refer to:* Potency , a measure of the activity of a drug in a biological system* Virility* Potency is a measure of the differentiation potential of stem cells...
200 fold. These ligands can be structurally diverse and form van der Waals
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
bonds to the surface of the superpocket.
From the S3 pocket stretches a binding site that is distinct for renin, the S3sp subpocket.
The S3sp subpocket can accommodate both hydrophobic and polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
residues, the pocket can accommodate three water molecules but has also lipophillic nature. The S3sp subpocket is not conformationally flexible so the residues occupying the pocket must have certain characteristics. They can not be sterically demanding and must have reasonably high number of rotatable bonds and be able to connect with hydrogen bonds.
The S2 pocket is large, bipartite
Bipartite
Bipartite means having two parts, or an agreement between two parties. More specifically, it may refer to any of the following:-Mathematics:* 2 * bipartite graph* bipartite cubic, a type of cubic function-Medicine:...
and hydrophobic but can accommodate both hydrophobic and polar ligands. This diversity of possible polarity offers the P2 residue opportunity of variation in its connection to the enzyme.
The S3-S1 and the S3sp subpocket have been the main target of drug design but recent discoveries have indicated that there are other sites of interest. Interactions to the pockets on the S′ site have been proven to be critical for affinity
Affinity
Affinity is a word used in a variety of fields, usually to indicate some kind of preference, relationship, or a potential or actual closeness between two entities.Articles dealing with various usages of the word: affinity include:-Commerce and law:...
, especially the S1′ and S2′, and in vitro test have indicated that interaction with the flap region could be important to affinity.
Interaction with both aspartic acids in the active site results in a higher affinity. Higher affinity also results by occupying more active site pockets. However some pockets contribute more to the affinity than others, see table 1. A hydrophobic interaction with the S3sp subpocket, S1 and S3 contribute to higher potency and affinity.
By having a large and aromatic residue in P3 increases inhibitory activity. Occupation of the S3sp subpocket can increase potency by 50 fold and results in tight binding.
Example of binding to the renin inhibitor:
Aliskiren is a peptide like renin inhibitor and unlike most it is rather hydrophilic. It blocks the catalytic function of the enzyme by occupying the S3 to S2′ pockets, except the S2 pocket. Aliskiren also binds to the S3sp subpocket and because that pocket is distinct for renin aliskiren doesn't inhibit other aspartic proteases such as cathepsin D and pepsin
Pepsin
Pepsin is an enzyme whose precursor form is released by the chief cells in the stomach and that degrades food proteins into peptides. It was discovered in 1836 by Theodor Schwann who also coined its name from the Greek word pepsis, meaning digestion...
.
The side chain of aliskiren that binds the S3sp subpocket is ideal and leads to its quality as an inhibitor of human renin.
The hydroxyl group in aliskiren forms a hydrogen bond with both oxygen atoms of the Asp
Aspartic acid
Aspartic acid is an α-amino acid with the chemical formula HOOCCHCH2COOH. The carboxylate anion, salt, or ester of aspartic acid is known as aspartate. The L-isomer of aspartate is one of the 20 proteinogenic amino acids, i.e., the building blocks of proteins...
32. The amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
group forms a hydrogen bond with the carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
group of Gly
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
217 and the oxygen atom of the Asp32. The methoxy
Methoxy
In chemistry , methoxy refers to the functional group consisting of a methyl group bound to oxygen. This alkoxy group has the formula O–CH3.The word is used in organic nomenclature usually to describe an ether...
group on the aromatic ring fills the S3 pocket and may possibly form a hydrogen bond with a secondary amine group of Tyr
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
14. The amide group forms a hydrogen bond with a secondary amine group of Ser
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
76. The S1 and S1′ pockets are occupied by the two propyl
Propyl
In organic chemistry, propyl is a three-carbon alkyl substituent with chemical formula -C3H7. It is the substituent form of the alkane propane...
groups in positions P1 and P1′. The terminal amide in position P2′ anchors the amide tail in the active site by forming a hydrogen bond with Arg
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
74 in the S2′ pocket.
Current status
The first renin inhibitor approved for the treatment of hypertension, aliskiren, has not only shown to be effective in lowering blood pressure but also has fewer side-effects compared with ACE inhibitors. Moreover the renin inhibitor indicates that it provides organ protection, especially for patient with concomitant renal dysfunctionRenal failure
Renal failure or kidney failure describes a medical condition in which the kidneys fail to adequately filter toxins and waste products from the blood...
and cardiovascular disease
Cardiovascular disease
Heart disease or cardiovascular disease are the class of diseases that involve the heart or blood vessels . While the term technically refers to any disease that affects the cardiovascular system , it is usually used to refer to those related to atherosclerosis...
.
Aliskiren has been in development as a monotherapy and in combination with other high blood pressure medicines. Aliskiren in combination with hydrochlorothiazide
Hydrochlorothiazide
Hydrochlorothiazide, abbreviated HCTZ, HCT, or HZT, is a first-line diuretic drug of the thiazide class that acts by inhibiting the kidneys' ability to retain water. This reduces the volume of the blood, decreasing blood return to the heart and thus cardiac output and, by other mechanisms, is...
was approved by the FDA in 2008 under the tradename Tekturna HCT.
In 2007 the companies Actelion/Merck and Speedel announced that they have the next generation of renin inhibitors in clinical research. The lead compound from Actelion/Merck has entered phase II trials. One compound from Speedel, SPP635, has completed phase IIa. The results showed that it was safe and well tolerated over four weeks period and it reduced the blood pressure by 9,8 mmHg – 17,9 mmHg. In 2008 SPP635 was continuing phase II development for hypertension in diabetic
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
patients. More renin inhibitors from Speedel are in clinical trials. Two of them, SPP1148 and SPP676, have entered phase I. Other are in preclinical phases
Pre-clinical development
In drug development, pre-clinical development is a stage of research that begins before clinical trials can begin, and during which important feasibility, iterative testing and drug safety data is collected....
, the compound SPP1234 and compounds from the SPP800 series.
The next generation of renin inhibitors have shown potential improvements over previous generations were bioavailability has increased up to 30% in humans, and they have better tissue distribution
Distribution (pharmacology)
Distribution in pharmacology is a branch of pharmacokinetics which describes the reversible transfer of drug from one location to another within the body....
.
See also
- ACE inhibitorACE inhibitorACE inhibitors or angiotensin-converting enzyme inhibitors are a group of drugs used primarily for the treatment of hypertension and congestive heart failure...
- ACE inhibitors drug designACE inhibitors drug designThe discovery of an orally inactive peptide from snake venom established the important role of angiotensin converting enzyme inhibitors in regulating blood pressure. This led to the development of Captopril, the first ACE inhibitor. When the adverse effects of Captopril became apparent new...
- AliskirenAliskirenAliskiren is the first in a class of drugs called direct renin inhibitor. Its current licensed indication is essential hypertension....
- AngiotensinAngiotensinAngiotensin, a peptide hormone, causes blood vessels to constrict, and drives blood pressure up. It is part of the renin-angiotensin system, which is a major target for drugs that lower blood pressure. Angiotensin also stimulates the release of aldosterone, another hormone, from the adrenal cortex...
- Angiotensin II receptor antagonistAngiotensin II receptor antagonistAngiotensin II receptor antagonists, also known as angiotensin receptor blockers , AT1-receptor antagonists or sartans, are a group of pharmaceuticals which modulate the renin-angiotensin-aldosterone system...
- Beta blockerBeta blockerBeta blockers or beta-adrenergic blocking agents, beta-adrenergic antagonists, beta-adrenoreceptor antagonists or beta antagonists, are a class of drugs used for various indications. They are particularly for the management of cardiac arrhythmias, cardioprotection after myocardial infarction ,...
- Circulatory systemCirculatory systemThe circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
- Discovery and development of angiotensin receptor blockersDiscovery and development of angiotensin receptor blockersThe angiotensin receptor blockers , also called angiotensin receptor antagonists or sartans, are a group of anti-hypertensive drugs that act by blocking the effects of the hormone angiotensin II in the body, thereby lowering blood pressure...
- Renin-angiotensin systemRenin-angiotensin systemThe renin-angiotensin system or the renin-angiotensin-aldosterone system is a hormone system that regulates blood pressure and water balance....
- Renin inhibitorRenin inhibitorRenin inhibitor, or inhibitors of renin, are a new group of pharmaceuticals that are used primarily in treatment of hypertension.They act on the juxtaglomerular cells of kidney, which produce renin in response to decreased blood flow....