Discovery and development of angiotensin receptor blockers/ja: Difference between revisions

The [[angiotensin receptor blockers]] (ARBs), also called angiotensin (AT1) receptor antagonists or sartans, are a group of [[antihypertensive]] drugs that act by blocking the effects of the [[hormone]] [[angiotensin II]] (Ang II) in the body, thereby lowering [[blood pressure]]. Their structure is similar to Ang II and they bind to [[Angiotensin receptor|Ang II receptors]] as inhibitors, e.g., [T24 from Rhys Healthcare].

ARBs are widely used drugs in the clinical setting today, their main [[Indication (medicine)|indications]] being mild to moderate [[hypertension]], [[chronic heart failure]], secondary [[stroke]] prevention and [[diabetic nephropathy]].

The discovery and development of ARBs is a demonstrative example of modern [[rational drug design]] and how design can be used to gain further knowledge of [[physiological]] systems, in this case, the characterization of the subtypes of Ang II receptors.

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==History==

In 1898, the [[physiologist]] [[Robert Tigerstedt]] and his student, Per Bergman, experimented with rabbits by injecting them with kidney extracts. Their results suggested the kidneys produced a [[protein]], which they named [[renin]], that caused a rise in blood pressure. In the 1930s, Goldblatt conducted experiments where he constricted the renal blood flow in dogs; he found the [[ischaemic]] kidneys did in fact secrete a chemical that caused [[vasoconstriction]]. In 1939, renin was found not to cause the rise in blood pressure, but was an [[enzyme]] which catalyzed the formation of the substances that were responsible, namely, [[angiotensin I]] (Ang I) and Ang II.

In the 1970s, scientists first observed Ang II to harm the heart and kidneys, and individuals with high levels of renin activity in [[Blood plasma|plasma]] were at increased risk of [[myocardial infarction]] and stroke.

With the introduction of [[angiotensin converting enzyme inhibitors|angiotensin converting enzyme (ACE) inhibitors]] in the late 1970s it was confirmed that Ang II plays an important role in regulating blood pressure and [[electrolyte]] and fluid balance.

Before that attempts had been made to develop useful Ang II receptor antagonists and initially, the main focus was on angiotensin [[peptide]] analogues. [[Saralasin]] and other Ang II analogues were potent Ang II receptor blockers but the main problem was a lack of oral [[bioavailability]].

In the early 1980s it was noted that a series of imidazole-5-[[acetic acid]] [[Derivative (chemistry)|derivatives]] diminished blood pressure responses to Ang II in rats. Two compounds, [[S-8307]] and [[S-8308]], were later found to be highly specific and promising non-peptide Ang II receptor antagonists but using [[molecular modeling]] it was seen that their structures would have to [[mimic]] more closely the [[pharmacophore]] of Ang II. Structural modifications were made and the orally active, potent and [[Ligand (biochemistry)#Selective and non-selective|selective]] nonpeptide AT₁ receptor blocker [[losartan]] was developed. In 1995 losartan was approved for clinical use in the United States and since then six additional ARBs have been approved. These drugs are known for their excellent [[Adverse effect (medicine)|side-effects]] profiles, which [[clinical trials]] have shown to be similar to those of [[placebos]].

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==The angiotensin II receptor==

The actions of Ang II are mediated by angiotensin receptors, [[Angiotensin II receptor type 1|AT₁]] and [[Angiotensin II receptor type 2|AT₂]]. These receptors are members of the [[G protein-coupled receptors]] family which are seven [[transmembrane]] [[helices]], connected by interchanging [[extracellular]] and [[intracellular]] loops.

Each [[G protein-coupled receptor]] couples to a specific [[G-protein]] which leads to activation of a special effector system. AT₁ receptors are for instance primarily coupled through the G_q/11 group of [[G-proteins]]. <br />

Two more angiotensin receptors have been described, AT₃ and AT₄, but their role is still unknown.

===Distribution in the body===

AT₁ receptors are mainly found in the heart, [[adrenal glands]], brain, liver and kidneys. Their main role is to regulate blood pressure as well as fluid and electrolyte balance. <br />

AT₂ receptors are highly expressed in the developing [[fetus]] but they decline rapidly after birth. In the adult, AT₂ receptors are present only at low levels and are mostly found in the heart, adrenal glands, uterus, ovaries, kidneys and brain.

===Functions===

Most of the known actions of Ang II are mediated through the AT₁ receptors, for example [[vasoconstriction]], [[aldosterone]] release, renal [[sodium reabsorption]] and [[vasopressin]] secretion. The AT₂ receptor also takes part in regulation of blood pressure and [[renal]] function but mediates [[Receptor antagonist|antagonistic]] effects compared to the AT₁ receptor.

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[[File:Bindingsite.jpg|left|thumb|300px|Fig 1. Losartan receptor binding]]

Ang II binds to AT₁ receptors via various [[binding sites]]. The primary binding site is at the extracellular region of the AT₁ receptor where Ang II interacts with residues in the [[N-terminus]] of the AT₁ receptor and its first and third extracellular loops. The transmembrane helices also contribute to the binding via the [[C-terminal]] [[carboxyl]] group that interacts with [[Lysine|Lys]]¹⁹⁹ in the upper part of helix 5 of the receptor; see figure 1 for details.<br />

The [[Salt bridge (protein and supramolecular)|ionic bridge]] formed between [[Lysine|Lys]]¹⁹⁹ and the carboxyl terminal group of the [[Phenylalanine|Phe]]⁸ residue of Ang II is most likely stabilized by the [[Tryptophan|Trp]]²⁵³ residue. In addition, [[Phenylalanine|Phe]]²⁵⁹ and [[Aspartic acid|Asp]]²⁶³ in transmembrane helix 6 and [[Lysine|Lys]]¹⁰² and [[Serine|Ser]]¹⁰⁵ in the outer region of transmembrane helix 3 have also been implicated in Ang II binding. This region may possibly participate in the stabilization of the receptor's ratification and in the formation of the intramembrane binding pocket.

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==Mechanism of action==

[[File:Renin ang pathway 2.jpg|right|thumb|350px|Fig 2. Renin angiotensin pathway]]

Blood pressure and fluid and electrolyte [[homeostasis]] is regulated by the [[renin–angiotensin–aldosterone system]].

[[Renin]], an enzyme released from the kidneys, converts the inactive plasma protein [[angiotensinogen]] into angiotensin I (Ang I).  Then Ang I is converted to Ang II with [[angiotensin converting enzyme]] (ACE), see figure 2. Ang II in plasma then binds to AT-receptors.

ARBs are blocking the last part of the [[renin–angiotensin system|renin–angiotensin pathway]] and block the pathway more specifically than [[ACE inhibitors]].

The AT₁ receptor mediates Ang II to cause increased [[cardiac contractility]], [[sodium reabsorption]] and vasoconstriction which all lead to increased blood pressure. By blocking AT₁ receptors, ARBs lead to lower blood pressure.

An insurmountable inhibition of the AT₁ receptor is achieved when the maximum response of Ang II cannot be restored in the presence of the ARB, no matter how high the [[concentration]] of Ang II is.

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==Drug discovery and development==

[[File:ARBdrugdevelopment.jpg|left|thumb|500px|Fig 3. Drug development of ARB]]

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===Development from saralasin to losartan and eprosartan===

Because of [[saralasin]], the first [[angiotensin II antagonist|Ang II antagonist]], and the development of the first [[ACE inhibitor]] [[captopril]], it was generally acknowledged that Ang II receptor antagonists might be promising as effective [[antihypertensive]] agents.

Saralasin was developed in the early 1970s and is an octapeptide analogue of Ang II, where the [[amino acids]] [[Aspartic acid|Asp]]¹, [[Isoleucine|Ile]]⁵ and [[Phenylalanine|Phe]]⁸ have been replaced with [[Serine|Ser]]¹, [[Valine|Val]]⁵ and [[Alanine|Ala]]⁸, respectively. Saralasin was not orally [[bioavailable]], had short duration of action and showed [[partial agonist]] activity and therefore it was not suitable as a drug.

Thus the goal was to develop a smaller nonpeptide substance with similar inhibition and binding features. At this time, a group at [[DuPont]] had already started the screening of nonpeptide mimics of Ang II using existing substances from chemical libraries.

Research investigators at [[Takeda Pharmaceutical Company|Takeda]] discovered in 1982 the weak nonpeptide Ang II antagonists S-8307 and S-8308 from a group of 1-[[Benzimidazole|benzylimidazole]]-5-acetic acid derivatives. S-8307 and S-8308 have moderate [[Potency (pharmacology)|potency]], short duration of action and limited oral bioavailability, however they are selective and competitive AT₁ receptor antagonists without partial agonist activity. A group at DuPont postulated that both Ang II and the Takeda leads were bound at the same receptor site. These two substances served as lead compounds for further optimization of AT₁ receptor blockers.

Using [[nuclear magnetic resonance]] studies on the spatial structure of Ang II, scientists at DuPont discovered that the Takeda structures had to be enlarged at a particular position to resemble more closely the much larger peptide Ang II.

Computer modeling was used to compare S-8308 and S-8307 with Ang II and it was seen that Ang II contains two [[acidic]] residues near the NH₂ terminus. These groups were not mimicked by the Takeda leads and therefore it was hypothesized that acidic [[functional groups]] would have to be added to the compounds.<br />

The 4-carboxy-derivative EXP-6155 had a binding activity which was ten-fold greater than that of S-8308 which further strengthened this [[hypothesis]].

By replacing the 4-carboxy-group with a 2-carboxy-benzamido-moiety the compound EXP-6803 was synthesized. It had highly increased binding affinity but was only active when administered [[intravenously]].

Replacing the 2-carboxy-benzamido-group with a 2-carboxy-[[phenyl]]-group created the [[lipophilic]] [[biphenyl]]-containing EXP-7711, which exhibited good oral activity but slightly less affinity for the AT₁ receptor. <br />

Then the [[chemical polarity|polar]] carboxyl group was replaced with a more lipophilic [[tetrazole]] group in order to increase oral bioavailability and duration of action further and the compound thus formed was named [[losartan]]. This development took place in 1986 and losartan became the first successful [[angiotensin II receptor antagonist|Ang II antagonist]] drug, approved as such in the United States in 1995 and was marketed by [[Merck & Co.|Merck]].

Using a different lead, optimization from S-8308, [[eprosartan]] was developed by [[SmithKline Beecham]] in 1992. Eprosartan does not have a biphenyl-methyl structure but in order to mimic the C-terminal end of Ang II the 5-acetic acid group was replaced with an ''a''-thienylacrylic acid and a 4-carboxy-moiety. Eprosartan is a selective, potent and competitive AT₁ antagonist and its binding to AT₁ receptors is rapid, reversible, saturable and of high affinity.

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===Development from losartan to other drugs===

Losartan, [[valsartan]], [[candesartan]], [[irbesartan]], [[telmisartan]] and [[olmesartan]] all contain a biphenyl-[[methyl]] group.

Losartan is partly metabolized to its 5-[[carboxylic acid]] [[metabolite]] EXP 3174, which is a more potent AT₁ receptor antagonist than its parent [[Chemical compounds|compound]]

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Valsartan, first marketed by [[Novartis]], is a non[[heterocyclic]] ARB, where the imidazole of losartan has been replaced by an [[acyl]]ated [[amino acid]].

Irbesartan was developed by [[Sanofi]] Research and is longer acting than valsartan and losartan and it has an imidazolinone ring where a [[carbonyl]] group functions as a [[hydrogen bond]] acceptor instead of the [[hydroxymethyl]] group in losartan. Irbesartan is a non-competitive inhibitor.

[[Candesartan cilexetil]] (TCV 116) is a benzimidazole which was developed at Takeda and is an [[ester]] [[carbonate]] [[prodrug]]. [[In vivo]], it is rapidly converted to the much more potent corresponding 7-carboxylic acid, candesartan. In the interaction of candesartan with AT₁ receptor the carboxyl group of the benzimidazole ring plays an important role. Candesartan and its prodrug have stronger blood pressure lowering effects than EXP 3174 and losartan.

Telmisartan, which was discovered and developed in 1991 by [[Boehringer Ingelheim]], has carboxylic acid as the biphenyl acidic group. It has the longest elimination [[half-life]] of the ARBs or about 24 hours.

[[Olmesartan medoxomil]] was developed by [[Daiichi Sankyo Co.|Sankyo]] in 1995 and is the newest ARB on the market, marketed in 2002. It is an ester prodrug like candesartan cilexetil. In vivo, the prodrug is completely and rapidly [[hydrolyzed]] to the active acid form, olmesartan (RNH-6270). It has a hydroxy[[isopropyl]] group connected to the imidazole ring in addition to the carboxyl group.

===Pharmacophore and structure-activity relationship===

'''Pharmacophore'''<br />

There are three functional groups that are the most important parts for the [[bioactivity]] of ARBs, see figure 1 for details.<br />

The first one is the imidazole ring that binds to amino acids in helix 7 ([[Asparagine|Asn]]²⁹⁵). The second group is the biphenyl-methyl group that binds to amino acids in both helices 6 and 7 ([[Phenylalanine|Phe]]³⁰¹, [[Phenylalanine|Phe]]³⁰⁰, [[Tryptophan|Trp]]²⁵³ and [[Histidine|His]]²⁵⁶). The third one is the [[tetrazole]] group that interacts with amino acids in helices 4 and 5 ([[Arginine|Arg]]¹⁶⁷ and [[Lysine|Lys]]¹⁹⁹). <br />

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Candesartan and olmesartan have the highest affinity for the AT₁ receptors, followed by irbesartan and eprosartan. Valsartan, telmisartan and EXP 3174 have similar affinities that are about ten-fold less than that of candesartan. Losartan has the least affinity. ARBs' affinity for the AT₂ receptor is generally much lower (or around 10,000 times less) than for the AT₁ subtype. Therefore, they allow unhindered stimulation of the AT₂ receptor.

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==Drug comparison and pharmacokinetics==

|+ Table 1: Comparison of ARB [[pharmacokinetics]]

! Drug

! Biological half-life [h]

! Protein binding [%]

! Bioavailability [%]

! Renal/hepatic [[Clearance (medicine)|clearance]] [%]

! Food effect

! Daily dosage [mg]

| [[Losartan]]

| Minimal

| [[EXP 3174]]

| [[Candesartan]]

| [[Valsartan]]

| 40-50% decreased by

| [[Irbesartan]]

| No

| [[Telmisartan]]

| No

| [[Eprosartan]]

| No

| [[Olmesartan]]

| No

| colspan="7" style="text-align: center;" |Sources:

ARBs have a large [[therapeutic index]] and therefore their (mostly low) oral bioavailability does not appear to be of clinical significance.

As can be seen in table 1, these drugs are highly plasma protein-bound and therefore oral administration once a day should provide sufficient [[antihypertensive]] effects.

Around 14% of orally ingested losartan is metabolized to its 5-carboxylic acid [[metabolite]] EXP 3174. As mentioned before, candesartan cilexetil and olmesartan medoxomil are inactive ester prodrugs that are completely hydrolyzed to their active forms by [[esterases]] during [[Absorption (pharmacokinetics)|absorption]] from the [[gastrointestinal tract]]. These three metabolites are more potent AT₁ receptor antagonists than their [[prodrugs]]. The other ARBs do not have active metabolites.

All of the ARBs, except for valsartan and olmesartan, are metabolized in some way by the [[cytochrome P450]] (CYP) enzyme [[CYP2C9|2C9]], that is found in the human liver. [[CYP2C9]] is for example responsible for the metabolizing of losartan to EXP 3174 and the slow metabolizing of valsartan and candesartan to their inactive metabolites. Telmisartan is, on the other hand, in part metabolized by [[glucuronidation]] and olmesartan is excreted as the unchanged drug.

Telmisartan is the only ARB that can cross the [[blood–brain barrier]] and can therefore inhibit centrally mediated effects of Ang II, contributing to even better blood pressure control.

All of the ARBs have the same [[mechanism of action]] and differences in their potency can be related to their different [[pharmacokinetic]] profiles. A few clinical head-to-head comparisons have been made and candesartan, irbesartan and telmisartan appear to be slightly more effective than losartan in lowering blood pressure. This difference may be related to different strengths of activity at the receptor level, such as duration and strength of receptor binding.

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==ARBs under development==

[[File:Pratosartan.svg|right|thumb|[[Pratosartan]] structure.]]

Several new nonpeptide ARBs are undergoing [[clinical trials]] or are at pre-clinical stages of development. Among these are [[embusartan]] (BAY 10-6734 or BAY 10-6734), KRH-594, [[fonsartan]] (HR 720) and [[pratosartan]] (KT3-671). Pratosartan, for example, has a novel structure: a seven-membered ring that bears an [[Ketone|oxo]] moiety (C=O) fused to the imidazole ring (figure 4), and its affinity for the AT₁ receptor is about 7 times higher than  losartan's. The purpose of the [[Ketone|oxo]] group is similar to that of the carboxylic acid groups on other ARBs.<br />

Other attributes of ARBs are also under investigation, such as the positive effects of telmisartan on [[lipid metabolism|lipid]] and [[glucose metabolism]] and losartan's effects of lowering [[uric acid]] levels. Such effects might lead to new indications for these drugs but further research is needed.

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==See also==

* [[Discovery and development of renin inhibitors]]

{{Drug design}}

{{Angiotensin receptor modulators}}