Angiotensin Converting Enzyme: A Target for Anti-Hypertensive Drugs

International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701________________________________________________________________Review PaperAngiotensin Converting Enzyme: A Target for Anti-Hypertensive DrugsSridevi P*, Prashanth KS and Bhagavan Raju MDepartment of Pharmaceutical Chemistry, CM College of Pharmacy,Dhulapally,Maisammaguda, Secunderabad, A. P., IndiaE-mail: sri_pingala@yahoo.co.in_______________________________________________________________________________________ABSTRACTThe renin-angiotensin system plays a major role in cardiovascular disease and during the past decadeextensive research investigated the possible clinical benefit of the use of angiotensin converting enzymeinhibitors (ACE-I) in different clinical conditions. Accordingly, these agents have been recommended for thetreatment of heart failure, hypertension, and acute and chronic myocardial infarction. The aim of thisdocument is to review the rationale and clinical evidence for the use of ACE-I in patients with cardiovasculardisease. Angiotensin converting enzyme inhibitors are a class of antihypertensive drugs used to lower the highblood pressure. The conversion of inactive Angiotensin I to potent Angiotensin II occurs rapidly during thepassage through the pulmonary circulation. Bradykinin is rapidly inactivated in the circulating blood and itdisappears completely in a single passage through the pulmonary circulation. Angiotensin I also disappears inthe pulmonary circulation due to its conversion to Angiotensin II. Furthermore Angiotensin II passes throughthe lungs without any loss. The inactivation of bradykinin and the conversion of Angiotensin I to Angiotensin IIin the lungs were thought to be caused by the same enzyme. ACE inhibitor compounds block either thesynthesis of Angiotensin II or the blocking of Angiotensin II to its receptor site. The essential effect of theseagents on the renin Angiotensin system is to inhibit the conversion of inactive Angiotensin I to activeAngiotensin II.______________________________________________________________________________________INTRODUCTIONAngiotensin converting enzyme inhibitors competitively inhibit the angiotensin converting enzyme. ACE isa non-specific enzyme involved in the metabolism of many small peptides, including the conversion ofAngiotensin-I, an inactive octapeptide to Angiotensin -II. Kininase, an enzyme that catalyses the degradation ofBradykinin and other potent vasodilator peptides, is also competitively inhibited by ACE-I. The treatment ofhypertension and congestive heart failure has improved significantly with the introduction of ACE inhibitorsand angiotensin receptor blockers. The SARs and structural modifications of these agents have produced majortherapeutic advances. These have become cornerstones of therapy today. For example, more than 25years agocaptopril was the first ACE inhibitor to be developed. Subsequent molecular modifications led to thedevelopment of newer agents, such as lisinopril. Although lisinopril exerts comparable ACE inhibition, itpossesses a superior pharmacokinetic profile. Instead of having captopril three times daily, lisinopril can beadministered once daily.Medication compliance is notoriously poor in cardiovascular patients. Administering an ACE inhibitor oncedaily results in greatly enhanced medication compliance. The therapeutic outcomes of patients withhypertension and CHF have improved immensely as a result.The application of basic science in modifying the chemical structure of these agents has resulted in patientsliving longer and suffering fewer cardiovascular events, such as MI or worsening CHF. Importantly, their dayto-dayquality of life is preserved as well.ACE inhibitors have achieved widespread usage in the treatment of cardiovascular and renal disease. ACEinhibitors alter the balance between the vasoconstrictive, salt-retentive, and hypertrophic properties ofangiotensin II (Ang II) and the vasodilatory and natriuretic properties of bradykinin and alter the metabolism ofa number of other vasoactive substances. ACE inhibitors differ in the chemical structure of their active moieties,in potency, in bioavailability, in plasma half-life, in route of elimination, in their distribution and affinity fortissue-bound ACE, and in whether they are administered as prodrugs. Thus, the side effects of ACE inhibitorscan be divided into those that are class specific and those that relate to specific agents. ACE inhibitors decreaseVol. 2 (1) Jan – Mar 2011 www.ijrpbsonline.com 63

International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701systemic vascular resistance without increasing heart rate and promote natriuresis. They have proved effective inthe treatment of hypertension, they decrease mortality in congestive heart failure and left ventricular dysfunctionafter myocardial infarction, and they delay the progression of diabetic nephropathy. Ongoing studies willelucidate the effect of ACE inhibitors on cardiovascular mortality in essential hypertension, the role of ACEinhibitors in patients without ventricular dysfunction after myocardial infarction, and the role of ACE inhibitorscompared with newly available angiotensin AT1 receptor antagonists.Angiotensin converting enzyme inhibitors occur in 1981, with FDA approval of Captopril, the first class drug tobe marketed for anti hypertensive activity. These compounds block either the synthesis of Angiotensin II or theblocking of Angiotensin II to its receptor site the essential effect of these agents on the renin Angiotensin systemis to inhibit the conversion of inactive Angiotensin I to active Angiotensin IIACE inhibitors are highly selective drugs because they attenuate or abolish responses to Angiotensin I butnot to Angiotensin II. Pharmacological effects apparently arise from suppression of synthesis of Angiotensin II.ACE inhibitors increase bradykinin levels and bradykinin stimulates prostaglandin biosynthesis.RENIN-ANGIOTENSIN-ALDOSTERONE PATHWAYThere are 11 ACE inhibitors approved for therapeutic use which can be sub classified into three groups based ontheir chemical composition:I. Sulfhydryl - containing inhibitors – Captopril, Zofenopril.II. Dicarboxylate - containing inhibitors – Enalapril, Lisinopril, Ramipril.III. Phosphate - containing inhibitors – Fosinopril.Majority of the inhibitors contain the dicarboxylate functionality. All of these compounds effectively blockthe conversion of Angiotensin I to Angiotensin II and have similar therapeutic and physiologic effects. Thecompounds differ primarily in their potency and pharmacokinetic profile. Additionally, the sulfhydryl group incaptopril is responsible for certain effects not seen with the other agents.DEVELOPMENT OF ACE INHIBITORSThe development of the nonapeptide teprotide (Glu-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro), which was originallyisolated from the venom of the Brazilian pit viper “Bothrops jararaca”, greatly clarified the importance of ACEin hypertension. However, its lack of oral activity limited its therapeutic utility. L-benzylsuccinic acid (2(R)-benzyl-3-carboxypropionic acid) was described to be the most potent inhibitor of carboxypeptidase A in theearly 1980s and a by-product analog, was proposed to bind to the active site of carboxypeptidase A via succinylcarboxyl group and a carbonyl group. These findings established that L-benzylsuccinic acid is bound at a singlelocus at the active site of carboxypeptidase A.Drug Design of Captopril (Sulfhydrils)Over 2000 compounds were tested randomly in a guinea pig ileum test and succinyl-L-proline was found tohave the properties of a specific ACE inhibitor. It showed inhibitory effect of angiotensinI and bradykinin without having any effects on angiotensin II. Then researchers started to search for a modelthat would explain inhibition on the basis of specific chemical interactions of compounds with the active site ofACE. Previous studies with substrates and inhibitors of ACE suggested that it was a zinc-Vol. 2 (1) Jan – Mar 2011 www.ijrpbsonline.com 64

International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701containing metalloprotein and a carboxypeptidase similar to pancreatic carboxypeptidase A. However ACEreleases dipeptides rather than single amino acids from the C-terminus of the peptide substrates. And it wasassumed that both their mechanism of action and their active site might be similar. Apositively charged Arg 145 at the active site was thought to bind with the negatively charged C-terminal carboxylgroup of the peptide substrate. It was also proposed that ACE binds by hydrogen bonding to the terminal, nonscissile, peptide bond of the substrate.But since ACE is a dipeptide carboxypeptidase, unlike carboxypeptidase A, the distance between thecationic carboxyl-binding site and the zinc atom should be greater, by approximately the length of one aminoacid residue. Proline was chosen as the amino acid moiety because of its presence as the carboxy terminal aminoacid residue in teprotide and other ACE inhibitors found in snake venoms. 11 other amino acids were tested butnone of them were more inhibitory. So it was proposed that succinyl amino acid derivative should be an ACEinhibitor and succinyl-L-proline was found to be such an inhibitor.It was also known that the nature of penultimate amino acid residue of a peptide substrate for ACEinfluences binding to the enzyme. The acyl group of the carboxyalkanoyl amino acid binds the zinc ion of theenzyme and occupies the same position at the active site of ACE as the penultimate. Therefore the substituent ofthe acyl group might also influence binding to the enzyme. A 2-methyl substituent with D configuration wasfound to enhance the inhibitory potency by about 15 fold of succinyl-L-proline. Then the search for a betterzinc-binding group started. Replacement of the succinyl carboxyl group by nitrogen-containing functionalities(amine, amide or guanidine) did not enhance inhibitory activity. However a potential breakthrough wasachieved by the replacement of the carboxyl group with a sulfhydril function (SH), a group withgreater affinity for the enzyme bound zinc ion. This yielded a potent inhibitor that was 1000 times more potentthan succinyl-L-proline. The optimal acyl chain length for mercapto-alkanoyl derivates of proline was found tobe 3-mercaptopropanoyl-L-proline, 5 times greater than that of 2-mercaptoalkanoyl derivates and 50 timesgreater that that of 4-mercaptoalkanoyl derivates. So the D-3-mercapto-2-methylpropanoyl-L-proline orCaptopril was the most potent inhibitor. Later, the researchers compared a few mercaptoacyl amino acidinhibitors and concluded that the binding of the inhibitor to the enzyme involved a hydrogen bond between adonor site on the enzyme and the oxygen of the amide carbonyl, much like predicted for the substratesCaptopril (Capoten)Drug Design of Other First Generation ACE InhibitorsThe most common adverse effects of Captopril, skin rash and loss of taste, are the same as caused bymercapto-containing penicillamine. Therefore a group of researchers aimed at finding potent, selective ACEinhibitors that wouldn’t contain a mercapto (SH) function and would have a weaker chelating function. Theyreturned to work with carboxyl compounds and started working with substituted N-carboxymethyl-dipeptides asa general structure (R-CHCOOH-A 1 -A 2 ). According to previous research they assumed that cyclic iminoacids would result in good potency if substituted on the carboxyl terminus of the dipeptide. Thereforesubstituting A 2 with proline gave good results. They also noted that according to the enzyme’s specificity iminoacids in the position next to the carboxyl terminus would not give a potent compound. Also noticeable is that bysubstituting R and A 1 groups with hydrophobic and basic residues would give a potent compound. Bysubstituting –NH in the general structure resulted in loss of potency which is consistent to the enzyme’s need fora –NH in corresponding position on the substrates. The results where 2 activeinhibitors: Enalaprilat and Lisinopril. These compounds both have phenylalanine in R position which occupiesthe S 1 groove in the enzyme. The result was thus these two new, potent tripeptide analogues with zinccoordinatingcarboxyl group: Enalaprilat and LisinoprilVol. 2 (1) Jan – Mar 2011 www.ijrpbsonline.com 65

International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701Ramipril (Altace)Enalapril (Vasotec)Lisinopril (Lopril)Fosinopril (Monopril)Discovery of 2 Active Sites: C-Domain and N-DomainMost of the ACE inhibitors on the market today are non-selective towards the two active sites of ACEbecause their binding to the enzyme is based mostly on the strong interaction between the zinc atom in theenzyme and the strong chelating group on the inhibitor. The resolution of the 3D structure of germinal ACE,which has only one active site that corresponds with C-domain of the somatic ACE, offers a structuralframework for structure-based design approach. Although N- and C-domain have comparable rates in vitro ofACE hydrolyzing, it seems like that in vivo the C-domain is mainly responsible for regulating blood pressure.This indicates that C-domain selective inhibitors could have similar profile to that of current non-selectiveinhibitors. Angiotensin I is mainly hydrolyzed by the C-domain in vivo but bradykinin is hydrolyzed by bothactive sites. Thus, by developing a C-domain selective inhibitor would permit some degradation of bradykininby the N-domain and this degradation could be enough to prevent accumulation of excess bradykinin which hasbeen observed during attacks of angioedema. C-domain selective inhibition could possibly result in specializedcontrol of blood pressure with less vasodilator-related adverse effects. N-domain selective inhibitors on theother hand give the possibility of opening up novel therapeutic areas. Apparently, the N-domain doesn’t have aVol. 2 (1) Jan – Mar 2011 www.ijrpbsonline.com 66

International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701big role in controlling blood pressure but it seems to be the principal metabolizing enzyme for AcSDKP, anatural haemoregulatory hormoneMechanism of actionFig: The effect of angiotensin converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers(ARBs) on renin angiotensin aldosterone pathwayACE AND ITS INHIBITORS, CONTINUING DEVELOPMENTSThe fact that ACE has two active N- and C-domains with different properties expands the potential use ofACE inhibitors. Blocking the hydrolysis of the myeloprotective tetrapeptideAcSer-Asp-Lys-Pro by the N-domain of ACE protects hematopoietic stem cells during aggressive chemotherapy and alleviates collagenaccumulation in rat hearts.Inclusion (II) or deletion (DD) of 287 base pairs in intron 16 can regulate human plasma ACE level. Therelevancein human pathology of this difference in genotype has been the subject of many clinical studies,frequently without a definite conclusion. Nevertheless, participation of plasma ACE in the therapeutic effects ofACE inhibitors is questionable.ACE inhibitors have activities that cannot be explained entirelyby blocking either Ang I or BK hydrolysis;this prompted usto study the cellular, subcellular, and molecular modes of theiractions. Stepwise increasedCaptopril concentrations continued to enhance smooth muscle stimulation by BK beyond the concentrationneeded to inhibit ACE. In cells expressing both ACE and BK B 2 receptors, ACE inhibitors augment BK activityindependently of blocking inactivation, shown also with ACE-resistant BK analogs. That is interpreted ascrosstalk between ACE and B 2 receptors, resulting in a more favorable conformational change. ACE inhibitorsthat do not directly affect B 2 receptors can be allosteric enhancers of the B 2 receptor agonists. Because of thespatially very close expression of the enzyme and receptors on cell membranes after transfection, they can formheterodimers.ANGIOTENSIN RECEPTOR ANTAGONISTSACE inhibitors share many common characteristics with another class of cardiovascular drugs calledAngiotensin receptor antagonists, which are often used when patients are intolerant of the adverse effectsproduced by ACE inhibitors. ACE inhibitors do not completely prevent the formation of angiotensin II, as thereare other conversion pathways, and so angiotensin receptor antagonists may be useful because they act toprevent the action of angiotensin II at the AT 1 receptor, leaving AT 2 receptor unblocked; the latter may haveconsequences needing further study.Angiotensin receptor antagonists, also known as angiotensin receptor blockers (ARBs), AT 1 -receptorantagonists or sartans, are a group of pharmaceuticals which modulate the renin-angiotensin-aldosterone system.Vol. 2 (1) Jan – Mar 2011 www.ijrpbsonline.com 67

International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701Their main use is in hypertension (high blood pressure), diabetic nephropathy (kidney damage due to diabetes)and congestive heart failure.Losartan, Irbesartan, Olmesartan, Candesartan and Valsartan include the tetrazole group (a ring with fournitrogen and one carbon). Losartan, Irbesartan, Olmesartan, Candesartan, and Telmisartan also include one ortwo imidazole groups.Telmisartan (Targit)Losartan (Cozaar)Irbesartan (Avapro)MECHANISM OF ACTIONThese substances are AT 1 -receptor antagonists – that is, they block the activation of angiotensin II AT 1receptors. Blockade of AT 1 receptors directly causes vasodilatation, reduces secretion of vasopressin, reducesproduction and secretion of aldosterone, amongst other actions – the combined effect of which is reduction ofblood pressure.Vol. 2 (1) Jan – Mar 2011 www.ijrpbsonline.com 68

International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701RECENT ADVANCES IN DESIGN AND DEVELOPMENT OF ACE INHIBITORSACE inhibitors have stimulated interest in the molecular biology of the RAS in the vascular and othersystems. The recent discovery of polymorphisms in both the ACE and angiotensinogen genes may prove to beof considerable clinical importance.Observational studies in cardiovascular disease are useful in generating hypotheses but give only partialinsight into the importance of pathophysiological systems in the generation of disease. ACE inhibitors are themeans by which the importance of the RAS in health, hypertension, and heart failure and in many other diseaseshas been elucidated. We now look forward to the development of new tools for the dissection of the RASsystem still further (renin inhibitors and angiotensin II receptor antagonists). When ACE inhibitors were firstdeveloped many believed that they would be of little more than novelty value, with perhaps a small role for thetreatment or diagnosis of renovascular hypertension. That indication is now questioned, but ACE inhibitors arean excellent choice for the treatment of most patients with hypertension. A series of landmark studies starting in1987 with CONSENSUS and culminating (so far) in the AIRE study have shown that ACE inhibitors should beused not only in all grades of heart failure associated with left ventricular dilatation and systolic dysfunction butalso in patients with substantial left ventricular dysfunction after infarction. The ISIS-4 and GISSI-3 studies nowsuggest that there is a case for treating all patients after myocardial infarction with an ACE inhibitor, at least forthe first six weeks.Mechanisms exist by which ACE inhibitors could retard the progression of atheroma, reduce the risks ofplaque rupture, or ameliorate the consequences of ruptured plaque. We need to know which of thesemechanisms is most important in this protective effect of ACE inhibitors. We now have the means to determinewhether the protective effect is mediated through reduction in angiotensin II or increases inbradykinin/prostaglandin/nitric oxide or other effects of ACE inhibitors. Undoubtedly, given the rightconditions, innovative new classes of drugs will continue to advance our knowledge in many areas of medicine.This opportunity is particularly welcome in an area such as heart failure that despite being widespread andimportant has been relatively neglected until now.ACEH, the first human homologue of angiotensin converting enzyme (ACE), and its relative collectrin,focus attention on novel roles for ACE-like proteins in peptide metabolism and in response to tissue injury.Modulation of the renin–angiotensin system (RAS), and particularly inhibition of angiotensin-convertingenzyme (ACE), a zinc metallopeptidase, has long been a prime strategy in the treatment of hypertension.However, other angiotensin metabolites are gaining in importance as our understanding of the RAS increases.Recently, genomic approaches have identified the first human homologue of ACE, termed ACEH (or ACE2).ACEH differs in specificity and physiological roles from ACE, which opens a potential new area for discoverybiology. The gene that encodes collectrin, a homologue of ACEH, is up regulated in response to renal injury.Collectrin lacks a catalytic domain, which indicates that there is more to ACE-like function than simple peptidehydrolysis.Vasopeptidase InhibitorsNew classes of agents termed vasopeptidase inhibitors (VPIs) have the potential to be the next majoradvance in the treatment of hypertension, congestive heart failure (CHF) and related cardiovascular diseases.VPIs are single molecular compounds that inhibit two distinct zinc metalloproteases: neutral endopeptidase(NEP) and angiotensin-converting enzyme (ACE). Vasopeptidase inhibition attenuates the formation ofangiotensin II while potentiating endogenous levels of atrial natriuretic peptide (ANP) and bradykinin (BK). Thecombined effect has proven to be highly efficacious for the treatment of hypertension and CHF in experimentalanimal models and recently in man.The human cardiovascular system is regulated by hemodynamic, neurohumoral and structural mechanisms.The endothelium and the neurohumoral system play a key role in modulating both vascular tone and structure byproducing vasoactive substances as well as influencing the modulation of blood cell adhesion. Although theneurohormonal systems are essential in the vascular homeostasis, they become maladaptive in conditions suchas hypertension, coronary disease and heart failure. The clinical success of blocking the renin-angiotensinsystem by angiotensin-converting enzyme (ACE) inhibitors and the sympathetic nervous system by β-blockersdemonstrates the importance of neurohumoral blockades. The inadequate effect of ACE or neutralendopeptidase (NEP) inhibitor monotherapy seen in some patients treated for hypertension or congestive heartfailure (CHF) and the promising effect seen after their combination led to the development of drugs thatsimultaneously inhibit both enzyme systems. NEP, like ACE, is an endothelial cell surface zincmetallopeptidase with a similar structure and catalytic site to ACE. NEP is the major enzymatic pathway for thedegradation of natriuretic peptides. Natriuretic peptides can be viewed as endogenous inhibitors of the reninangiotensinsystem. The dual metalloprotease inhibitors of ACE and NEP, called vasopeptidase inhibitors,represent a new class of drugs and an attractive therapeutic strategy for the treatment of cardiovascular diseasessuch as hypertension and CHF.Vol. 2 (1) Jan – Mar 2011 www.ijrpbsonline.com 69

International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701In Diabetic RetinopathyIt is known that antihypertensive therapy, especially the use of ACE inhibitors, slows down the progressionof diabetic nephropathy but whether these agents have a beneficial effect on retinopathy is not as clear.The relationship between the renin-angiotensin system (RAS) and the progression of diabetic renal diseasehas been a major focus of investigation over the past 20 years. More recently, experimental and clinical studieshave also suggested that the RAS may have a pathogenetic role at other sites of micro- and macrovascular injuryin diabetes as well as in the eye. Better understanding of the relationship of ACE inhibitors and diabeticretinopathy is contributed by the knowledge of the existence of an independent RAS in the eye. Majorcomponents of the RAS including angiotensin type 1 and angiotensin type 2 receptors have been identified inocular tissues. There is also evidence that ACE is produced locally by vascular endothelial cells and may havedirect detrimental effects on retinal flow and vascular structure, independent of changes in the systemic bloodpressure. Activation of angiotensin II type 1 receptors expressed on retinal endothelial cells and pericytes hasbeen implicated as contributing to the microvascular abnormalities found in diabetic retinopathy. This maytherefore account for the beneficial effect of ACE inhibition on this ocular complication; however, themechanism of their action is not yet fully understood. These observations imply that the use of ACE inhibitorsmay protect against the development and progression of diabetic retinopathy, the association between the RASand the development and progression of diabetic retinopathy is not straightforward. There is evidencesupporting the relationship of serum concentrations of ACE inhibitor and the presence and degree of diabeticretinopathy. In addition, it has been shown that captopril, an ACE inhibitor, limits the abnormal leakage offluorescein from retinal vessels, which is one of the elementary features of diabetic retinopathy. The EurodiabControlled Trial of Lisinopril in Insulin Dependent Diabetes (EUCLID) study found that the use of Lisinopril innormotensive type 1 diabetic patients decreased the progression of retinopathy and was associated with thereduction in PDR. However, some investigations showed non-significant benefits of other ACE inhibitors insubjects with type 1 and type 2 diabetes. Advanced diabetic retinopathy is characterized by neovascularizationand enhanced vascular permeability. The vascular endothelial growth factor VEGF glycoprotein is a potentvessel angiogenic and vasopermeability factor, which plays a key role in the pathogenesis of retinopathy,particularly its proliferative form. VEGF expression is induced by hypoxia and is considered to be a stimulus forneovascularization. Intraocular levels of this glycoprotein positively correlate with the degree of diabeticretinopathy and their elevated concentrations precede the onset of the proliferative form of retinopathy. It hasbeen shown that intraocular VEGF concentrations fall to near normal levels after photocoagulation. Inhibition ofVEGF activity also suppresses the development of retinopathy. Angiotensin II enhances VEGF-mediatedproliferation by inducing VEGF receptors in retinal endothelial cells, thus potentially accounting for theimprovement in permeability of the blood-retinal barrier observed with Captopril therapy and the beneficialeffect of ACE inhibition on diabetic ocular microvascular complications. The concentrations of bothAngiotensin II and VEGF in the vitreous fluid of patients with PDR were significantly higher than those in nondiabeticpatients and diabetics without retinopathy. Vitreous fluid levels of angiotensin II were found tosignificantly correlate with those of VEGF. Moreover, the levels of both VEGF and Angiotensin II were higherin active PDR than in quiescent PDR. These findings therefore suggest that Angiotensin II contributes to thedevelopment and progression of PDR in combination with VEGF or via stimulation of VEGF.Macroglial Müller cells are the likely site for pathophysiological processes involving the retinal RAS. Retinalcells able to produce VEGF are Müller glial cells, pigment epithelial cells, vascular smooth muscle cells andpericytes. One of the known factors implicated in the pathogenesis of diabetic retinopathy, which increasesretinal VEGF production, is also Angiotensin II. It is known that Angiotensin II, the effector molecule of theRAS, has angiogenic activity and may also induce neovascularization via paracrine effect on VEGF in diabeticpatients with PDR. The induction of VEGF by Angiotensin II requires hyperglycemic or oxidative conditionswhich accompany diabetes. If the RAS system plays a role in the VEGF over expression found in PDR inhumans, one would expect that the VEGF accumulation in the ocular fluid would be reduced upon medicinalintervention of Angiotensin II mediated processes. It is found that patients with PDR treated with an ACEinhibitor have relatively lower vitreous VEGF concentrations. Moreover, a strong negative correlation betweenvitreous VEGF concentration and the daily dose used was found in patients receiving an ACE inhibitor,Enalapril. In animal studies, ACE-inhibitor treatment reduced the retinal over expression of VEGF mRNA bothin Streptozotocin-induced diabetes and in retinopathy of prematurity models. It is feasible that a reduction inretinal Angiotensin II concentrations was involved in the lowering effect of ACE inhibition on the high vitreousVEGF concentrations in patients with PDR.An intracellular signaling pathway that could mediate the Angiotensin II-induced over expression of VEGFin the retina involves the hypoxia-inducible factor-1α (HIF-1α). In addition to hypoxia, it has been shown thatthis transcription factor can be increased by various agonists of which Angiotensin II is most potent. Themechanism of HIF-1α induction by Angiotensin II depends on the production of reactive oxygen species. Theobservation that hyperglycemia is required to detect Angiotensin II induced expression of VEGF in culturedsmooth muscle cells could be related to the formation of radicals, because of the fact that an altered cellularVol. 2 (1) Jan – Mar 2011 www.ijrpbsonline.com 70

International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701redox state is induced by hyperglycemia. These in vitro observations agree closely with earlier in vivoobservations in rat aorta, namely, that infusion of Angiotensin II increases the production of superoxide anionsby activation of NADPH oxidase. This effect could be completely blocked by Losartan, an Angiotensin type 1receptor antagonist. From this data on smooth muscle cells, a sequence of events is suggested that connectscellular activation by Angiotensin II via NADPH oxidase activation, superoxide production and HIF-1αactivation to the transcriptional activation of the VEGF gene. Although such a mechanism has yet to be verifiedin the eye and the retina, the fact that angiotensin II also increases VEGF transcription and production inpericytes and mesangial cells points to a broader implication of the suggested mechanism.It has been observed that the use of ACE inhibitors decreases the incidence of diabetic complication basedon microvascular dysfunction, i.e. nephropathy, neuropathy, and there is even more evidence pertaining to theirbeneficial effect on retinopathy. In diabetic patients, ACE-inhibition reduces microalbuminuria and normalizesthe increase in skin capillary permeability, and also attenuates the increased retinal blood flow whichaccompanies the progression of nonproliferative diabetic retinopathy. It has been considered that the beneficialeffects of ACE-inhibition on diabetic microangiopathy could involve VEGF activity at various stages. Highvitreous VEGF concentrations present in patients with PDR were found to be lower in those treated with anACE inhibitor. According to recent accessible data it is clear that ACE-inhibitors influence the role of VEGFgene activation influence the role of VEGF in microangiopathy either by attenuating its over expression or byreducing the number of VEGF receptors , or both. Hence, it is suggested that interference with a retinal effect onone or more of the ACE-related agonists, angiotensin II is most likely involved in the observed associationbetween ACE inhibiting therapy and lower vitreous VEGF concentrations in patients with PDR.CONCLUSIONACE inhibitors have activities that cannot be explained entirelyby blocking either Ang I or BK hydrolysis;this prompted us to study the cellular, subcellular, and molecular modes of their actions. For example, Captoprilshows ACE inhibition along with BK potentiation. Stepwise increased Captopril concentrations continued toenhance smooth muscle stimulation by BK beyond the concentration needed to inhibit ACE. In cells expressingboth ACE and BK B 2 receptors, ACE inhibitors augment BK activity independently of blocking inactivation,shown also with ACE-resistant BK analogs. That is interpreted as crosstalk between ACE and B 2 receptors,resulting in a more favorable conformational change. ACE inhibitors that do not directly affect B 2 receptorscan be allosteric enhancers of the B 2 receptor agonists. Because of the spatially very close expression of theenzyme and receptors on cell membranes after transfection, they can form heterodimers.The fact that ACE has two active N- and C-domains with different properties expands the potential use ofACE inhibitors. Blocking the hydrolysis of the myeloprotective tetrapeptideAcSer-Asp-Lys-Pro by the N-domain of ACE protects hematopoietic stem cells during aggressive chemotherapy, and alleviates collagenaccumulation in rat hearts. Inclusion (II) or deletion (DD) of 287 base pairs in intron 16 can regulate humanplasma ACE level. The relevance in human pathology of this difference in genotype has been the subject ofVol. 2 (1) Jan – Mar 2011 www.ijrpbsonline.com 71

International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701many clinical studies, frequently without a definite conclusion. Nevertheless, participation of plasma ACE inthe therapeutic effects of ACE inhibitors is questionable.Kinetics of ACE shows that it effectively hydrolyzes peptides only up to 12–13 amino acids. But after longerincubation, ACE cleaved the 30 residue long B chain of insulin or the 40–42 residue long amyloid ß protein,important in Alzheimer’s disease. Most circulating peptides have only a transient encounter with the ACE onendothelial surfaces, but after longer contact ACE could cleave larger active peptide substrates, not necessarilyat C-terminal end. ACE hydrolyzes the N-terminal tripeptide of luteinizing hormone releasing hormone and theamyloid ß-protein at Asp 7 -Ser 8 .As their putative, possible, or proven applications continue to evolve, itbecomes difficult to predict future developments.ACE-inhibitors are a good choice for diabetic patients because the drugs do not affect blood glucose levels;because they produce few side effects, they are an alternative to beta-Blockers and diuretics. Ace-inhibitors alsoprolong the lives of patients with heart failure; they slow the development of kidney failure in patients withdiabetes. The rate of ace-inhibitor usage for Patients with type-2 diabetes has risen from 13% in 1995 to 23% in2002 as more studies have revealed benefits to this population.REFERENCES1. Petrillo EW, Jr., Trippodo NC and DeForrest JM. Antihypertensive Agents. Ann Rep Med Chem.1990;25:51-60.2. Ferreira SH, Bartelt DC and Lewis LJ. Isolation of bradykinin-potentiating peptides from Bothropsjararaca venom. Biochemistry. 1970;9:2583-2593.3. Ondetti NA and Cushman DW. Enzymes of the renin- angiotensin system and their inhibitors. Ann RevBiochem 1982;51:283-308.4. Byers LD and Wolfenden R. binding of the by-product analog benzylsuccinic acid by carboxypeptidase A.Biochemistry. 1973;12:2070-2078.5. Timmermans PB, Wong PC, Chiu AT, et al. Angiotensin II receptors and angiotensin II receptorantagonists. Pharmacol Rev. 1993;45:205-213.6. Lacourciere Y, Brunner H, Irwin R, et al. effects of modulation of the rennin angitensin-aldosteronesystem on cough. Losartan Cough Study Group. J Hypertens. 199412:1387-1393.7. Annon. Trandolapril: an ACE inhibitor for treatment of hypertension. The Medical Letter. 1996;38:104-105.8. Drummer OH, Nicolaci J, Iakovidis D. biliary excretion and conjugation of diacid ACE inhibitors. JPharmacol Exp Ther. 1990;252:1202-1206.9. Krapcho J, Turk C, Cushman DW, et al. ACE inhibitors. Mercaptan, carboxyl dipeptide, and phosphonicacid inhibitors incorporating 4-substituted pralines. J Med Chem. 1988;31:1148-1160.10. Kim DH, Guinosso CJ, Buzby GC, Jr., et al. (Mercapto propanoyl) indoline-2-carboxylic acid and relatedcompounds as potent ACE inhibitors and antihypertensive agents. J Med Chem. 1983;26:394-403.11. Gross DM, Sweet, CS, UlmEH, et al. Effect of N-[(S)-1-carboxy-3-phenylpropyl]-L-Ala-L-Pro and itsethyl ester (MK-421) on ACE in vitro and angiotensin I pressor responses in vivo. J Pharmacol Exp Ther.1981;216:552-557.12. Patchett AA, Harris E, Tristram EW, et al. A new class of ACE inhibitors. Nature. 1980;288:280-283.13. Atkinson AB and Robertson JIS. Captopril in treatment of clinical hypertension and cardiac failure.Lancet. 1979;2:836-839.14. Bauer JH, Reams GP. The Angotensin II type 1 receptor antagonists: a new class of antihypertensivedrugs, Arch Intern Med. 1995;155:1361-1368.15. Carini DJ, Duncia JV, Aldrich PE, et al. Nanopeptide angiotensin II receptor antagonists: the discovery ofseries of N-(biphenylmethyl) imidazoles as potent orally active antihypertensives. J Med Chem.1991;31:2525-2547.16. Husser DA. New drugs of 1997. J Am Pharm Assoc.1998;38:166-16.Vol. 2 (1) Jan – Mar 2011 www.ijrpbsonline.com 72