Insurmountable angiotensin AT1 receptor antagonists: the role of tight antagonist binding

Distinct binding properties of the AT(1) receptor antagonist [(3)H]candesartan to intact cells and membrane preparations

[(3)H]-2-Ethoxy-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]-1H-benzimidazoline-7-carboxylic acid ([(3)H]candesartan), a non-peptide angiotensin II type 1 receptor (AT(1) receptor) antagonist bound with high affinity and specificity to intact adherent human AT(1) receptor transfected Chinese hamster ovary cells. The binding characteristics were preserved when cells were suspended, but the dissociation was 3-4-fold faster and the affinity 2-fold lower, while examining [(3)H]candesartan binding to cell membranes. These data suggested the role of the intracellular organisation of living CHO-hAT(1) cells in antagonist-AT(1) receptor interactions. Yet, a specific role of microtubule or actin filaments of the cytoskeleton, receptor phosphorylation by Protein Kinase C, membrane polarity, cytoplasmic components like ATP and the need of an intact cell membrane could be excluded. The potential effect of protease degradation or receptor oxidation during the membrane preparation was also unlikely. The dissociation rate and the equilibrium dissociation constant of [(3)H]candesartan increased with the temperature for both intact cells and membranes. Thermodynamic studies suggested that the bonds between candesartan and the hAT(1) receptor may be of different nature in intact CHO-hAT(1) cells and membranes thereof. Whereas the binding was almost completely enthalpy-driven on intact cells, there was a mixed contribution of both enthalpy and entropy on membranes.

Pharmacological differences among angiotensin II receptor antagonists

Angiotensin II AT1 receptor antagonists (AIIRAs) have demonstrated efficacy similar to other classes of antihypertensive agents as well as "placebo-level" tolerability at all doses. Pharmacokinetic and pharmacodynamic studies provide a framework for understanding important intra-class dissimilarities. Disparity in antagonistic effects may be determined by in vivo responses to challenges of exogenous angiotensin II (Ang II) and by ex vivo/in vitro responses to a drug's biological activity by radioligand receptor assay (RRA). Two independent studies have been conducted in which irbesartan exhibited a more pronounced and longer-lasting antagonism to the effects of exogenous Ang II than losartan and valsartan. Comparative trials have indicated that both irbesartan and candesartan show greater clinical efficacy in lowering blood pressure than losartan. Recently, we have compared the Ang II antagonistic properties of irbesartan 150 mg/day and candesartan 8 mg/day. Both drugs block AT1 receptors with "insurmountable" antagonism and demonstrate a long duration of action. While both irbesartan and candesartan showed a similar degree of antagonistic activity in vivo, distinctly higher antagonistic activity in plasma was found for irbesartan by RRA at all time-points. Furthermore, plasma renin activity during periods with high antagonistic activity was significantly higher, and aldosterone levels following Ang II stimulation were blunted to a greater extent, following administration of irbesartan. In summary, in the doses tested, irbesartan exhibits the strongest antagonism when compared with losartan, valsartan and candesartan. This finding may have clinical implications.

Dose-dependent blockade of the angiotensin II type 1 receptor with losartan in normal volunteers

Losartan, an angiotensin II type 1 receptor (AT1) antagonist, was developed as a more specific alternative to angiotensin-converting enzyme (ACE) inhibitors. At a daily dose of 50 mg, losartan is currently evaluated in large outcome trials involving patients with hypertension and postmyocardial infarction. The current study evaluated the level and duration of blockade of a pressor response to angiotensin II by 50 and 150 mg of losartan, compared with 32 mg of candesartan. Eight normotensive volunteers were randomly assigned to a single dose of losartan 50 or 150 mg, candesartan 32 mg, or placebo. Subjects were re-randomized after a 2-week washout period to complete all four study arms. Radial artery systolic pressure response to exogenous angiotensin II was measured at 2, 6, 12, and 24 h after administration of drug. Losartan 50 mg reduced the pressure response to exogenous angiotensin II significantly only at 6 h. In contrast, candesartan and losartan 150 mg produced a greater reduction in the pressure response to angiotensin II throughout the 24-h period. This suppression was not paralleled by a reduction in resting systemic arterial pressure. Higher doses than 50 mg of losartan might be evaluated to elicit optimal clinical effects.

Tight binding of the angiotensin AT(1) receptor antagonist

Angiotensin II induces angiotensin AT(1) receptor internalization via Clathrin coated pits formation. We investigated whether insurmountable inhibition by the non-peptide antagonist 2-ethoxy-1-[(2'-(1H-tetrazol-5-yl) biphenyl-4-yl) methyl]-1H-benzimidazoline-7-carboxylic acid (candesartan) was related to receptor internalization. Mild acid treatment can discriminate between internalized and cell surface bound [(3)H]angiotensin II. In contrast, it provides no information about the subcellular localization of bound [(3)H]candesartan since this binding is acid resistant. The internalization of [(3)H]angiotensin II is rapidly inhibited in the presence of 0.4 M sucrose. Yet, no such rapid effect was noticed for [(3)H]candesartan. [(3)H]candesartan displays insurmountable/long lasting binding to the vast majority of both wild type and L(314) truncated rat angiotensin AT(1A) receptors with impaired receptor internalization. In agreement with previously published AT(1) angiotensin receptor visualization experiments, the present data suggest that non-peptide antagonist-angiotensin AT(1) receptor complexes remain at the cell surface. Insurmountable antagonism of candesartan is therefore independent from receptor internalization via clathrin-coated pits.

Different types of antagonism by losartan and irbesartan on the effects of angiotensin II and its degradation products in rabbit arteries

A previous study by our group has demonstrated that the selective AT1-receptor antagonist losartan behaves as a noncompetitive antagonist in rabbit isolated renal artery (RA). In the present investigation, the influence of losartan and irbesartan on the contractile effects of angiotensin II (AII) and its degradation products angiotensin III (AIII) and angiotensin IV (AIV) was determined in the rabbit isolated RA and femoral artery (FA). The arteries were set up in organ chambers and changes in isometric force were recorded. In both rabbit isolated RA and FA preparations, AII, AIII and AIV elicited significant contractile responses with a similar efficacy. These effects were impaired by the presence of functional endothelium in RA preparations but not in FA preparations. In both preparations studied, the effects of AII, AIII and AIV were influenced neither by the aminopeptidase-A and -M inhibitor amastatin (10 microM), nor by the aminopeptidase-B and -M inhibitor bestatin (10 microM). In endothelium-denuded FA preparations, preincubation with losartan (3-300 nM) antagonized AII-, AIII- and AIV-induced contractions in a competitive manner. However, in endothelium-denuded RA preparations, losartan depressed the maximal contractile responses induced by AII but not those induced by AIII and AIV. In the same preparations, preincubation of another selective AT1-receptor antagonist irbesartan (3-30 nM) concentration-dependently shifted AII and AIII curves to the right in an insurmountable manner. The reduction of the maximal response of AII is more potent when compared to that of AIII (47.7 +/- 1.51% vs. 66.7 +/- 1.88%, percentage of the initial maximal response; P < 0.05; n=5). The selective AT2-receptor antagonist PD123177 (1 microM) did not influence the responses to all three peptides in both RA and FA preparations. These heterogeneous antagonistic effects of the two AT1-receptor antagonists studied with respect to the contractile actions of AII, AIII and AIV suggest the possible existence of multiple, functionally relevant AT1-receptor subtypes in rabbit RA preparations.

Efficacy and tolerability of a combination tablet of candesartan cilexetil and hydrochlorothiazide in insufficiently controlled primary hypertension--comparison with a combination of losartan and hydrochlorothiazide

This randomized, double-blind study compared the antihypertensive effect, safety and tolerability of a candesartan cilexetil/hydrochlorothiazide (candesartan/HCT; 16/12.5 mg) combination tablet with that of a losartan/HCT (50/12.5 mg) combination tablet in patients with mild-to-moderate primary hypertension insufficiently controlled on previous monotherapy. Men and women, aged 20-80 years, with a sitting diastolic blood pressure (DBP) > or = 90 and < or = 110 mmHg and sitting systolic blood pressure (SBP) < or = 200 mmHg during treatment with any kind of antihypertensive monotherapy for at least 4 weeks were randomized to candesartan/HCT or losartan/HCT once daily for 12 weeks. All BP measurements were performed 24 h after previous dose. Mean values and standard deviations (SD) or confidence intervals (CI) are given. A total of 340 patients were enrolled, of whom 299 (144 women and 155 men, mean age 59.5 [10.5] years) were randomized to candesartan/HCT (n = 151) or losartan/HCT (n = 148). BPs at randomization were 159.5 (15.4)/98.4 (5.8)mmHg and 160.5 (16.1)/98.5 (5.4)mmHg, respectively. There was a greater reduction in BP with candesartan/HCT than with losartan/HCT: DBP -10.4 (-11.8; -8.9) vs -7.8 (-9.3; -6.3) mmHg, difference between treatments -2.6 (-4.7; -0.5) mmHg (p = 0.016); SBP -19.4 (-22.1; -16.7) vs - 13.7 (-16.5; - 10.9) mmHg, difference between treatments -5.7 (-9.6; -1.8) mmHg (p = 0.004). The proportion of patients achieving a DBP < or = 90 mmHg was greater in the candesartan/HCT group: 60.9 (53.1; 68.7) vs 49.3 (41.3; 57.4)% (p = 0.044). There were 12 withdrawals in the candesartan/HCT group, of which 8 were due to adverse events, and 17 and 12, respectively in the losartan/HCT group. We conclude that the combination of candesartan and HCT reduces BP effectively and is well tolerated. BP was normalized in 61% of these patients who had insufficient response to previous monotherapy. The reduction in BP and the proportion of patients with normalized BP were greater with the candesartan/HCT 16/12.5 mg combination than with the losartan/ HCT 50/12.5 mg combination.

A two-state receptor model for the interaction between angiotensin II type 1 receptors and non-peptide antagonists

The interaction between non-peptide antagonists and the human angiotensin II type 1 (AT1) receptor in CHO-K1 cells was investigated by incubating the cells with antagonist, followed by a brief exposure to angiotensin II and measurement of the resulting inositol phosphate accumulation. The experimental data, expressed either as angiotensin II concentration-response curves or as antagonist concentration-inhibition curves, were in good agreement with computer-generated data according to a single-state model for the surmountable antagonist losartan and according to a two-step, two-state receptor model for the insurmountable antagonists candesartan, EXP3174, and irbesartan. Experimental and computer-generated data concerning the simultaneous exposure of the receptors to EXP3174 and losartan indicated that losartan produced a concentration-dependent restoration of the maximal response (angiotensin II concentration-response curves) as well as a rightward shift of the insurmountable portion of the EXP3174 inhibition curves, thus counteracting the higher-affinity EXP3174 binding. In conclusion, these findings provide further support for the concept that insurmountable and surmountable AT1 antagonists are mutually competitive and that insurmountable antagonist-receptor complexes may adopt different states.

Peptide and non-peptide bradykinin B2 receptor agonists and antagonists: a reappraisal of their pharmacology in the guinea-pig ileum

We have compared the pharmacology of different antagonists, Icatibant (H-DArg-Arg-Pro-Hyp-Gly-Thi-Ser-DTic-Oic-Arg-OH), MEN 11270 (H-DArg-Arg-Pro-Hyp-Gly-Thi-c(Dab-DTic-Oic-Arg)c(7 gamma-10 alpha)), and FR173657 ((E)-3-(6-acetamido-3-pyridyl)-N-[N-[2, 4-dichloro-3-[(2-methyl-8-quinolinyl)oxymethyl]phenyl]-N-methyl aminocarbonylmethyl]acrylamide) at bradykinin B2 receptors expressed in the guinea-pig ileum by using bradykinin and the non-peptide FR190997 ((8-[2,6-dichloro-3-[N-[(E)-4-(N-methylcarbamoyl)cinnamidoacety l]-N -methylamino]benzyloxy]-2-methyl-4-(2-pyridylmethoxy)quinoline) as agonists. In organ bath experiments, Icatibant and FR173657 exerted a non-competitive antagonism (pKB 9.5 and 9.2, respectively) of the contractile response to bradykinin, whereas MEN 11270 showed competitive antagonism (pKB 8.3, slope -0.90). The profile of action and apparent affinities of the three antagonists did not change if contact time was prolonged. The inhibition by the three antagonists of the contractile response to bradykinin was differently reverted by washout (MEN 11270 <30 min, Icatibant <60 min, FR173657 >60 min). The non-peptide ligand FR190997 acted as partial agonist if applied cumulatively to the bath (pD2 8.06, Emax 43% of maximal contractility), but as a full agonist when a maximally effective concentration was added (Emax 83%). FR173657 produced non-competitive antagonism of the response to FR190997 with apparent affinity similar to that measured toward bradykinin. On the contrary, Icatibant and MEN 11270 (300 nM both) competitively antagonized the contractile activity exerted by FR190997 with lower apparent pA2 value (6.9 and 7.2, respectively). In radioligand binding experiments, MEN 11270 and Icatibant displaced the [3H]bradykinin binding with pKi of 10.2 and 10.5 (Hill slope not different from unity), respectively. The non-peptide ligands displaced the [3H]bradykinin binding with similar affinity, their pKi being 8.7 and 8.6 for FR173657 and FR190997, respectively (both Hill slopes <1). The present study indicates the difference in the antagonism type (competitive vs. non-competitive) by Icatibant, MEN 11270, and FR173657, as mainly ascribable to their different reversibility from the bradykinin B2 receptor, and affected by the kinetic of the response induced by the different agonists. Results are discussed in view of a different interaction of peptide and non-peptide agonist at the receptor.

Angiotensin II type 1 receptor antagonists. Why do some of them produce insurmountable inhibition?

Chinese hamster ovary (CHO) cells expressing human recombinant angiotensin II type 1 (AT(1)) receptors offer a useful experimental system in which antagonist binding and inhibition of AT-induced inositol mono-, bis-, and trisphosphate accumulation can be measured under identical experimental conditions. The major conclusions of the current work are: All investigated AT(1) antagonists are competitive with respect to AT. They bind to a common or overlapping binding site on the receptor in a mutually exclusive way. Reduction of the maximal angiotensin II response, i.e. insurmountable inhibition, is observed only when the cells are preincubated with candesartan, EXP3174, or irbesartan and is strictly related to the dissociation rate of the antagonist-receptor complex. On the other hand, inhibition by losartan is fully surmountable by AT, and its dissociation is very rapid. With respect to the binding kinetics, the antagonist-receptor complex can adopt a fast and a slow reversible state. The equilibrium between both states, which is dependent upon the nature of the antagonists, determines the extent of insurmountable inhibition. Consequently, the dissociation rate of the different antagonists correlates with the amount of insurmountable inhibition. In addition to the relatively slow dissociation of candesartan, reassociation to the receptor, which is measurable in CHO-AT(1) cells, likely contributes to its long-lasting blood pressure lowering effect in vivo.

Inhibition of angiotensin II-induced inositol phosphate production by triacid nonpeptide antagonists in CHO cells expressing human AT1 receptors

PURPOSE

The aim of the present work is to describe the inhibitory properties of LY301875 and LY303336, two polysubstituted 4-aminoimidazole AT1 receptor antagonists, on CHO cells expressing human recombinant AT1 receptors.

METHODS

The binding of [3H]-angiotensin II to intact cells as well as to angiotensin II induced inositol phosphate accumulation is measured.

RESULTS

Both antagonists inhibit specific [3H]-angiotensin II binding to AT1 receptors in these cells, with IC50 values of 5.9 and 5.2 nM, respectively. Preincubation of the cells with LY301875 results in a decline of up to 80% of the maximal angiotensin II-stimulated inositol phosphate (IP) production. A near complete decline of the maximal response is observed for LY303336. This insurmountable inhibition is attenuated for both antagonists when losartan is included during the preincubation of the cells.

CONCLUSIONS

Functional recovery experiments, in which antagonist-preincubated cells are washed and exposed to fresh media, suggest that the insurmountable inhibition by LY301875 and LY303336 is related to their relatively slow dissociation from the AT1 receptors. As already described for losartan and the derived insurmountable AT1 antagonists candesartan. EXP3174, and irbesartan, coincubation experiments reveal that LY301875 and LY303336 interact with the AT1 receptor in a manner that is competitive with angiotensin II.

Interaction between the partially insurmountable antagonist valsartan and human recombinant angiotensin II type 1 receptors

The interaction between the AT1 receptor-selective antagonist valsartan, and its human receptor, was investigated by direct radioligand binding as well as by its inhibition of angiotensin II induced inositol phosphate accumulation in CHO cells expressing human recombinant AT1 receptors. Specific binding of [3H]-valsartan rapidly reached equilibrium at 37 degrees C. It was saturable and occurred to a homogeneous class of sites with a K(D) of 0.88+/-0.076. It was inhibited by other AT1 receptor antagonists with the same potency order as previously described for the binding of [3H]-angiotensin II and [3H]-candesartan to human AT1 receptors (i.e. candesartan > or = EXP3174 > valsartan = irbesartan = angiotensin II > losartan). When valsartan and angiotensin II were applied simultaneously to the CHO-AT1 cells. the antagonist caused a rightward shift of the angiotensin II concentration response curve. Hence, valsartan interacts with the AT1 receptor in a manner that is competitive with angiotensin II. Pre-incubation of the cells with 0.5, 5 and 50 nM valsartan caused an additional, concentration-dependent, up to 55% decline of the maximal response. The partial nature of this insurmountable inhibition by valsartan was confirmed by biphasic antagonist concentration-inhibition curves. These data reflect the co-existence of a fast reversible/surmountable as well as a tight binding/insurmountable valsartan receptor complex. In agreement, pre-incubation of the CHO-AT1 cells with 5 and 50 nM valsartan produced a partial inhibition of the angiotensin II induced increase of the free intracellular calcium concentration. [3H]-Valsartan dissociated from its receptors with a half-life of 17 min. In functional recovery experiments with valsartan-pre-treated cells, the angiotensin II-mediated response was half-maximally restored within approximately 30 min. These kinetic data suggest that the insurmountable inhibition by valsartan is related to its relatively slow dissociation from the human AT1 receptors.

Dynamic mechanisms of non-classical antagonism by competitive AT(1) receptor antagonists

Selective competitive angiotensin AT(1) receptor antagonists exhibit diverse patterns of antagonism of angiotensin-II-mediated responses in functional assays. These range from the classical parallel rightward shift of agonist concentration-response curves with no depression of the maximum response to an apparently straightforward insurmountable antagonism with complete depression of the maximum response and no rightward shift. This article reviews some earlier equilibrium-based models that have been used to explain the insurmountable antagonism, and suggests that a kinetic model might provide a more satisfactory account of the observations. Such a model might provide deeper insights into the pharmacology of G-protein-coupled receptors than the more popular equilibrium models.

Lys(199) mutation of the human angiotensin type 1 receptor differentially affects the binding of surmountable and insurmountable non-peptide antagonists

Many slow dissociating (insurmountable) non-peptide angiotensin type 1 receptor (AT1) antagonists contain,besides the acidic biphenyltetrazole substructure of losartan, a second acidic group to stabilise antagonist-receptor complexes. To investigate the involved basic amino-acids of the human AT1-receptor, wild-type and mutant receptors were transiently transfected in CHO-K1 cells and characterised by [3H]candesartan binding. Lys199-->Gln substitution decreased the affinity 45-fold for candesartan (95% insurmountable),18-fold for EXP3174 (70% insurmountable), 10-fold for irbesartan (40% insurmountable) and 5-fold for losartan (surmountable). His256 -->Ala substitution had only minor effects. This suggests that Lys199 is important for the tight binding of non-peptide antagonists.

Irbesartan: an updated review of its use in cardiovascular disorders

Irbesartan interrupts the renin-angiotensin system via selective blockade of the angiotensin II subtype 1 receptor; the latter being responsible for the pressor related effects of angiotensin II. As treatment for mild to moderate hypertension, irbesartan 150 mg/day controlled diastolic BP in 56% of patients according to pooled data from several phase III studies and 77% of patients in a large phase IV study. in comparative trials, irbesartan was significantly more effective than losartan and valsartan as treatment for mild to moderate essential hypertension and as effective as enalapril or atenolol. Results from many studies show an additive antihypertensive effect when hydrochlorothiazide is added to irbesartan monotherapy. The drug also induces statistically significant regression of left ventricular mass in patients with hypertension and left ventricular hypertrophy, and preliminary evidence suggests it has beneficial haemodynamic effects in patients with heart failure. Irbesartan is very well tolerated, exhibiting an adverse event profile similar to that seen with placebo in comparative trials. In conclusion, although the role of irbesartan as a treatment for heart failure is little clearer than it was 2 years ago, the place of the drug in the management of hypertension is now better established. There is evidence to suggest the drug may have a role as initial therapy for hypertension, although formal recommendation in management guidelines will almost certainly not occur until long term morbidity and mortality benefits are established.

Reversible and syntopic interaction between angiotensin receptor antagonists on Chinese hamster ovary cells expressing human angiotensin II type 1 receptors

Evidence for a competitive type of interaction between angiotensin II type 1 (AT(1)) antagonists on Chinese hamster ovary cells expressing the human AT(1) receptor (CHO-AT(1)) was obtained by analyzing the binding of [(3)H]-2-ethoxy-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]-1H- ben zimidazoline-7-carboxylic acid ([(3)H]candesartan) and by measuring the AT-induced production of inositol phosphates. The AT(1) antagonists candesartan, 2-n-butyl-4-chloro-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]+ ++imid azole-5-carboxylic acid (EXP3174), or 2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)bip hen yl- 4-yl)methyl]imidazole (losartan) produced a concentration-dependent increase in the apparent K(d) values of [(3)H]candesartan in saturation binding experiments, while the B(max) values were unchanged. Furthermore, the dissociation rate of the radioligand initiated by 1 microM unlabelled candesartan was not changed in the presence of 10 microM losartan, 10 microM EXP3174, or 10 microM irbesartan (2-n-butyl-4-spirocyclopentane-1-[(2'-(1H-tetrazol-5-yl)b iph enyl-4-yl) methyl]2-imidazolin-5-one)). Preincubation of the CHO-AT(1) cells with candesartan, EXP3174, and irbesartan caused a reduction in the maximal AT-induced inositol mono-, bis-, and trisphosphate production. This insurmountable effect was reversed in the presence of 1 microM losartan. In line with this finding, the insurmountable antagonist concentration-inhibition curves at 10 microM AT were shifted to the right in the presence of losartan. For candesartan this effect was concentration-dependent, yielding a pK(B) value for losartan of 7.7, which is similar to the pK(B) from previously obtained AT concentration-response curves. Finally, the dissociation rate of candesartan, EXP3174, irbesartan, and losartan was determined by measuring the recovery of AT responses after antagonist pretreatment and washing of the cells with medium containing 1 microM losartan to prevent re-association of the insurmountable antagonists. In addition, similar kinetic data were obtained from the slowing of the [(3)H]candesartan association rate to antagonist preincubated cells.

Angiotensin II receptor antagonists

Blockade of the renin-angiotensin system began as a way of studying the pathogenesis of cardiovascular disease with specific pharmacological probes. Oral activity, achieved by shortening the original peptide structures, transformed the probes into therapeutic agents, the angiotensin-converting enzyme (ACE) inhibitors. However, ACE is a non-specific target for blocking the renin-angiotensin enzymatic cascade. The availability of orally active drugs turned ACE inhibition into a therapeutic breakthrough but more specific blockade always seemed desirable. This goal has now been achieved with the orally active angiotensin II receptor antagonists; six are on the market and more are under development. This new class of drugs is equal in efficacy to ACE inhibitors, at least in hypertensive patients. Trials now underway will demonstrate whether angiotensin II receptor antagonists can prevent target-organ damage and reduce cardiovascular morbidity and mortality. If they do, these compounds might one day replace ACE inhibitors.