M. de Gasparo

The AT2 receptor: fact, fancy and fantasy.

The angiotensin AT2 receptor subtype was recently cloned and pharmacologically characterized but its function still remains elusive and controversial. It is a member of the G-protein coupled receptor superfamily with a minimal sequence homology with the AT1 receptor, responsible for the known effect of angiotensin II. The AT2 receptor displays a totally different signaling mechanisms from the AT1 receptor and involves various phosphatases. It is expressed at low density in adult tissues but up-regulated in pathological circumstances. Clearly, the AT2 receptor has antiproliferative properties and therefore opposes the growth promoting effect linked to the AT1 receptor stimulation. It is also reported that the AT2 receptor regulates ionic fluxes, affects differentiation and nerve regeneration, has anti-angiogenic and anti-fibrotic properties and stimulates apoptosis. However, the results, although suggestive, are sometimes equivocal. Obviously, the AT2 receptor plays a role in the pathogenesis and remodeling of cardiovascular and renal diseases. A more extensive knowledge of the AT2 receptor could therefore contribute to the understanding of the clincial beneficial effects of the AT1 receptor antagonists.

Pharmacokinetics, disposition and biotransformation of [14C]-radiolabelled valsartan in healthy male volunteers after a single oral dose

1. The disposition of valsartan, a potent angiotensin II receptor antagonist, was investigated in six healthy male volunteers. They each received a single oral dose of 80 mg of a 14C-labelled preparation as a neutral buffered solution. 2. Peak concentrations of radioactivity and valsartan in plasma measured 1 h after dosing showed rapid onset of absorption. The results of this study combined with other available data indicate that at least 51% of the dose was absorbed. 3. Valsartan was the predominant radioactive compound in plasma. Elimination of valsartan and radioactivity was fast and multiexponential. beta-Half-lives of 6 +/- 1 h were observed. In a terminal elimination phase, low radioactivity levels decreased with a half-life of 81 +/- 33 h. A minor, pharmacologically inactive metabolite (valeryl-4-hydroxy-valsartan; M1) was detected in the plasma at time points later than 2 h after dosing, representing approximately 11% of the AUC(24 h) of plasma radioactivity. 4. The bulk of the dose was excreted within 4 days. The total excretion within 7 days amounted to 99 +/- 1% of dose. Faecal excretion was predominant (86 +/- 5% of dose). Valsartan was largely excreted unchanged (81 +/- 5% of the dose in the excreta). The predominant clearance mechanism appeared to be direct elimination via bile. 5. An inactive metabolite, M1, was formed by oxidative biotransformation and accounted for 9 +/- 3% of the dose in the excreta.

Antialdosterones: Incidence and prevention of sexual side effects☆

The use of spironolactone in the treatment of hypertension has been limited by the occurrence of sexual side effects, mainly menstrual disturbances in women and gynaecomastia in men. In order to minimize this limitation on the use of an effective potassium-sparing antihypertensive agent, two strategies can be proposed: (1) A decrease in the daily dose of spironolactone. In 182 patients with essential hypertension treated with spironolactone alone for a mean follow-up period of 23 months, daily doses of 75-100 mg were as effective on blood pressure as doses of 150-300 mg. In contrast, the development of gynaecomastia--91 cases among 699 men--was dose-related in 6.9% (50 mg/day) to 52.2% (150 mg or more/day) of the cases. (2) An improvement in the receptor-binding specificity of spironolactone. Three 9 alpha, 11 alpha-epoxy derivatives have been characterized in vitro in rats and in rabbits. They exhibited a 3- to 10-fold decrease of the antiandrogenic and progestagenic effect, compared with spironolactone. In humans, one of these derivatives counteracted the fall in urinary Na/K ratio induced by 9 alpha-fluorohydrocortisone at a 25 mg dose.

Angiotensin II receptor blockade with single doses of valsartan in healthy, normotensive subjects.

Valsartan (CGP 48933), a specific blocker of the angiotensin II (Ang II) receptor subtype 1 (AT1 receptor) was administered in single, oral doses of 40 mg and 80 mg to six healthy, normotensive male volunteers in a double-blind, placebo-controlled, randomized crossover trial. The aims of the study were a) to assess the extent, time course and dose-dependency of inhibition of the pressor effect of exogenous Ang II; and b) to attempt to correlate AT1 receptor blockade with the drug levels in plasma and with other markers of biological activity of the trial drug such as plasma renin activity (PRA).Using the Finapres device and i.v. bolus injections of exogenous Ang II, AT1 receptor blockade was assessed by measuring blood pressure (BP) and heart rate (HR) on a beat-by-beat basis. A dose-response curve for Ang II was obtained for each subject before and at 2,4, 6,8 and 24h after administration of placebo and of the two doses of valsartan. PRA was measured with a conventional radioimmunoassay method.Data evaluation included a) descriptive analysis of the changes of the Ang II dose-response curves after valsartan, as compared to the curve on placebo; b) calculation of the pressor dose D30 of Ang II at each timepoint, using linear regression; c) assessment of the effect of 4 μg Ang II on systolic BP and HR and the calculation of the percentage inhibition of these effects after valsartan; d) description of the relationship between drug levels in plasma and the measures of AT1 blockade, including pharmacokinetic-pharmacodynamic modeling with an Emax model for the percentage inhibition of systolic BP and HR.It is concluded that (a) the methodology used is suitable to evaluate AT1 receptor blockade in man; (b) instead of using a full dose-response curve for Ang II at each timepoint, an abbreviated approach with only one pre-determined Ang II dose may be adopted without substantial loss of information; (c) valsartan is an inhibitor of the AT1 receptor in man, with a mean Emax of 74% and a mean IC50 of 0.53 μmol·1−1 for the blood pressure response to exogenous Ang II; (d) receptor blockade after single oral doses of 40 mg and 80 mg valsartan reaches its maximum at 2 h and is detectable up to 24 h after administration; (e) despite a doubling of systemic exposure, the relationship between dose and receptor blockade appears to be rather flat in the dose range 40 to 80 mg; and (f) valsartan is well tolerated by healthy subjects in the dose range tested.

Pharmacokinetics and pharmacodynamic effects of the angiotensin II antagonist valsartan at steady state in healthy, normotensive subjects

AbstractObjective: Pharmacokinetics, pharmacodynamic effects and tolerability of 200 mg valsartan, once-daily for 8 days, were investigated in 16 healthy, normotensive volunteers on a normal sodium diet. Methods: This was a double-blind, placebo-controlled, randomized crossover study. Drug concentrations in plasma and urine, angiotensin II (Ang II) concentrations in plasma, systolic (SBP) and diastolic (DBP) blood pressure, heart rate (HR) in the supine position and 3 min after passive head-up tilting, as well as safety parameters (ECG, clinical chemistry and hematology, renal water and electrolyte excretion) were measured over 24 h after the first dose (day 1) and at steady state on day 8. Results: Absorption and distribution of valsartan were rapid (Cmax, 2 h; t½λ1 < 1 h), followed by a slower terminal elimination phase (t½λ2, 6 h) on days 1 and 8, with little accumulation in plasma (increase of 20% on day 8). Less than 10% of the dose was excreted unchanged in urine. The increase in plasma Ang II (Cmax, 6 h) was significantly enhanced at steady state. Supine SBP and DBP significantly decreased on day 8 only, by an average of −3.6 and −2.4 mmHg, respectively, versus placebo, without a concomitant increase in HR. Upon passive tilting, the increase in DBP, normally reinforced by sympathetic renin release, was slightly but significantly blunted on day 1 (−2.0 mmHg) and day 8 (−4.0 mmHg) of treatment with valsartan versus placebo. The orthostatic reflex increase in HR was slightly enhanced compared with placebo by an average of 2.8 beats · min−1 on day 1 and by 2.9 beats · min−1 on day 8. Valsartan was well tolerated and had no influence on ECG, clinical laboratory parameters, and water, electrolyte and uric acid excretion. Conclusions: Pharmacokinetics of valsartan are unchanged after multiple once-daily dosing, with little (expected) accumulation in plasma. Effects of 200 mg valsartan on blood pressure in healthy subjects on a normal sodium intake are small and become more prominent after repeated dosing. Indirect evidence of AT1 blockade by valsartan is demonstrated by an increase of plasma Ang II and by a blunted DBP response to passive tilting. The decrease in blood pressure at steady state enhances the increase in plasma Ang II. Valsartan is well tolerated and is devoid of effects on water, electrolyte and uric acid excretion at 200 mg per day in healthy normotensive volunteers.

Left ventricular wall stress and sarcoplasmic reticulum Ca2+-ATPase gene expression in renal hypertensive rats: dose-dependent effects of ACE inhibition and AT1-receptor blockade

BACKGROUND Cardiac hypertrophy is associated with altered Ca2+ handling and may predispose to the development of LV dysfunction and cardiac failure. At the cellular level, the re-expression of ANF represents a well-established marker of myocyte hypertrophy while the decreased expression of the sarcoplasmatic reticulum (SR) Ca(2+)-ATPase is thought o play a crucial role in the alterations of Ca2+ handling and LV function. We assessed the dose-dependent effect of chronic ACE inhibition or AT1 receptor blockade on cardiac function in relation to the cardiac expression of the SR Ca(2+)-ATPase and ANF. METHODS AND RESULTS Renal hypertensive rats (2K-1C) were treated for 12 weeks with three different doses of the ACE inhibitor benazepril, the AT1-receptor antagonist valsartan (each drug 0.3, 3, and 10 mg/kg per day i.p.) or placebo. LV dimensions, hypertrophy and wall stress were determined in vivo by magnetic resonance imaging and the gene expressions of ANF and SR Ca(2+)-ATPase were quantified by Northern blot. Low doses of both drugs did not affect blood pressure, hypertrophy, systolic wall stress and the ANF and SR Ca(2+)-ATPase gene expression. High doses of each drug reduced systolic blood pressure, wall stress, and LV hypertrophy to a similar extent and to values comparable to normotensive, age-matched rats. In addition, high dose treatment reduced LV end-systolic and end-diastolic volume as compared to untreated 2K-1C animals and normalized the mRNA levels of both ANF and SR Ca(2+)-ATPase (as compared to normotensive animals). CONCLUSIONS We conclude that in this model, high doses of ACE inhibition and AT1-receptor blockade are necessary to normalize systolic blood pressure, LV hypertrophy and systolic LV wall stress which, in turn, is associated with restoration of a normal cardiac phenotype with respect to SR Ca(2+)-ATPase and ANF and normalization of cardiac function.

Determination of renin, angiotensin converting enzyme and angiotensin II levels in human placenta, chorion and amnion from women with pregnancy induced hypertension

OBJECTIVE Pregnancy induced hypertension has been shown to be associated with a normal or low activity of the maternal circulating renin–angiotensin system (RAS) but little is known of the local RAS in placenta and fetal membranes. The present study attempts to determine, at full term of human preeclamptic pregnancies, the activity of the chorioplacental renin–angiotensin system.

Monoclonal Antibodies to Human Renin: Properties and Applications

A series of 11 different monoclonal antibodies generated against human kidney renin have been characterised. Their binding affinity, inhibition of renin activity, epitope distribution, crossreactivity with related enzymes and finally in vivo pharmacological effects were analysed. All antibodies were found to be specific for primate renin recognising 6 independent antigenic structures on the renin molecule. They expressed different effects on renin activity namely (1) no inhibition, (2) only partial, or (3) complete inhibition. Partially inhibiting antibodies demonstrated specific degrees of inhibition (30, 60 or 80%). One antibody, R-36-16, demonstrated an IC 50 of 1.3 X 10(-11) M/L and, when injected into marmosets, induced complete inhibition of plasma renin activity and reduction of blood pressure. Using a selected pair of antibodies a radioimmunoassay has been established providing a fast and highly reproducible determination of human and marmoset immunoreactive renin, detecting both active and inactive renin down to concentrations of 10 pg/ml (1.25 X 10(-17) moles of renin per 50 microliter sample).

Down regulation of the angiotensin II receptor subtype AT2 in human myometrium during pregnancy

Samples of human myometrium have been collected during pregnancy and from non-pregnant women. Binding studies revealed the presence of a 50-fold higher density of angiotensin II AT2 receptor in the non-pregnant state than during gestation. Low levels of the AT1 receptor subtype (approx. 20 fmol/mg protein) were detected in both pregnant and non-pregnant myometrium. Outside pregnancy, the AT2 receptor accounted for greater than 95% of all angiotensin receptors, whereas during pregnancy the value dropped to about 40%. The down regulation of the human myometrial AT2 receptor during pregnancy may be related to the high hormonal content of the environment induced by gestation. The mechanism by which the AT2 receptor is regulated appears to be different to that of the AT1 receptor.

Mechanism of inhibition of human renin by monoclonal antibodies.

The mechanism by which monoclonal antibodies directed against human renin (R3-36-16 and R3-47-10) inhibit renin activity was investigated using various substrates. Both antibodies acted as potent inhibitors of human renin activity when human angiotensinogen was used as a substrate. However, their effects differed clearly in the presence of synthetic tetradecapeptide. When low concentrations of tetradecapeptide were used as substrate, renin activity was only partially inhibited by R3-47-10, whereas it was stimulated by R3-36-16. At higher synthetic substrate concentrations, both antibodies stimulated angiotensin I production. This effect was independent of the pH. Both antibodies exerted their effects in the presence of CGP 29287, a peptidic transition-state competitive renin inhibitor, indicating that their binding sites differed from that of CGP 29287. In combination, the stimulatory effect of R3-36-16 was not blocked by R3-47-10, but the inhibition produced by R3-47-10 was reversed by R3-36-16. Both antibodies may prevent the large natural substrate angiotensinogen from entering the enzymatic cleft by steric hindrance. At a low substrate concentration, R3-47-10 may also partially hinder the access of synthetic tetradecapeptide into the active cleft by steric hindrance. In contrast, the stimulating effect of both antibodies may be due to a conformational change in the renin molecule, allowing an increased access of tetradecapeptide or a more rapid release of the product from the enzymatic cleft.