The New Angiotensin II Receptor Antagonist, Irbesartan: Pharmacokinetic and Pharmacodynamic Considerations

This article reviews the pharmacokinetics and pharmacodynamics of angiotensin II (AII) receptor antagonists (AIIRA), with particular focus on the novel compound irbesartan. Irbesartan has the highest oral bioavailability in its class (60% to 80%) and, unlike valsartan, its absorption is not affected by food. Irbesartan displays linear, dose related pharmacokinetics and, with the exception of tasosartan's active metabolite, has the longest elimination half-life of the AIIRA (11 to 15 h). Irbesartan exhibits the lowest amount of protein binding, limiting its potential for drug interactions. No drug interactions with irbesartan have been identified. Unlike losartan, candesartan, and tasosartan, irbesartan does not require biotransformation for AII blockade. The pharmacokinetics of irbesartan are not altered in renally or hepatically impaired patients, probably owing to excretion characteristic by both biliary and renal routes, or by differences in gender or age. Within its therapeutic dose range (150 to 300 mg), irbesartan shows sustained, dose related blockade 24 h after dosing. Irbesartan lowers blood pressure in a dose related manner up to 300 mg daily. Some clear differences in pharmacokinetics and pharmacodynamics exist among the AIIRA, which may have clinical implications. Am J Hypertens 1997;10:311S-317S ©1997 American Journal of Hypertension, Lt

Endothelin Inhibition as a Biologic Target for Treating Hypertension

Endothelin, a 21-amino-acid peptide, binds to a specific receptor on vascular smooth muscle cells, thereby inducing vasoconstriction. Although plasma levels are not consistently elevated in hypertension, there is evidence that endothelin has an important role in its pathogenesis. Administration of endothelin antagonists has lowered blood pressure and reduced end-organ damage in some animal models. It has also reduced the cross-sectional area of neointima due both to hypertension and vascular injury. Coadministration of endothelin and angiotensin II to rats produced a synergistic hypertensive effect. Similarly, coadministration of an endothelin antagonist with an angiotensin converting enzyme inhibitor resulted in a synergistic lowering of blood pressure. Several preliminary clinical studies have been done. The endothelin antagonist bosentan has decreased vascular resistance and blood pressure and increased cardiac index in patients with congestive heart failure. Plasma endothelin levels are elevated in the acute phase of myocardial infarction and in chronic heart failure. The magnitude of this increase, measured 3 days after patients experienced myocardial infarction, had a significance at least equal to known risk factors in predicting 1 year survival. Thus, there are reasons to believe that endothelin antagonists may become a useful tool in the management of various cardiovascular disorder

Olmesartan medoxomil: current status of its use in monotherapy

Olmesartan medoxomil is an angiotensin II receptor antagonist. In pooled analyses of seven randomized, double-blind trials, 8 weeks’ treatment with olmesartan medoxomil was significantly more effective than placebo in terms of the response rate, proportion of patients achieving target blood pressure (BP) and mean change from baseline in diastolic (DBP) and systolic blood pressure (SBP). Olmesartan medoxomil had a fast onset of action, with significant between-group differences evident from 2 weeks onwards. The drug was well tolerated with a similar adverse event profile to placebo. In patients with type 2 diabetes, olmesartan medoxomil reduced renal vascular resistance, increased renal perfusion, and reduced oxidative stress. In several large, randomized, double-blind trials, olmesartan medoxomil 20 mg has been shown to be significantly more effective, in terms of primary endpoints, than recommended doses of losartan, valsartan, irbesartan, or candesartan cilexetil, and to provide better 24 h BP protection. Olmesartan medoxomil was at least as effective as amlodipine, felodipine and atenolol, and significantly more effective than captopril. The efficacy of olmesartan medoxomil in reducing cardiovascular risk beyond BP reduction is currently being investigated in trials involving patients at high risk due to atherosclerosis or type 2 diabetes

Combination Antihypertensive Therapy: Does It Have a Role in Rational Therapy?

The pharmacological treatment of hypertension allows one to reduce substantially the risk of developing a cardiovascular complication. It appears more and more important to bring blood pressure to normal values in order to get the maximal benefit from antihypertensive therapy. Blood pressure lowering drugs make it possible to control blood pressure in about half of the patients when administered as monotherapy. The fraction of patients with a normal blood pressure can be markedly increased by combining drugs acting by different mechanisms. Low doses of antihypertensive agents are generally enough when coadministered. This helps to keep the incidence of side effects minimal and facilitates the patient's compliance with long-term treatment. Low-dose, fixed-dose combination therapy may therefore represent a valuable option not only to treat hypertensive patients unresponsive to drugs given as monotherapy, but also to initiate the treatment. Am J Hypertens 1997;10:131S-137S ©1997 American Journal of Hypertension, Lt

Measurement of Converting Enzyme Activity by Antibody-Trapping of Generated Angiotensin II: Comparison With Two Other Methods

Activity of the angiotensin converting enzyme (ACE) is usually measured in vitro by estimation of products cleaved by the enzyme from synthetic substrates. These substrates have affinities for ACE different from the natural substrate angiotensin I, and insensitive detection systems necessitate milli-molar substrate concentrations while physiological angiotensin I concentrations are in the picomolar range. A new assay for ACE activity measurement was developed which reliably quantitates femtomoles of generated angiotensin II in plasma from angiotensin I added at a 17 pmol/mL concentration. The production of high affinity monoclonal antibodies against angiotensin II (Kd = 7 × 10-11 mol/L) allowed a quantitative trapping (and thus protection from degrading enzymes) of angiotensin II generated during the incubation step and subsequent ra-dioimmunoassay by simple dilution with labelled angiotensin II. Using 40 µL plasma, the detection limit was 20 fmol/mL/min. Normal human plasma has an ACE activity of 335 ± 83 fmol/mL/min (mean ± SD). Precision was characterized by coefficients of variation of ^ 11% both within-assay and between-assays. Accuracy of the new method was established by comparing ACE activity with the ratio of plasma angiotensin II/angiotensin I in plasma obtained from normal volunteers 0.5 to 24 h after oral administration of 20 mg enalapril. The percentage of ACE inhibition indicated by both methods was almost identical (r = 0.93, n = 60, P < .001). Since the latter ratio appears to reflect in vivo ACE activity, these results indicate that accurate measurement in vitro of ACE activity in vivo has been achieved. Am J Hypertens 1992;5:393-39

Cardiac mass in glucocorticoid-hypertensive rats with and without circulating adrenaline

There is evidence that catecholamines may promote the development of cardiac hypertrophy in hypertension. To test the hypothesis that adrenaline directly determines left ventricular mass, normotensive Wistar rats were made adrenaline-deficient by adrenalectomy and hypertensive by administration of glucocorticoid. Blood pressure, heart rate, and body weight of the adrenalectomised group were not significantly different from a glucocorticoid treated control group with intact adrenals. Heart weight was significantly lower in the adrenalectomised rats, but this difference disappeared when heart weight was adjusted for body weight. It appears therefore that the presence or absence of adrenaline does not significantly affect cardiac mass in the presence of hypertension in this animal mode

O-36: The orally active renin inhibitor SPP100 blocks the renin-angiotensin system in humans equally well as enalapril

The activity of the renin-angiotensin system (RAS) is mainly determined by the concentration of active renin. Direct inhibition of renin is therefore a primary goal for blocking the RAS. So far, all specific renin inhibitors lacked potency or were clinically ineffective after oral administration. We tested the new orally active non-peptidic renin inhibitor SPP100 in 18 healthy volunteers on a constant sodium diet (100 mmol/day)using a double-blind, threeway crossover protocol. In 3 periods of 8 days, separated by wash-outs of 1 week, each volunteer received 2 dosage levels of SPP100 once a day (40,80,160 or 640mg) and placebo or 20mg enalapril. SPP100 was well tolerated. Not surprisingly, blood pressure and heart rate remained unchanged in these normal volunteers. SPP100 plasma levels showed that steady state was reached after 8 days of dosing. The table below summarizes median plasma levels at peak (P, 0.5-6h) and trough (T, 24h after dosing)on day 8: In conclusion, the renin inhibitor SPP100 dose-dependently blocks the RAS and decreases angiotensin levels in human subjects following oral administration. The effect is long-lasting and at 160mg at least equivalent to that of 20mg enalapril. SPP100 has the clear potential to become the first renin inhibitor that provides a true alternative to ACE-inhibitors and Ang II receptor antagonists in the therapy of hypertension, cardiovascular and renal disease. Placebo SPP 100 Enalapril 40mg 80mg 160mg 640mg 20mg Renin pg/ml P 12 34 95 130 670 330 T 11 19 39 64 373 58 PRA ng/ml/h P 1.0 0.28 0.21 0.16 0.08 27 T 1.4 0.89 0.60 0.41 0.39 3.5 Ang I fmol/ml P 3.2 1.6 1.4 0.33 0.70 350 T 7.0 7.1 6.1 4.8 3.1 34 Ang II fmol/ml P 3.0 1.5 1.5 0.61 0.27 0.88 T 4.5 3.7 3.5 3.2 2.5 4.

Rapid Site-Directed Mutagenesis Using Two-PCR-Generated DNA Fragments Reproducing the Plasmid Template

We describe a new rapid and efficient polymerase chain reaction (PCR)-based site-directed mutagenesis method. This procedure is effective with any plasmid and it employs four oligonucleotide primers. One primer contains the desired mutation, the second is oriented in the opposite direction (one of these two primers should be phosphorylated), and the third and fourth should be coding in complementary fashion for a unique restriction site to be introduced in a nonessential region. The method consists of two simultaneous PCR reactions; the PCR products are digested with the enzyme that recognizes the newly introduced unique restriction site and then ligased and used to transform competent bacteria. Additionally, the use of Dpn I facilitates the elimination of template DNA. The newly introduced restriction site is essential for ligation in the correct orientation of the two-PCR products and is further used for mutant screening. Resulting plasmids carry both the new restriction site and the desired mutation. Using this method, more than 20 mutants have already been generated (using two different kinds of templates); all these mutants were sequenced for the desired mutation and transfected into AtT-20 cells and the expressed mutant proteins encoded by the vector were assayed

Vascular acetylcholine response during chronic NO synthase inhibition: in vivo versus in vitro

Objective: The aim of this study was to compare the response to NO-mediated vasodilators in vivo and in vitro during chronic NO synthase inhibition. Methods:NG-Nitro-l-arginine-methyl ester (l-NAME, 0.4 g/l) or vehicle was administered in the drinking water for 6 weeks to male Wistar rats weighing 220-240 g. The effect of acetylcholine and sodium nitroprusside was examined in vivo, on systemic blood pressure and heart rate and in vitro, on the precontracted isolated mesenteric artery. The in vivo response to both vasodilators was examined in awake rats monitored by an indwelling catheter in the femoral artery. Isolated segments of the third-generation mesenteric artery were examined in vitro with a Mulvany dual myograph after precontraction with noradrenaline. Results: In isolated mesenteric arteries obtained from rats chronically treated with l-NAME, the initial relaxant response to acetylcholine was significantly decreased whereas that to sodium nitroprusside was enhanced. A late acetylcholine-induced contractile response was present and abolished by indomethacin. In vivo, the hypotensive action of sodium nitroprusside was also enhanced in the l-NAME-treated rats. Acetylcholine reduced blood pressure in the l-NAME-treated hypertensive animals more than in normotensive controls, but less than in control rats infused intravenously with noradrenaline at a dose increasing their blood pressure to hypertensive levels. Conclusions: The NO-mediated vasodilation induced by acetylcholine is attenuated during chronic NO synthase inhibition, both in vivo and in vitro. The blunted hypotensive response to acetylcholine can be demonstrated only if blood pressure of control rats is acutely increased to hypertensive level