DuP 532, an angiotensin II receptor antagonist: first administration and comparison with losartan

Does protein binding modulate the effect of angiotensin II receptor antagonists?

Introduction

Angiotensin II AT 1-receptor antagonists are highly bound to plasma proteins (≥ 99%). With some antagonists, such as DuP-532, the protein binding was such that no efficacy of the drug could be demonstrated clinically. Whether protein binding interferes with the efficacy of other antagonists is not known. We have therefore investigated in vitro how plasma proteins may affect the antagonistic effect of different AT1-receptor antagonists.

Methods

A radio-receptor binding assay was used to analyse the interaction between proteins and the ability of various angiotensin II (Ang II) antagonists to block AT1-receptors. In addition, the Biacore technology, a new technique which enables the real-time monitoring of binding events between two molecules, was used to evaluate the dissociation rate constants of five AT1-receptor antagonists from human serum albumin.

Results

The in vitro AT 1-antagonistic effects of different Ang II receptor antagonists were differentially affected by the presence of human plasma, with rightward shifts of the IC50 ranging from one to several orders of magnitude. The importance of the shift correlates with the dissociation rate constants of these drugs from albumin. Our experiments also show that the way that AT1-receptor antagonists bind to proteins differs from one compound to another. These results suggest that the interaction with plasma proteins appears to modulate the efficacy of some Ang II antagonists.

Conclusion

Although the high binding level of Ang II receptor antagonist to plasma proteins appears to be a feature common to this class of compounds, the kinetics and characteristics of this binding is of great importance. With some antagonists, protein binding interferes markedly with their efficacy to block AT1-receptors.

The importance of plasma protein binding in drug discovery

Plasma protein binding of drugs is a well-recognised phenomena, but it is only recently that the implications for drug action in vivo have been fully appreciated. Plasma proteins, by virtue of their high concentration, control the free drug concentration in plasma and in compartments in equilibrium with plasma, thereby, effectively attenuating drug potency in vivo. The historical background and thermodynamic basis for the 'Free Drug Principle' is presented, along with special considerations for intracellular targets, deep compartments and α1-acid glycoprotein binding. Real and apparent exceptions to the principle are discussed along with a survey of citations from the recent medicinal chemistry literature.

An automated electrophysiology serum shift assay for K(V) channels

The presence of serum in biological samples often negatively impacts the quality of in vitro assays. However, assays tolerant of serum are useful for assessing the in vivo availability of a small molecule for its target. Electrophysiology assays of ion channels are notoriously sensitive to serum because of their reliance on the interaction of the plasma membrane with a recording electrode. Here we investigate the tolerance of an automated electrophysiology assay for a voltage-gated potassium (K(V)) channel to serum and purified plasma proteins. The delayed rectifier channel, K(V)2.1, stably expressed in Chinese hamster ovary cells produces large, stable currents on the IonWorks Quattro platform (MDS Analytical Technologies, Sunnyvale, CA), making it an ideal test case. K(V)2.1 currents recorded on this platform are highly resistant to serum, allowing recordings in as high as 33% serum. Using a set of compounds related to the K(V) channel blocker, 4-phenyl-4-[3-(2-methoxyphenyl)-3-oxo-2-azaprop-1-yl]cyclohexanone, we show that shifts in compound potency with whole serum or isolated serum proteins can be reliably measured with this assay. Importantly, this assay is also relatively insensitive to plasma, allowing the creation of a bioassay for inhibitors of K(V)2.1 channel activity. Here we show that such a bioassay can quantify the levels of the gating modifier, guangxitoxin-1E, in plasma samples from mice dosed with the peptide. This study demonstrates the utility of using an automated electrophysiology platform for measuring serum shifts and for bioassays of ion channel modulators.

Evaluation of the angiotensin challenge methodology for assessing the pharmacodynamic profile of antihypertensive drugs acting on the renin-angiotensin system

AIMS

The performance of the experimental paradigm of angiotensin challenges with continuous non-invasive blood pressure measurement was evaluated. Angiotensin dose-response relationships were characterized, along with the influence of clinical covariates. The stability of angiotensin-induced peaks and the variability of the angiotensin doses were assessed. Finally, the predictive value of studies based on angiotensin challenges to determine drug doses effective in therapeutics was evaluated.

METHODS

The data were gathered from 13 clinical studies on nine angiotensin II receptor antagonists, one ACE inhibitor and one dual ACE-NEP inhibitor, using Finapres for measuring the response to exogenous angiotensin challenges. Modelling of angiotensin dose-response curves and determination of the inter and intrasubject variability were performed by nonlinear regression (NONMEM). The different sources of variations in angiotensin I and II doses and angiotensin-induced peaks were evaluated by analyses of variance. The dose of ACE inhibitors and angiotensin II receptor antagonists inhibiting blood pressure increase by at least 75%, as measured by this method, was chosen for comparison with the labelled starting dose.

RESULTS

Angiotensin challenges exhibited a clear dose-response relationship which can be characterized both by an Emax or a log linear model. The log linear model gave an average systolic/diastolic response of 24+/-6/20+/-5 mmHg for a unit dose of 1 microgram of angiotensin II equivalents, and an increase of 6/6 mmHg for each doubling of the dose. The angiotensin ED50 calculated values were 0.67 microgram for systolic and 0.84 microgram for diastolic blood pressure. The angiotensin doses for eliciting a given response and the angiotensin induced peaks were fairly constant between period and subject, but vary significantly between studies. Based on an inhibition of blood pressure by 75%, the agreement was good between the doses of ACE inhibitors and angiotensin receptor antagonists predicted from studies using the methodology of angiotensin challenges and the doses shown to be clinically efficacious, in spite of high intersubject and intrasubject variabilities.

CONCLUSIONS

This experimental method represents a valid surrogate for the therapeutic target and a useful tool for the pharmacokinetic and pharmacodynamic profiling of drugs acting on the renin-angiotensin system.

Pharmacologic profile of TA-606, a novel angiotensin II-receptor antagonist in the rat

This study was carried out to characterize a novel angiotensin II-receptor antagonist, TA-606, (3-pentyloxy) carbonyloxymethyl-5-acetyl-2-n-propyl-3-[2'(1H-tetrazole-5-yl) biphenyl-4-yl]methyl-4,5,6,7-tetrahydro imidazo [4,5-c] pyridine-4-carboxylate hydrochloride, which is a newly synthesized prodrug of 606A. In anesthetized rats, 606A inhibited angiotensin II-induced pressor response with a median inhibitory concentration (IC50) of 6 microg/kg, i.v., and was 8 times more potent than EXP3174, an active metabolite of losartan. Bioavailability of TA-606 was 11 times higher than that of 606A in Sprague-Dawley rats, with consistent hypotensive potencies in spontaneously hypertensive rats (SHRs). In conscious renal hypertensive rats (RHRs) and conscious SHRs, TA-606 lowered the blood pressure without any effects on the heart rate, and its effective dose for 30 mm Hg (ED30) values were 0.14 and 0.21 mg/kg, p.o., respectively. The effect of TA-606 lasted > 10 h in both models. Moreover, the effect of TA-606 was approximately 30 and 10 times more potent than those of losartan in RHRs and SHRs, respectively. TA-606 did not affect the blood pressure in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. TA-606 given for 12 weeks attenuated the development of hypertension in stroke-prone SHRs. These results indicate that TA-606 is a potent angiotensin II-receptor antagonist with antihypertensive efficacy. Thus TA-606 is suggested to be a possible useful agent in the treatment of hypertension.