Abstract
The present study describes the pharmacological profile of ((E)-alpha-[[1-butyl-5-[2-[(2-carboxyphenyl)methoxy]-4-methoxy-phenyl]-1H-pyrazol-4-yl]methlene]-6-methoxy-1,3-benzodioxole-5-propanoic acid) (SB 234551), a high-affinity, nonpeptide endothelin type A (ETA)-selective receptor antagonist. In human cloned ETA and endothelin type B (ETB) receptors, SB 234551 produced a concentration-dependent displacement of [125I]-endothelin-1 with Ki values of 0.13 and 500 nM, respectively. SB 234551 elicited concentration-dependent, rightward competitive shifts in the endothelin-1 concentration-response curves in isolated rat aorta and isolated human pulmonary artery (ETA receptor-mediated vascular contraction) with Kb values of 1.9 and 1.0 nM, respectively. SB 234551 antagonized ETBreceptor-mediated vasoconstriction in the isolated rabbit pulmonary artery, as demonstrated by concentration-dependent, rightward shifts in the sarafotoxin S6c concentration-response curves (Kb = 555 nM). SB 234551 produced weak functional inhibition of sarafotoxin S6c-mediated endothelium-dependent relaxation (IC50 = 7 μM). SB 234551 (10 μM) had no significant effect against contraction produced by several other vasoactive agents and did not significantly influence radioligand binding to a number of diverse receptors. SB 234551 (0.1–1.0 mg/kg i.v.) dose-dependently inhibited the pressor response to exogenous endothelin-1 in conscious rats. In vivo pharmacokinetic analysis in the rat demonstrated that SB 234551 was rapidly absorbed from the GI tract with a bioavailability of 30%. SB 234551 had a plasma half-life of 125 min and a systemic clearance of 25.0 ml/min/kg. The present study demonstrates that SB 234551 is an antagonist with high affinity for the ETA receptor, while sparing the ETB receptor. SB 234551 is a new pharmacological tool that should assist in the elucidation of the role of endothelin in pathophysiology.
A critical need in the search for endothelin-based therapeutics is clarification of the physiological and pathophysiological functions of the endothelin receptor subtypes. Effects associated with ETA receptor activation include vasoconstriction, mitogenic activity, electrolyte excretion, microvascular permeability and release of biological mediators (Ohlstein et al., 1995). In contrast, the ETB receptor is associated with both vasodilator and vasoconstrictor actions, as well as effects on neuronal function (Ohlstein et al., 1995). On the basis of thein vivo effects of ETB receptor agonists, such as S6c and IRL-1620, studies from a number of laboratories have suggested the presence of two ETB receptor subtypes, one mediating vasodilation and the other mediating vasoconstriction (Warneret al., 1993a,b; MacLean et al., 1994; Ohlsteinet al., 1994a; Douglas et al., 1995b). These ETB receptor subtypes have been tentatively designated as ETB1 and ETB2 (Douglas et al., 1995a). The endothelin receptor antagonists RES-701 and PD 142893 have both been reported to be ETB1-selective antagonists in functional assays (Warner et al., 1993a; Douglas et al., 1995b). These compounds show no functional antagonist activity, even at concentrations up to 10 μM, against responses apparently mediated by the ETB2 receptor (Warner et al., 1993a; Douglas et al., 1995a; Hay et al., 1996). In radioligand binding studies using cloned human ETB receptors, however, these antagonists haveKi values in the submicromolar range. Accordingly, on the basis of pharmacological functional studies, the cloned human ETB receptor most closely resembles the ETB1-mediated response functionally linked to vasodilation. However, inasmuch as the available pharmacological data suggest the presence of heterogeneous functional ETB receptor subtypes, confirmation by conventional protein purification/molecular cloning is necessary before their existence can be firmly established.
Although the recent identification of peptide and nonpeptide receptor antagonists represents an important milestone in endothelin research, it is likely that elucidation of the role of ET-1 in the pathophysiology of diseases will require the clinical testing of high-affinity, structurally distinct compounds with a range of selectivities for the ETA and the ETB receptors and their subtypes. We have reported recently on the discovery and characterization of a high-affinity mixed ETA/ETB receptor antagonist, SB 217242 (Ohlstein et al., 1996). However, a high-affinity ETA-selective antagonist that exerts reduced effects on the ETB1 receptor mediating vasodilation, while maintaining the inhibition of the ETB2 vasoconstrictor effects demonstrated by SB 217242, would appear to have an attractive compound profile. In this study, we report on the discovery and characterization of SB 234551, the lead compound from a new chemical series of high-affinity nonpeptide endothelin receptor antagonists with such a profile.
Materials and Methods
Membrane preparation and radioligand binding.
Chinese hamster ovary cells stably transfected with cloned human ETA and ETB receptors were cultured and cell membranes prepared as reported previously (Nambi et al., 1994). [125I]ET-1 binding to membrane preparations was performed as described previously (Nambi et al., 1994). Briefly, assay volumes were 50 μl, and the concentrations of membrane proteins were 0.50 and 0.05 μg/tube for human ETA and ETB receptors, respectively. The concentrations of the radioligands were 30 to 1000 pM for saturation-binding and 300 pM for competition-binding experiments using the cloned human ETAand ETB receptors. Nonspecific binding was measured in the presence of 1 μM unlabeled ET-1. The incubations (performed for 60 min at 30°C) were stopped by dilution with cold buffer and filtration through Whatman GF/C filters presoaked in 0.1% bovine serum albumin. The filters were washed three times (5 ml each time) and counted using a gamma counter.
In vitro endothelin receptor antagonist activity.
Male Sprague-Dawley rats (300–325 g) or New Zealand White rabbits (2–3 kg) were euthanized with sodium pentobarbital (100 mg/kg i.p.). Rat thoracic aortae and rabbit pulmonary arteries were excised, cleaned of adherent tissue and the vascular endothelium denuded by gently rubbing the intimal surface of the vessel with a stainless steel probe (Ohlstein et al., 1989). Rabbit saphenous veins were prepared as described previously (Douglas et al., 1995b). Isolated vessels were cut into 4-mm rings and suspended in 10-ml organ bath chambers containing Krebs-bicarbonate solution (mM: NaCl, 112.0; KCl, 4.7; KH2PO4, 1.2; MgSO4, 1.2; CaCl2, 2.5; NaHCO3, 25.0 and dextrose, 11.0. Tissue baths were maintained at 37°C and aerated continuously with 95% O2/5% CO2, pH = 7.35. Resting tensions of 1 gm for rat aorta, rabbit pulmonary artery and saphenous vein were maintained throughout the experiments. Endothelial integrity was assessed pharmacologically in terms of the ability of ACh (0.1 μM) to produce relaxation of tissues precontracted with norepinephrine (10 μM). Tissues were equilibrated for 1 hr before the start of experiments, and isometric tensions were recorded on Beckman R-611 dynographs using Grass FTO3c force-displacement transducers.
Human lung tissue, from organ donors with no known history of respiratory disorders, was obtained from the International Institute for the Advancement of Medicine (Exton, PA) and the National Disease Research Interchange (Philadelphia, PA). Lungs were received within 24 hr of removal. A glass probe was placed in the pulmonary artery, and 4-mm ring preparations were prepared; the endothelium was removed as described above. The preparations were then placed in 10-ml organ baths containing Krebs solution and connected via silk suture to Grass FT03C force-displacement transducers. Mechanical responses were recorded isometrically by MP100WS/Acknowledge data acquisition system (BIOPAC Systems, Goleta, CA). Tissues were equilibrated under 2 gm resting load for 1 hr, and washed every 15 min with fresh Krebs solution, before the start of each experiment.
To test for viability, after the equilibration period and before construction of cumulative concentration-response curves, tissues were exposed to phenylephrine (10 μM) for human pulmonary artery or to KCl (60 mM) for rat aorta and rabbit pulmonary artery. After plateau of this reference contraction, tissues were washed several times over 60 min. In experiments examining the effects of receptor antagonists, tissues were exposed to the compound for 30 min before the addition of contractile agonists. Only one agonist concentration-response curve was generated per tissue, and paired tissues were used for evaluation of receptor antagonists. Experiments evaluating ETB-mediated vasodilation in norepinephrine (1 μM)-precontracted isolated rabbit saphenous vein were performed as described previously (Douglas et al., 1995b).
Agonist-induced responses were expressed as a percentage of the reference contraction obtained at the beginning of the experiment. Geometric mean EC50 values were calculated from linear regression analyses of data. Using nonlinear least-squares regression, concentration-response curves were analyzed by fitting the experimental data to the following logistic equation:
In vivo hemodynamics.
Male Sprague-Dawley rats (300–350 g) were prepared with chronic indwelling catheters, following the protocol of Ohlstein et al., (1994b). Briefly, rats were anesthetized with sodium methohexitone (100 mg/kg i.p.) and catheters placed in the abdominal aorta (to record systemic arterial blood pressure) and vena cava (for i.v. bolus administration of drugs)via the left femoral artery and vein, respectively. All tubing was tunneled under the skin and exited at the midscapular region. Tubing was filled with a dextrose:heparin solution (0.5 g/ml dextrose and 1000 units/ml heparin) to prevent obstructive thrombus formation. After completion of the surgery, animals were housed in Plexiglas cages under a 12-hr light-dark cycle with access to standard laboratory chow and drinking water ad libitum. Animals were allowed at least 3 days to recover from surgical intervention before undergoing experimentation.
On the day of experimentation, animals received a bolus i.v. dose of 100 pmol/kg ET-1 (∼ED75 dose). Thirty minutes later, immediately prior to a repeat dose of ET-1, either saline vehicle or SB 234551 (0.1–1 mg/kg) was administered. Repeat doses of ET-1 were administered every 30 min over a subsequent 4-hr period.
Pharmacokinetics of SB 234551 in rats.
The pharmacokinetic evaluation of SB 234551 was performed as described previously (Ohlsteinet al., 1996). Briefly, male Sprague-Dawley rats (350 gm) were surgically prepared with indwelling cannulas in the vena cava, femoral artery and duodenum. SB 234551 was administered as a 2-hr intraduodenal infusion (in saline vehicle) via the duodenal cannula at a rate of 50 μg/kg/hr (total dose, 6 mg/kg). Blood samples (110 μl) were collected from the femoral arterial cannula at various time intervals over 1440 min. The animals received approximately 2 ml of heparinized blood from untreated donor rats upon completion of this leg of the study. One week later, rats were crossed over to receive SB 234551 as an i.v. infusion. SB 234551 was administered viathe femoral vein cannula at a rate of 0.20 μg/hr/kg for 2 hr.
Pharmacokinetic analysis.
SB 234551 was isolated from rat plasma by liquid-liquid extraction and was quantitated by reverse-phase HPLC with MS/MS detection performed on an API III tandem triple quadrupole mass spectrometer (Perkin Elmer Sciex Instruments, Rochester, NY). The assay provided a lower limit of quantification of 5 ng/ml based on 0.05 ml plasma and was linear up to 1000 ng/ml. Plasma concentration-time profiles were analyzed using noncompartmental methods. The area under the plasma concentration-time curve (AUC) was estimated by a combination of linear and log trapezoidal methods. Plasma clearance was calculated as dose/AUC. Bioavailability was determined as dose-normalized AUC (intraduodenal)/dose-normalized AUC (intravenous). The apparent terminal half-life was estimated by least-squares linear regression analysis of the log-transformed concentration-time data.
All experiments were performed in accordance with the guidelines of the Animal Care and Use Committee, SmithKline Beecham Pharmaceuticals and AALAC.
Calculations and statistics.
Values are expressed as mean ± S.E.M., and n represents the number of animals or separate experiments studied in a particular group. Statistical analysis was conducted using ANOVA or two-tailed Student’st test for paired samples, where appropriate, P ≤ .05 being accepted as significant.
Materials.
All solutions were prepared daily. Endothelin isopeptides and BQ-123 were purchased from American Peptide Co. (Santa Clara, CA). [125I]ET-1 (2200 Ci/mmol) was obtained from New England Nuclear (Boston, MA). All other chemicals were of the highest grades available. SB 234551, SB 209670, SB 217242, PD 156707, L-749,329, BQ-788 and bosentan were synthesized in the Department of Medicinal Chemistry, SmithKline Beecham Pharmaceuticals (King of Prussia, PA). RES-701 (Matsuda et al., 1993) was kindly provided by Kyowa Hakko Kogyo (Tokyo, Japan).
Results
Radioligand binding to human ETA/ETBreceptors.
Competition of [125I]-ET-1 binding to recombinant human ETA and ETB receptors by unlabeled ET-1, ET-3 and the subtype-selective ligands S6c, BQ-123, and SB 234551 is shown in figure 1. Whereas ET-1 displayed similar affinities for ETA and ETB receptors (IC50 = 0.3 and 0.2 nM, respectively), ET-3 had approximately 500-fold lower affinity for ETA receptors, with IC50 values of 100 and 0.2 nM for ETA and ETB, respectively (fig. 1). Similarly, S6c had an IC50 of 0.5 nM for the ETB receptor (fig. 1, bottom panel), whereas BQ-123 displayed an IC50 of 50 nM for the ETA receptor (fig. 1, top panel). In addition, BQ-123 displayed weak affinity for the cloned human ETB receptors, and S6C displayed weak affinity toward the cloned human ETA receptor.
SB 234551 was a high-affinity ligand at the cloned human ETA receptor, with approximately 5000-fold higher affinity for this receptor than for the human cloned ETB receptor:Ki = 0.13 nM and 500 nM, respectively (fig. 1). The affinities of SB 234551 and other nonpeptide endothelin receptor antagonists are summarized in table 1. These data demonstrate that SB 234551 is one of the highest-affinity nonpeptide receptor antagonists yet described for the cloned human ETA receptor but has low affinity for the cloned human ETB receptor.
In vitro functional activity.
SB 234551 (10–1000 nM) produced concentration-dependent, parallel rightward shifts in the ET-1 concentration-response curves in the isolated rat aorta (fig.2). The contractile response elicited by ET-1 in this tissue is mediated by the ETA receptor (Ohlstein et al., 1996). The Kb value for inhibition of ET-1-induced contraction for SB 234551 was 1.9 ± 0.1 nM (fig. 2). Schild analysis of these concentration-response curves yielded a slope of the regression line of 0.97, which was not significantly different from unity. ET-1-induced maximal contraction in the isolated rat aorta was not significantly affected by SB 234551. No agonist activity was observed with SB 234551 at the highest concentration studied (10 μM).
Characterization of ET-1-induced contraction in isolated human pulmonary artery was performed. The responses to ET-1 in the human large pulmonary artery are mediated predominantly, if not exclusively,via ETA receptor activation (Hay et al., 1993). In human isolated large pulmonary arteries, ET-1 (1–300 nM) produced a concentration-dependent contraction with an EC50 of 5 nM (fig. 3). SB 234551 (10 nM) elicited high-affinity, surmountable antagonism of ET-1induced contractions with a Kb of 1.0 nM.
The relative affinities of SB 234551 and other nonpeptide endothelin receptor antagonists were evaluated against ET-1-induced contractions in human pulmonary artery (table 2). The affinity of SB 234551 was similar to those for SB 209670 and L-749,329 (Kb = 0.62 and 2.2 nM, respectively), approximately 5-fold greater than that of SB 217242 (Kb = 5.3 nM) and approximately 200-fold greater than that of BQ-123 (Kb = 221 nM; table 2). It is interesting to note that in the isolated human pulmonary artery, both bosentan (1 μM) and PD 156707 (100 nM) had no significant effect on ET-1 induced contraction.
SB 234551 produced functional inhibition of ETB2receptor-mediated vasoconstriction, as demonstrated by antagonism of S6c-mediated contraction in the isolated rabbit pulmonary artery. S6c produces contractile responses in this tissue viaETB receptor activation (Warner et al., 1993a,b;Ohlstein et al., 1994a). SB 234551 (10 μM) produced inhibition in the S6c concentration-response curves, with aKb of 555 ± 119 nM (table3). Comparison with other endothelin receptor antagonists shows that SB 234551 had about the same functional blockade for the ETB2 receptor in rabbit pulmonary artery as SB 217242 (Kb = 353 nM) and had approximately 35-fold less affinity at this receptor than SB 209670. SB 234551 had at least 2-fold more affinity than either bosentan or BQ-788, whereas RES-701 (10 μM) had no affinity for this receptor.
S6c produces ETB1-mediated vasodilation in the rabbit saphenous vein (Douglas et al., 1995b). ET-3 produced similar vasodilation responses in the isolated rabbit saphenous vein (data not shown). SB 234551 was a weak antagonist of ETB1receptor-mediated vasodilation, as demonstrated by its limited ability to inhibit ET-3-mediated vasodilation (IC50 = 7 μM) in the isolated endothelium-intact rabbit saphenous vein (fig.4; table 3). Thus an approximately 3600-fold difference was apparent between the Kb value for inhibition of the ETA receptor-mediated functional response, with respect to inhibition of the ETBreceptor-mediated functional response (table 3). In contrast, both SB 209670 and SB 217242 produced greater inhibition of ETB2receptor-mediated dilation in the rabbit saphenous vein, with respective IC50 values of 4 and 70 nM (table 3). Furthermore, bosentan was a weak inhibitor of ETB2receptor-mediated dilation, with an IC50 value of 2 μM.
The selectivity of SB 234551 was evaluated in several in vitro assays. SB 234551 (10 μM), a concentration four orders of magnitude greater than the Ki orKb at the ETA receptor, did not significantly affect radiolabeled ligands binding to a number of receptors with diverse ligands, including CGRP, IL-8, NK-1, NK-2, NK-3, arginine vasopressin and thrombin. However, SB 234551 produced weak inhibition of radioligand binding of [125I]angiotensin II to rat adrenal cortex (Ki = 5.2 μM) and angiotensin II-induced vasoconstriction in the rabbit aorta (Kb = 2.9 μM). Nevertheless, this degree of affinity was over three orders of magnitude lower at the AT1 receptor than at the ETA receptor, and it is unlikely that this activity contributes significantly to the compound’s pharmacological activity.
Inhibition of endothelin-induced pressor responses in conscious normotensive rats.
Changes in mean arterial pressure in response to bolus i.v. injections of a submaximal dose of ET-1 (100 pmol/kg, the approximate ED75) were measured at 30-min intervals before and after bolus i.v. administration of vehicle or SB 234551 (0.1, 0.1 and 1.0 mg/kg) in conscious, chronically catheterized male Sprague-Dawley rats. Unlike responses in anesthetized rats, the change in blood pressure after the bolus injection of ET-1 is of brief duration (3–5 min) and repeatable in short intervals without tachyphylaxis, as illustrated by the values for the vehicle-treated group in figure 5. Basal mean arterial blood pressure averaged 111 ± 2.5 mmHg for all rats and was not statistically different between groups; neither was it altered by the injection of various doses of SB 234551. The change in blood pressure after injections of 100 pmol/kg ET-1 was 35 ± 2 mmHg during the control period. SB 234551 (0.1–1 mg/kg) inhibited the pressor response to ET-1 in a dose-dependent manner (fig. 5). Maximal inhibition (80%) was observed with a dose of 1.0 mg/kg at 30 min after dosing, and the response to ET-1 gradually returned to control levels by 4 hr.
Oral pharmacokinetics of SB 234551 in conscious normotensive rats.
Disposition kinetics and oral performance of SB 234551 were assessed in conscious male Sprague-Dawley rats. These investigations involved cross-over studies in multiple-cannulated rats to quantify the extent of absorption (intraduodenal infusion) and pharmacokinetic linearity. The plasma concentration-time profile demonstrated that SB 234551 was rapidly absorbed after intraduodenal infusion. The pharmacokinetic parameters are summarized in table4. The systemic plasma clearance of SB 234551 was 25.0 ml/min/kg, and the intraduodenal bioavailability was 30%. The terminal plasma half-life values after i.v. and intraduodenal routes were similar: approximately 129 and 125 min, respectively.
Discussion
The development of high-affinity and subtype-selective receptor antagonists is expanding our understanding of the role of endothelin in pathophysiology. We have recently reported the development of a novel indane series of compounds that are high-affinity antagonists of both ETA and ETB receptors (Ohlstein et al., 1994a,b, 1996). Although the prototype molecule, SB 209670, is a high-affinity, selective endothelin receptor antagonist, there is only a 50-fold difference between the affinity at the ETAreceptor and that at the ETB receptor. It is still not clear what is the optimal endothelin receptor subtype selectivity for endothelin-based therapeutics. On the basis of current understanding, however, it appears that receptors that are defined as ETB1mediate vasodilation and can be regarded as “beneficial,” whereas stimulation of the purported ETB2 receptors produces undesired effects such as smooth muscle contraction (Douglas et al., 1995a,b). This hypothesis has to be clarified, so compounds that differentiate between the vasodilator ETB1 and the vasoconstrictor ETB2 receptors would help to delineate further the role of endothelin in the etiology of cardiovascular and other diseases. Such compounds would also provide information on the optimal profile for a therapeutically useful endothelin receptor antagonist for specific disorders.
In this report, we describe the pharmacological characterization of SB 234551, the lead compound from a new series of pyrazole endothelin receptor antagonists. SB 234551 has a unique receptor selectivity profile; binding studies indicate that the compound has approximately 5000-fold more affinity for the cloned human ETA receptor than for the cloned human ETB receptor (table 1). There was a good correlation between the affinity of SB 234551 for recombinant human ETA receptors from the results of radioligand binding and functional studies: in the former the Ki for SB 234551 was 0.13 nM, and the Kb values for inhibition of ET-1-mediated vasoconstriction in the isolated rat aorta and human pulmonary artery were 1.9 and 1.0 nM, respectively. These data indicate that SB 234551 is one of the highest-affinity endothelin receptor antagonists yet reported in human vascular tissue. In contrast, it is interesting to note that in human pulmonary artery, neither bosentan nor PD 156707 produced significant inhibition of ET-1-mediated vasoconstriction, despite high affinity in the cloned human ETA receptor. In particular, convincing evidence exists that the responses in human large pulmonary artery appear to be mediated predominantly, if not exclusively, viaETA receptors: 1) S6c, the ETB receptor ligand, has no effect on basal vascular tone (Hay et al., 1993), and 2) ET-1-induced responses are inhibited potently by BQ-123 (Hayet al., 1993; Buchan et al., 1994). The lack of activity of bosentan or PD 156707 has not been explained, but it may indicate the presence of an ETA receptor subtype that has differential sensitivity to the available endothelin receptor antagonists. Alternatively, it may be due to differences in antagonist affinities, which have also been reported for cultured human pulmonary smooth muscle cells (Hatakeyama et al., 1994) and in functional studies in this same tissue (Buchan et al., 1994). Nonetheless, the present data support the interpretation that ET-1 produces vascular contraction of the human pulmonary artery by stimulating the ETA receptor, an effect antagonized significantly by SB 234551.
Numerous reports have demonstrated that multiple ETBreceptor subtypes exist (Warner et al., 1993a,b; Sudjarwoet al., 1993; Karaki et al., 1994, MacLeanet al., 1994; Douglas et al., 1995b; Gellaiet al., 1996; McCulloch and MacLean, 1995, 1996). For example, an ETB receptor found on the vascular endothelium has been associated with endothelium-dependent vasorelaxation. This receptor has been termed “ETB1-like.” In the isolated rabbit saphenous vein, SB 234551 weakly inhibits ETB-mediated release of endothelium-dependent vasodilation (IC50 = 7 μM), in contrast to SB 209670 (3 nM), BQ-788 (300 nM) and RES 701 (300 nM) (table 3). The other ETBreceptor, designated “ETB2-like,” has been implicated in endothelin-mediated vasoconstriction (Warner et al., 1993a,b; Douglas et al., 1995b; Hay et al., 1996). The rabbit pulmonary artery possesses ETB2-like receptors, and SB 234551 produces functional inhibition of the ETB2-like receptor, as demonstrated by antagonism of S6c-mediated contraction of the isolated rabbit pulmonary artery (Kb = 555 nM). Although the affinity of SB 234551 in this tissue is modest, experiments with SB 209670 and SB 217242 also show a disparity between inhibition of rabbit and human ETB receptor-mediated constriction, the latter two compounds being more potent in human bronchial ETBreceptors (Hay et al., 1996).
The functional roles of ETA and ETB receptors have been studied previously in the canine kidney (Brooks et al., 1994, 1995). In these studies renal vasoconstriction was shown to be mediated by ETA receptors, and ETBreceptor stimulation inhibited sodium reabsorption. Furthermore, the ETB1 receptor subtype was shown to mediate tubular sodium reabsorption, because RES-701 antagonized ETB1-mediated natriuresis induced by S6c or ET-1 (in the presence of BQ-123). In addition, ETB1 receptors may induce renal vasodilation. Thein vivo demonstration of the ETB1 receptor sparing activity of SB 234551 has been demonstrated in this same model (Brooks et al., 1998). The i.v. infusion of SB 234551 (30 μg/kg/min) in anesthetized dogs was demonstrated to inhibit the vasoconstrictor responses to exogenously administered ET-1 and significantly to increase renal plasma flow and urinary sodium excretion (Brooks et al., 1998). These data indicate that SB 234551 unmasks ETB1 receptor-induced renal vasodilation and inhibition of sodium reabsorption.
In summary, this is the first report on the pharmacological characterization of SB 234551, the lead molecule from a new pyrazole series of endothelin receptor antagonists. SB 234551 is a high-affinity antagonist of ETA/ETB2-mediated functional responses. The optimal receptor profile (i.e., ETAvs. ETB) for the most therapeutically useful endothelin receptor antagonist is not yet known, but it is likely that different diseases require compounds with different receptor subtype profiles. However, because both ETA and ETB2 receptors are involved in vasoconstriction, an antagonist with high affinity for these receptors might be predicted to provide beneficial effects.
Acknowledgments
The authors thank A. Gao, A. Konalian-Beck, S. Atkinson, M. Darcy and D. Shah for synthesis of the compounds used in this study; M. Pullen and M. Luttmann for excellent technical assistance; Dr. Kei-lei Fong for assisting in the pharmacokinetic studies and Dr. Stephen Douglas for critical reading of the manuscript.
Footnotes
-
Send reprint requests to: Eliot H. Ohlstein, Ph.D., Department of Cardiovascular Pharmacology, UW 2511, SmithKline Beecham Pharmaceuticals, 709 Swedeland Road, King of Prussia, PA 19406.
- Abbreviations:
- ET-1
- endothelin-1
- ET-3
- endothelin-3
- S6c
- sarafotoxin S6c
- ETA receptor
- endothelin type A receptor
- ETB receptor
- endothelin type B receptor
- SB 234551
- ((E)-alpha-[[1-butyl-5-[2-[(2-carboxyphenyl)methoxy]-4-methoxy-phenyl]-1H-pyrazol-4-yl]methlene]-6-methoxy-1,3-benzodioxole-5-propanoic acid)
- Received August 1, 1997.
- Accepted April 29, 1998.
- The American Society for Pharmacology and Experimental Therapeutics