2,4-Diarylpyrrolidine-3-carboxylic acids--potent ETA selective endothelin receptor antagonists. 1. Discovery of A-127722

We have discovered a novel class of endothelin (ET) receptor antagonists through pharmacophore analysis of the existing non-peptide ET antagonists. On the basis of this analysis, we determined that a pyrrolidine ring might replace the indian ring in SB 209670. The resultant compounds were readily prepared and amenable to extensive SAR studies. Thus a series of N-substituted trans,trans-2-(4-methoxyphenyl)-4-(1,3-benzodioxol-5-yl)pyrroli din e-3- carboxylic acids (8) have been synthesized and evaluated for binding at ET(A) and ET(B) receptors. Compounds with N-acyl and simple N-alkyl substituents had weak activity. Compounds with N-alkyl substituents containing ethers, sulfoxides, or sulfones showed increased activity. Much improved activity resulted from compounds where the N-substituents were acetamides. Compound 17u (A-127722) with the N,N-dibutylacetamide substituent is the best of the series. It has an IC(50)=0.36 nM for inhibition of ET-1 radioligand binding at the ET(A) receptor, with a 1000-fold selectivity for the ET(A) vs the ET(B) receptor. It is also a potent inhibitor (IC(50)=0.16 nM) of phosphoinositol hydrolysis stimulated by ET-1, and it antagonized the ET-1-induced contraction of the rabbit aorta with a pA(2)=9.20. The compound has 70% oral bioavailability in rats.

Endothelin attenuates apoptosis in human smooth muscle cells

An imbalance between proliferation and apoptosis is an important causal factor for disorders involving abnormal cell accumulation. Endothelin (ET)-1, a 21-amino-acid peptide with mitogenic and vasoconstricting activities, not only acts as a mitogen, but also attenuates paclitaxel-induced apoptosis in smooth muscle cells. In both human pericardial and prostatic smooth muscle cells, addition of ET-1 reduced paclitaxel-induced DNA fragmentation and phosphatidylserine on the cell surface, two characteristics of apoptosis. By comparison, angiotensin II, another vasoactive peptide, did not have a significant effect on apoptosis. The effect of ET-1 was dose-dependent with an EC50 of 1 nM. These results suggest that ET is a potential survival factor for smooth muscle cells, and that altered activity of the ET system in disease states has potential to contribute to aberrant cell growth.

Pyrrolidine-3-carboxylic acids as endothelin antagonists. 4. Side chain conformational restriction leads to ET(B) selectivity

When the dialkylacetamide side chain of the ET(A)-selective antagonist ABT-627 is replaced with a 2,6-dialkylacetanilide, the resultant analogues show a complete reversal of receptor selectivity, preferring ET(B) over ET(A). By optimizing the aniline substitution pattern, as well as the alkoxy group on the 2-aryl substituent, it is possible to prepare antagonists with subnanomolar affinity for ET(B) and with selectivities in excess of 4000-fold. A number of these compounds also show promising pharmacokinetic profiles; a useful balance of properties is found in A-192621 (38). Pharmacology studies with A-192621 serve to reveal the role of the ET(B) receptor in modulating blood pressure; the observed hypertensive response to persistent ET(B) blockade is consistent with previous postulates and indicates that ET(B)-selective antagonists may not be suitable as agents for long-term systemic therapy.

Discovery and Metabolic Stabilization of Potent and Selective 2-Amino-N-(adamant-2-yl) Acetamide 11β-Hydroxysteroid Dehydrogenase Type 1 Inhibitors

Starting from a rapidly metabolized adamantane 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) inhibitor 22a, a series of E-5-hydroxy-2-adamantamine inhibitors, exemplified by 22d and (+/-)-22f, was discovered. Many of these compounds are potent inhibitors of 11beta-HSD1 and are selective over 11beta-HSD2 for multiple species (human, mouse, and rat), unlike other reported species-selective series. These compounds have good cellular potency and improved microsomal stability. Pharmacokinetic profiling in rodents indicated moderate to large volumes of distribution, short half-lives, and a pharmacokinetic species difference with the greatest exposure measured in rat with 22d. One hour postdose liver, adipose, and brain tissue 11beta-HSD1 inhibition was confirmed with (+/-)-22f in a murine ex vivo assay. Although 5,7-disubstitued-2-adamantamines provided greater stability, a single, E-5-position, polar functional group afforded inhibitors with the best combination of stability, potency, and selectivity. These results indicate that adamantane metabolic stabilization sufficient to obtain short-acting, potent, and selective 11beta-HSD1 inhibitors has been discovered.

Endothelin stimulates glucose uptake and GLUT4 translocation via activation of endothelin ETA receptor in 3T3-L1 adipocytes

Endothelin-1 (ET-1) is a 21-amino acid peptide that binds to G-protein-coupled receptors to evoke biological responses. This report studies the effect of ET-1 on regulating glucose transport in 3T3-L1 adipocytes. ET-1, but not angiotensin II, stimulated glucose uptake in a dose-dependent manner with an EC50 value of 0.29 nM and a 2.47-fold stimulation at 100 nM. ET-1 stimulated glucose uptake in differentiated 3T3-L1 cells but had no effect in undifferentiated cells, although ET-1 stimulated phosphatidylinositol hydrolysis to a similar degree in both. The 3T3-L1 cells expressed approximately 560,000 sites/cell of ETA receptor, which was not altered during differentiation. Western blot analysis and immunofluorescence staining show that ET-1 stimulated the translocation of insulin-responsive aminopeptidase and GLUT4 to the plasma membrane. The effect of ET-1 on glucose uptake was blocked by A-216546, an antagonist selective for the ETA receptor. ET-1 treatment did not induce phosphorylation of insulin receptor beta-subunit, insulin receptor substrate-1, or Akt but stimulated the tyrosyl phosphorylation of a 75-kDa protein. Genistein (100 microM), an inhibitor of tyrosine kinases, inhibited ET-1-stimulated glucose uptake. Our results show that ET-1 stimulates GLUT4 translocation and glucose uptake in 3T3-L1 adipocytes via activation of ETA receptor.

Oxidant-induced activation of protein kinase C in UC11MG cells

Free radical formation and subsequent lipid peroxidation may participate in the pathogenesis of tissue injury, including the brain injury induced by hypoxia or trauma and cardiac injury arising from ischemia and reperfusion. However, the exact cellular mechanisms by which the initial oxidative insult leads to the ultimate tissue damage are not known. A number of reports have indicated that protein kinase C (PKC) may be activated following oxidative stress and that this enzyme may play an important role in the steps leading to cellular damage. In this work, we have examined in a cell model whether PKC is activated following oxidative exposure. UC11MG cells, a human astrocytoma cell line, were treated with H2O2. Incubation with 0.5 mM H2O2 increased malondialdehyde levels by as early as 15 minutes. To assess the effects of H2O2 treatment on PKC activation, we measured phosphorylation of an endogenous PKC substrate, the MARCKS (myristoylated alanine-rich C kinase substrate) protein. Treatment of cells with 0.2-1.0 mM H2O2 resulted in a rapid increase in MARCKS phosphorylation. Phosphorylation was stimulated approximately 2.5-fold following treatment with 0.5 mM H2O2 for ten minutes. Treatment with phorbol 12-myristate 13-acetate, a PKC activator, increased MARCKS phosphorylation approximately 4-fold. The H2O2-induced MARCKS phosphorylation was inhibited by the addition of the kinase inhibitors H-7 and staurosporine. Furthermore, specific down-regulation of PKC by phorbol ester also inhibited H2O2-induced MARCKS phosphorylation. These results indicate that PKC is rapidly activated in cells following an oxidative exposure and that this cell system may be a good model to further investigate the role of PKC in regulating oxidative damage in the cell.

Prediction of clinical drug-drug interactions of veliparib (ABT-888) with human renal transporters (OAT1, OAT3, OCT2, MATE1, and MATE2K)

Veliparib (ABT-888) is largely eliminated as parent drug in human urine (70% of the dose). Renal unbound clearance exceeds glomerular filtration rate, suggesting the involvement of transporter-mediated active secretion. Clinically relevant pharmacokinetic interactions in the kidney have been associated with OAT1, OAT3, OCT2, MATE1, and MATE2K. In the present study, interactions of veliparib with these transporters were investigated. Veliparib inhibited OAT1, OAT3, OCT2, MATE1, and MATE2K with IC50 values of 1371, 505, 3913, 69.9, and 69.5 μM, respectively. The clinical unbound maximum plasma concentration of veliparib after single oral dose of 50 mg (0.45 μM) is manyfold lower than IC50 values for OAT1, OAT3, OCT2, MATE1, or MATE2K. These results indicate a low potential for drug-drug interaction (DDI) with OAT1/3, OCT2, or MATE1/2K. Additional studies demonstrated that veliparib is a substrate of OCT2. In Oct1/Oct2 double-knockout mice, the plasma exposure of veliparib was increased by 1.5-fold, and the renal clearance was decreased by 1.8-fold as compared with wild-type mice, demonstrating that organic cation transporters contribute to the renal elimination in vivo. In summary, the in vitro transporter data for veliparib predicts minimal potential for an OAT1/3-, OCT2-, and MATE1/2K-mediated DDI given the clinical exposure after single oral dose of 50 mg.

New antimitotic agents with activity in multi-drug-resistant cell lines and in vivo efficacy in murine tumor models

During a screen for compounds that could inhibit cell proliferation, a series of new tubulin-binding compounds was identified with the discovery of oxadiazoline 1 (A-105972). This compound showed good cytotoxic activity against non-multi-drug-resistant and multi-drug-resistant cancer cell lines, but its utility in vivo was limited by a short half-life. Medicinal chemistry efforts led to the discovery of indolyloxazoline 22g (A-259745), which maintained all of the in vitro activity seen with oxadiazoline 1, but also demonstrated a better pharmacokinetic profile, and dose-dependent in vivo activity. Over a 28 day study, indolyloxazoline 22g increased the life span of tumor-implanted mice by up to a factor of 3 upon oral dosing. This compound, and others of its structural class, may prove to be useful in the development of new chemotherapeutic agents to treat human cancers.

Potent and selective non-benzodioxole-containing endothelin-A receptor antagonists

The benzodioxole ((methylenedioxy)benzene) group is present in a number of endothelin (ET) receptor antagonists thus far reported. As part of our own endothelin antagonist program we have developed (2R*,3R*,4S*)-1-(N,N-dibutylacetamido)-4-(1,3-benzodioxol-5- yl)-2-(4-methoxyphenyl)pyrrolidine-3-carboxylic acid (A-127722). This is a potent antagonist, binding to the ETA and ETB receptor subtypes with affinities (IC50) of 0.4 and 520 nM, respectively, and also contains the aforementioned benzodioxole. While this compound was seemingly optimized at its N-terminus, no effort had been directed toward understanding the contributions to binding affinity or receptor subtype selectivity conferred by the benzodioxole. Substitution by 1- or 2-naphthyl yielded weak antagonists. Oxygenated benzenes, such as p-anisyl, were potent compounds with IC50s in the low-nanomolar range. Simple deletion of either of the two oxygen atoms (dihydrobenzofurans) yielded extremely potent agents, possessing subnanomolar affinity for the ETA receptor. Additionally, the compounds showed enhanced selectivity, binding to the ETB receptor subtype in the micromolar range. This paper describes the development of this novel class of compounds.

In vitro OATP1B1 and OATP1B3 inhibition is associated with observations of benign clinical unconjugated hyperbilirubinemia

1.  Transient benign unconjugated hyperbilirubinemia has been observed clinically with several drugs including indinavir, cyclosporine, and rifamycin SV. Genome-wide association studies have shown significant association of OATP1B1 and UGT1A1 with elevations of unconjugated bilirubin, and OATP1B1 inhibition data correlated with clinical unconjugated hyperbilirubinemia for several compounds. 2.  In this study, inhibition of OATP1B3 and UGT1A1, in addition to OATP1B1, was explored to determine whether one measure offers value over the other as a potential prospective tool to predict unconjugated hyperbilirubinemia. OATP1B1 and OATP1B3-mediated transport of bilirubin was confirmed and inhibition was determined for atazanavir, rifampicin, indinavir, amprenavir, cyclosporine, rifamycin SV and saquinavir. To investigate the intrinsic inhibition by the drugs, both in vivo Fi (fraction of intrinsic inhibition) and R-value (estimated maximum in vivo inhibition) for OATP1B1, OATP1B3 and UGT1A1 were calculated. 3.  The results indicated that in vivo Fi values >0.2 or R-values >1.5 for OATP1B1 or OATP1B3, but not UGT1A1, are associated with previously reported clinical cases of drug-induced unconjugated hyperbilirubinemia. 4.  In conclusion, inhibition of OATP1B1 and/or OATP1B3 along with predicted human pharmacokinetic data could be used pre-clinically to predict potential drug-induced benign unconjugated hyperbilirubinemia in the clinic.

Pharmacology of endothelin receptor antagonists ABT-627, ABT-546, A-182086 and A-192621: in vitro studies

Endothelins (ETs), 21-amino-acid peptides involved in the pathogenesis of various diseases, bind to ET(A) and ET(B) receptors to initiate their effects. Based on the same core structure, we have developed four small-molecule ET receptor antagonists, ABT-627, ABT-546, A-182086 and A-192621, which exhibit difference in selectivity for ET(A) and ET(B) receptors. In this report, we compare the potency and selectivity of these four antagonists in inhibiting (125)I-labelled ET-1 binding to cloned human ET(A) and ET(B) receptors, and in blocking ET-1-induced functional responses (arachidonic acid release and phosphatidylinositol hydrolysis).

Pyrrolidine-3-carboxylic acids as endothelin antagonists. 5. Highly selective, potent, and orally active ET(A) antagonists

The synthesis and structure-activity relationships (SAR) of a series of pyrrolidine-3-carboxylic acids as endothelin antagonists are described. The data shows an increase in selectivity when the methoxy of Atrasentan (ABT-627) is replaced with methyl, and the benzodioxole is replaced with dihydrobenzofuran. Adding a fluorine further increases the binding activity and provides a metabolically stable and orally bioavailable ET(A)-selective antagonist.

Endothelins and endothelin receptor antagonists: binding to plasma proteins

Endothelins (ET) are 21-amino acid peptides that bind to membrane receptors to initiate a wide range of pathophysiological effects. PD-156707, L-749329, Ro-470203, and A-127722 are potent non-peptide ET receptor antagonists developed recently. When tested in human and rat plasma, both ET-1 and -3 and the four aforementioned antagonists exhibited a high degree (> 98%) of plasma protein binding. When ET-1 binding to the receptors was examined, 5% (v/v) of human plasma inhibited ET-1 binding to both ETA and ETB receptors by 80 - 90%. Similarly, 5% (w/v) of human serum albumin inhibited ET-1 binding by 82%, suggesting that the major protein component in plasma which interfered with ET-1 binding to the receptors was serum albumin. Competition studies show that, in the absence of human serum albumin, the IC50 values of PD-156707, L-749329, Ro-47-0203, and A-127722 were 0.37, 0.29, 5.7, and 0.22 nM, respectively. Addition of increasing doses of human serum albumin incrementally decreased the potency of the antagonists; in the presence of 5% of human serum albumin, the IC50 values increased to 62.8, 50.2, 122.7, and 6.72 nM for PD-156707, L-749329, Ro-47-0203, and A-127722, respectively. In conclusion, ET and ET receptor antagonists exhibit a high degree of binding to plasma proteins, especially serum albumin. Consequently, serum albumin inhibits ET binding to its receptors, and also decreases the potency of ET receptor antagonists. Our findings may explain the discrepancy observed for ET receptor antagonists between in vitro and in vivo potencies.

Endothelin receptors in human smooth muscle cells: antagonist potency differs on agonist-evoked responses

The present study characterized endothelin (ET) receptors in human pericardium smooth muscle cells (HPSMC) and examined the potency of antagonists on ET-evoked signal transduction and DNA synthesis. HPSMC contain both ETA and ETB receptors. ET-1 binding was completely inhibited by a nonselective antagonist (Ro-46-2005) or a combination of ETA-selective and ETB-selective ligands (BQ-123 and ET-3). The molecular masses for ETA and ETB receptors were 69 and 42 kDa, respectively. ET-1, but not ET-3, stimulated phosphatidylinositol hydrolysis and arachidonic acid release in a dose- and time-dependent manner, reaching a plateau within 20-40 min. These immediate effects of ET-1 on signal transduction were completely inhibited by 1 microM, BQ-123, ET-1, but not ET-3, stimulated DNA synthesis in a dose-dependent manner, and the effect became prominent after 24 h. BQ-123 (1 microM) or Ro-46-2005 (10 microM) did not completely inhibit this mitogenic effect of ET-1. The reduced potency of BQ-123 and Ro-46-2005 on the delayed effect of ET-1 was not the result of ligand degradation or a difference in receptor internalization; rather, the decrease in potency was due to the fact that antagonist binding was more reversible than ET-1 binding.

Design, synthesis, and activity of a series of pyrrolidine-3-carboxylic acid-based, highly specific, orally active ET(B) antagonists containing a diphenylmethylamine acetamide side chain

The endothelin (ET)-B receptor subtype is expressed on vascular endothelial and smooth muscle cells and mediates both vasodilation and vasoconstriction. On the basis of the pharmacophore of the previously reported ET(A)-specific antagonist 1, (ABT-627), we are reporting the discovery of a novel series of highly specific, orally active ET(B) receptor antagonists. Replacing the dibutylaminoacetamide group of 1 with a diphenylmethylaminoacetamide group resulted in antagonist 2 with a complete reversal of receptor specificity. Structure-activity relationship studies revealed that ortho-alkylation of the phenyl rings could further increase ET(B) affinity and also boost the ET(A)/ET(B) activity ratio of the resulting antagonists. A similar antagonism selectivity profile could also be achieved when one of the phenyl rings of the acetamide side chain was replaced with an alkyl group, preferably a tert-butyl group (10h). Combining these features with modification of the 2-aryl group of the pyrrolidine core, we have identified a potent antagonist (9k, A-308165) with over 27 000-fold selectivity favoring the ET(B) receptor and an acceptable pharmacokinetic profile (F = 24%) in rats.

Endothelin receptor agonists and antagonists exhibit different dissociation characteristics

Endothelins (ETs) are vasoconstricting peptides that bind to membrane receptors to initiate their physiological effects. This report compares the dissociation characteristics of selected ET agonists and antagonists, and studies the effects of any difference in dissociation characteristics on the potency of antagonists. Competition studies using various ET receptor ligands against [125I]ET-1 or [125I]ET-3 binding demonstrated that porcine cerebellum membranes contain predominantly ETB receptor. [125I]IRL1620 associated with the receptors in a time-dependent manner. Although bound [125I]IRL1620 was easier to dissociate than bound [125I]ET-3, both agonists exhibited a dissociation half life > 20 h. For non-radiolabeled ligands, bind-and-wash studies were employed in which membranes were pre-incubated with unlabeled ligand followed by extensive washing before assaying for [125I]ET-1 binding. Results from bind-and-wash studies confirmed that bound non-radiolabeled IRL1620 and ET were as difficult to dissociate as [125I]ligands. In contrast, bound PD142893 and Ro46-2005 were easily dissociated from ETB receptors. Consequently, the inhibitory effects of PD142893 and Ro46-2005 on [125I]agonist binding diminished following incubation time. In cloned human ETA and ETB receptors, bound ET-1 was also more difficult to dissociate than bound antagonists. These results suggest that the differences in the dissociation characteristics of ET receptor agonists vs. antagonists may account for the diminished potency of Ro46-2005 and PD142893 as a function of incubation time.

A possible role for dephosphorylation in glucocorticoid receptor transformation

Addition of bovine intestinal alkaline phosphatase to mouse AtT-20 cell cytosol increases the rate of glucocorticoid receptor transformation, as evidenced by a change in sedimentation rate from 9.1S to 5.2S. Acid phosphatases are completely ineffective in this regard. Alkaline phosphatase-promoted receptor transformation is both time- and dose-dependent. A variety of phosphatase inhibitors are effective in inhibiting this process, the most potent being transition metal oxyanions such as molybdate, tungstate, and arsenate. The ability of the various inhibitors to suppress alkaline phosphatase-promoted receptor transformation does not correspond well with their potencies for inhibiting para-nitrophenyl phosphate hydrolysis. However, a better correspondence between the inhibition of endogenous receptor transformation and total cytosolic phosphatase activity is observed, and both sodium fluoride and glucose-1-phosphate inhibit endogenous receptor transformation. The protease inhibitors phenyl-methylsulfonyl fluoride and antipain have no effect on receptor transformation. Surprisingly, leupeptin is effective in inhibiting alkaline phosphatase-promoted receptor transformation. Although this raises the possibility of a contaminating protease activity in the alkaline phosphatase enzyme preparation, treatment of covalently affinity-labeled receptor with the enzyme shows no proteolysis of the receptor or any other non-specifically labeled cytosolic protein. Thus, it is possible that a novel action of leupeptin, unrelated to its protease-inhibitory activity, may be involved in the suppression of receptor transformation. The studies presented here suggest that dephosphorylation of some component in cytosol is involved in the destabilization of receptor subunit interactions, resulting in glucocorticoid receptor transformation.

Pyrrolidine-3-carboxylic acids as endothelin antagonists. 3. Discovery of a potent, 2-nonaryl, highly selective ETA antagonist (A-216546)

Previously we have reported the discovery of ABT-627 (1, A-147627, active enantiomer of A-127722), a 2,4-diaryl substituted pyrrolidine-3-carboxylic acid based endothelin receptor-A antagonist. This compound binds to the ETA receptor with an affinity (Ki) of 0. 034 nM and with a 2000-fold selectivity for the ETA receptor versus the ETB receptor. We have expanded our structure-activity studies in this series, in an attempt to further increase the ETA selectivity. When the p-anisyl group of 1 was replaced by an n-pentyl group, the resultant antagonist 3 exhibited substantially increased ETB/ETA activity ratio, but a decreased ETA affinity. Structure-activity studies revealed that substitution and geometry of this alkyl group, and substitution on the benzodioxolyl ring, are important in optimizing this series of highly ETA selective antagonists. In particular, the combination of a (E)-2,2-dimethyl-3-pentenyl group and a 7-methoxy-1,3-benzodioxol-5-yl group provided hydrophobic compound 10b with subnanomolar affinity for human ETA receptor subtype and with an ETB/ETA activity ratio of over 130000. Meanwhile, synthetic efforts en route to olefinic compounds led to the discovery that 2-pyridylethyl (9o) and 2-(2-oxopyrrolidinyl)ethyl (9u) replacement of the p-anisyl group of 1yielded very hydrophilic ETA antagonists with potency and selectivity equal to those of 10b. On the basis of overall superior affinity, high selectivity for the ETA receptor (Ki, 0.46 nM for ETA and 13000 nM for ETB), and good oral bioavailability (48% in rats), A-216546 (10a) was selected as a potential clinical backup for 1.

Endothelin receptor antagonists exhibit diminishing potency following incubation with agonist

Endothelins (ET) are 21-amino acid peptides that bind to membrane receptors to initiate a wide range of pathophysiological effects. ET binding to receptors has been shown to be almost irreversible because bound ET is difficult to dissociate. This report studies the dissociation characteristics of receptor antagonists and further examines the effects of ET's difficult-to-dissociate binding on the potency of antagonists. In membranes prepared from porcine cerebellum, [125I]ET-1 binding was effectively blocked by ET-1 and ET-3 with similar IC50 values (0.08 nM vs. 0.17 nM), suggesting that porcine cerebellum contains predominantly the ETB receptor subtype. [125I]ET-3 binding was inhibited by Ro46-2005 and PD142893, two non-selective antagonists, with IC50 values of 570 +/- 50 nM and 410 +/- 100 nM, respectively. Consistent with previous observations, bound [125I]ET-1 in porcine cerebellum membranes was also difficult to dissociate. In contrast, bound Ro46-2005 or PD142893, but not bound ET-1, could be readily washed away from membranes, suggesting that antagonist binding was more reversible than ET-1 binding. Although Ro46-2005 or PD142893 at 0.5 microM inhibited 0.1 nM [125I]ET-1 binding by > 80% after 15 min of incubation, the inhibitory effect decreased to approximately 50% after 3 h of incubation, and further decreased to < 10% at 24 h. This decrease in antagonizing potency was further confirmed by the results that the IC50 values of the two antagonists against [125I]ET-3 binding increased with increasing incubation time. Control experiments indicate that the observed decrease in the potency of Ro46-2005 and PD142893 was not the result of ligand degradation. These results suggest that the potency of antagonists is critically dependent on the incubation time because antagonist binding is more reversible than ET binding.

Pharmacology of A-216546: a highly selective antagonist for endothelin ET(A) receptor

Endothelins, 21-amino acid peptides involved in the pathogenesis of various diseases, bind to endothelin ET(A) and ET(B) receptors to initiate their effects. Here, we characterize the pharmacology of A-216546 ([2S-(2,2-dimethylpentyl)-4S-(7-methoxy-1,3-benzodioxol-5-yl )-1-(N,N-di(n-butyl) aminocarbonylmethyl)-pyrrolidine-3R-carboxylic acid), a potent antagonist with > 25,000-fold selectivity for the endothelin ET(A) receptor. A-216546 inhibited [125I]endothelin-1 binding to cloned human endothelin ET(A) and ET(B) receptors competitively with Ki of 0.46 and 13,000 nM, and blocked endothelin-1-induced arachidonic acid release and phosphatidylinositol hydrolysis with IC50 of 0.59 and 3 nM, respectively. In isolated vessels, A-216546 inhibited endothelin ET(A) receptor-mediated endothelin-1-induced vasoconstriction, and endothelin ET(B) receptor-mediated sarafotoxin 6c-induced vasoconstriction with pA2 of 8.29 and 4.57, respectively. A-216546 was orally available in rat, dog and monkey. In vivo, A-216546 dose-dependently blocked endothelin-1-induced pressor response in conscious rats. Maximal inhibition remained constant for at least 8 h after dosing. In conclusion, A-216546 is a potent, highly endothelin ET(A) receptor-selective and orally available antagonist, and will be useful for treating endothelin-1-mediated diseases.

Quantitative measurements of corticosteroids in ex vivo samples using on-line SPE-LC/MS/MS

Abnormal elevation of 11beta-HSD1 activities in tissues, such as fat and brain, may contribute to the development of the abdominal obesity and Alzheimer disease, and the inhibition of 11beta-HSD1 might be beneficial to the management of these diseases. To assess the effects of pharmacologic inhibitors of 11beta-HSD1, we developed a fast LC/MS/MS method to quantify corticosteroids in minced tissue samples in the presence of 11beta-HSD substrates. The novel on-line SPE-LC/MS/MS method was developed with dual binary gradient and a throughput of 4.5 min/sample. A total of six corticosteroids (cortisol, cortisone, corticosterone, dehydrocorticosterone, dexamethasone, and dehydrodexamethasone) were studied. The lower limit of quantitation from 0.40 to 11.4 fmol and 4.5 orders magnitude of dynamic range were obtained for these six compounds. Three novel enzymatic bi-products, all isomers of cortisol, were observed in the liver or fat samples. Two of them were identified by matching the HPLC retention times and MS/MS spectra with authentic compounds. The potential interferences of these isomers and their removal are discussed.

Discovery of a series of pyrrolidine-based endothelin receptor antagonists with enhanced ET(A) receptor selectivity

Endothelins, ET-1, ET-2, and ET-3 are potent vasoconstricting and mitogenic 21-amino acid bicyclic peptides, which exert their effects upon binding to the ET(A) and ET(B) receptors. The ET(A) receptor mediates vasoconstriction and smooth muscle cell proliferation, and the ET(B) receptor mediates different effects in different tissues, including nitric oxide release from endothelial cells, and vasoconstriction in certain vascular cell types. Selective antagonists of endothelin receptor subtypes may prove useful in determining the role of endothelin in various tissue types and disease states, and hence as therapeutic agents for such diseases. The pyrrolidine carboxylic acid A-127722 has been disclosed as a potent and ET(A)-selective antagonist, and is currently undergoing clinical trials. In our efforts to find antagonists with altered selectivity (ET(A)-selective, ET(B)-selective, or nonselective), we investigated the SAR of the 2-substituent on the pyrrolidine. Compounds with alkyl groups at the 2-position possessed ET(A) selectivity improved over A-127722 (1400-fold selective), with the best of these compounds showing nearly 19,000-fold selectivity.

'Irreversible' endothelin-1 binding does not prohibit ABT-627 from reversing endothelin-1-induced effects

Endothelin-1 (ET-1) is thought to play a role in a wide range of pathological conditions. One of the distinct characteristics of ET-1 is its long-lasting vasoconstrictor action, which is presumably caused by the irreversible binding of ET-1 to ET receptors and by the functional effects of internalized ET receptors. ABT-627 is a potent endothelin-A (ET(A))-selective antagonist with a Ki value at 0.034 nM for the human ET(A) receptor, and is currently being used in clinical studies for prostate cancer. Unlike ET-1, the binding of 125I-labeled ABT-627 to human ET(A) receptors expressed in Chinese hamster ovary (CHO) cells is reversible, and the dissociation half-life for the ligand/receptor complex is 2 h. Interestingly, the binding of both ET-1 and ABT-627 to the ET(A)-receptor results in partial receptor internalization but only ET-1 is capable of triggering intracellular functional responses. Although ABT-627 binding to membranes is more reversible than ET-1 binding, ABT-627 is able to reverse an ET-1-induced contraction in rat aortic rings in a dose-dependent manner, and at 1 microM produces nearly complete reversal of the constrictor effects of 10 nM ET-1 within 60 min. Similarly, in vivo studies show that ABT-627 (0.01 and 0.1 mg/kg/min i.v.) reverses the ET-1-induced increase in arterial pressure in anesthetized, ganglionic-blocked rats, and after 60 min, ABT-627 essentially normalizes pressure. Our data show that ABT-627 is capable of reversing an established response induced by ET-1 and is useful in reversing pathological conditions involving ET-1.

Dissociation characteristics of endothelin receptor agonists and antagonists in cloned human type-B endothelin receptor

The human type-B endothelin receptor (h-ETB) was cloned from human lung poly A+RNA and stably expressed in CHO cells. Endothelin (ET) receptor binding and stimulation of PI hydrolysis demonstrated that the cloned h-ETB receptor is functional and linked to intracellular signal transduction pathways in CHO cells. The molecular mass of the h-ETB receptor was determined to be 65 KDa, and Bmax and Kd were 0.36 pmol/mg and 80 pM, respectively. Competition studies employing receptor ligands revealed that the potencies of the test ligands (IRL1620, PD142893, and Ro46-2005) were dependent on the length of the incubation time, whereas the natural agonists (ET-1 and ET-3) were not. When competing with ET-1 in the h-ETB receptor binding, the IC50 increased from 1.2 nM to 8.2 nM for IRL1620, 0.068 microM to 1.9 microM for PD142893, and 0.76 microM to 12.7 microM for Ro46-2005, as the incubation time increased from 1 hr to 24 hr. These time-induced changes are likely due to differences in the dissociation characteristics between the artificial ligands and the natural ligands.

Internalization of type-A endothelin receptor

Endothelins (ETs) are 21 amino acid peptides which bind to ET(A)- and ET(B)-receptors to evoke diverse physiological responses. This report studies the internalization of ET(A)-receptor in Chinese hamster ovary (CHO) cells which were stably transfected with ET(A)-receptor cDNA. ET-1 binding induced ET(A) internalization in a time-dependent manner with 40% of ET(A)-receptors internalized at 37 degrees C after 30 min. To localize internalized ET(A)-receptor, cells were immunostained using a polyclonal antibody against the extracellular loop between IV and V transmembrane segments of the ET(A)-receptor. To examine the fate of internalized ET-1, cells were treated with 10 nM biotinylated ET-1 coupled with Texas Red-labeled streptavidin. In the absence of ET-1, a majority of ET(A) was localized on the surface of cells. After ET-1 treatment for 60 min, internalized ET(A)-receptors were localized in a perinuclear structure. ET-1 remained bound to ET(A)-receptor after internalization for up to 60 min and then dissociated from the receptor. After dissociation, ET-1 possibly became degraded and ET(A) recycled back to the cell surface. Protein kinase inhibitors such as KT5926 and staurosporine partially inhibited ET(A)-receptor internalization. The results of this study may facilitate the understanding of pathways involved in ET-1-induced receptor internalization.