Characterization of SR 121463A, a highly potent and selective, orally active vasopressin V2 receptor antagonist

Effects of YM218, a nonpeptide vasopressin V1A receptor-selective antagonist, on human vasopressin and oxytocin receptors

The binding and signal transduction characteristics of YM218 ((Z)-4'-{4,4-difluoro-5-[2-oxo-2-(4-piperidinopiperidino)ethylidene]-2,3,4,5-tetrahydro-1H-1-benzoazepine-1-carbonyl}-2-methyl-3-furanilide hemifumarate), a newly synthesized, potent arginine vasopressin (AVP) V(1A) receptor-selective antagonist, were examined using cloned human AVP receptors (V(1A), V(1B) and V(2)) stably expressed in Chinese hamster ovary (CHO) cells and human uterine smooth muscle cells (USMCs) expressing oxytocin receptors. YM218 potently inhibited specific binding of [(3)H] AVP to V(1A) receptors, exhibiting a K(i) value of 0.30 nM. In contrast, YM218 exhibited much lower affinity for V(1B), V(2) and oxytocin receptors, exhibiting K(i) values of 25,500 nM, 381 nM and 71.0 nM, respectively. In CHO cells expressing V(1A) receptors, YM218 potently inhibited the AVP-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), exhibiting an IC(50) value of 0.25 nM. However, in human USMCs expressing oxytocin receptors, YM218 exhibited a much lower potency in inhibiting the oxytocin-induced [Ca(2+)](i) increase, showing an IC(50) value of 607 nM, and had no effect on the AVP-induced [Ca(2+)](i) increase in CHO cells expressing V(1B) receptors. Furthermore, in CHO cells expressing V(2) receptors, YM218 did not potently inhibit the production of cAMP stimulated by AVP, showing an IC(50) value of 62.2 nM. In all assays used, YM218 did not exhibit any agonistic activity. These results demonstrate that YM218 is a potent, nonpeptide human V(1A) receptor-selective antagonist, and that YM218 will be a valuable new tool to gain further insight into the physiologic and pharmacologic actions of AVP.

Influence of enantiomers of 1-naphthylalanine in position 2 of VAVP and dVAVP on their pharmacological properties

In this study, we described the synthesis and some pharmacological properties of four new analogues of arginine vasopressin (AVP). Two peptides are substituted in position 2 with L-1-naphthylalanine (L-1-Nal) or its D-enantiomer and in position 4 with valine. In the further two compounds, we combined the above modifications with placement into position 1 of 3-mercaptopropionic acid residue (Mpa). All new peptides were tested for vasopressor and antidiuretic activities. We also estimated the uterotonic activities of these compounds in vitro. Urine samples prior and after peptide administration were analyzed for electrolytes excretion. All analogues are potent oxytocin antagonists. One of them, namely [L-1-Nal2,Val4]AVP, which appears practically not to interact with V1a and V2 receptors, is exceptionally selective. Our results open new possibilities for the design of very potent and selective oxytocin antagonists in vitro.

Pharmacologic properties of YM218, a novel, potent, nonpeptide vasopressin V1A receptor-selective antagonist

The pharmacologic profile of YM218, (Z)-4'-{4,4-difluoro-5-[2-oxo-2-(4-piperidinopiperidino)ethylidene]-2,3,4,5-tetrahydro-1H-1-benzoazepine-1-carbonyl}-2-methyl-3-furanilide hemifumarate, a newly synthesized, nonpeptide vasopressin (AVP) receptor antagonist, was investigated using several in vitro and in vivo methods. YM218 exhibited high affinity for V1A receptors isolated from rat liver, with a Ki value of 0.50 nM. In contrast, YM218 exhibited much lower affinity for rat pituitary V1B, kidney V2, and uterus oxytocin receptors, with Ki values of 1510 nM, 72.2 nM, and 150 nM, respectively. In vivo studies revealed that YM218 dose-dependently inhibited pressor response to exogenous AVP in pithed rats (intravenous) and in conscious normotensive rats (intravenous or oral) with a long duration of action (>8 h at 3 mg/kg, p.o.). In contrast, oral administration of YM218 did not increase urine excretion in conscious rats. These results demonstrate that YM218 is a potent nonpeptide AVP V1A receptor-selective antagonist that will be useful in future studies to help clarify the physiologic and pathophysiologic roles of AVP.

Intravenous administration of conivaptan hydrochloride improves cardiac hemodynamics in rats with myocardial infarction-induced congestive heart failure

We investigated the effects of intravenously administered conivaptan hydrochloride, a dual vasopressin V1A and V2 receptor antagonist, on cardiac function in rats with congestive heart failure following myocardial infarction, and compared results with those for the selective vasopressin V2 receptor antagonist SR121463A. Rats were subjected to left coronary artery occlusion to induce myocardial infarction, which in turn led to congestive heart failure. At 4 weeks after coronary occlusion, conivaptan (0.03, 0.1 and 0.3 mg/kg i.v.) dose-dependently increased urine volume and reduced urine osmolality in both myocardial infarction and sham-operated rats. SR121463A (0.3 mg/kg i.v.) also increased urine volume and decreased urine osmolality in myocardial infarction rats, to a degree comparable to that by conivaptan (0.3 mg/kg i.v.). At 6 weeks after surgery, myocardial infarction rats showed increases in right ventricular systolic pressure, right atrial pressure, left ventricular end-diastolic pressure and relative weights of the heart and the lungs, and a decrease in first derivative of left ventricular pressure (dP/dt(max))/left ventricular pressure, showing that congestive heart failure was well established. Conivaptan (0.3 mg/kg i.v.) significantly reduced right ventricular systolic pressure, left ventricular end-diastolic pressure, lung/body weight and right atrial pressure in myocardial infarction rats. Moreover, conivaptan (0.3 mg/kg i.v.) significantly increased dP/dt(max)/left ventricular pressure. SR121463A at a dose of 0.3 mg/kg i.v. significantly decreased left ventricular end-diastolic pressure and right atrial pressure, and tended to decrease right ventricular systolic pressure and relative lung weight in myocardial infarction rats. Although the aquaretic and preload-reducing effects of SR121463A were similar to those of conivaptan, SR121463A failed to improve dP/dt(max)/left ventricular pressure. These results suggest that dual vasopressin V1A and V2 receptor antagonists provide greater benefit than selective vasopressin V2 receptor antagonists in the treatment of congestive heart failure.

Key Amino Acids Located within the Transmembrane Domains 5 and 7 Account for the Pharmacological Specificity of the Human V1b Vasopressin Receptor

In mammals, the vasopressin V(1b) receptor (V(1b)-R) is known to regulate ACTH secretion and, more recently, stress and anxiety. The characterization of the molecular determinant responsible for its pharmacological selectivity was made possible by the recent discovery of the first V(1b) antagonist, SSR149415. Based upon the structure of the crystallized bovine rhodopsin, we established a three-dimensional molecular model of interaction between the human V(1b)-R (hV(1b)-R) and SSR149415. Four amino acids located in distinct transmembrane helices (fourth, fifth, and seventh) were found potentially responsible for the hV(1b)-R selectivity. To validate these assumptions, we selectively replaced the leucine 181, methionine 220, alanine 334, and serine 338 residues of hV(1a)-R by their corresponding amino acids present in the hV(1b)-R (phenylalanine 164, threonine 203, methionine 324, and asparagine 328, respectively). Four mutants, which all exhibited nanomolar affinities for vasopressin and good coupling to phospholipase C pathway, were generated. hV(1a) receptors mutated at position 220 and 334 exhibited striking increase in affinity for SSR149415 both in binding and phospholipase C assays at variance with the hV(1a)-R modified at position 181 or 338. In conclusion, this study provides the first structural features concerning the hV(1b)-R and highlights the role of few specific residues in its pharmacological selectivity.

SSR126768A (4-chloro-3-[(3R)-(+)-5-chloro-1-(2,4-dimethoxybenzyl)-3-methyl-2-oxo-2,3-dihydro-1H-indol-3-yl]-N-ethyl-N-(3-pyridylmethyl)-benzamide, hydrochloride): a new selective and orally active oxytocin receptor antagonist for the prevention of preterm labor

4-chloro-3-[(3R)-(+)-5-chloro-1-(2,4-dimethoxybenzyl)-3-methyl-2-oxo-2,3-dihydro-1H-indol-3-yl]-N-ethyl-N-(3-pyridylmethyl)benzamide, hydrochloride (SSR126768A), a new potent and selective, orally active oxytocin (OT) receptor antagonist was characterized in several biochemical and pharmacological models. In binding studies, SSR126768A showed nanomolar affinity for rat and human recombinant and native OT receptors (K(i) = 0.44 nM) and exhibited much lower affinity for V(1a), V(1b), and V(2) receptors. In addition, it did not interact with a large number of other receptors, enzymes, and ion channels (1 microM). In autoradiographic experiments performed on at-term human pregnant uterus sections, SSR126768A dose dependently displaced [I(125)]d(CH(2))(5)[Tyr(Me)(2), Thr(4), Orn(8) (125)I-Tyr-NH(2)(9)]VT in situ labeling to OT receptors highly expressed in these tissues. In functional studies, SSR126768A behaved as a full antagonist and potently antagonized OT-induced intracellular Ca(2+) increase (K(i) = 0.50 nM) and prostaglandin release (K(i) = 0.45 nM) in human uterine smooth muscle cells. In rat isolated myometrium, OT-induced uterine contractions were competitively antagonized by SSR126768A (pA(2) = 8.47). Similarly, in human pregnant myometrial strips, SSR126768A inhibited the contractile uterine response to OT. In conscious telemetrated rats, oral administration of SSR126768A (1-10 mg/kg) produced a competitive inhibition of the dose response to OT on uterine contractions up to 24 h at 3 mg/kg p.o.; no tachyphylaxis was observed after 4-day repeated treatment. Finally, SSR126768A (30 mg/kg p.o.) significantly delayed parturition in pregnant rats in labor similar to ritodrine (10 mg/kg p.o.). Thus, SSR126768A is a potent, highly selective, orally active OT receptor antagonist with a long duration of action. This molecule could find therapeutic application as a tocolytic agent for acute and chronic oral management of preterm labor.

Disorders of body water homeostasis

Disorders of body fluids are among the most commonly encountered problems in the practice of clinical medicine. This is in large part because many different disease states can potentially disrupt the finely balanced mechanisms that control the intake and output of water and solute. It therefore behoves clinicians treating such patients to have a good understanding of the pathophysiology, the differential diagnosis and the management of these disorders. Because body water is the primary determinant of the osmolality of the extracellular fluid, disorders of body water homeostasis can be divided into hypo-osmolar disorders, in which there is an excess of body water relative to body solute, and hyperosmolar disorders, in which there is a deficiency of body water relative to body solute. The classical hyperosmolar disorder is diabetes insipidus (DI), and the classical hypo-osmolar disorder is the syndrome of inappropriate antidiuretic hormone secretion (SIADH). This chapter first reviews the regulatory mechanisms underlying water and sodium metabolism, the two major determinants of body fluid homeostasis. The major disorders of water metabolism causing hyperosmolality and hypo-osmolality, DI and SIADH, are then discussed in detail, including the pathogenesis, differential diagnosis and treatment of these disorders.

Vasopressin-independent renal urinary concentration: increased rBSC1 and enhanced countercurrent multiplication

BACKGROUND

A close association between the expression of the sodium transporter, rat bumetanide sensitive cotransporter (rBSC), in thick ascending limb of Henle and urinary concentration has been reported. However, direct evidence for this association and the mechanism of rBSC1 expression are still to be elucidated.

METHODS

Brattleboro (BB) rats weighing approximately 200 g were dehydrated by water restriction for 4 hours, which induced around a 5% body weight reduction. Although plasma arginine vasopressin (AVP) was undetectable even after the water restriction, BB rats concentrated urine from 182 +/- 23 (mean +/- SD) at baseline to 404 +/- 65 mOsm/kg. H2O.

RESULTS

Urinary volume was reduced from 5.8 +/- 1.8 to 1.4 +/- 0.6 mL/h. This treatment significantly increased sodium and urea accumulation in the renal medulla and reduced urinary sodium excretion. rBSC1 signals for both mRNA and protein were increased in dehydrated rats, although aquaporin type 2 (AQP2) expression was not enhanced in dehydrated BB rats. Subcutaneous infusion of desmopressin acetate (DDAVP) intensified rBSC1 signals of BB rats more than those in dehydrated condition.

CONCLUSION

Dehydration increased rBSC1 expression and enhanced countercurrent multiplication even in AVP deficiency. These results supply strong evidence for the association between rBSC1 expression and urinary concentration, and indicate the presence of an AVP-independent mechanism for urine concentration.

Pancreatic vasopressin V1b receptors: characterization in In-R1-G9 cells and localization in human pancreas

Vasopressin (AVP) receptors present in In-R1-G9 cells, a hamster glucagon-secreting alpha-pancreatic cell line, were characterized using SSR-149415, a selective nonpeptide V1b receptor antagonist, and reference AVP compounds. Binding experiments, using [3H]AVP as a ligand, identified a single population of high-affinity binding sites. SSR-149415 competitively inhibited this binding and exhibited nanomolar and stereospecific affinity for these sites. The affinity of various AVP/oxytocin ligands confirmed a V1b binding profile. In functional studies, AVP was a potent stimulant in inducing intracellular Ca2+ increase, glucagon secretion, and cell proliferation. These effects were fully antagonized by SSR-149415 with a nanomolar potency, whereas its diasteroisomer as well as two selective V1a and V2 receptor antagonists were much less potent. Additionally, the order of potency of AVP agonists and antagonists was in agreement with V1b-mediated effects. By RT-PCR, we confirmed the presence of V1b receptor mRNA in both In-R1-G9 cells and in human pancreas. The distribution pattern of V1b receptors investigated in human pancreas by immunohistochemistry showed strong labeling in islets of Langerhans, and colocalization studies indicated that this receptor was expressed in alpha-glucagon, beta-insulin, and somatostatin pancreatic cells. Thus, in In-R1-G9 cells, AVP mediates intracellular Ca2+ increase, glucagon secretion, and cell proliferation by activating V1b receptors, and these effects are potently antagonized by SSR-149415. Moreover, the presence of V1b receptors also found in human Langerhans islets could suggest hormonal control of AVP in human pancreas.

Distúrbios na secreção e ação do hormônio antidiurético

A manutenção da constância da osmolalidade plasmática e do equilíbrio hidroeletrolítico deve-se à regulação do volume extracelular e da natremia, através da integração entre as ações do hormônio antidiurético (ADH), o sistema renina-angiotensina-aldosterona (SRAA) e o mecanismo da sede. Distúrbios na síntese, secreção ou ação do ADH podem resultar em síndromes poliúricas, em que ocorre excreção aumentada de urina hipotônica, resultante da ingestão excessiva de água, secreção ou ação inadequadas do ADH ou alterações nos canais de água da aquaporina-2 (AQP2). A hiponatremia pode ocorrer por depleção de sal, mecanismos dilucionais ou metabólicos, além de ser uma freqüente ocorrência após a cirurgia hipofisária, tendo sido descrita em 9 a 35% de pacientes operados. A causa pode ser devida a uma lesão transitória ou definitiva da hipófise posterior, a síndrome de secreção inapropriada do hormônio antidiurético (SIADH) ou, mais freqüentemente, a síndrome cerebral perdedora de sal (SCPS). A apresentação clínica de ambas as síndromes é similar e o diagnóstico diferencial pode apresentar dificuldades. A determinação do estado volêmico é essencial para o diagnóstico, já que os pacientes portadores de SIADH caracterizam-se por serem euvolêmicos ou hipervolêmicos, enquanto aqueles com a SCPS são hipovolêmicos. Os critérios para o diagnóstico incluem parâmetros clínicos, determinação da osmolalidade plasmática e urinária e os testes de restrição hídrica, sobrecarga hipertônica e o da furosemida, importante na discriminação entre SIADH e SCPS. O tratamento das síndromes poliúricas depende da etiologia e inclui o uso de análogos do ADH, diuréticos, ou outras drogas tais como clorpropramida, clofibrato, corticóides e carbamazepina. O tratamento da SIADH necessita de restrição de líqüidos e/ou furosemida para diminuir o volume de água extracelular. O tratamento da SCPS, ao contrário, implica em reposição de volume com fornecimento de suplementação de sódio e líqüidos, sendo que a fludrocortisona pode ser uma boa alternativa terapêutica.

Vasopressin: a new target for the treatment of heart failure

BACKGROUND

Arginine vasopressin is a peptide hormone that modulates a number of processes implicated in the pathogenesis of heart failure. Numerous vasopressin antagonists are currently under development for the treatment of this syndrome.

METHODS

Preclinical and clinical data describing the effects of vasopressin and the vasopressin antagonists on both normal physiology and heart failure were reviewed.

RESULTS

Through activation of V(1a) and V(2) receptors, vasopressin regulates various physiological processes including body fluid regulation, vascular tone regulation, and cardiovascular contractility. Vasopressin synthesis is significantly and chronically elevated in patients with heart failure despite the volume overload and reductions in plasma osmolality often observed in these patients. Vasopressin also appears to adversely effect hemodynamics and cardiac remodeling, while potentiating the effects of norepinephrine and angiotensin II. The selective V(2) and dual V(1a)/V(2) receptor antagonists tolvaptan and conivaptan, respectively, substantially increase free water excretion and plasma osmolality, reduce body weight, improve symptoms of congestion, and moderately increase serum sodium concentrations in patients with heart failure who present with symptoms of fluid overload. Tolvaptan effectively normalizes serum sodium concentrations in hyponatremic heart failure patients. Conivaptan significantly reduces pulmonary capillary wedge pressure without affecting systemic vascular resistance or cardiac output. The clinical significance of V(1a) receptor antagonism requires further investigation.

CONCLUSIONS

Current preclinical and clinical findings with the vasopressin antagonists appear promising, however further evaluation in phase III clinical trials is necessary to define the role of vasopressin antagonism in the treatment of heart failure.

Vasopressin-induced facilitation of adrenergic responses in the rat mesenteric artery is V1-receptor dependent

1. The present study was designed to analyse the possible involvement of V1- and V2-receptors in vasopressin (AVP)-induced facilitation of the sympathetic nervous system. Furthermore, we aimed to determine whether the site of facilitation by AVP is located pre- or postsynaptically. 2. Electrical field stimulation (EFS) was applied on the rat mesteric artery to activate the sympathetic nervous system. In addition, we evaluated the direct vascular effects of AVP. The postsynaptic effect of AVP on the sympathetic nervous system was investigated by exposing the vessels to exogenous noradrenaline. These experiments were performed in the absence or presence of selective V1 and V2 receptor antagonists SR 49059 and SR 121463, respectively. Desmopressin was applied as a selective V2 agonist. 3. The direct vasoconstrictor effect of AVP was antagonized by SR 49059 and not by SR 121463. Desmopressin neither showed any direct vasoconstrictor effect nor produced vasodilatation after a precontraction induced by noradrenaline (10 microM). The EFS-induced rise in vascular tone could be increased by a sub-pressor concentration of AVP. This fascilitation could be antagonized by SR 49059, but not by SR 121463. Desmopressin did not influence the increase in vascular tone during EFS. Vasoconstriction induced by exogenous noradrenaline could be facilitated by a sub-pressor concentration of AVP and this selective postsynaptic effect could be antagonized by V1-receptor blockade. 4. In conclusion, the AVP-induced facilitation of the sympathetic nervous system is completely V1-receptor dependent and at least partly postsynaptically mediated.

Cyclic AMP is sufficient for triggering the exocytic recruitment of aquaporin-2 in renal epithelial cells

The initial response of renal epithelial cells to the antidiuretic hormone arginine vasopressin (AVP) is an increase in cyclic AMP. By applying immunofluorescence, cell membrane capacitance and transepithelial water flux measurements we show that cAMP alone is sufficient to elicit the antidiuretic cellular response in primary cultured epithelial cells from renal inner medulla, namely the transport of aquaporin-2 (AQP2)-bearing vesicles to, and their subsequent fusion with, the plasma membrane (AQP2 shuttle). The AQP2 shuttle is evoked neither by AVP-independent Ca(2+) increases nor by AVP-induced Ca(2+) increases. However, clamping cytosolic Ca(2+) concentrations below resting levels at 25 nM inhibited exocytosis. Exocytosis was confined to a slow monophasic response, and readily releasable vesicles were missing. Analysis of endocytic capacitance steps revealed that cAMP does not decelerate the retrieval of AQP2 from the plasma membrane. Our data suggest that cAMP initiates an early step, namely the transport of AQP2-bearing vesicles towards the plasma membrane, and do not support a regulatory function for Ca(2+) in the AQP2 shuttle.

Expression of human vasopressin and oxytocin receptors in Escherichia coli

In order to produce large amounts of human vasopressin and oxytocin receptors compatible with direct structural biology approaches such as X-ray crystallography, NMR or mass spectrometry, we have expressed these neurohypophysial hormone receptors in Escherichia coli. To facilitate the level of expression, the coding sequence for the V1a vasopressin receptor and the oxytocin receptor were first optimized for bacterial expression. The resulting 'bacterial receptor cDNAs' were then subcloned into pET/T7-driven prokaryotic expression vectors. Different constructs have been prepared: each cDNA was incorporated alone or in fusion with a T7 tag sequence or a glutathione-S-transferase tag sequence at the N-terminus end. Moreover, a 6 x His tag sequence has been added at the C-terminus end for one-step purification of the receptors. Screening of BL21(DE3) and BL21(DE3)pLysS bacterial strains transformed with the different constructions was achieved by Coomassie blue-stained SDS-polyacrylamide gels and by 6 x His antibody Western blotting. Several clones were selected for purification of the receptors. Expression levels of the receptors are now encouraging and will be optimized for further structural and functional studies. Moreover, at the same time, the construction of the bacterial-optimized sequence of the V2 vasopressin receptor and its expression will be performed.

Nonpeptide vasopressin receptor antagonists: development of selective and orally active V1a, V2 and V1b receptor ligands

The involvement of vasopressin (AVP) in several pathological states has been reported recently and the selective blockade of the different AVP receptors could offer new clinical perspectives. During the past few years, various selective, orally active AVP V1a (OPC-21268, SR49059 (Relcovaptan)), V2 (OPC-31260, OPC-41061 (Tolvaptan), VPA-985 (Lixivaptan), SR121463, VP-343, FR-161282) and mixed V1a/V2 (YM-087 (Conivaptan), JTV-605, CL-385004) receptor antagonists have been intensively studied in various animal models and have reached, Phase IIb clinical trials for some of them. For many years now, our laboratory has focused on the identification of nonpeptide vasopressin antagonists with suitable oral bioavailability. Using random screening on small molecule libraries, followed by rational SAR and modelization, we identified a chemical series of 1-phenylsulfonylindolines which first yielded SR49059, a V1a receptor antagonist prototype. This compound displayed high affinity for animal and human V1a receptors and antagonized various V1a AVP-induced effects in vitro and in vivo (intracellular [Ca2+] increase, platelet aggregation, vascular smooth muscle cell proliferation, hypertension and coronary vasospasm). We and others have used this compound to study the role of AVP in various animal models. Recent findings from clinical trials show a potential interest for SR49059 in the treatment of dysmenorrhea and in Raynaud's disease. Structural modifications and simplifications performed in the SR49059 chemical series yielded highly specific V2 receptor antagonists (N-arylsulfonyl-oxindoles), amongst them SR121463 which possesses powerful oral aquaretic properties in various animal species and in man. SR121463 is well-tolerated and dose-dependently increases urine output and decreases urine osmolality. It induces free water-excretion without affecting electrolyte balance in contrast to classical diuretics (e.g. furosemide and hydrochlorothiazide). Notably, in cirrhotic rats with ascites and impaired renal function, a 10-day oral treatment with SR121463 (0.5 mg/kg) totally corrected hyponatremia and restored normal urine excretion. This compound also displayed interesting new properties in a rabbit model of ocular hypertension, decreasing intraocular pressure after single or repeated instillation. Thus, V2 receptor blockade could be of interest in several water-retaining diseases such as the syndrome of inappropriate antidiuretic hormone secretion (SIADH), liver cirrhosis and congestive heart failure and deserves to be widely explored. Finally, further chemical developments in the oxindole family have led to the first specific and orally active V1b receptor antagonists (with SSR149415 as a representative), an awaited class of drugs with expected therapeutic interest mainly in ACTH-secreting tumors and various emotional diseases such as stress-related disorders, anxiety and depression. However, from the recently described tissue localization for this receptor, we could also speculate on other unexpected uses. In conclusion, the development of AVP receptor antagonists is a field of intensive pharmacological and clinical investigation. Selective and orally active compounds are now available to give new insight into the pathophysiological role of AVP and to provide promising drugs.

Pharmacodynamic effects of a nonpeptide antidiuretic hormone V2 antagonist in cirrhotic patients with ascites

Water retention and dilutional hyponatremia, mainly attributable to an impairment of free water excretion and increased vasopressin activity, are well-documented complications in cirrhotic patients with ascites. VPA-985 is a selective, nonpeptide, orally active, vasopressin-2-receptor antagonist. The aim of this study was to determine the pharmacodynamics, safety, and pharmacokinetics of ascending single doses (25, 50, 100, 200, and 300 mg) in cirrhotic patients with ascites in a randomized, double-blind, placebo-controlled trial. Each dose level was studied in 5 patients (4 active and 1 placebo). After an overnight fast and fluid restriction (continued for 4 hours after dose administration), all patients were given placebo on baseline day and an oral suspension of VPA or placebo on the following day. VPA produced a significant dose-related increase in daily urine output (1,454 +/- 858 mL to 4,568 +/- 4,385 mL with VPA 300 mg) and a dose-related decrease in urine osmolality. The free water clearance reached greater than 3 mL/min for doses 100 mg or greater. Simultaneously, significant increases in serum osmolality, sodium, and vasopressin levels were found. There was a significant increase in sodium urine excretion. VPA was rapidly absorbed and maximum serum concentrations were achieved within 1 hour after administration. Elimination half-life ranged from 9.0 hours after 100 mg to 22.6 hours after 200 mg. In conclusion, VPA induced a dose-related aquaretic response, suggesting a therapeutic potential in managing water retention in patients with liver cirrhosis with ascites.

Effect of YM471, a nonpeptide AVP receptor antagonist, on human coronary artery smooth muscle cells

The antagonistic properties of YM471, a potent nonpeptide vasopressin (AVP) V(1A) and V(2) receptor antagonist, were characterized using human coronary artery smooth muscle cells (CASMC). YM471 potently inhibited specific binding of 3H-AVP to V(1A) receptors on human CASMC, exhibiting a K(i) value of 0.49 nM. Furthermore, YM471 inhibited the AVP-induced increase in intracellular free Ca(2+) concentration with an IC(50) value of 1.42 nM, but exerted no agonistic activity on CASMC. Additionally, while AVP concentration-dependently induced hyperplasia and hypertrophy in CASMC, YM471 prevented these AVP-induced growth effects, exhibiting IC(50) values of 0.93 and 2.64 nM, respectively. These results indicate that YM471 has high affinity for V(1A) receptors on, and high potency in inhibiting AVP-induced physiologic responses of, human CASMC.

Arginine vasopressin modulates expression of neuronal NOS in rat renal medulla

Arginine vasopressin (AVP) plays a central role in water balance. In principal cells of the collecting duct system, AVP controls the expression of several genes, including aquaporin-2. Because nitric oxide (NO) participates in the regulation of water reabsorption by the collecting duct system, we analyzed the effect of AVP on the expression of NO synthase (NOS) isoforms in the kidney. Rats were either water restricted or water loaded to modify the circulating AVP levels, and expressions of NOS isoforms were assessed by Western blot analysis. In water-restricted rats, endothelial NOS (eNOS) expression increased in the outer medulla, and neuronal NOS (nNOS) expression rose in both the outer medulla and the papilla. Conversely, water loading induced a decrease in expression of nNOS in the outer medulla and papilla but did not alter eNOS expression. Oral administration of the specific V(2)-receptor antagonist SR-121463B decreased nNOS expression in the outer medulla and papilla but did not alter eNOS expression levels. Finally, the very low nNOS expression levels observed in AVP-deficient Brattleboro rats was restored by AVP infusion for 1 wk. Thus AVP specifically increases nNOS expression levels in the renal outer medulla and papilla. Because nNOS is specifically expressed in principal cells of the collecting duct system, the stimulation of nNOS expression by AVP may participate in the control of water reabsorption.

Pharmacological characterization of YM471, a novel potent vasopressin V(1A) and V(2) receptor antagonist

The pharmacologic profile of YM471 ((Z)-4'-[4,4-difluoro-5-[2-(4-dimethylaminopiperidino)-2-oxoethylidene]-2,3,4,5-tetrahydro-1H-1-benzoazepine-1-carbonyl]-2-phenylbenzanilide monohydrochloride), a novel potent vasopressin V(1A) and V(2) receptor antagonist, was investigated using several in vitro and in vivo techniques. YM471 showed high affinity for rat vasopressin V(1A) and V(2) receptors, exhibiting K(i) values of 0.16 and 0.77 nM, respectively. In contrast, YM471 exhibited much lower affinity for rat vasopressin V(1B) and oxytocin receptors, with K(i) values of 10.5 microM and 31.0 nM, respectively. In conscious rats, oral administration of YM471 (0.1-3.0 mg/kg) produced dose-dependent inhibition of the pressor response caused by exogenous vasopressin and increased urine excretion and decreased urine osmolality; this effect lasted more than 8 h. In all biological assays used, YM471 exhibited no agonistic activity. These results demonstrate that YM471 exerts potent and long-lasting antagonistic activity on both vasopressin V(1A) and V(2) receptors, and that this compound may be a useful tool for clarifying the physiologic and pathophysiologic roles of vasopressin and the therapeutic usefulness of the vasopressin receptor antagonist.

Vasopressin-induced presynaptic facilitation of sympathetic neurotransmission in the pithed rat

OBJECTIVE

Several studies have shown that arginine vasopressin (AVP) potentiates the sympathetic nervous transmission in isolated vessels. The present study investigates such a potentiation in the pithed rat model.

METHODS

Male Wistar rats weighing 270-310 g were used. Spinal-cord stimulation was applied, with frequencies of 0.25-4 Hz, in the presence or absence of a subpressor dose of intravenous (i.v.) AVP (1 pmol/kg per min). In addition, the effect of AVP on postsynaptic alpha-adrenoceptor-mediated responses was studied using exogenously administered noradrenaline (NA). For this purpose dose-response curves (DRCs) for NA (i.v.) were constructed.

RESULTS

In the pithed rat model endogenously generated angiotensin II facilitates neurally mediated increments in vascular resistance. Without the administration of the angiotensin II type 1 (AT1) antagonist, irbesartan, the facilitating effect of AVP was not visible. However, after the administration of the AT1 antagonist, irbesartan, the facilitating effect of AVP became apparent. The stimulation-induced rise in diastolic blood pressure (DBP) was enhanced in the presence of AVP from 63.7 +/- 4.5 to 78.6 +/- 4.2 mmHg, at a stimulation frequency of 4 Hz. The vasopressin receptor V1 antagonist, SR-49059, completely inhibited this AVP-induced facilitation, whereas the V2 antagonist, SR-121463B, or the V2 agonist, desmopressin, did not. The DRC of exogenously administered NA was not influenced by AVP.

CONCLUSION

The stimulating effect of AVP on sympathetic neurotransmission is completely dependent on the stimulation of presynaptically located V1 receptors. The facilitating effect of angiotensin II on the sympathetic nervous system (SNS) in the pithed rat model masks the facilitating effect of AVP in this preparation.

Pharmacological characterization of F-180: a selective human V(1a) vasopressin receptor agonist of high affinity

1. The pharmacological properties of F-180, a vasopressin (VP) structural analogue, were determined on CHO cells expressing the different human vasopressin and oxytocin (OT) receptor subtypes. Binding experiments revealed that F-180 exhibited a high affinity for the human V(1a) receptor subtype (K(i)=11 nM) and was selective for this receptor subtype. 2. Functional studies performed on CHO cells expressing human V(1a) receptors indicate that similarly to AVP, F-180 can stimulate the accumulation of inositol phosphate. The activation constant (K(act)) for both F-180 and AVP was 1.7 nM. F-180 was also an agonist for the human V(2) and V(1b) receptor subtypes and an antagonist for the human OT receptor. 3. Since marked species pharmacological differences for vasopressin receptors have been described, we studied the properties of F-180 on various mammalian species. F-180 showed high affinity and good selectivity for human and bovine V(1a) receptors, but weak affinity and non selective properties for rat V(1a) receptors. 4. To assess the functional properties of F-180 on a native biological model, we performed studies on primary cultures of cells from bovine zona fasciculata (ZF). As AVP, F-180 stimulated inositol phosphate accumulation and cortisol secretion with similar efficiency. 5. In conclusion, we demonstrate that F-180 is the first selective V(1a) agonist described for human and bovine vasopressin receptors. Therefore F-180 can be used as a powerful pharmacological tool to characterize the actions of vasopressin that are mediated by V(1a) receptor subtypes.

Long term regulation of aquaporin-2 expression in vasopressin-responsive renal collecting duct principal cells

Fine regulation of water reabsorption by the antidiuretic hormone [8-arginine]vasopressin (AVP) occurs in principal cells of the collecting duct and is largely dependent on regulation of the aquaporin-2 (AQP2) water channel. AVP-inducible long term AQP2 expression was investigated in immortalized mouse cortical collecting duct principal cells. Combined RNase protection assay, Western blot, and immunofluorescence analyses revealed that physiological concentrations of AVP added to the basal side, but not to the apical side, of cells grown on filters induced both AQP2 mRNA and apical protein expression. The stimulatory effect of AVP on AQP2 expression followed a V(2) receptor-dependent pathway because [deamino-8-d-arginine]vasopressin (dDAVP), a specific V(2) receptor agonist, produced the same effect as AVP, whereas the V(2) antagonist SR121463B antagonized action of both AVP and dDAVP. Moreover, forskolin and cyclic 8-bromo-AMP fully reproduced the effects of AVP on AQP2 expression. Analysis of protein degradation pathways showed that inhibition of proteasomal activity prevented synthesis of AVP-inducible AQP2 mRNA and protein. Once synthesized, AQP2 protein was quickly degraded, a process that involves both the proteasomal and lysosomal pathways. This is the first study that delineates induction and degradation mechanisms of AQP2 endogenously expressed by a renal collecting duct principal cell line.

Characterization of (2S,4R)-1-[5-chloro-1-[(2,4-dimethoxyphenyl)sulfonyl]-3-(2-methoxy-phenyl)-2-oxo-2,3-dihydro-1H-indol-3-yl]-4-hydroxy-N,N-dimethyl-2-pyrrolidine carboxamide (SSR149415), a selective and orally active vasopressin V1b receptor antagonist

(2S,4R)-1-[5-Chloro-1-[(2,4-dimethoxyphenyl)sulfonyl]-3-(2-methoxy-phenyl)-2-oxo-2,3-dihydro-1H-indol-3-yl]-4-hydroxy-N,N-dimethyl-2-pyrrolidine carboxamide (SSR149415), the first selective, nonpeptide vasopressin V1b receptor antagonist yet described, has been characterized in vitro and in vivo. SSR149415 showed competitive nanomolar affinity for animal and human V1b receptors and exhibited much lower affinity for rat and human V1a, V2, and oxytocin receptors. Moreover, this compound did not interact with a large number of other receptors, enzymes, or ion channels. In vitro, SSR149415 behaved as a full antagonist and potently inhibited arginine vasopressin (AVP)-induced Ca2+ increase in Chinese hamster ovary cells expressing rat or human V1b receptors. The in vivo activity of SSR149415 has been studied in several models of elevated corticotropin secretion in conscious rats. SSR149415 inhibited exogenous AVP-induced increase in plasma corticotropin, from 3 mg/kg i.p. and 10 mg/kg p.o. upwards. Similarly, this compound antagonized AVP-potentiated corticotropin release provoked by exogenous corticoliberin at 3 mg/kg p.o. The effect lasted for more than 4 h at 10 mg/kg p.o. showing a long-lasting oral effect. SSR149415 (10 mg/kg p.o.) also blocked corticotropin secretion induced by endogenous AVP increase subsequent to body water loss. Moreover, 10 mg/kg i.p SSR149415 inhibited plasma corticotropin elevation after restraint-stress in rats by 50%. In the four-plate test, a mouse model of anxiety, SSR149415 (3 mg/kg p.o. upwards) displayed anxiolytic-like activity after acute and 7-day repeated administrations. Thus, SSR149415 is a potent, selective, and orally active V1b receptor antagonist. It represents a unique tool for exploring the functional role of V1b receptors and deserves to be clinically investigated in the field of stress and anxiety.

The effects of several vasopressin receptor antagonists on normal intraocular pressure and the intraocular distribution of vasopressin receptor subtypes

The aim of the present paper is to study the relation between vasopressin antagonism and the regulation of intraocular pressure (IOP). From the studies on the effect of several vasopressin receptor antagonists, VP-343, OPC-21268, YM087, OPC-31260 and SR121463, on normal IOP and the effect of VP-343 on pupil diameter in rabbit, it was shown that some vasopressin antagonists decreased normal IOP and VP-343 had no influence on pupil diameter. A vasopressin receptor mapping study in normal cynomolgus monkey eye revealed a high density binding site for a [H3]vasopressin V1 antagonist in the region of iris. These findings suggest that a vasopressin antagonist should decrease normal IOP without miosis and that vasopressin V1 receptors are present in iris.

An overview of SR121463, a selective non-peptide vasopressin V(2) receptor antagonist

SR121463 is a selective, orally active, non-peptide antagonist of vasopressin (AVP) V(2) receptors with powerful aquaretic properties in various animal species and humans. SR121463 belongs to a new class of drugs, called aquaretics, which are capable of inducing free-water excretion without affecting electrolyte balance. SR121463 displays high affinity for animal and human V(2) receptors and exhibits a remarkably selective V(2) receptor profile. SR121463 and [(3)H]SR121463 are used, therefore, as selective probes for characterization and labeling of V(2) receptors. In various functional studies in vitro, SR121463 behaves as a potent antagonist. It inhibits AVP-stimulated human renal adenylyl cyclase and dDAVP (1-desamino, 8-D arginine-vasopressin)-induced relaxation of rat aorta. SR121463 also behaves as an inverse agonist in cells expressing a constitutively activated human V(2) receptor mutant. In vitro, SR121463 rescued misfolded V(2) AVP receptor mutants by increasing cell surface expression and restoring V(2) function. In normally hydrated conscious rats, dogs and monkeys, SR121463, by either i.v. or p.o. administration, induced a dose-dependent aquaresis with no major changes in urinary Na+ and K+ excretion (unlike classical diuretics). In cirrhotic rats with ascites and impaired renal function, a 10-day treatment with SR121463 totally corrected hyponatremia and restored normal urine excretion. In a model of diabetic nephropathy in rats, SR121463 strongly reduced albumin excretion. SR121463 was also effective at extrarenal V(2) (or V(2)-like) receptors involved in vascular relaxation or clotting factor release in vitro and in vivo. In the rabbit model of ocular hypertension, SR121463 by either single or repeated instillation, decreased intraocular pressure. After acute and chronic administration to rats, dogs or healthy human volunteers, SR121463 was well absorbed and well tolerated. In all species studied the drug produced pronounced aquaresis without any agonist effect. Thus, SR121463 is a potent, orally active and selective antagonist at V(2) receptors with powerful aquaretic properties. It is a useful tool for further exploration of function of renal or extrarenal V(2) receptors. Pure V(2) receptor antagonists are likely to be therapeutically useful in several water-retaining diseases such as hyponatremia, Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH), congestive heart failure, liver cirrhosis, and other disorders possibly mediated by V(2) receptors (e.g., glaucoma).

Molecular modeling of interactions of the non-peptide antagonist YM087 with the human vasopressin V1a, V2 receptors and with oxytocin receptors

The nonapeptide hormones arginine vasopressin (CYFQNCPRG-NH2, AVP) and oxytocin (CYIQNCPLG-NH2, OT), control many essential functions in mammals. Their main activities include the urine concentration (via stimulation of AVP V2 receptors, V2R, in the kidneys), blood pressure regulation (via stimulation of vascular V1a AVP receptors, V1aR), ACTH control (via stimulation of V1b receptors, V1bR, in the pituitary) and labor and lactation control (via stimulation of OT receptors, OTR, in the uterus and nipples, respectively). All four receptor subtypes belong to the GTP-binding (G) protein-coupled receptor (GPCR) family. This work consists of docking of YM087, a potent non-peptide V1aR and V2R - but not OTR - antagonist, into the receptor models based on relatively new theoretical templates of rhodopsin (RD) and opiate receptors, proposed by Mosberg et al. (Univ. of Michigan, Ann Arbor, USA). It is simultaneously demonstrated that this RD template satisfactorily compares with the first historical GPCR structure of bovine rhodopsin (Palczewski et al., 2000) and that homology-modeling of V2R, V1aR and OTR using opiate receptors as templates is rational, based on relatively high (20-60%) sequence homology among the set of 4 neurophyseal and 4 opiate receptors. YM087 was computer-docked to V1aR, V2R and OTR using the AutoDock (Olson et al., Scripps Research Institute, La Jolla, USA) and subsequently relaxed using restrained simulated annealing and molecular dynamics, as implemented in AMBER program (Kollman et al., University of California, San Francisco, USA). From about 80 diverse configurations, sampled for each of the three ligand/receptor systems, 3 best energy-relaxed complexes were selected for mutual comparisons. Similar docking modes were found for the YM087/V1aR and YM087/V2R complexes, diverse from those of the YM087/OTR complexes, in agreement with the molecular affinity data.

Chronic exposure to vasopressin upregulates ENaC and sodium transport in the rat renal collecting duct and lung

Vasopressin is known to acutely stimulate sodium transport in the renal collecting duct. We investigated the long-term regulation by vasopressin of the epithelial sodium channel (ENaC) in the rat kidney. Five-day infusion of dDAVP (a V(2) receptor agonist) to Brattleboro rats lacking vasopressin induced a marked increase in beta- and gamma-subunit ENaC mRNA levels in the renal cortex (beta, 85%; gamma, 100%), with no change in alpha-ENaC mRNA. Expression of beta- and gamma-ENaC mRNAs was also enhanced in lung (beta, 49%; gamma, 33%) but not in distal colon (an organ devoid of V(2) receptors). Similar results were obtained in Sprague Dawley rats after either partial water restriction or dDAVP infusion for 5 days. Transepithelial voltage and transepithelial sodium and water net fluxes were measured in isolated perfused cortical collecting ducts of Brattleboro rats. Acute addition of 2x10(-10) mol/L dDAVP to the bath increased sodium and water fluxes in the same proportion, and to a far greater extent in dDAVP-infused than in control Brattleboro rats (change in Na(+) net flux, 337+/-30 versus 49+/-11 pmol. min(-1). mm(-1), respectively; P<0.001). These effects were abolished by amiloride. Extrarenal water losses, partly originating from the lung, were reduced by high plasma vasopressin level. This study shows that vasopressin increases sodium transport in the renal collecting duct and probably in the lung, through a differential transcriptional regulation of ENaC subunits. This effect is followed by isoosmotic water reabsorption and likely contributes to the process of water conservation. It could lead to less efficient sodium excretion, however, and thus participate in some forms of salt-sensitive hypertension.

Effects of YM471, a nonpeptide AVP V(1A) and V(2) receptor antagonist, on human AVP receptor subtypes expressed in CHO cells and oxytocin receptors in human uterine smooth muscle cells

YM471, (Z)-4'-[4,4-difluoro-5-[2-(4-dimethylaminopiperidino)-2-oxoethylidene]-2,3,4,5-tetrahydro-1H-1-benzoazepine-1-carbonyl]-2-phenylbenzanilide monohydrochloride, is a newly synthesized potent vasopressin (AVP) receptor antagonist. Its effects on binding to and signal transduction by cloned human AVP receptors (V(1A), V(1B) and V(2)) stably expressed in Chinese hamster ovary (CHO) cells, and oxytocin receptors in human uterine smooth muscle cells (USMC) were studied. YM471 potently inhibited specific [(3)H]-AVP binding to V(1A) and V(2) receptors with K(i) values of 0.62 nM and 1.19 nM, respectively. In contrast, YM471 exhibited much lower affinity for V(1B) and oxytocin receptors with K(i) values of 16.4 microM and 31.6 nM, respectively. In CHO cells expressing V(1A) receptors, YM471 potently inhibited AVP-induced intracellular Ca(2+) concentration ([Ca(2+)](i)) increase, exhibiting an IC(50) value of 0.56 nM. However, in human USMC expressing oxytocin receptors, YM471 exhibited much lower potency in inhibiting oxytocin-induced [Ca(2+)](i) increase (IC(50)=193 nM), and did not affect AVP-induced [Ca(2+)](i) increase in CHO cells expressing V(1B) receptors. Furthermore, in CHO cells expressing V(2) receptors, YM471 potently inhibited the production of cyclic AMP stimulated by AVP with an IC(50) value of 1.88 nM. In all assays, YM471 showed no agonistic activity. These results demonstrate that YM471 is a potent, nonpeptide human V(1A) and V(2) receptor antagonist which will be a valuable tool in defining the physiologic and pharmacologic actions of AVP.

Glomerular injury and tubular loss in adriamycin nephrosis

Glomerular injury manifested by sustained proteinuria usually leads to tubule injury and reduction of the GFR. The current study explored the link between these processes in rats with adriamycin nephrosis. One group of nephrotic rats received a vasopressin V2 receptor blocker (V2X) from 4 to 16 wk after injection of adriamycin, whereas a second group received no treatment (NoRx). V2 receptor blockade increased urine volume without affecting protein excretion. At 16 wk, both groups of nephrotic rats exhibited a marked reduction in GFR in comparison with normal controls (V2X, 0.22 +/- 0.19 ml/min; NoRx, 0.20 +/- 0.11 ml/min; control, 1.23 +/- 0.11 ml/min). Morphologic studies revealed that the majority of glomeruli in nephrotic rats were no longer connected to normal tubule segments (V2X, 81 +/- 21%; NoRx, 85 +/- 18%; control, 1 +/- 2%). Glomeruli without tubules were not, however, globally sclerosed. Disruption of the glomerular tubular junction was associated with the presence of amorphous material separating damaged tubule cells from the basement membrane. Serial sections revealed that this material spread from extensive areas of adhesion between the glomerular tuft and capsule to invest the tubular neck. Reduction of the GFR was strongly correlated with the fraction of glomeruli not connected to normal tubules (r(2) = 0.82; P < 0.0001). V2 receptor blockade did not preserve renal function or structure. These findings suggest that local extension of glomerular injury to destroy the tubule neck is an important cause of loss of renal function in adriamycin nephrosis.

The basic and clinical pharmacology of nonpeptide vasopressin receptor antagonists

The neurohypophysial hormone arginine vasopressin (AVP) is a cyclic nonpeptide whose actions are mediated by the stimulation of specific G protein--coupled membrane receptors pharmacologically classified into V1-vascular (V1R), V2-renal (V2R) and V3-pituitary (V3R) AVP receptor subtypes. The random screening of chemical compounds and optimization of lead compounds recently resulted in the development of orally active nonpeptide AVP receptor antagonists. Potential therapeutic uses of AVP receptor antagonists include (a) the blockade of V1-vascular AVP receptors in arterial hypertension, congestive heart failure, and peripheral vascular disease; (b) the blockade of V2-renal AVP receptors in the syndrome of inappropriate vasopressin secretion, congestive heart failure, liver cirrhosis, nephrotic syndrome and any state of excessive retention of free water and subsequent dilutional hyponatremia; (c) the blockade of V3-pituitary AVP receptors in adrenocorticotropin-secreting tumors. The pharmacological and clinical profile of orally active nonpeptide vasopressin receptor antagonists is reviewed here.

Nephrogenic diabetes insipidus

Nephrogenic diabetes insipidus, which can be inherited or acquired, is characterized by an inability to concentrate urine despite normal or elevated plasma concentrations of the antidiuretic hormone arginine vasopressin. Polyuria, with hyposthenuria, and polydipsia are the cardinal clinical manifestations of the disease. About 90% of patients with congenital nephrogenic diabetes insipidus are males with the X-linked recessive form of the disease (OMIM 304800) who have mutations in the arginine vasopressin receptor 2 gene (AVPR2), which codes for the vasopressin V2 receptor. The gene is located in chromosomal region Xq28. In <10% of the families studied, congenital nephrogenic diabetes insipidus has an autosomal-recessive or autosomal-dominant (OMIM 222000 and 125800, respectively) mode of inheritance. Mutations have been identified in the aquaporin-2 gene (AQP2), which is located in chromosome region 12q13 and codes for the vasopressin-sensitive water channel. When studied in vitro, most AVPR2 mutations result in receptors that are trapped intracellularly and are unable to reach the plasma membrane. A few mutant receptors reach the cell surface but are unable to bind arginine vasopressin or to properly trigger an intracellular cyclic AMP signal. Similarly, aquaporin-2 mutant proteins are misrouted and cannot be expressed at the luminal membrane. Chemical or pharmacological chaperones have been found to reverse the intracellular retention of aquaporin-2 and arginine vasopressin receptor 2 mutant proteins. Because many hereditary diseases stem from the intracellular retention of otherwise functional proteins, this mechanism may offer a new therapeutic approach to the treatment of those diseases that result from errors in protein kinesis.

A nonpeptide vasopressin V(1a) receptor antagonist, SR 49059, does not prevent cisplatin-induced emesis in piglets

We determined the pharmacological and the antiemetic properties of SR 49059, a selective nonpeptide V(1a) receptor antagonist, on cisplatin-induced emesis in the piglet. Firstly, we clearly demonstrate that SR 49059 is a potent V(1a) receptor antagonist in vitro and in vivo in the piglet. In binding studies, [3H]-SR 49059 exhibited high affinity for V(1a) receptors in piglet liver membranes (K(d) of 0.76 +/- 0.12 nM and B(max) of 138 +/- 22 fmol/mg prot.). In vivo, in decerebrate piglets, SR 49059 (1 mg/kg iv) antagonized AVP (500 ng/kg iv)-induced hypertension for at least 150 min and also blocked, for at least 270 min at 3 mg/kg iv, the pressor responses to exogenous LVP. After single and repeated iv or icv administration, we studied the antiemetic properties of SR 49059 on cisplatin-induced emesis in piglets. Animals receiving an emetic dose of cisplatin (5.5 mg/kg, iv) were observed continuously for 60 h. Piglets acting as controls were iv administered with vehicle 15 min prior to cisplatin infusion (T0(-15min)), while experimental animals received a single iv administration of SR 49059 at the dose of 1 or 3 mg/kg. In additional piglets, we administered SR 49059 (3 mg/kg) every 12 h from T0(-15min) to T48(-15min) (cumulative dose, 15 mg/kg). Another set of animals - observed only during the acute phase - was administered with SR 49059 (10 mg/kg) every 3 h from T0(-15min) to T15(-15min) (cumulative dose, 60 mg/kg). Lastly, 10 piglets were given a bilateral icv injection of SR 49059 (500 microg and 1500 microg/side) 1 h prior to cisplatin infusion. In all groups treated with SR 49059, the latency of the first emetic episode and the incidence of vomiting during the acute, the delayed and the cumulative phases remained statistically similar to that observed in controls, suggesting that V(1a) receptors are not involved in the onset and completion of nausea and vomiting.

Single amino acid substitutions and deletions that alter the G protein coupling properties of the V2 vasopressin receptor identified in yeast by receptor random mutagenesis

To facilitate structure-function relationship studies of the V2 vasopressin receptor, a prototypical G(s)-coupled receptor, we generated V2 receptor-expressing yeast strains (Saccharomyces cerevisiae) that required arginine vasopressin-dependent receptor/G protein coupling for cell growth. V2 receptors heterologously expressed in yeast were unable to productively interact with the endogenous yeast G protein alpha subunit, Gpa1p, or a mutant Gpa1p subunit containing the C-terminal G alpha(q) sequence (Gq5). In contrast, the V2 receptor efficiently coupled to a Gpa1p/G alpha(s) hybrid subunit containing the C-terminal G alpha(s) sequence (Gs5), indicating that the V2 receptor retained proper G protein coupling selectivity in yeast. To gain insight into the molecular basis underlying the selectivity of V2 receptor/G protein interactions, we used receptor saturation random mutagenesis to generate a yeast library expressing mutant V2 receptors containing mutations within the second intracellular loop. A subsequent yeast genetic screen of about 30,000 mutant receptors yielded four mutant receptors that, in contrast to the wild-type receptor, showed substantial coupling to Gq5. Functional analysis of these mutant receptors, followed by more detailed site-directed mutagenesis studies, indicated that single amino acid substitutions at position Met(145) in the central portion of the second intracellular loop of the V2 receptor had pronounced effects on receptor/G protein coupling selectivity. We also observed that deletion of single amino acids N-terminal of Met(145) led to misfolded receptor proteins, whereas single amino acid deletions C-terminal of Met(145) had no effect on V2 receptor function. These findings highlight the usefulness of combining receptor random mutagenesis and yeast expression technology to study mechanisms governing receptor/G protein coupling selectivity and receptor folding.

Development and therapeutic indications of orally-active non-peptide vasopressin receptor antagonists

Vasopressin (AVP) is a cyclic nonapeptide hormone that exhibits many physiological effects including free water reabsorption, vasoconstriction, cellular proliferation and adrenocorticotrophic hormone (ACTH) secretion. In a healthy organism, AVP plays an important role in the homeostasis of fluid osmolality and volume status. However, in several diseases or conditions such as the syndrome of inappropriate secretion of AVP (SIADH), congestive heart failure, arterial hypertension, liver cirrhosis, nephrotic syndrome, dysmenorrhoea and ocular hypertension, AVP may play an important role in their pathophysiology. Recently, orally-active non-peptide AVP receptor antagonists were developed by random screening of chemical entities and optimisation of lead compounds. These include agents specific for the V(1)-vascular and V(2)-renal AVP receptor subtypes. Dual V(1)/V(2) AVP receptor antagonists are also being studied. Some of these non-peptide receptor antagonists have been studied extensively, while others are currently under investigation. Potential therapeutic indications for AVP receptor antagonists comprise: 1) The blockade of V(1)-vascular AVP receptors in arterial hypertension, congestive heart failure, Raynaud's syndrome, peripheral vascular disease and dysmenorrhea. 2) The blockade of V(2)-renal AVP receptors in the syndrome of inappropriate secretion of vasopressin, congestive hart failure, liver cirrhosis, nephrotic syndrome and any state of excessive retention of free water and subsequent dilutional hyponatraemia. 3) The blockade of V(3)-pituitary AVP receptors in ACTH-secreting tumours. This review examines the pharmacology of orally-active non-peptide AVP receptor antagonists and their clinical applications.

Total Synthesis of SR 121463 A, a Highly Potent and Selective Vasopressin V2 Receptor Antagonist

SR 121463 A, 1, is a promising nonpeptide prototype for potent and selective antagonism of the vasopressin V(2) receptor subtype and, thus, a candidate for control of the clinically debilitating condition of hyponatremia and its associated syndromes. In the present work, we present a novel and stereoselective synthesis that stems from the preparation of three key intermediates: the substituted benzenesulfonyl chloride 2, the N-protected oxindole 3, and protected dibromide 4. The synthesis of 1 has been achieved in good overall yield, each step proceeding in greater than 80% yield. In addition, intermediate 2 and the syn isomer of 1 were prepared with complete control of stereochemistry. The latter reduction appears to proceed by lithium cation mediated chelation control. Molecular mechanics calculations with the MM3* and MMFF force fields underscore geometric and energetic aspects of the reaction.

Selective blockade of vasopressin V2 receptors reveals significant V2-mediated water reabsorption in Brattleboro rats with diabetes insipidus

BACKGROUND

In a previous study we observed that acute administration of the selective antagonist of vasopressin (AVP) V2 receptors, SR 121463A (SR), aggravated the symptoms of diabetes insipidus (DI) in homozygous Brattleboro rats (an AVP-deficient strain). The present study investigates in more details the acute and chronic effects of SR in DI rats.

METHODS AND RESULTS

In experiment A, different groups of rats received acute i.p. injections of SR (0.001-10 mg/kg) or vehicle alone, and urine was collected for the next 24 h. SR dose-dependently increased urine flow rate and decreased urine osmolality with no significant change in solute excretion, thus confirming a pure 'aquaretic' effect. In experiments B and C, the chronic effects of orally administered SR were evaluated over 8 days in Brattleboro DI rats (experiment B, 1 mg/kg/day) and in adult Sprague-Dawley rats with normal AVP secretion (experiment C, 3 mg/kg/day). In DI rats, the aquaretic effects of SR persisted with the same intensity over the 8 days. In Sprague-Dawley rats, SR induced a sustained, stable aquaretic effect and also increased non-renal water losses, suggesting an effect of AVP on water conservation in extrarenal sites. Because oxytocin (OT) synthesis is elevated in DI rats and OT is known to bind to V2 receptors, we evaluated the antidiuretic effects of OT in DI rats in experiment D. Chronic infusion of OT (3 microg/kg/h, i.p.) induced a marked antidiuresis, and acute SR (1 mg/kg) in OT-treated DI rats completely abolished this antidiuretic effect, thus indicating that it was due to binding of OT to V2 receptors.

CONCLUSION

(i) SR is a potent orally active aquaretic and induces stable effects during 1 week in rats with or without endogenous AVP secretion. (ii) Significant V2 receptor-mediated water reabsorption occurs in collecting ducts of Brattleboro DI rats because their usual urine osmolality is about twofold higher than the minimum observed during SR-induced maximum diuresis. (iii) This V2 agonism could be mediated in part by OT binding to V2 receptors. Small amounts of endogenous AVP, known to be produced by adrenal and testis in DI rats, could also contribute to this V2 agonism, as well as a possible constitutive activation of the V2 receptors. (iv) In normal rats, AVP probably reduces water losses through extrarenal sites, probably the lungs.

Pharmacological chaperones: a new twist on receptor folding

Protein misfolding is at the root of several genetic human diseases. These diseases do not stem from mutations within the active domain of the proteins, but from mutations that disrupt their three-dimensional conformation, which leads to their intracellular retention by the quality control apparatus of the cell. Facilitating the escape of the mutant proteins from the quality control system by lowering the temperature of the cells or by adding chemicals that assist folding (chemical chaperones) can result in proteins that are fully functional despite their mutation. The discovery that ligands with pharmacological selectivity (pharmacological chaperones) can rescue the proper targeting and function of misfolded proteins, including receptors, might help to develop new treatments for 'conformational diseases'.

Generation and phenotype of mice harboring a nonsense mutation in the V2 vasopressin receptor gene

The V2 vasopressin receptor (V2R) plays a key role in the maintenance of a normal body water balance. To generate an in vivo model that allows the physiological and molecular analysis of the role of V2Rs in kidney function, we have created mouse lines that lack functional V2Rs by using targeted mutagenesis in mouse embryonic stem cells. Specifically, we introduced a nonsense mutation known to cause X-linked nephrogenic diabetes insipidus (XNDI) in humans (Glu242stop) into the mouse genome. V2R-deficient hemizygous male pups showed a decrease in basal urine osmolalities and were unable to concentrate their urine. These pups also exhibited an enlargement of renal pelvic space, failed to thrive, and died within the first week after birth due to hypernatremic dehydration. Interestingly, female mice heterozygous for the V2R mutation showed normal growth but displayed an XNDI-like phenotype, characterized by reduced urine concentrating ability of the kidney, polyuria, and polydipsia. Western blot analysis and immunoelectron microscopic studies showed that the loss of functional V2Rs had no significant effect on the basal expression levels of aquaporin-2 and the bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1). The V2R mutant mice described here should serve as highly useful tools for the development of novel therapeutic strategies for the treatment of XNDI.

Binding properties of a selective tritiated vasopressin V2 receptor antagonist, [H]-SR 121463

BACKGROUND

[3H]-SR 121463 is the first radiolabeled selective nonpeptide vasopressin V2 receptor antagonist ligand that has been reported to date. In the present work, we studied the binding properties of [3H]-SR 121463 for renal V2 receptors from animal and human origins.

METHODS

Binding studies were performed with [3H]-SR 121463 in Chinese hamster ovary (CHO) cells transfected with the human V2 receptor and in various kidney preparations expressing the native V2 receptors (rat, rabbit, dog, pig, monkey, and human). Autoradiographies were performed in rat and human kidney sections.

RESULTS

[3H]-SR 121463 binding to CHO cells stably transfected with the cloned human renal V2 receptor was specific, highly stable, time dependent, saturable, and reversible. A single population of high-affinity binding sites was identified (Kd = 0.94 +/- 0.34 nmol/L, Bmax = 9876 +/- 317 fmol/mg protein). Of note, [3H]-SR 121463 revealed a higher number (about 40%) of V2 sites than [3H]-AVP in the same preparation. Displacement of [3H]-SR 121463 binding by reference peptide and nonpeptide vasopressin/oxytocin compounds exhibited a typical AVP V2 profile. [3H]-SR 121463 also displayed a high affinity for native V2 receptors in several kidney preparations from rat, pig, dog, rabbit, bovine, monkey, and human. The autoradiographic experiments using rat and human kidney sections showed intense labeling in the medullopapillary region and lower intensity in the cortex, consistent with a main localization of V2 receptors on collecting tubules.

CONCLUSION

[3H]-SR 121463 is a useful ligand for the specific labeling of animal and human V2 receptors and could be a suitable probe for the search and in situ localization of V2 sites.

Vasopressin V(2)-receptor-dependent regulation of AQP2 expression in Brattleboro rats

The role of AVP-V(2) receptor (AVP-V(2)R)-dependent regulation of aquaporin-2 (AQP2) expression was evaluated in vasopressin-deficient Brattleboro (BB) rats. AQP2 levels were relatively high in BB rats (52 +/- 8% of levels in Wistar rats), and treatment with the AVP-V(2)R antagonist SR-121463A (0.8 mg/day) for 48 h was associated with 1) increased urine output (170 +/- 9%), 2), reduced AQP2 protein levels (42 +/- 10% in whole kidney and 53 +/- 8% in inner medulla), and 3) reduced AQP2 mRNA levels (36 +/- 7%). In addition, the levels of AQP2 phosphorylated in the protein kinase A (PKA) consensus site (Ser(256) of AQP2) was reduced to 3 +/- 1% of control levels. Lithium (Li) treatment of BB rats for 1 mo, known to reduce adenylyl cyclase (AC) activity, downregulated AQP2 protein levels (15 +/- 6%) and increased urine output (220%). Downregulation of AQP2 expression in response to SR-121463A or Li treatment indicates that AQP2 expression in BB rats depends in part on activation of AVP-V(2)Rs and that the signaling cascade(s) involves AC and hence cAMP. Complete water restriction of BB rats produced only a small increase in AQP2 mRNA (235 +/- 33%) and AQP2 protein (156 +/- 22%) levels. Immunoelectron microscopy confirmed the increase in AQP2 abundance but revealed no change in AQP2 apical plasma membrane labeling in response to thirsting. In conclusion, the expression and phosphorylation of AQP2 in BB rats are in part dependent on AVP-V(2)R signaling, and AVP-V(2)-mediated regulation of AQP2 trafficking and expression is effectively decoupled in BB rats, indicating differences in AVP-V(2)R-mediated regulation of AQP2 trafficking and expression.

Conserved aromatic residues in the transmembrane region VI of the V1a vasopressin receptor differentiate agonist vs. antagonist ligand binding

Despite their opposite effects on signal transduction, the nonapeptide hormone arginine-vasopressin (AVP) and its V1a receptor-selective cyclic peptide antagonist d(CH2)5[Tyr(Me)2]AVP display homologous primary structures, differing only at residues 1 and 2. These structural similarities led us to hypothesize that both ligands could interact with the same binding pocket in the V1a receptor. To determine receptor residues responsible for discriminating binding of agonist and antagonist ligands, we performed site-directed mutagenesis of conserved aromatic and hydrophilic residues as well as nonconserved residues, all located in the transmembrane binding pocket of the V1a receptor. Mutation of aromatic residues of transmembrane region VI (W304, F307, F308) reduced affinity for the d(CH2)5[Tyr(Me)2]AVP and markedly decreased affinity for the unrelated strongly hydrophobic V1a-selective nonpeptide antagonist SR 49059. Replacement of these aromatic residues had no effect on AVP binding, but increased AVP-induced coupling efficacy of the receptor for its G protein. Mutating hydrophilic residues Q108, K128 and Q185 in transmembrane regions II, III and IV, respectively, led to a decrease in affinity for both agonists and antagonists. Finally, the nonconserved residues T333 and A334 in transmembrane region VII, controlled the V1a/V2 binding selectivity for both nonpeptide and cyclic peptide antagonists. Thus, because conserved aromatic residues of the V1a receptor binding pocket seem essential for antagonists and do not contribute at all to the binding of agonists, we propose that these residues differentiate agonist vs. antagonist ligand binding.

Vasopressin-induced von Willebrand factor secretion from endothelial cells involves V2 receptors and cAMP

Vasopressin and its analogue 1-deamino-8-D-arginine vasopressin (DDAVP) are known to raise plasma von Willebrand factor (vWF) levels. DDAVP is used as a hemostatic agent for the treatment of von Willebrand's disease. However, its cellular mechanisms of action have not been elucidated. DDAVP, a specific agonist for the vasopressin V2 receptor (V2R), exerts its antidiuretic effect via a rise in cAMP in kidney collecting ducts. We tested the hypothesis that DDAVP induces vWF secretion by binding to V2R and activating cAMP-mediated signaling in endothelial cells. vWF secretion from human umbilical vein endothelial cells (HUVECs) can be mediated by cAMP, but DDAVP is ineffective, presumably due to the absence of V2R. We report that DDAVP stimulates vWF secretion in a cAMP-dependent manner in HUVECs after transfection of the V2R. In addition, vasopressin and DDAVP induce vWF secretion in human lung microvascular endothelial cells (HMVEC-L). These cells (but not HUVECs) express endogenous V2R, as shown by RT-PCR. Vasopressin-induced vWF secretion is mimicked by DDAVP and inhibited by the selective V2R antagonist SR121463B. It is mediated by cAMP, since it is inhibited by the protein kinase A inhibitor Rp-8CPT-cAMPS. These results indicate that vasopressin induces cAMP-mediated vWF secretion by a direct effect on endothelial cells. They also demonstrate functional expression of V2R in endothelial cells, and provide a cellular mechanism for the hemostatic effects of DDAVP.

Effect of SR121463, a selective non-peptide vasopressin V2 receptor antagonist, in a rabbit model of ocular hypertension

The activity on intraocular pressure (IOP) of SR121463, a selective non-peptide arginin-vasopressin (AVP) V2 receptor antagonist, was investigated in a rabbit model of ocular hypertension. We first demonstrated that, in vitro, SR121463 displayed high competitive affinity for rabbit vasopressin V2 receptors (Ki = 2.1 +/- 1.2 nM). In vivo, SR121463 was instilled once (at concentrations ranging from 0.1 to 3%), or for 10 days (20 instillations) at 1% concentration, in the eye of ocular hypertensive rabbits (intraocular injection of 0.14 mg alpha-chymotrypsin). SR121463 also was instilled at 1% in the normotensive eye or intravenously injected (100 microg/kg) to ocular hypertensive rabbits. SR121463 was compared to timolol 0.5% or to clonidine 0.25%. Additionally, local and systemic safety aspects were examined. Results showed that SR121463 was locally well-tolerated and had no anesthetic effect. A significant decrease in IOP of the hypertensive eye was observed for concentrations of SR121463 > or =1%. This decrease was comparable to that obtained with reference compounds. A similar activity was found after intravenous administration. No tachyphylaxis was observed after 10 days, and no contralateral or systemic effect was noted. Also, when applied on the normotensive eye or when intravenously injected, SR121463 had no effect on the normotensive eye. These results on IOP and the good local and systemic safety profile, suggest that a potent vasopressin V2 receptor antagonist, SR121463, could be of value for the treatment of glaucoma, through a mechanism of action that remains to be elucidated.