Pharmacology of vasopressin antagonists

Lixivaptan, a New Generation Diuretic, Counteracts Vasopressin-Induced Aquaporin-2 Trafficking and Function in Renal Collecting Duct Cells

Vasopressin V2 receptor (V2R) antagonists (vaptans) are a new generation of diuretics. Compared with classical diuretics, vaptans promote the excretion of retained body water in disorders in which plasma vasopressin concentrations are inappropriately high for any given plasma osmolality. Under these conditions, an aquaretic drug would be preferable over a conventional diuretic. The clinical efficacy of vaptans is in principle due to impaired vasopressin-regulated water reabsorption via the water channel aquaporin-2 (AQP2). Here, the effect of lixivaptan—a novel selective V2R antagonist—on the vasopressin-cAMP/PKA signaling cascade was investigated in mouse renal collecting duct cells expressing AQP2 (MCD4) and the human V2R. Compared to tolvaptan—a selective V2R antagonist indicated for the treatment of clinically significant hypervolemic and euvolemic hyponatremia—lixivaptan has been predicted to be less likely to cause liver injury. In MCD4 cells, clinically relevant concentrations of lixivaptan (100 nM for 1 h) prevented dDAVP-induced increase of cytosolic cAMP levels and AQP2 phosphorylation at ser-256. Consistent with this finding, real-time fluorescence kinetic measurements demonstrated that lixivaptan prevented dDAVP-induced increase in osmotic water permeability. These data represent the first detailed demonstration of the central role of AQP2 blockade in the aquaretic effect of lixivaptan and suggest that lixivaptan has the potential to become a safe and effective therapy for the treatment of disorders characterized by high plasma vasopressin concentrations and water retention.

Arginine vasopressin modulates electrical activity and calcium homeostasis in pulmonary vein cardiomyocytes

Background

Atrial fibrillation (AF) frequently coexists with congestive heart failure (HF) and arginine vasopressin (AVP) V1 receptor antagonists are used to treat hyponatremia in HF. However, the role of AVP in HF-induced AF still remains unclear. Pulmonary veins (PVs) are central in the genesis of AF. The purpose of this study was to determine if AVP is directly involved in the regulation of PV electrophysiological properties and calcium (Ca²⁺) homeostasis as well as the identification of the underlying mechanisms.

Methods

Patch clamp, confocal microscopy with Fluo-3 fluorescence, and Western blot analyses were used to evaluate the electrophysiological characteristics, Ca²⁺ homeostasis, and Ca²⁺ regulatory proteins in isolated rabbit single PV cardiomyocytes incubated with and without AVP (1 μM), OPC 21268 (0.1 μM, AVP V1 antagonist), or OPC 41061 (10 nM, AVP V2 antagonist) for 4–6 h.

Results

AVP (0.1 and 1 μM)-treated PV cardiomyocytes had a faster beating rate (108 to 152%) than the control cells. AVP (1 μM) treated PV cardiomyocytes had higher late sodium (Na⁺) and Na⁺/Ca²⁺ exchanger (NCX) currents than control PV cardiomyocytes. AVP (1 μM) treated PV cardiomyocytes had smaller Ca²⁺_i transients, and sarcoplasmic reticulum (SR) Ca²⁺ content as well as higher Ca²⁺ leak. However, combined AVP (1 μM) and OPC 21268 (0.1 μM) treated PV cardiomyocytes had a slower PV beating rate, larger Ca²⁺_i transients and SR Ca²⁺ content, smaller late Na⁺ and NCX currents than AVP (1 μM)-treated PV cardiomyocytes. Western blot experiments showed that AVP (1 μM) treated PV cardiomyocytes had higher expression of NCX and p-CaMKII, and a higher ratio of p-CaMKII/CaMKII.

Conclusions

AVP increases PV arrhythmogenesis with dysregulated Ca²⁺ homeostasis through vasopressin V1 signaling.

The V2 receptor antagonist tolvaptan raises cytosolic calcium and prevents AQP2 trafficking and function: an in vitro and in vivo assessment

Tolvaptan, a selective vasopressin V2 receptor antagonist, is a new generation diuretic. Its clinical efficacy is in principle due to impaired vasopressin‐regulated water reabsorption via aquaporin‐2 (AQP2). Nevertheless, no direct in vitro evidence that tolvaptan prevents AQP2‐mediated water transport, nor that this pathway is targeted in vivo in patients with syndrome of inappropriate antidiuresis (SIAD) has been provided. The effects of tolvaptan on the vasopressin–cAMP/PKA signalling cascade were investigated in MDCK cells expressing endogenous V2R and in mouse kidney. In MDCK, tolvaptan prevented dDAVP‐induced increase in ser256‐AQP2 and osmotic water permeability. A similar effect on ser256‐AQP2 was found in V1aR −/− mice, thus confirming the V2R selectively. Of note, calcium calibration in MDCK showed that tolvaptan per se caused calcium mobilization from the endoplasmic reticulum resulting in a significant increase in basal intracellular calcium. This effect was only observed in cells expressing the V2R, indicating that it requires the tolvaptan–V2R interaction. Consistent with this finding, tolvaptan partially reduced the increase in ser256‐AQP2 and the water permeability in response to forskolin, a direct activator of adenylyl cyclase (AC), suggesting that the increase in intracellular calcium is associated with an inhibition of the calcium‐inhibitable AC type VI. Furthermore, tolvaptan treatment reduced AQP2 excretion in two SIAD patients and normalized plasma sodium concentration. These data represent the first detailed demonstration of the central role of AQP2 blockade in the aquaretic effect of tolvaptan and underscore a novel effect in raising intracellular calcium that can be of significant clinical relevance.

Vasopressin receptor antagonists: Characteristics and clinical role

Hyponatremia, the most common electrolyte disorder in hospitalized patients is associated with increased risk of mortality even when mild and apparently asymptomatic. Likewise morbidity manifested as attention deficits, gait disturbances, falls, fractures, and osteoporosis is more prevalent in hyponatremic subjects. Hyponatremia also generates a significant financial burden. Therefore, it is important to explore approaches that effectively and safely treat hyponatremia. Currently available strategies are physiologically sound and affordable but lack evidence from clinical trials and are limited by variable efficacy, slow response, and/or poor compliance. The recent emergence of vasopressin receptor antagonists provides a class of drugs that target the primary pathophysiological mechanism, namely vasopressin mediated impairment of free water excretion. This review summarizes the historical development, pharmacology, clinical trials supporting efficacy and safety, shortcomings, as well as practical suggestions for the use of vasopressin receptor antagonists.

Randomized, double blinded, placebo-controlled trial to evaluate the efficacy and safety of tolvaptan in Chinese patients with hyponatremia caused by SIADH

To study the effect of tolvaptan on non-acute, non-hypovolemic hyponatremia in inappropriate secretion of antidiuretic hormone (SIADH) syndrome in Chinese patients. Hyponatremic SIADH patients received placebo (N = 18) or tolvaptan (N = 19) at an initial dose of 15 mg/day with further titration to 30 mg/day and 60 mg/day based on serum sodium concentrations. Randomized, double-blind, placebo-controlled trial. Primary endpoint was the change of the serum sodium from baseline to days 4 and 7. Analysis of covariance (ANCOVA) was used for statistical analysis. At day 4, average daily changes in serum sodium levels from baseline was 1.9 ± 2.9 mmol/L (1.9 ± 2.9 mEq/L) in the placebo group and 8.1 ± 3.6 mmol/L (8.1 ± 3.6 mEq/L) in the tolvaptan group; at day 7, the values were 2.5 ± 3.9 mmol/L (2.5 ± 3.9 mEq/L) and 8.6 ± 3.9 mmol/L (8.6 ± 3.9 mmEq/L) for the placebo and tolvaptan groups (ANCOVA, P < 0.001). At days 4 and 7, daily urine output and proportions of patients with normalized serum sodium were significantly superior in the tolvaptan group. The most common adverse events occurring in the tolvaptan group were dry mouth and thirst. Tolvaptan demonstrated superiority to placebo in the treatment of Chinese SIADH patients with hyponatremia by elevating serum sodium concentration with acceptable safety profile.

Clinical characteristics of responders to treatment with tolvaptan in patients with acute decompensated heart failure: Importance of preserved kidney size

BACKGROUND

Recent clinical trials have demonstrated the efficacy of short-term treatment with tolvaptan, an oral vasopressin V2 receptor antagonist, in patients with heart failure. However, the response to tolvaptan varies among patients. The aim of this study was to determine factors associated with response to tolvaptan in patients with acute decompensated heart failure (ADHF).

METHODS

The Tolvaptan Registry, a prospective, observational, multicenter cohort study performed in Japan, aims to determine factors affecting the responsiveness of tolvaptan in patients with ADHF. We enrolled ADHF patients treated with tolvaptan and they were divided into two groups: responders and non-responders. Responders were defined as subjects who met all of the following three conditions: (1) increasing urine volume during a 24-hour period after the start of tolvaptan treatment; (2) improvement in New York Heart Association functional class; and (3) decrease in cardiothoracic ratio assessed by chest X-ray on day 3 of tolvaptan administration.

RESULTS

Among the 114 patients, treatment with tolvaptan improved three conditions of heart failure in more than half of all the cohorts (71 patients, 62%). As for baseline characteristics, estimated glomerular filtration rate, urine osmolality, and kidney size were significantly greater in responders than in non-responders. Multivariate logistic analysis revealed that kidney size was independently associated with responders (odds ratio: 1.083, p=0.001, 95% confidence interval 1.031-1.137).

CONCLUSIONS

The main clinical characteristic of responders to treatment with tolvaptan is that kidney size is preserved.

Resolution of severe hyponatraemia is associated with improved survival in patients with cancer

Background

Hyponatraemia is a common finding in patients with cancer, and has been shown to be associated with poor prognosis in different settings. We have analysed the impact of severe hyponatraemia in patients with cancer.

Methods

A retrospective review of all patients admitted to a specialist cancer hospital with a plasma sodium of less than 115 mmol/l and a diagnosis of malignancy was undertaken. Patient and tumour characteristics were analysed as well as impact of hyponatraemia management on overall survival and number of lines of cancer treatment received.

Results

57 patients were identified. 84% had advanced Stage 3 or 4 cancer and approximately 85% with data available had symptoms attributable to hyponatraemia. Mean length of hospital stay was 12 days, and overall survival (OS) was 5.1 months. Plasma sodium level corrected in 56% of patients and here OS was 13.6 months compared to 16 days in those whose sodium did not correct (p < 0.001). Those whose sodium corrected were more likely to receive further lines of anti-cancer treatment.

Conclusions

Severe hyponatraemia in cancer is associated with very poor survival, but correction of the sodium level leads to additional treatment and significantly greater overall survival (although it is not possible to determine if this is due to specific therapy of the hyponatraemia or the resolving hyponatraemia reflects an improvement in the clinical condition). Aggressive treatment of hyponatraemia may allow more anti-cancer treatment and improve survival.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1156-6) contains supplementary material, which is available to authorized users.

Urea minimizes brain complications following rapid correction of chronic hyponatremia compared with vasopressin antagonist or hypertonic saline

Hyponatremia is a common electrolyte disorder that carries significant morbidity and mortality. However, severe chronic hyponatremia should not be corrected rapidly to avoid brain demyelination. Vasopressin receptor antagonists (vaptans) are now being widely used for the treatment of hyponatremia along with other alternatives like hypertonic saline. Previous reports have suggested that, in some cases, urea can also be used to correct hyponatremia. Correction of severe hyponatremia with urea has never been compared to treatment with a vaptan or hypertonic saline with regard to the risk of brain complications in the event of a too rapid rise in serum sodium. Here, we compared the neurological outcome of hyponatremic rats corrected rapidly with urea, lixivaptan, and hypertonic saline. Despite similar increase in serum sodium obtained by the three drugs, treatment with lixivaptan or hypertonic saline resulted in a higher mortality than treatment with urea. Histological analysis showed that treatment with urea resulted in less pathological change of experimental osmotic demyelination than was induced by hypertonic saline or lixivaptan. This included breakdown of the blood-brain barrier, microglial activation, astrocyte demise, and demyelination. Thus, overcorrection of hyponatremia with urea resulted in significantly lower mortality and neurological impairment than the overcorrection caused by lixivaptan or hypertonic saline.

Tolvaptan attenuates left ventricular fibrosis after acute myocardial infarction in rats

Tolvaptan, a non-peptide V2-receptor antagonist, is a newly developed diuretic agent. Recently, we reported that tolvaptan has diuretic as well as anti-inflammatory and anti-fibrotic actions in chronic heart failure. In this study, we investigated whether tolvaptan has a cardioprotective effect in acute heart failure after myocardial infarction (MI). After MI induction, rats were randomized into 6 groups as follows: vehicle group, group treated with 15 mg∙kg⁻¹∙day⁻¹ furosemide, 2 groups treated with 3 or 10 mg∙kg⁻¹∙day⁻¹ tolvaptan, and 2 groups treated with 15 mg∙kg⁻¹∙day⁻¹ furosemide combined with 3 or 10 mg∙kg⁻¹∙day⁻¹ tolvaptan. Each agent was administered for 2 weeks, and blood pressure levels and infarct sizes were similar in all MI groups. Lower left ventricular end-systolic volumes and greater improvement of left ventricular ejection fraction were observed in the tolvaptan-treated groups compared with the vehicle group. In contrast, furosemide alone did not improve them. Sirius red staining revealed that tolvaptan significantly repressed MI-induced interstitial fibrosis in the left ventricle. MI-induced mRNA expressions related to cardiac load, inflammation, and fibrosis were significantly attenuated in the combination group. The combination treatment also repressed MI-induced mineralocorticoid receptor expression. Tolvaptan, or combination of furosemide and tolvaptan, may improve cardiac function in acute MI.

Tolvaptan Improves Left Ventricular Dysfunction after Myocardial Infarction in Rats

Background—

Arginine vasopressin, which promotes the reabsorption of renal water is increased in chronic heart failure. Here, we compared the effects of tolvaptan, a newly developed nonpeptide V 2 receptor antagonist, with those of furosemide, a loop diuretic, and a combination of these 2 agents in rats with left ventricular dysfunction after myocardial infarction (MI).

Methods and Results—

After 10 weeks of MI induction, the rats were separated them into the following 6 groups adjusted to the infarct size: a vehicle group, a group treated with 15 mg·kg -1 ·day -1 of furosemide, 2 groups treated with 3 or 10 mg·kg −1 ·day −1 of tolvaptan; and 2 groups treated with 15 mg·kg −1 ·day −1 of furosemide plus 3 or 10 mg·kg −1 ·day −1 tolvaptan. Each treatment agent was administered for 4 weeks, and all groups had similar blood pressure levels and infarct size. The tolvaptan-treated groups were found to have lower levels of left ventricular end-diastolic and systolic cardiac volumes than the vehicle group did. Furthermore, the improvement in the ejection fraction in the tolvaptan-treated groups was significantly greater than those in the vehicle group. ED-1 immunostaining and Sirius red staining revealed that tolvaptan significantly repressed MI-induced macrophage infiltration and interstitial fibrosis in the left ventricle, respectively. Tolvaptan attenuated the MI-induced mRNA expressions of atrial and brain natriuretic peptides, monocyte chemotactic protein-1, transforming growth factor-β1, arginine vasopressin V 1a receptor, and endothelin-1 in the marginal infarct region.

Conclusions—

Tolvaptan may improve cardiac dysfunction after MI, which is partially mediated by the suppression of V 1a receptor, neurohumoral activation and inflammation.

V2 receptor antagonist; tolvaptan

Hyponatremia is the most common electrolyte disorder in hospitalized patients. Many studies documented that it was related to increased morbidity and mortality in patients with congestive heart failure, liver cirrhosis, and neurologic diseases. Although knowledge of hyponatremia has been cumulated, the optimal management of hyponatremia remains incompletely established in clinical practice because of the diversity of underlying disease states, and its multiple causes with differing pathophysiologic mechanisms. Since vasopressin receptor antagonists have unique aquaretic effect to selectively increase electrolytes-free water excretion, clinicians could apply a more effective method to treat hyponatremia. Tolvaptan has significant evidence that it improves serum sodium levels in patients with euvolemic or hypervolemic hyponatremia related with heart failure, cirrhosis or syndrome of inappropriate anti-diuretic hormone. Tolvaptan has acceptable safety and tolerability for long-term usage in chronic hyponatremia, and the beneficial effects on serum Na⁺ occurred in patients with both mild and marked hyponatremia.

Tolvaptan for the treatment of hyponatremia secondary to the syndrome of inappropriate antidiuretic hormone secretion

Hyponatremia is prevalent in hospitalized patients and predicts a poor prognosis. The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is perceived as one of the most frequent causes of hyponatremia. Traditionally, chronic hyponatremia has been treated with fluid restriction and demeclocycline. However, these treatment options have been unsatisfactory due to problems with treatment compliance and/or safety concerns. In recent years, several vasopressin-receptor antagonists, the vaptans, were introduced into clinical practice. One of these vaptans - tolvaptan - is an oral vasopressin V2-receptor antagonist that induces free water excretion without increasing sodium excretion. Few studies have assessed the role of vaptans in treating hyponatremia in a population with only SIADH. Current data shows that vaptans may safely correct mild or moderate hyponatremia in patients with SIADH. However, further clinical trials are needed to determine the optimal dosing, proper monitoring and adequate precautions for the use of vaptans in this patient population.

The Therapeutic Use of Vaptans for the Treatment of Dilutional Hyponatremia

Hyponatremia is a very common electrolyte abnormality. Dilutional hyponatremia is very difficult to treat effectively due to the complications of conventional treatment. Arginine-vasopressin (AVP) plays an integral role in circulatory and water homeostasis. AVP is a hormone released in response to increases in plasma tonicity or decreases in plasma volume in an attempt to maintain the plasma osmolality between 284 and 295 mOsm/L. AVP receptor antagonists or “vaptans” are a new class of drugs that allow for the safe and efficacious treatment of dilutional hyponatremia. Conivaptan, a mixed V1a/V2 receptor antagonist, and tolvaptan, a selective V2 receptor antagonist, are the only 2 vaptans approved by the US Food and Drug Administration.

Tolvaptan inhibits ERK-dependent cell proliferation, Cl⁻ secretion, and in vitro cyst growth of human ADPKD cells stimulated by vasopressin

In autosomal dominant polycystic kidney disease (ADPKD), arginine vasopressin (AVP) accelerates cyst growth by stimulating cAMP-dependent ERK activity and epithelial cell proliferation and by promoting Cl(-)-dependent fluid secretion. Tolvaptan, a V2 receptor antagonist, inhibits the renal effects of AVP and slows cyst growth in PKD animals. Here, we determined the effect of graded concentrations of tolvaptan on intracellular cAMP, ERK activity, cell proliferation, and transcellular Cl(-) secretion using human ADPKD cyst epithelial cells. Incubation of ADPKD cells with 10(-9) M AVP increased intracellular cAMP and stimulated ERK and cell proliferation. Tolvaptan caused a concentration-dependent inhibition of AVP-induced cAMP production with an apparent IC(50) of ∼10(-10) M. Correspondingly, tolvaptan inhibited AVP-induced ERK signaling and cell proliferation. Basolateral application of AVP to ADPKD cell monolayers grown on permeable supports caused a sustained increase in short-circuit current that was completely blocked by the Cl(-) channel blocker CFTR(inh-172), consistent with AVP-induced transepithelial Cl(-) secretion. Tolvaptan inhibited AVP-induced Cl(-) secretion and decreased in vitro cyst growth of ADPKD cells cultured within a three-dimensional collagen matrix. These data demonstrate that relatively low concentrations of tolvaptan inhibit AVP-stimulated cell proliferation and Cl(-)-dependent fluid secretion by human ADPKD cystic cells.

Arginine vasopressin (AVP) and treatment with arginine vasopressin receptor antagonists (vaptans) in congestive heart failure, liver cirrhosis and syndrome of inappropriate antidiuretic hormone secretion (SIADH)

Arginine vasopressin (AVP) is the major physiological regulator of renal water excretion and blood volume. The AVP pathways of V(1a)R-mediated vasoconstriction and V(2)R-induced water retention represent a potentially attractive target of therapy for edematous diseases. Experimental and clinical evidence suggests beneficial effects of AVP receptor antagonists by increasing free water excretion and serum sodium levels. This review provides an update on the therapeutic implication of newly developed AVP receptor antagonists in respective disorders, such as chronic heart failure, liver cirrhosis and syndrome of inappropriate antidiuretic hormone secretion.

The syndrome of inappropriate antidiuresis: pathophysiology, clinical management and new therapeutic options

Hyponatremia is a marker of different underlying diseases and it can be a cause of morbidity itself; this implies the importance of a correct approach to the problem. The syndrome of inappropriate antidiuresis (SIAD) is one of the most common causes of hyponatremia: it is a disorder of sodium and water balance characterized by urinary dilution impairment and hypotonic hyponatremia, in the absence of renal disease or any identifiable non-osmotic stimulus able to induce antidiuretic hormone (ADH) release; according to its definition, it is diagnosed through an exclusion algorithm. SIAD is usually observed in hospitalized patients and its prevalence may be as high as 35%. The understanding of the syndrome has notably evolved over the last years, as reflected by the significant change in the name, once the syndrome of inappropriate secretion of ADH (SIADH), today SIAD. This review is up to date and it analyses the newest notions about pathophysiological mechanisms, classification, management and therapy of SIAD, including vaptans.

Vasopressin-receptor antagonists

Despite a crucial role in body fluid homeostasis, elevated vasopressin levels can also be pathological in conditions such as congestive heart failure, liver cirrhosis and the syndrome of inappropriate antidiuretic hormone secretion. The result of elevated vasopressin is renal water retention and hyponatremia, a low serum sodium concentration. Hyponatremia is associated with excess morbidity and mortality. Nonpeptide vasopressin-receptor antagonists represent a new drug class of small molecules that competitively inhibit one or more of the vasopressin receptors. There are three vasopressin receptors in humans, including V1a, V1b and V2. Selective V2- and combined V1a/V2-receptor antagonists have been developed for the treatment of hyponatremia resulting from congestive heart failure, liver cirrhosis and the syndrome of inappropriate antidiuretic hormone secretion. Two nonpeptide vasopressin-receptor antagonists, conivaptan and tolvaptan, have recently been approved by American and European drug authorities for clinical use. This article aims to provide a succinct and clinical update on nonpeptide vasopressin-receptor antagonists, including their mechanism of action, performance in randomized clinical trials and current clinical status.

Tolvaptan

Tolvaptan is an orally administered, nonpeptide, selective arginine vasopressin V(2) receptor antagonist that increases free water clearance, thereby correcting low serum sodium levels. SALT-1 and -2, two identical, randomized, double-blind, placebo-controlled, multicentre trials, included patients with hypervolaemic or euvolaemic hyponatraemia (serum sodium <135 mmol/L) associated with heart failure, cirrhosis or the syndrome of inappropriate antidiuretic hormone secretion. In both trials, patients receiving (in addition to standard medical treatment) tolvaptan 15-60 mg once daily (titrated according to response) for up to 30 days (n = 95 and 118) experienced significantly greater improvements than those receiving placebo (n = 89 and 114) for the co-primary endpoints of the change in average daily area under the curve for the serum sodium level from baseline to day 4 and from baseline to day 30. This beneficial effect of tolvaptan on serum sodium levels in SALT-1 and -2 was observed in patients with mild (serum sodium <135 mmol/L) and in those with marked (serum sodium <130 mmol/L) hyponatraemia at baseline. Tolvaptan was also superior to placebo in increasing serum sodium levels from baseline to day 7 in a subgroup of 323 patients with hyponatraemia (serum sodium <134 mmol/L) in the randomized, double-blind, multicentre EVEREST trials, which included patients who were hospitalized for worsening heart failure. Tolvaptan was generally well tolerated in clinical trials. The most frequently reported adverse events were thirst and dry mouth, which result from the pharmacodynamic effects of the drug.