Characteristics of the K+-competitive H+,K+-ATPase inhibitor AZD0865 in isolated rat gastric glands

Absorption, distribution, metabolism and excretion of linaprazan glurate in rats

Linaprazan (AZD0865, TX07) is one of potassium-competitive acid blockers. However, linaprazan is rapidly excreted from the body, shortening its acid inhibition property. Linaprazan glurate (X842) is a prodrug of linaprazan with a prolonged inhibitory effect on gastric acid secretion. Linaprazan glurate has entered clinical trials, but few studies have reported its metabolism in non-clinical and clinical settings. In this study, we studied the pharmacokinetics, tissue distribution, mass balance, and metabolism of linaprazan glurate in rats after a single oral dose of 2.4 mg/kg (100 µCi/kg) [14C]linaprazan glurate. The results demonstrated that linaprazan glurate was mainly excreted via feces in rats with 70.48% of the dose over 168 h. The plasma AUC0-∞ of linaprazan glurate in female rats was 2 times higher than that in male rats. Drug-related substances were mainly concentrated in the stomach, eyes, liver, small intestine, and large intestine after administration. In blood, drug-related substances were mostly distributed into plasma instead of hemocytes. In total, 13 metabolites were detected in rat plasma, urine, feces, and bile. M150 (2,6-dimethylbenzoic acid) was the predominant metabolite in plasma, accounting for 80.65% and 67.65% of AUC0-24h in male and female rats, respectively. Based on the structures, linaprazan glurate was mainly hydrolyzed into linaprazan, followed by a series of oxidation, dehydrogenation, and glucuronidation in rats. Besides, CES2 is the main metabolic enzyme involved in the hydrolysis of linaprazan glurate to linaprazan.

Potassium-competitive acid blockers and gastroesophageal reflux disease

Proton pump inhibitors (PPIs), the most commonly used antisecretory medi-cations in the management of reflux illness, virtually eliminate elective surgery for ulcer disease, and relegate anti-reflux surgery to patients with gastroesophageal reflux disease (GERD) who are inadequately managed by medical therapy. However, PPI medications still leave some therapeutic demands of GERD unmet. Furthermore, up to 40%-55% of daily PPI users have chronic symptoms, due to PPI refractoriness. Potassium-competitive acid blockers (P-CABs) transcend many of the problems and limits of PPIs, delivering quick, powerful, and extended acid suppression and allowing for treatment of numerous unmet needs. Recently, it has become clear that compromised mucosal integrity plays a role in the etiology of GERD. As a result, esophageal mucosal protection has emerged as a novel and potential treatment approach. An increasing body of research demonstrates that when P-CABs are used as primary drugs or add-on drugs (to regular treatment), they provide a considerable extra benefit, particularly in alleviating symptoms that do not respond to PPI therapy.

Novel Therapies for Gastroesophageal Reflux Disease: Beyond Proton Pump Inhibitors

PURPOSE OF REVIEW

Despite the many areas of unmet needs in gastroesophageal reflux disease (GERD), proton pump inhibitors (PPIs) remain the cornerstone of medical therapy. However, since their introduction, the therapeutic limitations of PPIs in GERD management have been increasingly recognized.

RECENT FINDINGS

In this review we discuss the new medical, endoscopic, and surgical therapeutic modalities that have been developed over the last decade. They include the potassium-competitive acid blockers (P-CABs) which provide a rapid onset, prolonged, and profound acid suppression, mucosal protectants which promote the physiological protective barrier of the esophageal mucosa, new prokinetics and neuromodulators. There are growing numbers of novel therapeutic endoscopic techniques that are under investigation or were recently introduced into the market, further expanding our therapeutic armamentarium for GERD. The development of diverse therapeutic modalities for GERD, despite the availability of PPIs, suggests that there are many areas of unmet need in GERD that will continue and drive future exploration for novel therapies.

[Gastroesophageal Reflux Disease]

Gastroesophageal reflux disease (GERD) is a condition that develops when reflux of stomach contents causes troublesome symptoms and/or complications. The prevalence of GERD is increasing worldwide and in Asia-Pacific. The latest Korean guidelines for GERD were published in 2012, and several international guidelines and consensus statements for the management of GERD have also been recently published. Here, we review these guidelines and consensus statements in order to provide a better understanding of the diagnosis and treatment of GERD.

Intestinal response to salinity challenge in the Senegalese sole (Solea senegalensis)

Fish are continuously forced to actively absorb or expel water and ions through epithelia. Most studies have focused on the gill due to its role in Na⁺ and Cl^- trafficking. However, comparatively few studies have focused on the changing function of the intestine in response to external salinity. Therefore, the present study investigated the main intestinal changes of long-term acclimation of the Senegalese sole (Solea senegalensis) to 5, 15, 38 and 55ppt. Through the measurement of short-circuit current (Isc) in Ussing chambers and biochemical approaches, we described a clear anterior/posterior functional regionalization of the intestine in response to salinity. The use of specific inhibitors in Ussing chamber experiments, revealed that the bumetanide-sensitive Na⁺/K⁺/Cl^- co-transporters are the main effectors of Cl^- uptake in both anterior intestine and rectum. Additionally, the use of the anion exchanger specific inhibitor, DIDS, showed a salinity/region dependency of anion exchanger function. Moreover, we also described ouabain-sensitive Na⁺/K⁺-ATPase (NKA) and Bafilomycin A1-sensitive H⁺-ATPase activities (HA), which displayed changes related to salinity and intestinal region. However, the most striking result of the present study is the description of an omeprazole-sensitive H⁺/K⁺-ATPase (HKA) in the rectum of Senegalese sole. Its activity was consistently measurable and increased at lower salinities, reaching rates even higher than those of the NKA. Together our results provide new insights into the changing role of the intestine in response to external salinity in teleost fish. The rectal activity of HKA offers an alternative/cooperative mechanism with the HA in the final processing of intestinal water absorption by apical titration of secreted bicarbonate.

The binding selectivity of vonoprazan (TAK-438) to the gastric H+, K+ -ATPase

BACKGROUND

The gastric H(+) ,K(+) -ATPase is the preferred target for acid suppression. Until recently, the only drugs that effectively inhibited this ATPase were the proton pump inhibitors (PPIs). PPIs are acid-activated prodrugs that require acid protection. Once acid-activated, PPIs bind to cysteines of the ATPase, resulting in covalent, long-lasting inhibition. The short plasma half-life of PPIs and continual de novo synthesis of the H(+) ,K(+) -ATPase result in difficulty controlling night-time acid secretion. A new alternative to PPIs is the pyrrolo-pyridine, vonoprazan (TAK-438), a potassium-competitive acid blocker (PCAB) that does not require acid protection. In contrast to other PCABs, vonoprazan has a long duration of action, resulting in 24-h control of acid secretion, a high pKa of 9.37 and high affinity (Ki = 3.0 ηmol/L).

AIM

To determine binding selectivity of vonoprazan for the gastric H(+) ,K(+) -ATPase and to explain its slow dissociation.

METHODS

Gastric gland and parietal cell binding of vonoprazan was determined radiometrically. Molecular modelling explained the slow dissociation of vonoprazan from the H(+) ,K(+) -ATPase.

RESULTS

Vonoprazan binds selectively to the parietal cell, independent of acid secretion. Vonoprazan binds in a luminal vestibule between the surfaces of membrane helices 4, 5 and 6. Exit of the drug to the lumen is hindered by asp137 and asn138 in the loop between TM1 and TM2, which presents an electrostatic barrier to movement of the sulfonyl group of vonoprazan. This may explain its slow dissociation from the H(+) ,K(+) -ATPase and long-lasting inhibition.

CONCLUSION

The binding model provides a template for design of novel potassium-competitive acid blockers.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Evolving pharmacological approaches in gastroesophageal reflux disease

INTRODUCTION

Proton pump inhibitors (PPIs) have considerably improved quality of life in patients with gastroesophageal reflux disease (GERD). However, many patients remain symptomatic despite standard PPI therapy.

AREAS COVERED

This review focuses on evolving therapeutic strategies related to the pathophysiological processes of GERD and insufficient response to PPIs. Several clinical trials evaluated new PPI formulations and newer types of acid-suppressive drugs. These studies have evaluated traditional end points in GERD, but have not shown clinical superiority to current PPIs. Novel therapeutic strategies targeting underlying mechanisms of GERD, such as transient lower esophageal sphincter relaxations (TLESRs) and esophageal hypersensitivity, are being developed for add-on therapy to PPIs. Prokinetic drugs may also have some potential in the add-on treatment of GERD with insufficient response to PPIs. Add-on studies are hampered by insufficient information on optimal patient selection and lack of established end points.

EXPERT OPINION

Newer drugs for symptomatic control in GERD have largely focused on improved acid suppression, without evidence of clinical superiority. Drugs targeting esophageal motility and sensitivity to be used as add-onc therapy in PPI insufficient responders have not reached Phase III trials to date, due to difficulties with patient selection, tolerability and end points.

Update on the mechanisms of gastric acid secretion

Acid-related disorders represent a major healthcare concern. In recent years, our understanding of the physiologic processes underlying gastric acid secretion has improved notably. The identity of several apical ion transport proteins, which are necessary for acid secretion to take place, has been resolved. The recent developments have uncovered potential therapeutic targets for the treatment of acid-related disorders. This brief review provides an update on the mechanisms of gastric acid secretion, with a particular focus on apical ion transport.

Revisiting the parietal cell

The parietal cell is responsible for secreting concentrated hydrochloric acid into the gastric lumen. To fulfill this task, it is equipped with a broad variety of functionally coupled apical and basolateral ion transport proteins. The concerted scientific effort over the last years by a variety of researchers has provided us with the molecular identity of many of these transport mechanisms, thereby contributing to the clarification of persistent controversies in the field. This article will briefly review the current model of parietal cell physiology and ion transport in particular and will update the existing models of apical and basolateral transport in the parietal cell.

Medical management of gastroesophageal reflux disease

Antisecretory therapies that raise intragastric pH provide the best healing of the esophageal mucosal damage that occurs in gastroesophageal reflux disease. Continuous maintenance therapy is also effective to reduce the likelihood of recurrence of esophagitis and control symptoms in the long term. Proton pump inhibitor (PPI) therapy is an effective approach for healing esophagitis and controlling symptoms. Endoscopic and surgical treatments may provide an option for patients who are refractory to PPIs in whom reflux has been clearly demonstrated. Long-term antireflux medication is often needed after surgical treatment because of persisting or recurrent pathologic reflux and symptoms. An alternative approach to controlling transient lower esophageal sphincter relaxations, such as the GABA-B agonists, deserves further study.

The gastric HK-ATPase: structure, function, and inhibition

The gastric H,K-ATPase, a member of the P(2)-type ATPase family, is the integral membrane protein responsible for gastric acid secretion. It is an alpha,beta-heterodimeric enzyme that exchanges cytoplasmic hydronium with extracellular potassium. The catalytic alpha subunit has ten transmembrane segments with a cluster of intramembranal carboxylic amino acids located in the middle of the transmembrane segments TM4, TM5,TM6, and TM8. Comparison to the known structure of the SERCA pump, mutagenesis, and molecular modeling has identified these as constituents of the ion binding domain. The beta subunit has one transmembrane segment with N terminus in cytoplasmic region. The extracellular domain of the beta subunit contains six or seven N-linked glycosylation sites. N-glycosylation is important for the enzyme assembly, maturation, and sorting. The enzyme pumps acid by a series of conformational changes from an E(1) (ion site in) to an E(2) (ion site out) configuration following binding of MgATP and phosphorylation. Several experimental observations support the hypothesis that expulsion of the proton at 160 mM (pH 0.8) results from movement of lysine 791 into the ion binding site in the E(2)P configuration. Potassium access from the lumen depends on activation of a K and Cl conductance via a KCNQ1/KCNE2 complex and Clic6. K movement through the luminal channel in E(2)P is proposed to displace the lysine along with dephosphorylation to return the enzyme to the E(1) configuration. This enzyme is inhibited by the unique proton pump inhibitor class of drug, allowing therapy of acid-related diseases.

Gastric secretion

PURPOSE OF REVIEW

This review summarizes the past year's literature regarding the regulation of gastric exocrine and endocrine secretion.

RECENT FINDINGS

Gastric acid secretion is tightly regulated by overlapping neural, hormonal, paracrine, and intracellular pathways in order to achieve the correct amount of acid secretion required by the specific situation. Too little acid can interfere with the absorption of iron, calcium, vitamin B12, and certain drugs as well as predispose to enteric infection, bacterial overgrowth, and gastric malignancy. Too much acid can induce esophageal, gastric, and duodenal injury. Gastrin, histamine, acetylcholine, and ghrelin stimulate whereas somatostatin, cholecystokinin, atrial natriuretic peptide, and nitric oxide inhibit acid secretion. Most patients infected with Helicobacter pylori manifest a pangastritis and produce less than normal amounts of acid; those with antral predominant gastritis, however, are hypergastrinemic and produce increased amounts of acid. Improved understanding of the channels and receptors that are required for and regulate H+K+-ATPase activity should lead to the development of novel antisecretory agents.

SUMMARY

A better understanding of the pathways regulating gastric secretions should lead to new strategies to prevent and treat a variety of gastric disorders such as gastroesophageal reflux disease, autoimmune gastritis, gastric cancer, and functional dyspepsia.

Medical therapy of gastroesophageal reflux disease

PURPOSE OF REVIEW

Proton pump inhibitors remain the mainstay of medical therapy in gastroesophageal reflux disease. Despite their increasing use, up to 40% of patients are not fully satisfied with their antireflux therapy. Recent data on efficacy and safety are reviewed and causes of failure are discussed.

RECENT FINDINGS

Several randomized studies and a metaanalysis have shown marginal differences in efficacy between various proton pump inhibitor regimens. In subgroups, however, such as severe esophagitis, esomeprazole may be superior. Poor compliance is one of the main causes of failure. Nonacid reflux is likely to play an important role, especially in patients with regurgitation or cough persisting on therapy. Genetic polymorphisms involved in proton pump inhibitor metabolism, Helicobacter pylori infection or nocturnal acid breakthrough during therapy are probably less important than initially suspected. Recent pharmacological developments include new proton pump inhibitor isomers, potassium competitive acid blockers and inhibitors of transient lower esophageal sphincter relaxations.

SUMMARY

There are still important unmet needs in the treatment of gastroesophageal reflux disease. Optimizing acid control is unlikely to improve the condition of the majority of patients with incomplete proton pump inhibitor response. Inhibition of transient lower esophageal sphincter relaxations remains the major pharmacological target for future drug development.

SLC26A7 can function as a chloride-loading mechanism in parietal cells

To date three potential candidates for parietal cell basolateral Cl(-) entry have been described: the highly 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS)-sensitive Cl(-)/HCO(3)(-) exchanger AE2, the HCO(3)(-) and lowly DIDS-sensitive SLC26A7 protein, and the Na(+)-2Cl(-)K(+) cotransporter (NKCC1). In this study we investigate the contribution of these pathways to secretagogue stimulated acid secretion. Individually hand-dissected rat gastric glands were microfluorimetrically monitored for Cl(-) influx and pH(i) changes. Transporter activity was determined by varying ion content and through the use of pharmacological inhibitors. Expression of SLC26A7 in rat parietal cells was shown by immunohistochemistry and Western blot. SLC26A7 was inhibited by 5-Nitro-2-(3-phenylpropyl-amino)benzoic acid (NPPB) (100 microM) in the Xenopus laevis oocyte expression system. Cl(-) influx in parietal cells was enhanced by histamine, depended partially on endogenous HCO(3)(-) synthesis and completely on extracellular Na(+). Removal and subsequent readdition of Cl(-) revealed a low and a high DIDS-sensitive HCO(3)(-) extrusion system contributing to Cl(-) uptake. At acidic pH(i), however, H(+) extrusion via the H(+),K(+)-ATPase depending on Cl(-) uptake was abolished only in the presence of 100 microM (NPPB) and at high (250 microM) DIDS concentration. There was no effect of the NKCC inhibitor bumetanide on stimulated H(+) extrusion. These results would be compatible with SLC26A7 as a Cl(-) uptake system under histamine stimulation.