Studies on the mechanism of action of the gastric H+,K(+)-ATPase inhibitor SPI-447

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.

Inhibition of gastric H+,K+-ATPase by new dihydropyrazole derivative KYY-008

The gastric proton pump (H⁺,K⁺-ATPase) responsible for the H⁺ secretion of gastric acid is an essential therapeutic target for acid-related diseases. H⁺,K⁺-ATPase belongs to a P₂-type ATPase family. Here, we examined the effects of a newly synthesized dihydropyrazole derivative KYY-008 on the H⁺,K⁺-ATPase. KYY-008 concentration-dependently inhibited the enzyme activity of the ATPase in the membrane fractions prepared from isolated hog gastric mucosa and from human kidney HEK293 cells in which gastric H⁺,K⁺-ATPase is exogenously expressed. The IC₅₀ values in these samples were 3.4 μM and 3.7 μM, respectively. In addition, KYY-008 significantly inhibited the H⁺,K⁺-ATPase-derived H⁺ uptake into the tightly sealed vesicles prepared from the hog gastric mucosa. In contrast, KYY-008 has no effect on the activities of other P₂-type ATPases such as Na⁺,K⁺-ATPase and Ca²⁺-ATPase. KYY-008 did not change the ionic currents of voltage-dependent Ca²⁺ channels, that were potential targets for some dihydropyrazole derivatives. Together, we discovered a new dihydropyrazole derivative which acts as a selective inhibitor of gastric H⁺,K⁺-ATPase.

Potassium-competitive acid blockers: Advanced therapeutic option for acid-related diseases

Acid-related diseases (ARDs), such as peptic ulcers and gastroesophageal reflux disease, represent a major health-care concern. Some major milestones in our understanding of gastric acid secretion and ARD treatment reached during the last 50years include 1) discovery of histamine H₂-receptors and development of H₂-receptor antagonists, 2) identification of H⁺,K⁺-ATPase as the parietal cell proton pump and development of proton pump inhibitors (PPIs), and 3) identification of Helicobacter pylori (H. pylori) as the major cause of peptic ulcers and development of effective eradication regimens. Although PPI treatments have been effective and successful, there are limitations to their efficacy and usage, i.e. short half-life, insufficient acid suppression, slow onset of action, and large variation in efficacy among patients due to CYP2C19 metabolism. Potassium-competitive acid blockers (P-CABs) inhibit H⁺,K⁺-ATPase in a reversible and K⁺-competitive manner, and exhibit almost complete inhibition of gastric acid secretion from the first dose. Many pharmaceutical companies have tried to develop P-CABs, but most of their clinical development has been discontinued due to safety concerns or a similar efficacy to PPIs. Revaprazan was developed in Korea and was the first P-CAB approved for sale. Vonoprazan, approved in 2014 in Japan, has a completely different chemical structure and higher pKa value compared to other P-CABs, and exhibits rapid onset of action and prolonged control of intragastric acidity. Vonoprazan is an effective treatment for ARDs that is especially effective in healing reflux esophagitis and for H. pylori eradication. P-CABs, such as vonoprazan, promise to further improve the management of ARDs.

High-throughput screening of potassium-competitive acid blockers

H(+),K(+)-ATPase is a key enzyme in the process of gastric acid secretion, and proton pump inhibitors (PPIs) have been accepted as one of the most effective treatments for peptic ulcer and gastroesophageal reflux disease. To discover a novel class of PPIs, the authors screened a low-molecular-weight compound library and identified two prospective acid blockers that were pyrrole derivatives. Both compounds inhibited H(+),K(+)-ATPase in a reversible and potassium-competitive manner. These compounds led to the development of TAK-438 (1-[5-(2-fluorophenyl)-1-(pyridin-3-ylsulfonyl)-1H-pyrrol-3-yl]-N-methylmethanamine monofumarate), which is currently undergoing clinical trials as a novel potassium-competitive acid blocker for the treatment of acid-related diseases.

1-[5-(2-Fluorophenyl)-1-(pyridin-3-ylsulfonyl)-1H-pyrrol-3-yl]-N-methylmethanamine monofumarate (TAK-438), a novel and potent potassium-competitive acid blocker for the treatment of acid-related diseases

Proton pump inhibitors (PPIs) are widely used in the treatment of acid-related diseases. However, several unmet medical needs, such as suppression of night-time acid secretion and rapid symptom relief, remain. In this study, we investigated the pharmacological effects of 1-[5-(2-fluorophenyl)-1-(pyridin-3-ylsulfonyl)-1H-pyrrol-3-yl]-N-methylmethanamine monofumarate (TAK-438), a novel potassium-competitive acid blocker (P-CAB), on gastric acid secretion in comparison with lansoprazole, a typical PPI, and SCH28080 [3-(cyanomethyl)-2-methyl,8-(phenylmethoxy)imidazo(1,2-a)pyridine], a prototype of P-CAB. TAK-438, SCH28080, and lansoprazole inhibited H(+),K(+)-ATPase activity in porcine gastric microsomes with IC(50) values of 0.019, 0.14, and 7.6 μM, respectively, at pH 6.5. The inhibitory activity of TAK-438 was unaffected by ambient pH, whereas the inhibitory activities of SCH28080 and lansoprazole were weaker at pH 7.5. The inhibition by TAK-438 and SCH28080 was reversible and achieved in a K(+)-competitive manner, quite different from that by lansoprazole. TAK-438, at a dose of 4 mg/kg (as the free base) orally, completely inhibited basal and 2-deoxy-d-glucose-stimulated gastric acid secretion in rats, and its effect on both was stronger than that of lansoprazole. TAK-438 increased the pH of gastric perfusate to a higher value than did lansoprazole or SCH28080, and the effect of TAK-438 was sustained longer than that of lansoprazole or SCH28080. These results indicate that TAK-438 exerts a more potent and longer-lasting inhibitory action on gastric acid secretion than either lansoprazole or SCH28080. TAK-438 is a novel antisecretory drug that may provide a new option for the patients with acid-related disease that is refractory to, or inadequately controlled by, treatment with PPIs.

Mechanism of action of AZD0865, a K+-competitive inhibitor of gastric H+,K+-ATPase

AZD0865 is a member of a drug class that inhibits gastric H(+),K(+)-ATPase by K(+)-competitive binding. The objective of these experiments was to characterize the mechanism of action, selectivity and inhibitory potency of AZD0865 in vitro. In porcine ion-leaky vesicles at pH 7.4, AZD0865 concentration-dependently inhibited K(+)-stimulated H(+),K(+)-ATPase activity (IC(50) 1.0+/-0.2 microM) but was more potent at pH 6.4 (IC(50) 0.13+/-0.01 microM). The IC(50) values for a permanent cation analogue, AR-H070091, were 11+/-1.2 microM at pH 7.4 and 16+/-1.8 microM at pH 6.4. These results suggest that the protonated form of AZD0865 inhibits H(+),K(+)-ATPase. In ion-tight vesicles, AZD0865 inhibited H(+),K(+)-ATPase more potently (IC(50) 6.9+/-0.4 nM) than in ion-leaky vesicles, suggesting a luminal site of action. AZD0865 inhibited acid formation in histamine- or dibutyryl-cAMP-stimulated rabbit gastric glands (IC(50) 0.28+/-0.01 and 0.26+/-0.003 microM, respectively). In ion-leaky vesicles at pH 7.4, AZD0865 (3 microM) immediately inhibited H(+),K(+)-ATPase activity by 88+/-1%. Immediately after a 10-fold dilution H(+),K(+)-ATPase inhibition was 41%, indicating reversible binding of AZD0865 to gastric H(+),K(+)-ATPase. In contrast to omeprazole, AZD0865 inhibited H(+),K(+)-ATPase activity in a K(+)-competitive manner (K(i) 46+/-3 nM). AZD0865 inhibited the process of cation occlusion concentration-dependently (IC(50) 1.7+/-0.06 microM). At 100 microM, AZD0865 reduced porcine renal Na(+),K(+)-ATPase activity by 9+/-2%, demonstrating a high selectivity for H(+),K(+)-ATPase. Thus, AZD0865 potently, K(+)-competitively, and selectively inhibits gastric H(+),K(+)-ATPase activity and acid formation in vitro, with a fast onset of effect.

Gastric type H+,K+-ATPase in the cochlear lateral wall is critically involved in formation of the endocochlear potential

Cochlear endolymph has a highly positive potential of approximately +80 mV known as the endocochlear potential (EP). The EP is essential for hearing and is maintained by K(+) circulation from perilymph to endolymph through the cochlear lateral wall. Various K(+) transport apparatuses such as the Na(+),K(+)-ATPase, the Na(+)-K(+)-2Cl(-) cotransporter, and the K(+) channels Kir4.1 and KCNQ1/KCNE1 are expressed in the lateral wall and are known to play indispensable roles in cochlear K(+) circulation. The gastric type of the H(+),K(+)-ATPase was also shown to be expressed in the cochlear lateral wall (Lecain E, Robert JC, Thomas A, and Tran Ba Huy P. Hear Res 149: 147-154, 2000), but its functional role has not been well studied. In this study we examined the precise localization of H(+),K(+)-ATPase in the cochlea and its involvement in formation of EP. RT-PCR analysis showed that the cochlea expressed mRNAs of gastric alpha(1)-, but not colonic alpha(2)-, and beta-subunits of H(+),K(+)-ATPase. Immunolabeling of an antibody specific to the alpha(1) subunit was detected in type II, IV, and V fibrocytes distributed in the spiral ligament of the lateral wall and in the spiral limbus. Strong immunoreactivity was also found in the stria vascularis. Immunoelectron microscopic examination exhibited that the H(+),K(+)-ATPase was localized exclusively at the basolateral site of strial marginal cells. Application of Sch-28080, a specific inhibitor of gastric H(+),K(+)-ATPase, to the spiral ligament as well as to the stria vascularis caused prominent reduction of EP. These results may imply that the H(+),K(+)-ATPase in the cochlear lateral wall is crucial for K(+) circulation and thus plays a critical role in generation of EP.

An update on acid secretion

Gastric acid secretion is a complex process that requires hormonal, neuronal, or calcium-sensing receptor activation for insertion of pumps into the apical surface of the parietal cell. Activation of any or all these pathways causes the parietal cell to secrete concentrated acid with a pH at or close to 1. This acidic fluid combines with enzymes that are secreted from neighbouring chief cells and passes out of the gland up through a mucous gel layer covering the surface of the stomach producing a final intragastric pH of less than 4 during the active phase of acid secretion. Defects in either the mucosal barrier or in the regulatory mechanisms that modulate the secretory pathways will result in erosion of the barrier and ulcerations of the stomach or esophagus. The entire process of acid secretion relies on activation of the catalytic cycle of the gastric H+,K+-ATPase, resulting in the secretion of acid into the parietal cell canaliculus, with K+ being the important and rate-limiting ion in this activation process. In addition to K+ as a rate limiter for acid production, Cl- secretion via an apical channel must also occur. In this review we present a discussion of the mechanics of acid secretion and a discussion of recently identified transporter proteins and receptors. Included is a discussion of some of the recent candidates for the apical K' recycling channel, as well as two recently identified apical proteins (NHE-3, PAT-1), and the newly characterized calcium-sensing receptor (CaSR). We hope that this review will give additional insight into the complex process of acid secretion.

Role of potassium in acid secretion

Potassium (K⁺) ions are critical for the activation and catalytic cycle of the gastric H⁺,K⁺-ATPase, resulting in the secretion of hydrochloric acid into the parietal cell canaliculus. As both symptom, severity and esophageal mucosal damage in gastro-esophageal reflux disease (GERD) are related to the degree of acid exposure, K⁺ is a logical target for approaches to inhibit acid production. The probable K⁺ binding site on the gastric H⁺,K⁺-ATPase has recently been described and studies are elucidating how K⁺ activates the enzyme. K⁺ channels in the apical membrane of the parietal cell are implicated in the recycling of K⁺ and, to date, three potential K⁺ channels (KCNQ1, Kir2.1 and Kir4.1) have been identified. The channels represent theoretical sites for agents to control acid secretion but it will be difficult to develop selective blockers. An alternative strategy is to prevent K⁺ from activating gastric H⁺,K⁺-ATPase; the potassium-competitive acid blocker (P-CAB) class inhibits acid secretion by binding at or near the K⁺ binding site. Ongoing research is further defining the role of K⁺ in the functioning of the gastric H⁺,K⁺-ATPase, as well as determining the clinical utility of agents directed toward this important cation.

Potassium-competitive acid blockade: a new therapeutic strategy in acid-related diseases

Current therapies to treat gastroesophageal reflux disease (GERD), peptic ulcer disease (PUD), and other acid-related diseases either prevent stimulation of the parietal cell (H2 receptor antagonists, H2RAs) or inhibit gastric H+,K+-ATPase (e.g., proton pump inhibitors, PPIs). Of the 2 approaches, the inhibition of the final step in acid production by PPIs provides more effective relief of symptoms and healing. Despite the documented efficacy of the PPIs, therapeutic doses have a gradual onset of effect and do not provide complete symptom relief in all patients. There is scope for further improvements in acid suppressive therapy to maximize healing and offer more complete symptom relief. It is unlikely that cholecystokinin2 (CCK2, gastrin) receptor antagonists, a class in clinical trials, will be superior to H2RAs or PPIs. However, a new class of acid suppressant, the potassium-competitive acid blockers (P-CABs), is undergoing clinical trials in GERD and other acid-related diseases. These drugs block gastric H+,K+-ATPase by reversible and K+-competitive ionic binding. After oral doses, P-CABs rapidly achieve high plasma concentrations and have linear, dose-dependent pharmacokinetics. The pharmacodynamic properties reflect the pharmacokinetics of this group (i.e., the effect on acid secretion is correlated with plasma concentrations). These agents dose dependently inhibit gastric acid secretion with a fast onset of action and have similar effects after single and repeated doses (i.e., full effect from the first dose). Animal studies comparing P-CABs with PPIs suggest some important pharmacodynamic differences (e.g., faster and better control of 24-hr intragastric acidity). Studies in humans comparing PPIs with P-CABs will help to define the place of this new class in the management of acid-related diseases.

The cavity structure for docking the K(+)-competitive inhibitors in the gastric proton pump

2-Methyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine-3-acetonitrile (SCH 28080) is a reversible inhibitor specific for the gastric proton pump. The inhibition pattern is competitive with K(+). Here we studied the binding sites of this inhibitor on the putative three-dimensional structure of the gastric proton pump alpha-subunit that was constructed by homology modeling based on the structure of sarcoplasmic reticulum Ca(2+) pump. Alanine and serine mutants of Tyr(801) located in the fifth transmembrane segment of the gastric proton pump alpha-subunit retained the (86)Rb transport and K(+)-dependent ATPase (K(+)-ATPase) activities. These mutants showed 60-80-times lower sensitivity to SCH 28080 than the wild type in the (86)Rb transport activity. The K(+)-ATPase activities of these mutants were not completely inhibited by SCH 28080. The sensitivity to SCH 28080 was dependent on the bulkiness of the side chain at this position. Therefore, the side chain of Tyr(801) is important for the interaction with this inhibitor. In the three-dimensional structure of the E(2) form (conformation with high affinity for K(+)) of the gastric proton pump, Tyr(801) faces a cavity surrounded by the first, fourth, fifth, sixth, and eighth transmembrane segments and fifth/sixth, seventh/eighth, and ninth/tenth loops. SCH 28080 can dock in this cavity. However, SCH 28080 cannot dock in the same location in the E(1) form (conformation with high affinity for proton) of the gastric proton pump due to the drastic rearrangement of the transmembrane helices between the E(1) and E(2) forms. These results support the idea that this cavity is the binding pocket of SCH 28080.