Inhibition of gastric H+,K+-ATPase and acid secretion by SCH 28080, a substituted pyridyl(1,2a)imidazole

Neuroendocrine mechanism of gastric acid secretion: Historical perspectives and recent developments in physiology and pharmacology

The physiology of gastric acid secretion is one of the earliest subjects in medical literature and has been continuously studied since 1833. Starting with the notion that neural stimulation alone drives acid secretion, progress in understanding the physiology and pathophysiology of this process has led to the development of therapeutic strategies for patients with acid-related diseases. For instance, understanding the physiology of parietal cells led to the developments of histamine 2 receptor blockers, proton pump inhibitors (PPIs), and recently, potassium-competitive acid blockers. Furthermore, understanding the physiology and pathophysiology of gastrin has led to the development of gastrin/CCK2 receptor (CCK2 R) antagonists. The need for refinement of existing drugs in patients have led to second and third generation drugs with better efficacy at blocking acid secretion. Further understanding of the mechanism of acid secretion by gene targeting in mice has enabled us to dissect the unique role for each regulator to leverage and justify the development of new targeted therapeutics for acid-related disorders. Further research on the mechanism of stimulation of gastric acid secretion and the physiological significances of gastric acidity in gut microbiome is needed in the future.

Deep learning driven de novo drug design based on gastric proton pump structures

Existing drugs often suffer in their effectiveness due to detrimental side effects, low binding affinity or pharmacokinetic problems. This may be overcome by the development of distinct compounds. Here, we exploit the rich structural basis of drug-bound gastric proton pump to develop compounds with strong inhibitory potency, employing a combinatorial approach utilizing deep generative models for de novo drug design with organic synthesis and cryo-EM structural analysis. Candidate compounds that satisfy pharmacophores defined in the drug-bound proton pump structures, were designed in silico utilizing our deep generative models, a workflow termed Deep Quartet. Several candidates were synthesized and screened according to their inhibition potencies in vitro, and their binding poses were in turn identified by cryo-EM. Structures reaching up to 2.10 Å resolution allowed us to evaluate and re-design compound structures, heralding the most potent compound in this study, DQ-18 (N-methyl-4-((2-(benzyloxy)-5-chlorobenzyl)oxy)benzylamine), which shows a Ki value of 47.6 nM. Further high-resolution cryo-EM analysis at 2.08 Å resolution unambiguously determined the DQ-18 binding pose. Our integrated approach offers a framework for structure-based de novo drug development based on the desired pharmacophores within the protein structure.

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.

Green Synthesis and Antimicrobial Activity of Some Novel N-Arylimidazo[1,2-a]pyrazine-2-Carboxamide Derivatives

The article deals with the synthesis of some novel N-arylimidazo[1,2-a]pyrazine-2-carboxamides (7a-l) by condensation reaction of imidazo[1,2-a]pyrazine-2-carboxylic acid (5) with different aliphatic/aromatic amines (6a-l) by using 1-methylimidazole, Mukaiyama’s reagent and 2-chloro-1-methylpyridinium iodide under microwave irradiation conditions. A new series of compounds 7 have been prepared from 2-iodopyrazine (1). Compound 1 on purged with ammonia gas in the presence of Cu2O and K2CO3 furnishes pyrazin-2-amine (2), which is treated with ethyl 3-bromo-2-oxopropanoate (3) to produce ethyl imidazo[1,2-a]pyrazine-2-carboxylate (4), which on hydrolysis with NaOH yields imidazo[1,2-a]pyrazine-2-carboxylic acid (5). The structures of the newly synthesized compounds have been elucidated on the basis of spectral (IR, 1H and 13C NMR and MS) and analytical data. Compounds 7a-l have also been screened for their antimicrobial activity. Some of the compounds exhibit promising antimicrobial activity

In vivo antiulcer activity, phytochemical exploration, and molecular modelling of the polyphenolic-rich fraction of Crepis sancta extract

Bioactivity-guided investigation of the methanol extract of Crepis sancta aerial parts, collected off Al-Tafilah, South Jordan, was applied, and in this study, the extract was explored for its phytochemical components and in vivo antiulcer activity. In addition, a docking study involving the purified compounds with the newly crystalized gastric proton pump (PDB # 5YLU) was performed. In-depth phytochemical investigation using the state-of-the-art chromatographic and analytical techniques was implemented resulting in the identification of two eudesmane-type sesquiterpenoids, 3-oxo-γ-costic acid (1) and its methyl ester (2) together with seven different methoxylated flavonols (3-9) as the extract's major components. The in vivo antiulcer study at three different doses (50, 100, and 200 mg/kg) against ethanol-induced gastric ulcer in male albino rats, compared to omeprazole (20 mg/kg) as a standard proton pump inhibitor antiulcer drug, revealed that the tested extract, at the middle and the highest doses, featured comparable or even superior activities relative to omeprazole as deduced from histopathological examination, in particular with regard to reducing inflammatory cell infiltration and ceasing mucosal haemorrhage. The tested extract revealed also a dose-dependent reduction in the volume and titrable acidity of the gastric juice together with a dose-dependent increase in the protective gastric mucin content which may explain the noticeable gastroprotective effect. Molecular modelling study of the isolated compounds showed a binding mode similar to the co-crystallized substrate vonoprazan in 5YLU which strengthens the importance of the tested extract as a potential natural remedy for treating gastric ulcer.

Editorial: control of acid secretion

Linked Content

This article is linked to Sunwoo et al paper. To view this article visit https://doi.org/10.1111/apt.14818.

Designed inhibitors with hetero linkers for gastric proton pump H+,K+-ATPase: Steered molecular dynamics and metadynamics studies

Acid suppressant SCH28080 and its derivatives reversibly reduce acid secretion activity of the H⁺,K⁺-ATPase in a K⁺ competitive manner. The results on homologation of the SCH28080 by varying the linker chain length suggested the improvement in efficacy. However, the pharmacokinetic studies reveal that the hydrophobic nature of the CH₂ linker units may not help it to function as a better acid suppressant. We have exploited the role of linker unit to enhance the efficacy of such reversible acid suppressant drug molecules using hetero linker, i.e., disulfide and peroxy linkers. The logarithm of partition coefficient defined for a drug molecule relates to the partition coefficient, which allows the optimum solubility characteristics to reach the active site. The logarithm of partition coefficient calculated for the designed inhibitors suggests that inhibitors would possibly reach the active site in sufficient concentration like in the case of SCH28080. The steered molecular dynamics studies have revealed that the Inhibitor-1 with disulfide linker unit is more stable at the active site due to greater noncovalent interactions compared to the SCH28080. Centre of mass distance analysis suggests that the Cysteine-813 amino acid residue selectively plays an important role in the inhibition of H⁺,K⁺-ATPase for Inhibitor-1. Furthermore, the quantum chemical calculations with M11L/6-31+G(d,p) level of theory have been performed to account the noncovalent interactions responsible for the stabilization of inhibitor molecules in the active site gorge of the gastric proton pump at different time scale. The hydrogen bonding and hydrophobic interaction studies corroborate the center of mass distance analysis as well. Well-tempered metadynamics free energy surface and center of mass separation analysis for the Inhibitor-1 is in good agreement with the steered molecular dynamics results. The torsional angle of the linker units seems to be crucial for better efficacy of drug molecules. The torsional angle of linker units of SCH28080 (COCH₂C) and of Inhibitor 1 (CSSC) prefers to lie within ∼60°-90° for a longer time during the simulations, whereas, the peroxy linker (COOC) of Inhibitor 2 prefers to adopt ∼120-160°. Therefore, it appears that the smaller torsion angle of linker units can achieve better interactions with the active site residues of H⁺,K⁺-ATPase to inhibit the acid secretion activity. The reversible drug molecules with disulfide linker unit would be a promising candidate as proton pump antagonist to H⁺,K⁺-ATPase.

The cryo-EM structure of gastric H+,K+-ATPase with bound BYK99, a high-affinity member of K+-competitive, imidazo[1,2-a]pyridine inhibitors

The gastric proton pump H⁺,K⁺-ATPase acidifies the gastric lumen, and thus its inhibitors, including the imidazo[1,2-a]pyridine class of K⁺-competitive acid blockers (P-CABs), have potential application as acid-suppressing drugs. We determined the electron crystallographic structure of H⁺,K⁺-ATPase at 6.5 Å resolution in the E2P state with bound BYK99, a potent P-CAB with a restricted ring structure. The BYK99 bound structure has an almost identical profile to that of a previously determined structure with bound SCH28080, the original P-CAB prototype, but is significantly different from the previously reported P-CAB-free form, illustrating a common conformational change is required for P-CAB binding. The shared conformational changes include a distinct movement of transmembrane helix 2 (M2), from its position in the previously reported P-CAB-free form, to a location proximal to the P-CAB binding site in the present BYK99-bound structure. Site-specific mutagenesis within M2 revealed that D137 and N138, which face the P-CAB binding site in our model, significantly affect the inhibition constant (K_i) of P-CABs. We also found that A335 is likely to be near the bridging nitrogen at the restricted ring structure of the BYK99 inhibitor. These provide clues to elucidate the binding site parameters and mechanism of P-CAB inhibition of gastric acid secretion.

The gastric H,K-ATPase in stria vascularis contributes to pH regulation of cochlear endolymph but not to K secretion

BACKGROUND

Disturbance of acid-base balance in the inner ear is known to be associated with hearing loss in a number of conditions including genetic mutations and pharmacologic interventions. Several previous physiologic and immunohistochemical observations lead to proposals of the involvement of acid-base transporters in stria vascularis.

RESULTS

We directly measured acid flux in vitro from the apical side of isolated stria vascularis from adult C57Bl/6 mice with a novel constant-perfusion pH-selective self-referencing probe. Acid efflux that depended on metabolism and ion transport was observed from the apical side of stria vascularis. The acid flux was decreased to about 40 % of control by removal of the metabolic substrate (glucose-free) and by inhibition of the sodium pump (ouabain). The flux was also decreased a) by inhibition of Na,H-exchangers by amiloride, dimethylamiloride (DMA), S3226 and Hoe694, b) by inhibition of Na,2Cl,K-cotransporter (NKCC1) by bumetanide, and c) by the likely inhibition of HCO3/anion exchange by DIDS. By contrast, the acid flux was increased by inhibition of gastric H,K-ATPase (SCH28080) but was not affected by an inhibitor of vH-ATPase (bafilomycin).  K flux from stria vascularis was reduced less than 5 % by SCH28080.

CONCLUSIONS

These observations suggest that stria vascularis may be an important site of control of cochlear acid-base balance and demonstrate a functional role of several acid-base transporters in stria vascularis, including basolateral H,K-ATPase and apical Na,H-exchange. Previous suggestions that H secretion is mediated by an apical vH-ATPase and that basolateral H,K-ATPase contributes importantly to K secretion in stria vascularis are not supported. These results advance our understanding of inner ear acid-base balance and provide a stronger basis to interpret the etiology of genetic and pharmacologic cochlear dysfunctions that are influenced by endolymphatic pH.

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.

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 H+,K+-ATPase

The gastric H(+),K(+)-ATPase is responsible for gastric acid secretion. This ATPase is composed of two subunits, the catalytic α subunit and the structural β subunit. The α subunit with molecular mass of about 100 kDa has 10 transmembrane domains and is strongly associated with the β subunit with a single transmembrane segment and a peptide mass of 35 kDa. Its three-dimensional structure is based on homology modeling and site-directed mutagenesis resulting in a proton extrusion and K(+) reabsorption model. There are three conserved H3O(+)-binding sites in the middle of the membrane domain and H3O(+) secretion depends on a conformational change involving Lys(791) insertion into the second H3O(+) site enclosed by E795, E820, and D824 that allows export of protons at a concentration of 160 mM. K(+) countertransport involves binding to this site after the release of protons with retrograde displacement of Lys(791) and then K(+) transfer to E343 and exit to the cytoplasm. This ATPase is the major therapeutic target in treatment of acid-related diseases and there are several known luminal inhibitors allowing analysis of the luminal vestibule. One class contains the acid-activated covalent, thiophilic proton pump inhibitors, the most effective of current acid-suppressive drugs. Their binding sites and trypsinolysis allowed identification of all ten transmembrane segments of the ATPase. In addition, various K(+)-competitive inhibitors of the ATPase are being developed, with the advantage of complete and rapid inhibition of acid secretion independent of pump activity and allowing further refinement of the structure of the luminal vestibule of the E2 form of this ATPase.

Control of gastric H,K-ATPase activity by cations, voltage and intracellular pH analyzed by voltage clamp fluorometry in Xenopus oocytes

Whereas electrogenic partial reactions of the Na,K-ATPase have been studied in depth, much less is known about the influence of the membrane potential on the electroneutrally operating gastric H,K-ATPase. In this work, we investigated site-specifically fluorescence-labeled H,K-ATPase expressed in Xenopus oocytes by voltage clamp fluorometry to monitor the voltage-dependent distribution between E₁P and E₂P states and measured Rb⁺ uptake under various ionic and pH conditions. The steady-state E₁P/E₂P distribution, as indicated by the voltage-dependent fluorescence amplitudes and the Rb⁺ uptake activity were highly sensitive to small changes in intracellular pH, whereas even large extracellular pH changes affected neither the E₁P/E₂P distribution nor transport activity. Notably, intracellular acidification by approximately 0.5 pH units shifted V_0.5, the voltage, at which the E₁P/E₂P ratio is 50∶50, by −100 mV. This was paralleled by an approximately two-fold acceleration of the forward rate constant of the E₁P→E₂P transition and a similar increase in the rate of steady-state cation transport. The temperature dependence of Rb⁺ uptake yielded an activation energy of ∼90 kJ/mol, suggesting that ion transport is rate-limited by a major conformational transition. The pronounced sensitivity towards intracellular pH suggests that proton uptake from the cytoplasmic side controls the level of phosphoenzyme entering the E₁P→E₂P conformational transition, thus limiting ion transport of the gastric H,K-ATPase. These findings highlight the significance of cellular mechanisms contributing to increased proton availability in the cytoplasm of gastric parietal cells. Furthermore, we show that extracellular Na⁺ profoundly alters the voltage-dependent E₁P/E₂P distribution indicating that Na⁺ ions can act as surrogates for protons regarding the E₂P→E₁P transition. The complexity of the intra- and extracellular cation effects can be rationalized by a kinetic model suggesting that cations reach the binding sites through a rather high-field intra- and a rather low-field extracellular access channel, with fractional electrical distances of ∼0.5 and ∼0.2, respectively.

Ilaprazole, a new proton pump inhibitor, is primarily metabolized to ilaprazole sulfone by CYP3A4 and 3A5

Ilaprazole is a new proton pump inhibitor, designed for treatment of gastric ulcers, and developed by Il-Yang Pharmaceutical Co (Seoul, Korea). It is extensively metabolised to the major metabolite ilaprazole sulfone. In the present study, several in vitro approaches were used to identify the cytochrome P450 (CYP) enzymes responsible for ilaprazole sulfone formation. Concentrations of ilaprazole sulfone were determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Incubation of ilaprazole with cDNA-expressed recombinant CYPs indicated that CYP3A was the major enzyme that catalyses ilaprozole to ilaprazole sulfone. This reaction was inhibited significantly by ketoconazole, a CYP3A inhibitor, and azamulin, a mechanism-based inhibitor of CYP3A, while no substantial effect was observed using selective inhibitors for eight other P450s (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP2E1). In addition, the formation of ilaprazole sulfone correlated well with CYP3A-catalysed testosterone 6β-hydroxylation and midazolam 1'-hydroxylation in 20 different human liver microsome panels. The intrinsic clearance of the formation of ilaprazole sulfone by CYP3A4 was 16-fold higher than that by CYP3A5. Collectively, these results indicate that the formation of the major metabolite of ilaprazole, ilaprazole sulfone, is predominantly catalysed by CYP3A4/5.

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.

Characterization of a novel potassium-competitive acid blocker of the gastric H,K-ATPase, 1-[5-(2-fluorophenyl)-1-(pyridin-3-ylsulfonyl)-1H-pyrrol-3-yl]-N-methylmethanamine monofumarate (TAK-438)

Inhibition of the gastric H,K-ATPase by the potassium-competitive acid blocker (P-CAB) 1-[5-(2-fluorophenyl)-1-(pyridin-3-ylsulfonyl)-1H-pyrrol-3-yl]-N-methylmethanamine (TAK-438), is strictly K(+)-competitive with a K(i) of 10 nM at pH 7. In contrast to previous P-CABs, this structure has a point positive charge (pK(a) 9.06) allowing for greater accumulation in parietal cells compared with previous P-CABs [e.g., (8-benzyloxy-2-methyl-imidazo(1,2-a)pyridin-3-yl)acetonitrile (SCH28080), pK(a) 5.6]. The dissociation rate of the compound from the isolated ATPase is slower than other P-CABs, with the t(1/2) being 7.5 h in 20 mM KCl at pH 7. The stoichiometry of binding of TAK-438 to the H,K-ATPase is 2.2 nmol/mg in the presence of Mg-ATP, vanadate, or MgP(i). However, TAK-438 also binds enzyme at 1.3 nmol/mg in the absence of Mg(2+). Modeling of the H,K-ATPase to the homologous Na,K-ATPase predicts a close approach and hydrogen bonding between the positively charged N-methylamino group and the negatively charged Glu795 in the K(+)-binding site in contrast to the planar diffuse positive charge of previous P-CABs. This probably accounts for the slow dissociation and high affinity. The model also predicts hydrogen bonding between the hydroxyl of Tyr799 and the oxygens of the sulfonyl group of TAK-438. A Tyr799Phe mutation resulted in a 3-fold increase of the dissociation rate, showing that this hydrogen bonding also contributes to the slow dissociation rate. Hence, this K(+)-competitive inhibitor of the gastric H,K-ATPase should provide longer-lasting inhibition of gastric acid secretion compared with previous drugs of this class.

Conformational rearrangement of gastric H(+),K(+)-ATPase induced by an acid suppressant

Acid-related gastric diseases are associated with disorder of digestive tract acidification. The gastric proton pump, H(+),K(+)-ATPase, exports H(+) in exchange for luminal K(+) to generate a highly acidic environment in the stomach, and is a main target for acid suppressants. Here, we report the three-dimensional structure of gastric H(+),K(+)-ATPase with bound SCH28080, a representative K(+)-competitive acid blocker, at 7 Å resolution based on electron crystallography of two-dimensional crystals. The density of the bound SCH28080 is found near transmembrane (TM) helices 4, 5 and 6, in the luminal cavity. The SCH28080-binding site is formed by the rearrangement of TM helices, which is in turn transmitted to the cytoplasmic domains, resulting in a luminal-open conformation. These results represent the first structural evidence for a binding site of an acid suppressant on H(+),K(+)-ATPase, and the conformational change induced by this class of drugs.

Mineralocorticoids stimulate the activity and expression of renal H+,K+-ATPases

In the renal collecting duct, mineralocorticoids drive Na(+) reabsorption, K(+) secretion, and H(+) secretion through coordinated actions on apical and basolateral transporters. Whether mineralocorticoids act through H(+),K(+)-ATPases to maintain K(+) and acid-base homeostasis is unknown. Here, treatment of mice with the mineralocorticoid desoxycorticosterone pivalate (DOCP) resulted in weight gain, a decrease in blood [K(+)] and [Cl(-)], and an increase in blood [Na(+)] and [HCO(3)(-)]. DOCP treatment increased the rate of H(+),K(+)-ATPase-mediated H(+) secretion in intercalated cells of the inner cortical collecting duct. mRNA expression of the catalytic subunit HKα(1) did not significantly change, whereas HKα(2) mRNA expression dramatically increased in the outer and inner medulla of DOCP-treated mice. A high-K(+) diet abrogated this increase in renal HKα(2) expression, showing that DOCP-mediated stimulation of HKα(2) expression depends on dietary K(+) intake. DOCP treatment of mice lacking HKα(1) (HKα(1)(-/-)) resulted in greater urinary Na(+) retention than observed in either wild-type mice or mice lacking both HKα(1) and HKα(2) (HKα(1,2)(-/-)). DOCP-treated HKα(1,2)(-/-) mice exhibited a lower blood [HCO(3)(-)] and less Na(+) and K(+) retention than either wild-type or HKα(1)(-/-) mice. Taken together, these results indicate that H(+),K(+)-ATPases-especially the HKα(2)-containing H(+),K(+)-ATPases-play an important role in the effects of mineralocorticoids on K(+), acid-base, and Na(+) balance.

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.

Pharmacodynamic and pharmacokinetic evaluation of CS-526 in cynomolgus monkeys

In the present study, we evaluated the effect of the novel acid pump antagonist 7-(4-fluorobenzyloxy)-2,3-dimethyl-1-{[(1S,2S)-2-methylcyclopropyl]methyl}-1H-pyrrolo[2,3-d]pyridazine (CS-526) on the intragastric acidity of cynomolgus monkeys. The study was performed in a crossover manner with five male animals. CS-526 was administered orally or intravenously at doses of 3.0, 10 and 30 mg/kg, or 0.3, 1.0 and 3.0 mg/kg, respectively. The time period in which the intragastric pH was 4.0 or more (Time(pH > or = 4.0)) and the median pH were calculated for 24 h after the administration. The intragastric pH was elevated after CS-526 treatment. The Time(pH > or = 4.0) was increased in a dose-dependent manner (p = 0.0292) in the oral administration, and the median pH was also increased in a dose-dependent fashion (p = 0.0491) in the intravenous administration. The plasma concentration of CS-526 and its metabolite R-130185 was increased after oral and intravenous administration of CS-526, except for one animal which did not show any detectable amount of R-130185 after intravenous administration at the lowest dose. The area under the time-concentration curve of the active component was increased in the dose proportional manner after oral and intravenous administration. The absolute bioavailability of the active component was estimated to be approximately 1%. Correlation between the pharmacodynamic parameters and the pharmacokinetic parameters was observed in oral (p = 0.0029-0.0745), but not in intravenous administration (p = 0.0558-0.2789). In conclusion, oral and intravenous administration of CS-526 showed inhibition on gastric acidity in cynomolgus monkeys using intragastric pH-metry and some pharmacokinetic and pharmacodynamic parameters were well correlated.

Heterogeneity of H-K-ATPase-mediated acid secretion along the mouse collecting duct

In the collecting duct (CD), H-K-ATPases function in cation reabsorption and H secretion. This study evaluated H-K-ATPase-mediated H secretion along the mouse CD, measured as EIPA- and luminal bafilomycin A(1)-insensitive intracellular pH (pH(i)) recovery from acute H loading (NH(4)) using BCECF. pH(i) recovery was measured in 1) microperfused cortical, outer medullary, and inner medullary CDs (CCD, OMCD, and IMCD) from C57BL/6J mice fed a normal diet and 2) common murine CD cell lines. H-K-ATPase activity along the native, microperfused CD was greatest in the CCD, less in the OMCD, and least in the IMCD (0.10 +/- 0.02, 0.04 +/- 0.01, and 0.01 +/- 0.002 U/min, respectively). H-K-ATPase activity was 0.30 +/- 0.03 and 0.26 +/- 0.03 in A- and B-type ICs, respectively, and was sensitive to Sch-28080 or ouabain. pH(i) recovery was greatest in the OMCD(1) cell line (0.25 +/- 0.01) and less in mpkCCD(c14) (0.17 +/- 0.01), mIMCD-K2 (0.12 +/- 0.01), and mIMCD-3 (0.05 +/- 0.01) cells. EIPA inhibited the majority of pH(i) recovery in these cells (100%, 64%, 75%, and 80% in mpkCCD(c14), OMCD(1), mIMCD-K2, and mIMCD-3, respectively). In OMCD(1) cells, where EIPA-insensitive pH(i) recovery was greatest, H-K-ATPase activity was 0.10 +/- 0.01 and was significantly inhibited (80%) by Sch-28080. We conclude that 1) H-K-ATPase-mediated H secretion in the native mouse CD is greatest in the ICs of the CCD, 2) A- and B-type ICs possess HKalpha(1) and HKalpha(2) H-K-ATPase activity, and 3) the OMCD(1) cell line best exhibits H-K-ATPase.

The renal H+-K+-ATPases: physiology, regulation, and structure

The H(+)-K(+)-ATPases are ion pumps that use the energy of ATP hydrolysis to transport protons (H(+)) in exchange for potassium ions (K(+)). These enzymes consist of a catalytic alpha-subunit and a regulatory beta-subunit. There are two catalytic subunits present in the kidney, the gastric or HKalpha(1) isoform and the colonic or HKalpha(2) isoform. In this review we discuss new information on the physiological function, regulation, and structure of the renal H(+)-K(+)-ATPases. Evaluation of enzymatic functions along the nephron and collecting duct and studies in HKalpha(1) and HKalpha(2) knockout mice suggest that the H(+)-K(+)-ATPases may function to transport ions other than protons and potassium. These reports and recent studies in mice lacking both HKalpha(1) and HKalpha(2) suggest important roles for the renal H(+)-K(+)-ATPases in acid/base balance as well as potassium and sodium homeostasis. Molecular modeling studies based on the crystal structure of a related enzyme have made it possible to evaluate the structures of HKalpha(1) and HKalpha(2) and provide a means to study the specific cation transport properties of H(+)-K(+)-ATPases. Studies to characterize the cation specificity of these enzymes under different physiological conditions are necessary to fully understand the role of the H(+)-K(+) ATPases in renal physiology.

Time-dependent inactivation of human phenylethanolamine N-methyltransferase by 7-isothiocyanatotetrahydroisoquinoline

Inhibitors of phenylethanolamine N-methyltransferase [PNMT, the enzyme that catalyzes the final step in the biosynthesis of epinephrine (Epi)] may be of use in determining the role of Epi in the central nervous system. Here we describe the synthesis and characterization of 7-SCN tetrahydroisoquinoline as an affinity label for human PNMT.

N-(2-hydroxyethyl)-N,2-dimethyl-8-{[(4R)-5-methyl-3,4-dihydro-2H-chromen-4-yl]amino}imidazo[1,2-a]pyridine-6-carboxamide (PF-03716556), a novel, potent, and selective acid pump antagonist for the treatment of gastroesophageal reflux disease

Inhibition of H(+),K(+)-ATPase is accepted as the most effective way of controlling gastric acid secretion. However, current acid suppressant therapy for gastroesophageal reflux disease, using histamine H(2) receptor antagonists and proton pump inhibitors, does not fully meet the needs of all patients because of their mechanism of action. This study sought to characterize the in vitro and in vivo pharmacology of a novel acid pump antagonist, N-(2-Hydroxyethyl)-N,2-dimethyl-8-{[(4R)-5-methyl-3,4-dihydro-2H-chromen-4-yl]amino}imidazo[1,2-a]pyridine-6-carboxamide (PF-03716556), and to compare it with other acid suppressants. Porcine, canine, and human recombinant gastric H(+),K(+)-ATPase activities were measured by ion-leaky and ion-tight assay. The affinities for a range of receptors, ion channels, and enzymes were determined to analyze selectivity profile. Acid secretion in Ghosh-Schild rats and Heidenhain pouch dogs were measured by titrating perfusate and gastric juice samples. PF-03716556 demonstrated 3-fold greater inhibitory activity than 5,6-dimethyl-2-(4-fluorophenylamino)-4-(1-methyl-1,2,3,4-tetrahydroisoquinoline-2-yl)pyrimidine (revaprazan), the only acid pump antagonist that has been available on the market, in ion-tight assay. The compound did not display any species differences, exhibiting highly selective profile including the canine kidney Na(+),K(+)-ATPase. Kinetics experiments revealed that PF-03716556 has a competitive and reversible mode of action. More rapid onset of action than 5-methoxy-2-{[(4-methoxy-3,5-dimethyl-2-pyridyl)methyl]-sulfinyl}-benzimidazole (omeprazole) and 3-fold greater potency than revaprazan were observed in Ghosh-Schild rats and Heidenhain pouch dogs. PF-03716556, a novel acid pump antagonist, could improve upon or even replace current pharmacological treatment for gastroesophageal reflux disease.

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.

Inhibition of gastric acid secretion by a standardized aqueous extract of Cecropia glaziovii Sneth and underlying mechanism

Cecropia glazioui Sneth (Cecropiaceae) is used in folk medicine in tropical and subtropical Latin America as cardiotonic, diuretic, hypotensive, anti-inflammatory and anti-asthmatic. The hypotensive/antihypertensive activity of the plant aqueous extract (AE) and isolated butanolic fraction (BuF) has been confirmed and putatively related to calcium channels blockade in vascular smooth musculature [Lapa, A.J., Lima-Landman, M.T.R., Cysneiros, R.M, Borges, A.C.R., Souccar, C., Barreta, I.P., Lima, T.C.M., 1999. The Brazilian folk medicine program to validate medicinal plants - a topic in new antihypertensive drug research. In: Hostettman, K., Gupta, M.P., Marston, A. (Eds.), Proceedings Volume, IOCD/CYTED Symposium, Panamá City, Panamá, 23-26 February 1997. Chemistry, Biological and Pharmacological Properties of Medicinal Plants from the Americas. Harwood Academic Publishers, Amsterdam, pp. 185-196; Lima-Landman, M.T., Borges, A.C., Cysneiros, R.M., De Lima, T.C., Souccar, C., Lapa, A.J., 2007. Antihypertensive effect of a standardized aqueous extract of Cecropia glaziovii Sneth in rats: an in vivo approach to the hypotensive mechanism. Phytomedicine 14, 314-320]. Bronchodilation and antidepressant-like activities of both AE and BuF have been also shown [Delarcina, S., Lima-Landman, M.T., Souccar, C., Cysneiros, R.M., Tanae, M.M., Lapa, A.J., 2007. Inhibition of histamine-induced bronchospasm in guinea pigs treated with Cecropia glaziovi Sneth and correlation with the in vitro activity in tracheal muscles. Phytomedicine 14, 328-332; Rocha, F.F., Lima-Landman, M.T., Souccar, C., Tanae, M.M., De Lima, T.C., Lapa, A.J., 2007. Antidepressant-like effect of Cecropia glazioui Sneth and its constituents -in vivo and in vitro characterization of the underlying mechanism. Phytomedicine 14, 396-402]. This study reports the antiulcer and antisecretory gastric acid activities of the plant AE, its BuF and isolated compounds with the possible mechanism involved. Both AE and BuF were assayed on gastric acid secretion of pylorus-ligated mice, on acute models of gastric mucosal lesions, and on rabbit gastric H(+), K(+)-ATPase preparations. Intraduodenal injection of AE or BuF (0.5-2.0g/kg, i.d) produced a dose-related decrease of the basal gastric acid secretion in 4-h pylorus-ligated mice. At 1.0g/kg, BuF decreased the volume (28%) and total acidity (33%) of the basal acid secretion, and reversed the histamine (2.5mg/kg, s.c.)- or bethanecol (1.0mg/kg, s.c.)-induced acid secretion to basal values, indicating inhibition of the gastric proton pump. Pretreatment of mice with the BuF (0.05-0.5g/kg, p.o.) protected against gastric mucosal lesions induced by 75% ethanol, indomethacin (30mg/kg, s.c.) or restraint at 4 degrees C. BuF also decreased the gastric H(+), K(+)-ATPase activity in vitro proportionately to the concentration (IC(50)=58.8microg/ml). The compounds isolated from BuF, consisting mainly of cathechins, procyanidins and flavonoids [Tanae, M.M., Lima-Landman, M.T.R., De Lima, T.C.M., Souccar, C., Lapa, A.J., 2007. Chemical standardization of the aqueous extract of Cecropia glaziovii Sneth endowed with antihypertensive, bronchodilator, antacid secretion and antidepressant-like activities. Phytomedicine 14, 309-313], inhibited the in vitro gastric H(+), K(+)-ATPase activity at equieffective concentrations to that of BuF. The results indicate that C. glazioui constituents inhibit the gastric proton pump; this effect may account for the effective antisecretory and antiulcer activities of the standardized plant extract.

Insight into the structural requirements of proton pump inhibitors based on CoMFA and CoMSIA studies

In the present study, a series of 179 quinoline and quinazoline heterocyclic analogues exhibiting inhibitory activity against Gastric (H+/K+)-ATPase were investigated using the comparative molecular field analysis (CoMFA) and comparative molecular similarity indices (CoMSIA) methods. Both the models exhibited good correlation between the calculated 3D-QSAR fields and the observed biological activity for the respective training set compounds. The most optimal CoMFA and CoMSIA models yielded significant leave-one-out cross-validation coefficient, q(2) of 0.777, 0.744 and conventional cross-validation coefficient, r(2) of 0.927, 0.914 respectively. The predictive ability of generated models was tested on a set of 52 compounds having broad range of activity. CoMFA and CoMSIA yielded predicted activities for test set compounds with r(pred)(2) of 0.893 and 0.917 respectively. These validation tests not only revealed the robustness of the models but also demonstrated that for our models r(pred)(2) based on the mean activity of test set compounds can accurately estimate external predictivity. The factors affecting activity were analyzed carefully according to standard coefficient contour maps of steric, electrostatic, hydrophobic, acceptor and donor fields derived from the CoMFA and CoMSIA. These contour plots identified several key features which explain the wide range of activities. The results obtained from models offer important structural insight into designing novel peptic-ulcer inhibitors prior to their synthesis.

Impaired acid secretion in cortical collecting duct intercalated cells from H-K-ATPase-deficient mice: role of HKalpha isoforms

Two classes of H pumps, H-K-ATPase and H-ATPase, contribute to luminal acidification and HCO(3) transport in the collecting duct (CD). At least two H-K-ATPase alpha-subunits are expressed in the CD: HKalpha(1) and HKalpha(2). Both exhibit K dependence but have different inhibitor sensitivities. The HKalpha(1) H-K-ATPase is Sch-28080 sensitive, whereas the pharmacological profile of the HKalpha(2) H-K-ATPase is not completely understood. The present study used a nonpharmacological, genetic approach to determine the contribution of HKalpha(1) and HKalpha(2) to cortical CD (CCD) intercalated cell (IC) proton transport in mice fed a normal diet. Intracellular pH (pH(i)) recovery was determined in ICs using in vitro microperfusion of CCD after an acute intracellular acid load in wild-type mice and mice of the same strain lacking expression of HKalpha(1), HKalpha(2), or both H-K-ATPases (HKalpha(1,2)). A-type and B-type ICs were differentiated by luminal loading with BCECF-AM and peritubular chloride removal from CO(2)/HCO(3)-buffered solutions to identify the membrane locations of Cl/HCO(3) exchange activity. H-ATPase- and Na/H exchange-mediated H transport were inhibited with bafilomycin A(1) (100 nM) and EIPA (10 microM), respectively. Here, we report 1) initial pH(i) and buffering capacity were not significantly altered in the ICs of HKalpha-deficient mice, 2) either HKalpha(1) or HKalpha(2) deficiency resulted in slower acid extrusion, and 3) A-type ICs from HKalpha(1,2)-deficient mice had significantly slower acid extrusion compared with A-type ICs from HKalpha(1)-deficient mice alone. These studies are the first nonpharmacological demonstration that both HKalpha(1) and HKalpha(2) contribute to H secretion in both A-type and B-type ICs in animals fed a normal diet.

A randomized, comparative study of three doses of AZD0865 and esomeprazole for healing of reflux esophagitis

BACKGROUND & AIMS

AZD0865 belongs to a new class of acid-suppressing agents with rapid onset of action and potent acid inhibition. We evaluated its effectiveness for healing reflux esophagitis.

METHODS

One thousand five hundred twenty-one patients with Los Angeles A-D esophagitis and heartburn of moderate or severe intensity for > or = 4 days/week were randomized to AZD0865 25, 50, or 75 mg or esomeprazole 40 mg once daily for 4-8 weeks. The primary end point was esophagitis healing by AZD0865 at 4 weeks. Healing and control of heartburn were also assessed at 2, 4, and 8 weeks for AZD0865 and esomeprazole.

RESULTS

After 4 weeks of treatment, healing rates were similar among AZD0865 doses (76.9%; confidence interval [CI], 72.4%-81.1%); 78.2% (CI, 73.7%-82.3%), and 81.1% (CI, 76.7%-84.9%) for 25, 50, and 75 mg, respectively). The healing rate with esomeprazole at 4 weeks was similar (81.9%; CI, 77.6%-88.7%), and healing rates also were comparable among all treatments at 2 and 8 weeks. There were no significant differences in heartburn control among treatments. AZD0865 and esomeprazole were well-tolerated, although reversible increases in transaminases occurred in a small number of patients receiving AZD0865, especially at the 75-mg dose.

CONCLUSIONS

AZD0865 25, 50, and 75 mg provided similar efficacy to esomeprazole 40 mg in terms of esophagitis healing and heartburn control. These findings suggest that increasing the degree of acid inhibition beyond that already achieved by esomeprazole 40 mg (or AZD0865 25 mg) does not translate into increased clinical efficacy in esophagitis patients.

Systematic expression profiling of the gastric H+/K+ ATPase in human tissue

OBJECTIVE

The potassium-competitive acid blockers (P-CABs), comprise a new, innovative group of competitive and reversible inhibitors of the gastric H+/K+ ATPase. Our aim was to identify sites of expression of the H+/K+ ATPase that are potential targets of these compounds by examining the expression profile of the gastric H+/K+ ATPase in the human body from a broad range of tissues.

MATERIAL AND METHODS

Expression profiling was done by quantitative mRNA analysis (TaqMan PCR). Tissues that were mRNA-positive for the alpha subunit were investigated further by Western blot and immunohistochemistry (IHC) for the presence of gastric H+/K+ ATPase protein.

RESULTS

In addition to the very high expression levels in the stomach, the adrenal gland, cerebellum and pancreas gave unexpectedly positive mRNA signals for the alpha subunit of gastric H +/K+ ATPase. However, they were negative for mRNA of the beta subunit, and Western blot and IHC were negative for alpha and beta subunit protein. Another group of tissues with low alpha subunit mRNA expression including the frontal cortex, cortex grey matter, testis, thymus and larynx submucosa were also found negative for both alpha and beta subunit protein. In contrast to mouse kidney, no gastric H+/K+ ATPase could be detected in human kidney.

CONCLUSIONS

We therefore conclude that the only organ in humans expressing significant levels of the P-CAB target gastric H+/K+ ATPase is the stomach.

Pharmacological profile of novel acid pump antagonist 7-(4-fluorobenzyloxy)-2,3-dimethyl-1-{[(1S,2S)-2-methyl cyclopropyl]methyl}-1H-pyrrolo[2,3-d]pyridazine (CS-526)

The pharmacological profiles of the novel acid pump antagonist 7-(4-fluorobenzyloxy)-2,3-dimethyl-1-{[(1S,2S)-2-methylcyclopropyl]methyl}-1H-pyrrolo[2,3-d]pyridazine (CS-526) were investigated in terms of hog gastric H+,K+-ATPase activity, gastric acid secretion, and acute gastroesophageal lesions in comparison with other proton pump inhibitors (PPIs). CS-526 inhibited H+,K+-ATPase activity in a concentration-dependent manner, with an IC50 value of 61 nM. The inhibitory effect of CS-526 on H+,K+-ATPase activity was more potent than that of any of the other PPIs examined. The inhibitory mechanism of CS-526 on H+,K+-ATPase was a competitive antagonism to the K+ binding site of H+,K+-ATPase, and it was also a reversible inhibition. In pylorus-ligated rats, intraduodenal or oral administration of CS-526 inhibited gastric acid secretion in a dose-dependent manner, and the ID50 values were 2.8 or 0.7 mg/kg, respectively. In Heidenhain pouch dogs, intrapouch administration of CS-526 inhibited histamine-stimulated gastric acid secretion in a dose- and retention time-dependent manner. In a reflux esophagitis model, intraduodenal and oral administration of CS-526 prevented esophageal lesions with ID50 values of 5.4 and 1.9 mg/kg, respectively. Lansoprazole prevented esophagitis only by intraduodenal administration (ID50 = 2.2 mg/kg). Furthermore, CS-526 inhibited acute gastric mucosal lesions. These data demonstrate that the novel acid pump antagonist CS-526 has potent antisecretory and antiulcer effects. These findings indicate that CS-526 would have a curative effect on gastroesophageal reflux disease via its potent antisecretory and antiulcer actions.

The gastric H,K ATPase as a drug target: past, present, and future

The recent progress in therapy if acid disease has relied heavily on the performance of drugs targeted against the H,K ATPase of the stomach and the H2 receptor antagonists. It has become apparent in the last decade that the proton pump is the target that has the likelihood of being the most sustainable area of therapeutic application in the regulation of acid suppression. The process of activation of acid secretion requires a change in location of the ATPase from cytoplasmic tubules into the microvilli of the secretory canaliculus of the parietal cell. Stimulation of the resting parietal cell, with involvement of F-actin and ezrin does not use significant numbers of SNARE proteins, because their message is depleted in the pure parietal cell transcriptome. The cell morphology and gene expression suggest a tubule fusion-eversion event. As the active H,K ATPase requires efflux of KCl for activity we have, using the transcriptome derived from 99% pure parietal cells and immunocytochemistry, provided evidence that the KCl pathway is mediated by a KCQ1/KCNE2 complex for supplying K and CLIC6 for supplying the accompanying Cl. The pump has been modeled on the basis of the structures of different conformations of the sr Ca ATPase related to the catalytic cycle. These models use the effects of site directed mutations and identification of the binding domain of the K competitive acid pump antagonists or the defined site of binding for the covalent class of proton pump inhibitors. The pump undergoes conformational changes associated with phosphorylation to allow the ion binding site to change exposure from cytoplasmic to luminal exposure. We have been able to postulate that the very low gastric pH is achieved by lysine 791 motion extruding the hydronium ion bound to carboxylates in the middle of the membrane domain. These models also allow description of the K entry to form the K liganded form of the enzyme and the reformation of the ion site inward conformation thus relating the catalytic cycle of the pump to conformational models. The mechanism of action of the proton pump inhibitor class of drug is discussed along with the cysteines covalently bound with these inhibitors. The review concludes with a discussion of the mechanism of action and binding regions of a possible new class of drug for acid control, the K competitive acid pump antagonists.

Analysis of the gastric H,K ATPase for ion pathways and inhibitor binding sites

New models of the gastric H,K ATPase in the E1K and E2P states are presented as the first structures of a K+ counter-transport P2-type ATPase exhibiting ion entry and exit paths. Homology modeling was first used to generate a starting conformation from the srCa ATPase E2P form (PDB code 1wpg) that contains bound MgADP. Energy minimization of the model showed a conserved adenosine site but nonconserved polyphosphate contacts compared to the srCa ATPase. Molecular dynamics was then employed to expand the luminal entry sufficiently to allow access of the rigid K+ competitive naphthyridine inhibitor, Byk99, to its binding site within the membrane domain. The new E2P model had increased separation between transmembrane segments M3 through M8, and addition of water in this space showed not only an inhibitor entry path to the luminal vestibule but also a channel leading to the ion binding site. Addition of K+ to the hydrated channel with molecular dynamics modeling of ion movement identified a pathway for K+ from the lumen to the ion binding site to give E2K. A K+ exit path to the cytoplasm operating during the normal catalytic cycle is also proposed on the basis of an E1K homology model derived from the E12Ca2+ form of the srCa ATPase (PDB code 1su4). Autodock analyses of the new E2P model now correctly discriminate between high- and low-affinity K+ competitive inhibitors. Finally, the expanded luminal vestibule of the E2P model explains high-affinity ouabain binding in a mutant of the H,K ATPase [Qiu et al. (2005) J. Biol. Chem. 280, 32349-32355].

Inhibition of the gastric H+,K+ -ATPase by plectrinone A, a diterpenoid isolated from Plectranthus barbatus Andrews

This work assessed the mechanism underlying the antisecretory gastric acid effect of Plectranthus barbatus Andrews (Lamiaceae) and active constituents. Popularly known as "false-boldo", this plant is used in Brazilian folk medicine to treat gastrointestinal and hepatic ailments. The plant aqueous extract (AE) and isolated compounds were assayed in vivo in pylorus-ligated mice, and in vitro on acid secretion measured as [(14)C]-aminopyrine ([(14)C]-AP) accumulation in rabbit gastric glands and gastric H(+),K(+)-ATPase preparations. Injected into the duodenal lumen, the AE of the plant leaves (0.5 and 1.0 g/kg) decreased the volume (62 and 76%) and total acidity (23 and 50%) of gastric acid secretion in pylorus-ligated mice. Bioguided purification of the AE yielded an active fraction (IC(50)=24 microg/ml) that inhibited acid secretion in rabbit gastric glands with a potency 10 to 18 times greater than that of the originating extract, on both the basal and stimulated acid secretion by histamine (His) (1 microM) or bethanechol (100 microM). At the same concentrations the gastric H(+),K(+)-ATPase activity was also inhibited. The active constituent was chemically identified as the abietanoid dienedione plectrinone A which reduced the H(+),K(+)-ATPase activity with IC(50)=171 microM. The results indicate that inhibition of the gastric proton pump by this diterpenoid may account for the antisecretory acid effect and reputed anti ulcer activity of Plectranthus barbatus.

Soraprazan: setting new standards in inhibition of gastric acid secretion

After treatment of millions of patients suffering from gastroesophageal reflux disease (GERD) and other acid-related ailments with proton pump inhibitors, there are still unmet medical needs such as rapid and reliable pain relief, especially for nocturnal acid breakthrough. In this work, we introduce and characterize the biochemistry and pharmacology of the potassium-competitive acid blocker (P-CAB) soraprazan, a novel, reversible, and fast-acting inhibitor of gastric H,K-ATPase. Inhibitory and binding properties of soraprazan were analyzed together with its mode of action, its selectivity, and its in vivo potency. This P-CAB has an IC(50) of 0.1 microM if measured with ion leaky vesicles and of 0.19 microM in isolated gastric glands. With a K(i) of 6.4 nM, a K(d) of 26.4 nM, and a B(max) of 2.89 nmol/mg, this compound is a highly potent and reversible inhibitor of the H,K-ATPase. Soraprazan shows immediate inhibition of acid secretion in various in vitro models and in vivo and was found to be more than 2000-fold selective for H,K-ATPase over Na,K- and Ca-ATPases. Soraprazan is superior to esomeprazole in terms of onset of action and the extent and duration of pH elevation in vivo in the dog. Rapid and consistent inhibition of acid secretion by soraprazan renders the P-CABs a promising group of compounds for therapy of GERD.

Real time measurements of water flow in amphibian gastric glands: modulation via the extracellular Ca2+-sensing receptor

The mechanisms for the formation of the osmotic gradient driving water movements in the gastric gland and its modulation via the extracellular Ca(2+)-sensing receptor (CaR) were investigated. Real time measurements of net water flux in the lumen of single gastric glands of the intact amphibian stomach were performed using ion-selective double-barreled microelectrodes. Water movement was measured by recording changes in the concentration of impermeant TEA(+) ions ([TEA(+)](gl)) with TEA(+)-sensitive microelectrodes inserted in the lumen of individual gastric glands. Glandular K(+) (K(+)(gl)) and H(+) (pH(gl)) were also measured by using K(+)- and H(+)-sensitive microelectrodes, respectively. Stimulation with histamine significantly decreased [TEA](gl), indicating net water flow toward the gland lumen. This response was inhibited by the H(+)/K(+)-ATPase inhibitor, SCH 28080. Histamine also elicited a significant and reversible increase in [K(+)](gl) that was blocked by chromanol 293B, a blocker of KCQN1 K(+) channels. Histamine failed to induce net water flow in the presence of chromanol 293B. In the "resting state," stimulation of CaR with diverse agonists resulted in significant increase in [TEA](gl). CaR activation also significantly reduced histamine-induced water secretion and apical K(+) transport. Our data validate the strong link between histamine-stimulated acid secretion and water transport. We also show that cAMP-dependent [K(+)](gl) elevation prior to the onset of acid secretion generates the osmotic gradient initially driving water into the gastric glands and that CaR activation inhibits this process, probably through reduction of intracellular cAMP levels.

Molecular regulation and physiology of the H+,K+ -ATPases in kidney

Two H(+), K(+)-adenosine triphosphatase (ATPase) proteins participate in K(+) absorption and H(+) secretion in the renal medulla. Both the gastric (HKalpha(1)) and colonic (HKalpha(2)) H(+),K(+)-ATPases have been localized and characterized by a number of techniques, and are known to be highly regulated in response to acid-base and electrolyte disturbances. Both ATPases are dimers of composition alpha/beta that localize to the apical membrane and both interact with the tetraspanin protein CD63. Although CD63 interacts with the carboxy-terminus of the alpha-subunit of the colonic H(+),K(+)-ATPase, it interacts with the beta-subunit of the gastric H(+),K(+)-ATPase. Pharmacologically, both ATPases are distinct; for example, the gastric H(+),K(+)-ATPase is inhibited by Sch-28080, but the colonic H(+),K(+)-ATPase is inhibited by ouabain (a classic inhibitor of the Na(+)-pump) and is completely insensitive to Sch-28080. The alpha-subunit of the colonic H(+),K(+)-ATPase is the only subunit of the X(+),K(+)-ATPase superfamily that has 3 different splice variants that emerge by deletion or elongation of the amino-terminus. The messenger RNA and protein of one of these splice variants (HKalpha(2C)) is specifically up-regulated in newborn rats and becomes undetectable in adult rats. Therefore, HKalpha(2), in addition to its role in potassium and acid-base homeostasis, appears to play a significant role in early growth and development. Finally, because chronic hypokalemia appears to be the most potent stimulus for upregulation of HKalpha(2), we propose that the HKalpha(2) participates importantly in the maintenance of chronic metabolic alkalosis.

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.