Reversal of antisecretory activity of omeprazole by sulfhydryl compounds in isolated rabbit gastric glands

Lansoprazole inhibits the cysteine protease legumain by binding to the active site

Proton pump inhibitors (PPIs) are prodrugs used in the treatment of peptic ulcer diseases. Once activated by acidic pH, the PPIs subsequently inhibit the secretion of gastric acid by covalently forming disulphide bonds with the SH groups of the parietal proton pump, that is the H⁺ /K⁺ -ATPase. Long-term use of PPIs has been associated with numerous adverse effects, including bone fractures. Considering the mechanism of activation, PPIs could also be active in acidic micro-environments such as in lysosomes, tumours and bone resorption sites. We suggested that the SH group in the active site of cysteine proteases could be susceptible for inhibition by PPIs. In this study, the inhibition by lansoprazole was shown on the cysteine proteases legumain and cathepsin B by incubating purified proteases or cell lysates with lansoprazole at different concentrations and pH conditions. The mechanism of legumain inhibition was shown to be a direct interaction of lansoprazole with the SH group in the active site, and thus blocking binding of the legumain-selective activity-based probe MP-L01. Lansoprazole was also shown to inhibit both legumain and cathepsin B in various cell models like HEK293, monoclonal legumain over-expressing HEK293 cells (M38L) and RAW264.7 macrophages, but not in human bone marrow-derived skeletal (mesenchymal) stem cells (hBMSC-TERT). During hBMSC-TERT differentiation to osteoblasts, lansoprazole inhibited legumain secretion, alkaline phosphatase activity, but had no effects on in vitro mineralization capacity. In conclusion, lansoprazole acts as a direct covalent inhibitor of cysteine proteases via disulphide bonds with the SH group in the protease active site. Such inhibition of cysteine proteases could explain some of the off-target effects of PPIs.

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.

Differences in binding properties of two proton pump inhibitors on the gastric H+,K+-ATPase in vivo

Restoration of acid secretion after treatment with covalently-bound proton pump inhibitors may depend on protein turnover and on reversal of inhibition by reducing agents such as glutathione. Glutathione incubation of the H(+),K(+)-ATPase isolated from omeprazole or pantoprazole-treated rats reversed 88% of the omeprazole inhibition but none of the pantoprazole inhibition. The present study was designed to measure binding properties of omeprazole or pantoprazole in vivo. Rats were injected with (14)C-omeprazole or (14)C-pantoprazole after acid stimulation. The specific binding to the gastric H(+),K(+)-ATPase was measured at timed intervals as well as reversal of binding by glutathione reduction. The stoichiometry of omeprazole and pantoprazole binding to the catalytic subunit of the H(+),K(+)-ATPase was 2 moles of inhibitor per mole of the H(+),K(+)-ATPase phosphoenzyme. Omeprazole bound to one cysteine between transmembrane segments 5/6 and one between 7/8, pantoprazole only to the two cysteines in the TM5/6 domain. Loss of drug from the pump was biphasic, the fast component accounted for 84% of omeprazole binding and 51% of pantoprazole binding. Similarly, only 16% of omeprazole binding but 40% of pantoprazole binding was not reversed by glutathione. The residence time of omeprazole and pantoprazole on the ATPase in vivo depends on the reversibility of binding. Binding of pantoprazole at cysteine 822 is irreversible whereas that of omeprazole at cysteine 813 and 892 is reversible both in vivo and in vitro. This is consistent with the luminal exposure of cysteine 813 and 892 and the intra-membranal location of cysteine 822 in the 3D structure of the H(+),K(+)-ATPase.

Current trends in the treatment of upper gastrointestinal disease

The past 25 years have seen an amazing improvement in the treatment and understanding of acid-related disorders. In particular, the introduction of selective histamine receptor antagonists and proton pump inhibitors has made the medical control of acid secretion an effective means of therapy. The demonstration that infection with Helicobacter pylori is responsible for most cases of peptic ulcer disease resulted in another major improvement in therapy in these areas as a result of the eradication of the organism. Research continues in an attempt to find improved means of acid control and better methods for the eradication of H. pylori based on unique proteins expressed by the organism to resist gastric acidity.

Restoration of acid secretion following treatment with proton pump inhibitors

BACKGROUND & AIMS

Proton pump inhibitors (PPIs) are covalent inhibitors of the gastric H+,K+-adenosine triphosphatase (ATPase) forming disulfide bonds. Recovery of acid secretion after PPI inhibition may be due to de novo synthesis of pump protein and/or disulfide reduction and reactivation of inhibited pump. The half-time of recovery of acid secretion in rats following omeprazole treatment is approximately 15 hours, whereas pump protein half-life is 54 hours. In humans, the half-life of the inhibitory effect on acid secretion is approximately 28 hours for omeprazole and approximately 46 hours for pantoprazole. Whereas all PPIs bind to cysteine 813, pantoprazole additionally binds to cysteine 822, deeper in the membrane domain of TM6. Their different durations of action may reflect different rates of pump reactivation due to differing accessibility of the disulfides to glutathione.

METHODS

Rats were stimulated and treated with 30 mg/kg of each PPI. Gastric ATPase was prepared and reversal of inhibition of the H+,K+-ATPase was measured as the time-dependent restoration of activity by incubation with dithiothreitol or glutathione.

RESULTS

One hundred percent reactivation of ATPase following inhibition in vivo by omeprazole or its enantiomers was seen with dithiothreitol and 89% with glutathione. Similar data were found for lansoprazole or rabeprazole. No reactivation by either reducing agent was seen following inhibition by pantoprazole.

CONCLUSIONS

Recovery of acid secretion following inhibition by all PPIs, other than pantoprazole, may depend on both protein turnover and reversal of the inhibitory disulfide bond. In contrast, recovery of acid secretion after pantoprazole may depend entirely on new protein synthesis.

Duration of effect of lansoprazole on gastric pH and acid secretion in normal male volunteers

AIM

A double-blind, placebo-controlled study to assess the duration of effect of lansoprazole 30 mg o.m. on intragastric pH, acid secretion, gastrin levels, the potential for rebound acidity, and the relationship between gastric acid and drug pharmacokinetic parameters.

METHODS

Sixteen subjects were treated with lansoprazole 30 mg daily or placebo for 14 days, followed by a 7-day post-dosing period and a post-study evaluation on day 28. Ambulatory 24-h pH was recorded and pentagastrin-stimulated acid secretion measured. Plasma kinetics of lansoprazole were determined.

RESULTS

Mean intragastric pH in the lansoprazole group increased significantly (P < 0.05) from baseline to day 14 compared to placebo. After cessation of treatment, secretory activity, as measured by intragastric pH, basal acid output and stimulated acid output, returned to baseline in 2 to 4 days without any overshoot, indicating the absence of acid rebound. Lansoprazole's terminal disposition half-life was 1.11 h. Mean pH and serum gastrin returned to baseline with half-lives of 22 and 19 h, respectively.

CONCLUSIONS

Lansoprazole 30 mg daily significantly increases mean intragastric pH without producing acid rebound. Regeneration of acid production depends primarily on de novo synthesis of the acid pump.

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

3-Amino-5-methyl-2(2-methyl-3-thienyl)- imidazo[1,2-a]thieno[3,2-c]pyridine, SPI-447, is a potent gastric H+,K(+)-ATPase inhibitor, but a detailed mechanism of the inhibition is unknown. This study was designed to investigate the mechanism by which SPI-447 inhibits gastric H+,K(+)-ATPase. For this purpose, the inhibitory action of SPI-447 on gastric H+,K(+)-ATPase from porcine gastric mucosa was compared with that of omeprazole (an irreversible inhibitor) and SCH28080 (a reversible inhibitor). All compounds produced dose-dependent inhibition of gastric H+,K(+)-ATPase, and the inhibitory intensities were increased under acidic conditions. The anti-H+,K(+)-ATPase actions of SPI-447 and SCH28080 were attenuated by dilution, but not influenced by glutathione pretreatment. In contrast, that of omeprazole was not influenced by dilution, but was suppressed by glutathione pretreatment. KCl addition reversed the inhibition of H+,K(+)-ATPase-mediated H(+)-transport by SPI-447 and SCH28080, but had no effect on that by omeprazole. The anti-gastric H+,K(+)-ATPase action of SPI-447 was additive with that of SCH28080. SPI-447 and SCH28080 had no effect on Na+,K(+)-ATPase activity. These findings indicated that the inhibitory mechanism of SPI-447 on gastric H+,K(+)-ATPase was similar to that of SCH28080, but different from that of omeprazole; i.e., 1) reversible, 2) SH-group independent, 3) K(+)-competitive, and 4) highly specific against gastric H+,K(+)-ATPase.

Turnover of the gastric H+,K(+)-adenosine triphosphatase alpha subunit and its effect on inhibition of rat gastric acid secretion

BACKGROUND & AIMS

The rate of turnover and the effect of inhibition of acid secretion on the turnover of gastric H+,K(+)-adenosine triphosphatase (ATPase) is unknown. The aim of this study was to determine the turnover of the alpha subunit of gastric H+,K(+)-ATPase in rats under control conditions and during inhibition of acid secretion by ranitidine or omeprazole.

METHODS

The turnover of the alpha subunit of the ATPase was determined by measuring the loss of incorporated 35S-methionine. This was compared with the rate of recovery of K(+)-stimulated ATPase activity in the omeprazole-treated animals.

RESULTS

The half-life of the alpha subunit was 54 hours. A 1-week treatment with omeprazole had no significant effect, but the half-life increased to 125 hours (P < 0.01) after continuous ranitidine infusion. After omeprazole treatment, K(+)-stimulated ATPase activity recovered with a half-time of 15 hours.

CONCLUSIONS

The turnover of the gastric ATPase subunit was independent of omeprazole inhibition but was prolonged by ranitidine. The effect of ranitidine suggests that the resting pump in tubulovesicles may turn over more slowly than the stimulated pump in the secretory canaliculus. The rapid recovery of ATPase activity compared with turnover after omeprazole is caused by both H+,K(+)-ATPase synthesis and loss of covalently bound drug.

Gastric antisecretory activity of lansoprazole in different experimental models: comparison with omeprazole

1. The activity of the novel proton pump with inhibitor lansoprazole was examined in different gastric secretion models in vitro and in vivo, in comparison with omeprazole. 2. In the conscious cat with gastric fistula lansoprazole (0.25-2 mumol/kg i.v.) caused a dose-dependent reduction of the acid secretion induced by dimaprit, pentagastrin, 2-deoxy-D-glucose and bombesin, being approximately as potent as omeprazole (0.25-1.5 mumol/kg i.v.). Similar to omeprazole, lansoprazole was also more effective when administered in hyperacidic states. 3. In the anaesthetized rat with lumen perfused stomach lansoprazole (0.03-1 mumol/kg i.v.) was approximately 3 times more potent than omeprazole (0.1-3 mumol/kg i.v.) in inhibiting the acid secretion induced by histamine, 2-deoxy-D-glucose and forskolin. 4. In the isolated gastric fundus from the immature rat lansoprazole (1-30 microM) reduced basal acid secretion and the acid response to histamine and forskolin, with a potency not significantly different from that of omeprazole. 5. No significant differences were found in the different species between lansoprazole and omeprazole as for the duration of action. 6. In conclusion, lansoprazole exerts a marked antisecretory effect in a variety of gastric secretion models from different species. However, it did not significantly differ from omeprazole when considering either the potency or the duration of action.

Pharmacological aspects of acid secretion

The secretion of gastric acid is regulated both centrally and peripherally. The finding that H2-receptor antagonists are able to reduce or abolish acid secretion due to vagal, gastrinergic, and histaminergic stimulation shows that histamine plays a pivotal role in stimulation of the parietal cell. In the rat, the fundic histamine is released from the ECL cell, in response to gastrin, acetylcholine, or epinephrine, and histamine release is inhibited by somatostatin or by the H3-receptor ligand, R-alpha-methyl histamine. The parietal cell has a muscarinic, M3, receptor responsible for [Ca]i regulation. Blockade of muscarinic receptors by atropine can be as effective as H2-receptor blockade in controlling acid secretion. However, general effects on muscarinic receptors elsewhere produce significant side effects. The different receptor pathways converge to stimulate the gastric H+,K(+)-ATPase, the pump responsible for acid secretion by the stomach. This enzyme is an alpha,beta heterodimer, present in cytoplasmic membrane vesicles of the resting cell and in the canaliculus of the stimulated cell. It has been shown that acid secretion by the pump depends on provision of K+Cl- efflux pathway becoming associated with the pump. As secretion occurs only in the canaliculus, this K+Cl- pathway is activated only when the pump inserts into the canalicular membrane. Transport by the enzyme involves reciprocal conformational changes in the cytoplasmic and extracytoplasmic domain. These result in changes in sidedness and affinity for H3O+ and K+, enabling active H+ for K+ exchange. The acid pump inhibitors of the substituted benzimidazole class, such as omeprazole, are concentrated in the canaliculus of the secreting parietal cell and are activated there to form sulfenamides. The omeprazole sulfenamide, for example, reacts covalently with two cysteines in the extracytoplasmic loops between the fifth and sixth transmembrane and the seventh and eighth transmembrane segments of the alpha subunit of the H+,K(+)-ATPase, forming disulfide derivatives. This inhibits ATP hydrolysis and H+ transport, resulting in effective, long-lasting regulation of acid secretion. Therefore, this class of acid pump inhibitor is significantly more effective and faster acting than the H2 receptor antagonists. K+ competitive antagonists bind to the M1 and M2 transmembrane segments of the alpha subunit of the acid pump and also abolish ATPase activity. These drugs should also be able to reduce acid secretion more effectively than receptor antagonists and provide shorter acting but complete inhibition of acid secretion.

Cell biology of gastric acid secretion

The parietal cells, which are responsible for the production of gastric HCl acid, are uniquely equipped for high-gradient ion transport. Adequate energy is supplied by oxidative metabolism in the mitochondria, which occupy an exceptionally high proportion of the cytoplasmic volume. Another characteristic feature is the secretory canaliculi. These are tortuous small channels lined by microvilli which penetrate all parts of the cytoplasm and which expand during stimulation of secretion. The activity of the parietal cell is controlled by receptors for acetylcholine, histamine and gastrin on the basolateral cell membrane. Stimulation of these receptors modulates the levels of protein kinases in the cell and brings about the changes from resting to stimulated structure. A key role in the production of acid is played by the gastric acid pump, also known as the H+, K(+)-ATPase, which exports hydrogen ions in 1:1 exchange for potassium ions. This protein is a member of the P-type ATP-driven ion pumps and appears to be uniquely located in the parietal cell. The gastric acid pump is found in the tubulovesicular membranes of the resting cell and moves to the membrane lining the secretory canaliculus when acid secretion is stimulated. Functional acid secretion also requires the presence of KCl pathways in the secretory membrane in order to supply the acid pump with a source of potassium ions. For each hydrogen ion secreted across the secretory membrane, one bicarbonate ion is generated in the cytoplasm and is transported across the basolateral membrane in exchange for chloride. The movement of ions across the apical membrane is followed osmotically by water, resulting in the secretion of 160 mM HCl from the parietal cell.

Inhibitions of acid secretion by E3810 and omeprazole, and their reversal by glutathione

A substituted benzimidazole ([4-(3-methoxypropoxy)-3-methylpyridine-2-yl]methylsulfinyl)- 1H-benzimidazole sodium salt (E3810), is a gastric proton pump (H+, K(+)-ATPase) inhibitor. E3810 and omeprazole inhibited acid accumulation dose dependently as measured with aminopyrine uptake in isolated rabbit gastric glands, their IC50 values being 0.16 and 0.36 microM, respectively. The addition of exogenous reduced glutathione (GSH) to the gland suspension reactivated dose dependently the acid secretion which had been inhibited by 2 microM E3810 or omeprazole as a function of the incubation time. Furthermore, GSH at 1 and 3 mM reversed the antisecretory effect of E3810 more quickly than it did that of omeprazole. The antisecretory effect of E3810 was slightly greater than that of omeprazole in histamine-stimulated fistula dogs in vivo. The duration of the antisecretory activity of E3810 at concentrations of 2 and 4 mg/kg was shorter than that of omeprazole at the same concentrations in pentagastrin-stimulated fistula dogs. The reversal of the antisecretory activity of the inhibitors in dogs is suggested to be due to the action of endogenous extracellular GSH, in addition to de novo synthesis of the proton pump, because bullfrog gastric mucosae were found in the present study to secrete GSH into the mucosal solution at the rate of about 0.25 nmol/min/g tissue.

Substituted thieno[3,4-d]imidazoles, a novel group of H+/K(+)-ATPase inhibitors. Differentiation of their inhibition characteristics from those of omeprazole

The action of the H+/K(+)-ATPase inhibitors, Hoe 731 and S 4216, both thieno-imidazole derivatives, was compared with that of the benzimidazole derivative, omeprazole. In intact, gastric membrane vesicles under conditions shown to result in acidification of the vesicle interior. Hoe 731 and S 4216 inhibited H+/K(+)-ATPase activity with an IC50 value of about 1.0 microM. In the absence of a generated pH gradient the respective IC50 values were 5.5 and 2.1 microM. In contrast, omeprazole inhibited the enzyme only in the presence of proton accumulation (IC50: 0.7 microM). The inhibitory action of omeprazole on H+/K(+)-ATPase-mediated proton transport was prevented by the membrane permeable mercaptane, dithioerythritol, but not by the membrane impermeable, mercaptane glutathione, whereas both mercaptanes were able to prevent the effect of Hoe 731 and S 4216. These results indicate that the thienoimidazoles react with intravesicular (luminal) and extravesicular (cytosolic) SH groups of the H+/K(+)-ATPase, whereas omeprazole interacts uniquely with luminal SH groups of the enzyme. In isolated parietal cells all drugs caused a concentration-dependent inhibition of HCl production, as measured by [14C]aminopyrine uptake, during histamine and dibutyryl-cAMP stimulation. The IC50 value was 0.1 microM for Hoe 731 and omeprazole and 0.4 microM for S 4216 after 30-min incubation. The inhibitory action of Hoe 731 and S 4216 faded with increasing incubation time, whereas omeprazole caused an unchanged inhibition over the entire 120-min incubation period. We suggest that several factors, e.g. weaker chemical stability of the drugs or perturbation of cellular glutathione levels, may be responsible for the fading inhibitory action of thienoimidazoles in the parietal cell.

Direct hyposmotic stimulation of gastric acid secretion

Gastric glands isolated from rabbit stomach were incubated in isosmotic medium or media made hyposmotic by 50-100 mOsm/kg. As indicated by radiolabeled aminopyrine accumulation, acid secretion was nearly 3 times greater in 200 mOsm/kg hyposmotic than in isosmotic medium after a 30-min incubation. The hyposmotic stimulation appeared within 2 min, peaked at 10-15 min and declined almost to the isosmotic control by 45 min. As estimated by the wet weight corrected for inulin extracellular space, the intracellular water of the glands also peaked at 15 min and returned to the isosmotic norm by 45 min. Hyposmotic stimulation of acid secretion directly involved the parietal cell, since parietal cells obtained from gastric glands were also stimulated. That the hyposmotic response was direct was indicated by omeprazole inhibition of aminopyrine accumulation in hyposmotic medium.

Omeprazole: mode of action and effect on acid secretion in animals

H+,K+-ATPase constitutes the final step in the acid secretory process that takes place in the parietal cell, and this enzyme has recently been recognized as a target for inhibitors of acid secretion. The gastric H+,K+-ATPase is located in the uniquely acidic environment of the parietal cell. Omeprazole, a weak base, is concentrated within the acid canaliculus of the parietal cell and is rapidly converted to an inhibitor of the H+,K+-ATPase in the acid compartments of the parietal cell. Omeprazole is thus activated close to its target enzyme, where it is present in high concentrations, and binds selectively to the gastric H+,K+-ATPase. Omeprazole is the first H+,K+-ATPase inhibitor to be used in the treatment of acid-related diseases, and inhibition of acid secretion is closely correlated to inhibition of gastric H+,K+-ATPase activity. Both basal and stimulated acid secretion are inhibited by omeprazole in a dose-dependent manner, irrespective of the nature of the stimulus. Omeprazole exerts a prolonged antisecretory effect, and in dogs, 4 days are required for return to normal acid secretion following a maximal inhibitory dose.

Mechanisms of nonsteroidal anti-inflammatory drug-induced gastric damage. Actions of therapeutic agents

All nonsteroidal anti-inflammatory drugs (NSAIDs) used in the treatment of rheumatic diseases may cause gastric mucosal damage. Although the best-studied agent is aspirin, the mechanisms by which it damages the gastric mucosa are not fully understood. However, it is thought that the drug impairs mucosal defenses by penetrating the protective mucous and bicarbonate layers and damaging the epithelial lining cells. In turn, gastric acid is permitted to pour through the breached defenses. This "back-diffusion" of acid further injures cells and destroys capillaries and venules. This local damaging effect is pH dependent and is contributed to by the acid secretion of the stomach. Other mechanisms by which aspirin may induce or contribute to mucosal injury include inhibition of mucosal prostaglandin synthesis, reduction and alteration of mucus secretion, reduction of bicarbonate secretion, interference with cell turnover, as well as systemic effects such as platelet dysfunction. The mechanism by which nonaspirin NSAIDs cause gastrointestinal damage is uncertain. All are known to inhibit prostaglandin synthesis, which could contribute to their toxicity since prostaglandins found in the stomach both inhibit acid secretion and have mucosal defensive effects. Partial protections against aspirin-induced or other NSAID-induced gastric mucosal damage has been demonstrated, at least in some studies, by sucralfate, prostaglandins, omeprazole and histamine (H2)-receptor antagonists. Sucralfate appears to act primarily on local defensive mechanisms; its antisecretory effects are minimal. Prostaglandins exert a protective effect at both antisecretory and nonantisecretory (cytoprotective) doses, indicating that either or both mechanisms may be involved. The most recently studied agent, omeprazole, is the most potent of all acid inhibitors; it may also be cytoprotective, possibly as a result of its effects on sulfhydryl groups. Prostaglandins and omeprazole are not available in the United States and their potential side effects may limit their use in patients with chronic rheumatic diseases. Protection by H2-receptor antagonists is mostly related to reduction of acid secretion, though a cytoprotective effect may occur.

Studies on the mechanism of action of the gastric microsomal (H+ + K+)-ATPase inhibitors SCH 32651 and SCH 28080

The novel antisecretory agents SCH 32651 and SCH 28080 were evaluated for their antisecretory activities in vitro as well as for their abilities to inhibit the (H+ + K+)-ATPase enzyme activity in preparations of microsomal membranes from rabbit fundic mucosa. SCH 32651 and SCH 28080 inhibited both the histamine- and dibutyryl cAMP-stimulated uptake of [14C]-aminopyrine into isolated parietal cells with IC50 values of about 1.5 and 0.02 microM respectively. SCH 32651 and SCH 28080 competitively inhibited the K+-stimulated hydrolysis of ATP catalyzed by the (H+ + K+)-ATPase with Ki values of 16.3 and 0.12 microM respectively. The inhibition of the enzyme by both compounds was not affected by the addition of the sulfhydryl reducing agents dithiothreitol or beta-mercaptoethanol, was readily reversible by dilution or washing, and was dependent upon the concentration of KCl used to stimulate the enzyme. These data suggest that SCH 32651 and SCH 28080 are reversible, competitive inhibitors of the K+-stimulated hydrolysis of ATP.

Omeprazole. A preliminary review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in peptic ulcer disease and Zollinger-Ellison syndrome

Omeprazole is a substituted benzimidazole derivative which markedly inhibits basal and stimulated gastric acid secretion. It has a unique mode of action, irreversibly blocking the so-called proton pump of the parietal cell which is supposedly the terminal step in the acid secretory pathway. In animals, on a weight basis, omeprazole is 2 to 10 times more potent than cimetidine in inhibiting gastric acid secretion. Toxicological studies in rats have shown that very high doses of omeprazole administered for 2 years produce hyperplasia of gastric enterochromaffin-like cells and carcinoids, a few with proliferations into the submucosa. The significance of such findings to the clinical situation is wholly speculative and requires further research. Preliminary studies in patients with duodenal ulcers or Zollinger-Ellison syndrome have found no mucosal changes which would suggest that the drug represents a risk for development of carcinoid tumours at therapeutic dosages. In patients with duodenal ulcers omeprazole, at dosages of at least 20mg once daily, produced ulcer healing rates of between 60 and 100% after 2 weeks and between 90 and 100% after 4 weeks, even in patients resistant to treatment with H2-receptor antagonists. Comparative trials clearly demonstrated that omeprazole 20 to 40 mg administered once daily was significantly more effective than usual dosage regimens of cimetidine and ranitidine in healing duodenal ulcers during 2 to 4 weeks of treatment. At present no data are available evaluating omeprazole as maintenance therapy once ulcers have healed. Other clinical trials have also shown that omeprazole is effective for treating gastric ulcers, ulcerative peptic oesophagitis, and Zollinger-Ellison syndrome. In patients with Zollinger-Ellison syndrome the profound and long lasting antisecretory activity of omeprazole may make it the drug of choice for treating the massive acid hypersecretion associated with the disease, especially when H2-receptor antagonists are ineffective. During clinical trials reported to date omeprazole has been very well tolerated but further clinical experience is essential to fully evaluate its safety profile. Thus, omeprazole represents a pharmacologically unique antisecretory drug which is very effective for rapidly healing peptic ulcers and peptic oesophagitis, and for reducing gastric acid hypersecretion in patients with Zollinger-Ellison syndrome. If the apparent absence of undesirable mucosal morphological changes during treatment with usual doses in patients with peptic ulcer disease is confirmed, it may be a major advance in the treatment of these diseases.

The sulphoxide moiety of substituted benzimidazoles is essential for inhibition of parietal cell K+/H+-ATPase

The antisecretory action of the benzimidazole sulphoxide derivative B 823-10, 2[(4-methoxy-3-methyl-2-pyridylmethyl)-sulphinyl]- 5-trifluoromethyl(1H)-benzimidazole, was compared with the effect of the corresponding sulphide B 823-08 in several in vivo and in vitro and in vitro test systems. The sulphide B 823-08 and the sulphoxide B 823-10 were found to be equipotent in the Shay rat. The sulphide was found to inhibit H+ secretion in intact rabbit gastric glands and enriched guinea-pig parietal cells with lower potency than the corresponding sulphoxide. The relative potency in antisecretory activity (sulphide/sulphoxide) decreased in the following rank order: Shay rat: gastric glands: parietal cells. Purified K+/H+-ATPase was not blocked by the sulphide, whereas the sulphoxide inhibited the overall as well as the partial reactions of this enzyme. In all in vitro systems tested, inhibition of H+ secretion and enzyme activity by the sulphoxide, but not by the sulphide, was antagonized by SH-compounds such as dithiothreitol. It is concluded that in vivo sulphoxidation of the sulphide plays an important role in acid inhibition. In vitro an additional inhibitory mechanism of the sulphide has to be considered.