Specific proton pump inhibitors E3810 and lansoprazole affect the recovery process of gastric secretion in rats differently

High interpatient variability in response to mycophenolic acid maintenance therapy in patients with ANCA-associated vasculitis

BACKGROUND

Mycophenolic acid (MPA) is used in the maintenance therapy of antineutrophil cytoplasm antibody-associated systemic vasculitis (AASV). MPA exerts its immunosuppression by inhibiting inosine 5'-monophosphate dehydrogenase (IMPDH), depleting activated lymphocytes of guanine nucleotides and retarding their proliferation. The purpose of our study was to examine the correlation between clinical outcome and pharmacokinetic-pharmacodynamic (PD) relationships of MPA in patients with AASV.

METHODS

We studied 358 Caucasian control patients without any MPA therapy to examine basal IMPDH activity. Thirty Caucasian patients with AASV under maintenance therapy with mycophenolate mofetil (MMF) underwent therapeutic drug monitoring.

RESULTS

We observed a high interindividual variability with regard to basal IMPDH activity in patients without any MPA treatment (0.8-35 nmol/mg protein/h). Patients were followed for a mean (±SD) period of 22 ± 8 months. During the observation period, seven patients had a relapse with an elevated Birmingham Vasculitis Activity Score of 9.2 ± 6. The basal IMPDH activity (Abasal) in patients who subsequently relapsed was raised at baseline, before receiving their first dose of MMF, and further increased at the time of relapse, when compared with stable patients. Patients with a relapse during the maintenance therapy had significantly higher levels of IMPDH activity [IMPDH enzyme activity curve (AEC) (0-12)] than stable patients (P = 0.001), indicating inadequate IMPDH suppression. MPA-AUC (0-12) was significantly decreased in relapse patients, in contrast to stable patients (P < 0.05).

CONCLUSIONS

Due to the highly variable response to maintenance therapy with MPA, PD drug monitoring is a new tool for detecting inadequate immunosuppression in AASV patients.

Proton pump inhibitors interfere with the immunosuppressive potency of mycophenolate mofetil

OBJECTIVES

MMF is cleaved in the acidic milieu of the gastric compartment. However, its absorption might be impeded by proton pump inhibitors (PPIs), which suppress acid production and thus increase stomach pH. Since PPIs are widely used, it is useful to clarify whether the total drug amount of MMF is available in patients undergoing PPI treatment.

METHODS

We analysed 36 patients with autoimmune diseases under stable MMF maintenance therapy. Twenty-three patients received co-medication with pantoprazole; 13 patients received no treatment with PPIs or antacids. To assess the immunosuppressive potency, we measured mycophenolic acid levels and inosin monophosphate dehydrogenase (IMPDH) activity with a validated HPLC method in plasma samples collected pre-dose and at 0.5, 1, 1.5, 2, 3, 4, 5, 6, 8, 10 and 12 h after oral administration.

RESULTS

The mean MMF dosage of the non-PPI patients was 770 (249) mg/12 h and 771 (291) mg/12 h in pantoprazole-treated patients (NS). The total area under the curve of MMF showed a 37% reduction in PPI patients vs those treated with no PPIs (P < 0.01), and the maximum peak concentration of MMF was 60% lower in the pantoprazole patients (P < 0.001). The MMF exposure correlated with the inhibition of IMPDH activity. The area of enzyme activity curve was 42% higher in the PPI patients (P < 0.01).

CONCLUSIONS

The co-medication of pantoprazole with MMF significantly influences the drug exposure and immunosuppressive potency of MMF in patients with autoimmune diseases. This finding might at least partly explain the different outcomes in studies using MMF for maintenance therapy.

Proton pump inhibitors reduce mycophenolate exposure in heart transplant recipients-a prospective case-controlled study

This prospective study investigates the impact of proton pump inhibitors (PPI) on mycophenolic acid (MPA) pharmacokinetics in heart transplant recipients receiving mycophenolate mofetil (MMF) and tacrolimus. MPA plasma concentrations at baseline (C(0 h)), 30 min (C(0.5 h)), 1(C(1 h)) and 2 h (C(2 h)) were obtained by high-performance liquid chromatography (HPLC) in 22 patients treated with pantoprazole 40 mg and MMF 2000 mg. Measurements were repeated 1 month after pantoprazole withdrawal. A four-point limited-sampling strategy was applied to calculate the MPA area under the curve (MPA-AUC). Predose MPA concentrations with PPI were 2.6 +/- 1.6 mg/L versus 3.4 +/- 2.7 mg/L without PPI (p = ns). Postdose MPA concentrations were lower with PPI at C(0.5 h) (8.3 +/- 5.7 mg/L vs. 18.3 +/- 11.3 mg/L, p = 0.001) and C(1 h) (10.0 +/- 5.6 mg/L vs. 15.8 +/- 8.4 mg/L, p = 0.004), without significant differences at C(2 h) (8.3 +/- 6.5 mg/L vs. 7.6 +/- 3.9 mg/L). The MPA-AUC was significantly lower with PPI medication (51.2 +/- 26.6 mg x h/L vs. 68.7 +/- 30.3 mg x h/L; p = 0.003). The maximum concentration of MPA (MPA-C(max)) was lower (12.2 +/- 7.5 mg/L vs. 20.6 +/- 9.3 mg/L; p = 0.001) and the time to reach MPA-C(max) (t(max)) was longer with PPI (60.0 +/- 27.8 min vs. 46.4 +/- 22.2 min; p = 0.05). This is the first study to document an important drug interaction between a widely used immunosuppressive agent and a class of drugs frequently used in transplant patients. This interaction results in a decreased MMF drug exposure which may lead to patients having a higher risk for acute rejection and transplant vasculopathy.

Rabeprazole: a second-generation proton pump inhibitor in the treatment of acid-related disease

Rabeprazole is a proton pump inhibitor (PPI) presenting a very advantageous pharmacodynamic and pharmacokinetic profile over older PPIs. In particular, this drug has a very fast onset of action, due to a short activation time and a very high pKa, and may therefore be defined as a 'second generation' PPI. The aim of this article is to provide an update on the pharmacology and clinical profile of rabeprazole and its use in acid-related disorders, with a particular focus on its role in gastroesophageal reflux disease; in the treatment and prevention of duodenal and gastric ulcers and Zollinger-Ellison syndrome; in the therapy of the extraesophageal manifestations of gastroesophageal reflux disease (in particular the respiratory and ear, nose and throat ones); and in the eradication of Helicobacter pylori.

Influence of lansoprazole and rabeprazole on mycophenolic acid pharmacokinetics one year after renal transplantation

Peptic ulcer disease is a common complication after organ transplantation, and long-term administration of antiulcer agents is needed in many renal transplant recipients. Although several drug interactions with mycophenolic acid (MPA), the active metabolite of the prodrug mycophenolate mofetil (MMF), have been reported, little is known about the interaction between MPA and proton pump inhibitors (PPIs). The present study investigated the drug interaction between MMF and lansoprazole or rabeprazole and the impact of cytochrome (CYP) 2C19, and multidrug resistance (MDR)1 C3435T polymorphisms on these drug interactions at 1 year after renal transplantation. Retrospectively, 61 recipients were divided into 3 groups: MMF and tacrolimus as combination immunosuppressive therapy, together with either 30 mg lansoprazole (n = 22) or 10 mg rabeprazole (n = 17), or without PPI (n = 22). One year after transplantation, plasma concentrations of MPA were measured by high-performance liquid chromatography. The mean dose-unadjusted and -adjusted Cmax of MPA with 30 mg lansoprazole were significantly lower than those without PPI (11.8 vs. 17.8 microg/mL, P = 0.0197, and 22.6 vs. 33.1 ng/mL/mg MMF, P = 0.0222, respectively). In recipients having the CYP2C19 *1/*2+*1/*3 or MDR1 C3435T CC genotype, the mean dose-adjusted AUC0-12 of MPA with 30 mg lansoprazole was significantly smaller than that with 10 mg rabeprazole or without PPI. The plasma concentration of MPA was influenced by 30 mg lansoprazole but not 10 mg rabeprazole. Because of the greater gastric acid secretion-inhibitory effect of 30 mg lansoprazole in recipients having the CYP2C19 *1/*2+*1/*3 (intermediate metabolizer) or MDR1 C3435T CC genotype, the elution and hydrolysis of MMF might be decreased. Although the clinical relevance might be minor, the fact that administration of 30 mg lansoprazole in patients having the CYP2C19 *2 or *3 allele or the MDR1 C3435T CC genotype diminishes the absorption of MPA in the maintenance stage after renal transplantation should be taken into consideration with regard to the MPA pharmacokinetics.

Rabeprazole 10 mg twice daily is superior to 20 mg once daily for night-time gastric acid suppression

BACKGROUND

Insufficient acid suppression is one of the main causes of proton pump inhibitor-refractory gastro-oesophageal reflux disease.

AIM

To achieve more potent and long-lasting acid suppression, different dosage regimens of rabeprazole were compared in relation to the CYP2C19 genotype status.

METHODS

In a cross-over study, 18 healthy Helicobacter pylori-negative males (six homozygous extensive metabolizers, six heterozygous extensive metabolizers and six poor metabolizers) were given rabeprazole 10 mg once daily, 20 mg once daily or 10 mg twice daily, or water only (baseline data), for 7 days each. On day 7 of each regimen, 24-h intragastric pH-metry was performed.

RESULTS

No significant differences were observed in median pH values and pH>4 holding time ratios between rabeprazole 10 mg once daily and 20 mg once daily. However, with rabeprazole 10 mg twice daily, these parameters were significantly higher than those with 20 mg once daily. The potency of acid suppression by rabeprazole was influenced by the CYP2C19 genotype status. The differences were somewhat significant but not large. The incidence of nocturnal acid breakthrough was lowest with rabeprazole 10 mg twice daily.

CONCLUSIONS

Rabeprazole 10 mg twice daily, not 20 mg once daily, should be administered to achieve more potent and long-lasting acid suppression.

Effect of different proton pump inhibitors, differences in CYP2C19 genotype and antibiotic resistance on the eradication rate of Helicobacter pylori infection by a 1-week regimen of proton pump inhibitor, amoxicillin and clarithromycin

AIM

To investigate the effect of different proton pump inhibitors, S-mephenytoin 4'-hydroxylase (CYP2C19) genotype and antibiotic susceptibility on the eradication rate of Helicobacter pylori.

METHODS

One hundred and eighty-seven H. pylori-infected peptic ulcer patients were randomly treated with either rabeprazole (10 mg b.d.) or lansoprazole (30 mg b.d.) plus amoxicillin (750 mg b.d.) and clarithromycin (400 mg b.d.) for 1 week. The antibiotic susceptibility and CYP2C19 genotype (extensive or poor metabolizer) were investigated.

RESULTS

The eradication rates in the rabeprazole-amoxicillin-clarithromycin (RAC) and lansoprazole-amoxicillin-clarithromycin (LAC) groups were 75% and 69%, respectively, on an intention-to-treat basis, and 80% and 75%, respectively, on a per protocol basis. The eradication rate for clarithromycin-resistant strains was significantly lower than that for clarithromycin-sensitive strains (24% vs. 86%, P < 0.05). For clarithromycin-sensitive strains in the LAC group, there was a tendency for a lower eradication rate in extensive than poor metabolizers. The eradication rate in extensive metabolizers in the RAC group tended to be higher than that in extensive metabolizers in the LAC group (89% vs. 78%, P = 0.079726).

CONCLUSIONS

The success of the 1-week proton pump inhibitor-amoxicillin-clarithromycin regimen depends on the susceptibility of H. pylori to clarithromycin. Moreover, differences in CYP2C19 genotype influence the eradication rates of lansoprazole-based therapy, and the rabeprazole-based regimen has an advantage especially in extensive metabolizers.

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.

Eradication rates of clarithromycin-resistant Helicobacter pylori using either rabeprazole or lansoprazole plus amoxicillin and clarithromycin

BACKGROUND

The resistance of Helicobacter pylori to clarithromycin has become one of the primary reasons for eradication failure.

AIM

To compare the eradication rates of triple therapy using amoxicillin (A), clarithromycin (C) and rabeprazole (R) or lansoprazole (L) against clarithromycin-sensitive and clarithromycin-resistant strains.

METHODS

Two hundred and ninety-five patients were randomly divided into four groups and treated for 1 week: 147 cases were treated with RAC, i.e. 49 cases with R20C400 (10 mg R + 750 mg A + 200 mg C, twice daily), 48 cases with R40C400 (20 mg R + 750 mg A + 200 mg C, twice daily) and 50 cases with R40C800 (20 mg R + 750 mg A + 400 mg C, twice daily); 148 cases with treated with LAC (30 mg L + 750 mg A + 200 mg C, twice daily).

RESULTS

According to intention-to-treat and per protocol analyses, the eradication rates were 88% and 91% with RAC and 78% and 81% with LAC; the eradication rates with R20C400, R40C400 and R40C800 were 94%, 81% and 86%, respectively, in the intention-to-treat analysis. In addition, the eradication rates for clarithromycin-sensitive strains with RAC and LAC were 98% and 89%, respectively, and for clarithromycin-resistant strains with RAC and LAC were 8.1% and 0%, respectively.

CONCLUSIONS

The eradication rate was significantly higher with RAC than LAC. The eradication rate for clarithromycin-resistant strains was low in both groups, and an improved eradication rate could not be achieved by changing the dose of clarithromycin or proton pump inhibitor.

Effects of metabolite uptake on proton-equivalent elimination by two species of deep-sea vestimentiferan tubeworm,Riftia pachyptilaandLamellibrachiacfluymesi: proton elimination is a necessary adaptation to sulfide-oxidizing chemoautotrophic symbionts

Intracellular symbiosis requires that the host satisfy the symbiont's metabolic requirements, including the elimination of waste products. The hydrothermal vent tubeworm Riftia pachyptila and the hydrocarbon seep worm Lamellibrachia cf luymesi are symbiotic with chemolithoautotrophic bacteria that produce sulfate and protons as end-products. In this report, we examine the relationship between symbiont metabolism and host proton equivalent elimination in R. pachyptilaand L. cf luymesi, and the effects of sulfide exposure on proton-equivalent elimination by Urechis caupo, an echiuran worm that lacks intracellular symbionts (for brevity, we will hereafter refer to proton-equivalent elimination as `proton elimination'). Proton elimination by R. pachyptila and L. cf luymesi constitutes the worms' largest mass-specific metabolite flux, and R. pachyptilaproton elimination is, to our knowledge, the most rapid reported for any metazoan. Proton elimination rates by R. pachyptila and L.cf luymesi correlated primarily with the rate of sulfide oxidation. Prolonged exposure to low environmental oxygen concentrations completely inhibited the majority of proton elimination by R. pachyptila,demonstrating that proton elimination does not result primarily from anaerobic metabolism. Large and rapid increases in environmental inorganic carbon concentrations led to short-lived proton elimination by R. pachyptila, as a result of the equilibration between internal and external inorganic carbon pools. U. caupo consistently exhibited proton elimination rates 5-20 times lower than those of L. cf luymesi and R. pachyptila upon exposure to sulfide. Treatment with specific ATPase inhibitors completely inhibited a fraction of proton elimination and sulfide and inorganic carbon uptake by R. pachyptila, suggesting that proton elimination occurs in large part via K+/H+-ATPases and Na+/H+-ATPases. In the light of these results, we suggest that protons are the primary waste product of the symbioses of R. pachyptila and L. cf luymesi, and that proton elimination is driven by symbiont metabolism, and may be the largest energetic cost incurred by the worms.

Pharmacological and pharmacodynamic essentials of H(2)-receptor antagonists and proton pump inhibitors for the practising physician

The suppression of gastric acid secretion with anti-secretory agents has been the mainstay of medical treatment for patients with acid-related disorders. Although the majority of Helicobacter pylori -related peptic ulcers can be healed with antibiotics, ulcer healing and symptom control can be significantly improved when antibiotics are given with anti-secretory agents, especially with a proton pump inhibitor. There is a dynamic relationship between the suppression of intragastric acidity and the healing of peptic ulcer and erosive oesophagitis and control of acid-related symptoms. The suppression of gastric acid secretion achieved with H(2)-receptor antagonists has, however, proved to be suboptimal for effectively controlling acid-related disorders, especially for healing erosive oesophagitis and for the relief of reflux symptoms. H(2)-receptor antagonists are also not effective in inhibiting meal-stimulated acid secretion, which is required for managing patients with erosive oesophagitis. Furthermore, the rapid development of tolerance to H(2)-receptor antagonists and the rebound acid hypersecretion after the withdrawal of an H(2)-receptor antagonist further limit their clinical use. Although low-dose H(2)-receptor antagonists are currently available as over-the-counter medications for self-controlling acid-related symptoms, their pharmacology and pharmacodynamics have not been well studied, especially in the self-medicating population. Proton pump inhibitors have been proved to be very effective for suppressing intragastric acidity to all known stimuli, although variations exist in the rapidity of onset of action and the potency of acid inhibition after oral administration at the approved therapeutic doses, which may have important clinical implications for the treatment of gastro-oesophageal reflux disease and perhaps for eradicating H. pylori infection when a proton pump inhibitor is given with antibiotics. Once-daily dosing in the morning is more effective than dosing in the evening for all proton pump inhibitors with respect to the suppression of intragastric acidity and daytime gastric acid secretion in particular, which may result from a better bio-availability being achieved with the morning dose. When higher doses are needed, these drugs must be given twice daily to achieve the optimal suppression of 24 hour intragastric acidity. Preliminary results have shown that esomeprazole, the optical isomer of omeprazole, given at 40 mg, is significantly more effective than omeprazole 40 mg, lansoprazole 30 mg or pantoprazole 40 mg for suppressing gastric acid secretion. However, more studies in different patient populations are needed to compare esomeprazole with the existing proton pump inhibitors with regard to their efficacy, cost-effectiveness and long-term safety for the management of acid-related disorders.

Five-day proton pump inhibitor-based quadruple therapy regimen is more effective than 7-day triple therapy regimen for Helicobacter pylori infection

BACKGROUND

There have been no reports that describe whether 5-day quadruple therapy (rabeprazole + amoxicillin + clarithromycin + metronidazole; RACM) could substitute for standard 7-day triple therapy as a first-line therapy for Helicobacter pylori.

PATIENTS AND METHODS

This study was designed as a randomized prospective single centre study. A total of 160 H. pylori-positive patients who had not received therapy were given either a 5-day RACM regimen (n=80, rabeprazole 20 mg b.d., amoxicillin 750 mg b.d., clarithromycin 200 mg b.d. and metronidazole 250 mg b.d.) or a 7-day RAC regimen (n=80, rabeprazole 20 mg b.d., amoxicillin 750 mg b.d. and clarithromycin 200 mg b.d.). Cure of the infection was assessed by a (13)C urea breath test 1 month after the completion of therapy.

RESULTS

The eradication rates of the 5-day RACM regimen and the 7-day RAC regimen were 93% (95% CI: 84--97%) and 81% (95% CI: 71--89%) by intention-to-treat analysis, 94% (95% CI: 86--98%) and 83% (95% CI: 73--91%) by all-patients-treated analysis analysis and 95% (95% CI: 87--98%; P < 0.05) and 82% (95% CI: 72--90%) by per protocol analysis, respectively. No serious adverse effect was observed, and 99% of the patients reported complete compliance.

CONCLUSIONS

The cure rate of the 5-day RACM regimen was more effective than the 7-day RAC regimen, suggesting that this regimen could be preferable as a first-line therapy for H. pylori infection.

Addition of metronidazole to rabeprazole-amoxicillin-clarithromycin regimen for Helicobacter pylori infection provides an excellent cure rate with five-day therapy

BACKGROUND

New triple therapy for eradication of Helicobacter pylori based on a proton pump inhibitor (PPI) provides a cure rate of approximately 90% with few adverse effects. Recently, a PPI-based quadruple therapy, which consists of a PPI plus bismuth-based triple therapy for 7 days, has been studied, and a sufficient eradication rate has been achieved. However, a shorter duration results in improved compliance. In this study, newly developed short-term, simple twice-daily quadruple therapy consisting of rabeprazole, amoxicillin, clarithromycin, and metronidazole (RACM) was compared with a PPI-based triple-therapy regimen for eradication of H. pylori.

PATIENTS AND METHODS

This study was designed as a randomized open, prospective single-center study. Of a total of 105 H. pylori-positive patients, 55 received the RACM regimen for 5 days (rabeprazole, 10 mg bid; amoxicillin, 750 mg bid; clarithromycin, 200 mg bid; and metronidazole, 250 mg bid), and 50 received the RAC regimen for 5 days (rabeprazole, 10 mg bid; amoxicillin, 750 mg bid; and clarithromycin, 200 mg bid). Cure of the infection was assessed by HpSA (H. pylori stool antigen immunoassay) 1 month after completion of therapy.

RESULTS

The rates of eradication of H. pylori by RACM versus RAC were 94.5% (95% CI, 85-99) versus 80.0% (95% CI, 66-90) by intention-to-treat analysis; 98.1% (95% CI, 90-100) versus 87.0% (95% CI, 74-95) by all-patients-treated analysis; and 98.1% (95% CI, 90-100) versus 86.7% (95% CI, 73-95) by per-protocol analysis. No major adverse effects were reported, and 98.0% of patients reported complete compliance.

CONCLUSIONS

The simple twice-daily and short-term quadruple regimen for only 5 days provided an excellent eradication rate. Compliance with the regimen was high, and serious adverse effects were few. Therefore, the RACM regimen can be considered as safe and effective.

Rabeprazole produces rapid, potent, and long-acting inhibition of gastric acid secretion in subjects with Helicobacter pylori infection

AIM

To compare acid inhibiting activity and duration of action of different doses of rabeprazole, a substituted benzimidazole characterized as a highly potent and irreversible H+, K+-ATPase inhibitor, administered for 7 days to subjects infected with Helicobacter pylori.

METHODS

A total of 38 subjects (mean age 39.3 years) were enrolled in a single-centre, double-blind, randomized, crossover study. All subjects were confirmed positive for H. pylori by 14C urea breath test and ELISA serologies. Subjects were divided into two groups of 19 to receive two doses of rabeprazole, either 5 and 20 mg or 10 and 40 mg, and placebo, given in random order daily in the morning for 7 days. Peptone-stimulated acid, pH, and gastrin measurements were made for 24 h after the 1st dose and for 48 h after the 7th dose.

RESULTS

Peptone-stimulated acid secretion rates were decreased from 12.5 to 6.7, 4.0, 1.5, and 0.26 h after initial 5, 10, 20, and 40 mg doses, respectively; to 7.3, 4.3, 2.1, and 1.2 mmol/h 23 h after the initial dose; and to 2.4, 2.6, 0.6, and 0.8 mmol/h 23 h after the 7th dose. After 48 h, stimulated acid secretion had recovered less than 40% for all treatment groups compared to placebo. Median intragastric pH also increased from 2.0 with placebo to 4.9, 6.2, 6.6 and 6.9 during the 24-h period after the 7th dose of 5, 10, 20, and 40 mg. The 20 mg dose of rabeprazole produced equivalent acid inhibition to the 40 mg dose with less increase in plasma gastrin.

CONCLUSION

Rabeprazole in doses from 5 to 40 mg was a highly effective inhibitor of gastric acid secretion in subjects infected with H. pylori. The inhibition was rapid, dose-related, and long-acting, with less than 50% recovery of acid by 48 h after the 7th dose. The optimal acid inhibitory dose in these subjects appeared to be 20 mg daily, however 5 mg and 10 mg doses produced potent inhibition of gastric acid secretion.

Effect of rabeprazole on histamine synthesis in enterochromaffin-like cells of mast cell-deficient (Ws/Ws) rats

The effect of rabeprazole, the latest proton pump inhibitor, on the serum gastrin concentration, histidine decarboxylase activity and histamine content of the oxyntic mucosa in Wistar rats, mast cell-deficient (Ws/Ws) rats, and their normal type, +/+, rats was investigated. In Wistar rats, 2 weeks of treatment with rabeprazole (30 mg/kg/day, s.c.) induced a 1.8-fold increase in serum gastrin concentration and a 3.9-fold increase in histidine decarboxylase activity of the oxyntic mucosa over the control levels, whereas neither 2- nor 4-week treatment affected the histamine content of the oxyntic mucosa. In Ws/Ws and +/+ rats, the serum gastrin concentration, histidine decarboxylase activity and even histamine content of the oxyntic mucosa were increased significantly as compared with control levels after the 4-week treatment with rabeprazole. Immunohistochemistry using a histamine antibody confirmed the increase in the histamine content of the oxyntic mucosa after the 4-week treatment with rabeprazole. The finding that there were no differences in serum gastrin concentration and histidine decarboxylase activity between Ws/Ws and +/+ rats, both with and without the 4-week treatment, indicates that mast cells do not respond to endogenous hypergastrinemia elicited by acid-inhibitory treatment. Moreover, the present study clarified for the first time that enterochromaffin-like (ECL) cells in Ws/Ws rats synthesize and store histamine in response to gastrin.

Correlation between acid secretion and proton pump activity during inhibition by the proton pump inhibitors omeprazole and pantoprazole

Omeprazole and pantoprazole are known to be irreversible, SH-acting inhibitors of gastric H+,K+-adenosine triphosphatase (H+,K+-ATPase). Both drugs concentration-dependently and pH-dependently inhibited K+-dependent p-nitrophenyl phosphatase (K+-pNPPase) activity in purified rabbit gastric microsomes. The potency of omeprazole was about three times that of pantoprazole in the pH ranges tested. Both drugs also inhibited acid secretion, as determined by [14C]aminopyrine accumulation in isolated rabbit gastric glands, with the potency ratio being about 5 (omeprazole over that of pantoprazole). Under conditions in which acid secretion was inhibited completely by the drugs, the total K+-pNPPase activity in the digitonin-permeabilized glands was scarcely reduced, showing an apparent discrepancy between the acid secretion and the proton pump activity. The isolated glands were stimulated with secretagogues for 30 min in the presence of the inhibitors, homogenized, and then separated into fractions in which K+-pNPPase activity was measured. Omeprazole exclusively inhibited the activity in the low-speed fraction, which was rich in the apical membranes, whereas pantoprazole did not inhibit activity in any fraction. When the time of treatment with the inhibitors was increased up to 5 hr, the inhibition of the total K+-pNPPase activity in the glands reached a plateau at an inhibition rate lower than 50% within 2 hr. This suggested that no continuous recycling of the proton pump was occurring during stimulation. The inhibitory effect of both drugs on the permeabilized gland preparation was less potent than that on the purified enzyme, especially at the higher pH, and it appeared to be partially reversible. The extent of the reduction in potency was more prominent for pantoprazole. It is concluded that a lower amount of proton pump activity needs to be inhibited by pantoprazole than by omeprazole to achieve the same extent of acid secretion inhibition. This appears to be due to the nature of pantoprazole, i.e. the requirement of low pH for activation and the partial reversibility of the inhibition.

Review article: the pharmacology of rabeprazole

Rabeprazole sodium is a new substituted benzimidazole proton pump inhibitor with several differences compared with existing proton pump inhibitors. In vitro and animal studies have demonstrated that rabeprazole is a more potent inhibitor of H+,K(+)-ATPase and acid secretion than omeprazole, and is a more rapid inhibitor of proton pumps than omeprazole, lansoprazole, or pantoprazole. This probably reflects rabeprazole's faster activation in the parietal cell canaliculus. In human studies, once-daily doses of 5-40 mg of rabeprazole inhibit gastric acid secretion in a dose-dependent fashion. A once-daily dose of 20 mg has consistently achieved profound decreases in 24-h intragastric acidity in single and repeat dosing studies, in healthy volunteers and patients with either peptic ulcer disease or gastro-oesophageal reflux disease. Significantly greater decreases in intragastric acidity are achieved on day 1 of dosing with rabeprazole 20 mg than with omeprazole 20 mg. As with other proton pump inhibitors, rabeprazole has in vitro antibacterial activity against Helicobacter pylori, with greater activity against this organism than either lansoprazole or omeprazole. In addition to inhibiting bacterial urease activity, rabeprazole binds to several molecules on H. pylori. Clinical trials are needed to assess the clinical importance of these findings, as well as to assess whether the potential advantages of rabeprazole result in clinical benefit for patients with acid-related diseases.

Effect of rabeprazole (E3810), a novel proton pump inhibitor, on intragastric pH in healthy volunteers

In this study, we examined the effect of rabeprazole (E3810), a novel proton pump inhibitor, on gastric acidity under physiological conditions in healthy volunteers using 24-hour intragastric pH monitoring. Twenty-four-hour intragastric pH monitoring was performed three times to seven subjects randomly assigned in a cross over fashion to one of the following groups; without drug administration (basal), and with 10 mg or 20 mg of rabeprazole for four days. The median pH for 24 hours was 2.15 in the basal study, while the corresponding median pH were 5.05 and 5.90 after treatment with 10 mg and 20 mg of rabeprazole, respectively. Significant differences in the median pH were observed between the basal study and that after administration of the 10 mg and 20 mg doses of rabeprazole. The cumulative percentage of pH readings above the threshold pH 4 value, was 34.1% in the basal study, and 72.6% and 78.3% after treatment with 10 mg and 20 mg doses of rabeprazole, respectively. This result indicates that the efficacy of rabeprazole allows for substantial control of gastric acidity with once-daily dosing, and that both the 10 mg and 20 mg doses result in potent inhibition of gastric acid secretion.

Differences in pH-dependent activation rates of substituted benzimidazoles and biological in vitro correlates

Gastric proton pump inhibitors (PPIs) are substituted benzimidazole prodrugs that require an acid-induced activation. Its rate depends on the reactivity of the molecule relative to the environmental pH and determines the drug's tissue selectivity. Factors affecting the exposure of moderately acidic tissues to the activated PPI are the area under the serum concentration-time curve (AUC), serum protein binding, the partition coefficient logP and the serum elimination half-life relative to the chemical activation half-life at a critical tissue pH of about 5. These parameters have therefore been determined in a comparative fashion in the present study. The data shows that pantoprazole is less likely to undergo unwanted activation at moderately acidic targets as opposed to the parietal cell, compared to omeprazole. Actually, although 40 mg pantoprazole (steady state) gave a slightly higher serum AUC of the total parent compound than 40 mg omeprazole (10.5 vs. 7.1 micromol x h x l[-1]), a higher serum protein binding of pantoprazole versus omeprazole (98 vs. 96%) reversed the AUC values for the free drug in favor of a lower value for pantoprazole (0.19 vs. 0.28 micromol x h x l[-1]). It is the free parent compound that equilibrates across cell membranes to be activated in acidic tissue compartments. At pH 5.1, the activation half-life of pantoprazole was 4.7 versus 1.4 h for omeprazole, the latter being in the order of the common serum elimination half-life determined in an intraindividual comparison (1.24 vs. 1.25 h). Thus, pantoprazole is eliminated faster from blood than it is activated at a pH of about 5, while omeprazole is as quickly activated at this pH as it is eliminated from blood. Biological in vitro experiments confirmed that pantoprazole displays a lower liability to interfere with unwanted biological targets. This has been demonstrated in vitro for inhibition of both renal Na+/K+-ATPase, lysosomal acidification and the production of reactive oxygen species by neutrophils.

Rabeprazole

Rabeprazole is a proton pump inhibitor with antisecretory properties. In vitro animal experiments have indicated that the inhibition of the proton pump by rabeprazole is partially reversible. Rabeprazole has 2- to 10-fold greater antisecretory activity than omeprazole in vitro. However, it dissociates more readily from H+,K(+)-ATPase than omeprazole, resulting in a shorter duration of action. In comparative clinical trials rabeprazole was significantly more effective than placebo, famotidine or ranitidine and as effective as omeprazole in the treatment of patients with erosive or ulcerative gastro-oesophageal reflux disease or gastric or duodenal ulcers. Healing rates with rabeprazole were independent of Helicobacter pylori status. Rabeprazole in combination with either clarithromycin and metronidazole or clarithromycin and amoxicillin or amoxicillin and metronidazole or clarithromycin for 7 days produced eradication of H. pylori in 100, 95, 90 and 63% of patients. The tolerability profile of rabeprazole 20mg once daily was similar to that of famotidine 20mg twice daily, ranitidine 150mg 4 times daily or omeprazole 20mg once daily in comparative trials. The adverse events reported with once daily administration of rabeprazole 20mg include malaise, nausea, diarrhoea, headache, dizziness and skin eruptions in 0.7 to 2.2% of patients.

Mobilization of gastric histamine during repeated administration of a proton potassium adenosine triphosphatase inhibitor in intact and antrectomized rats

Intact and antrectomized female rats were treated with the potent proton pump inhibitor, E3810 (daily 40 mg/kg weight, s.c.) for 4 weeks. Plasma gastrin concentration and urinary excretion of N-terminal big gastrin increased until day 14 and persisted at a high level in intact rats treated with E3810, but did not increase in antrectomized rats. Urinary excretion of histamine increased progressively and reached 7 times the control value following 4 weeks of treatment with E3810 in intact rats, but not in antrectomized rats. At the termination of the treatment, the endocrine cell density in the oxyntic mucosa of intact rats had increased by 85% with increased histamine content and elevated histidine decarboxylase activity, while antrectomized rats showed a low histamine level and low histidine decarboxylase activity. Administration of gastrin-17 I (10 micrograms/kg weight, sc) itself caused a significant increase in urinary excretion of histamine, which was inhibited by the specific gastrin receptor antagonist, L-365,260. These results suggests that the massive urinary excretion of histamine caused by the treatment with E3810 reflects gastrin-induced mobilization of gastric histamine and that neither E3810 itself nor E3810-induced luminal pH elevation has direct effects on mobilization of oxyntic mucosal histamine.