CLINICAL PHARMACOLOGY
Mechanism of Action
Rabeprazole belongs to a class of antisecretory compounds (substituted benzimidazole proton-pump inhibitors) that do not exhibit anticholinergic or histamine H2-receptor antagonist properties, but suppress gastric acid secretion by inhibiting the gastric H+, K+ATPase at the secretory surface of the gastric parietal cell. Because this enzyme is regarded as the acid (proton) pump within the parietal cell, rabeprazole has been characterized as a gastric proton-pump inhibitor. Rabeprazole blocks the final step of gastric acid secretion.
In gastric parietal cells, rabeprazole is protonated, accumulates, and is transformed to an active sulfenamide. When studied in vitro, rabeprazole is chemically activated at pH 1.2 with a half-life of 78 seconds. It inhibits acid transport in porcine gastric vesicles with a half-life of 90 seconds.
Pharmacodynamics
Antisecretory Activity
The antisecretory effect begins within one hour after oral administration of 20 mg ACIPHEX. The median inhibitory effect of ACIPHEX on 24 hour gastric acidity is 88% of maximal after the first dose. ACIPHEX 20 mg inhibits basal and peptone meal-stimulated acid secretion versus placebo by 86% and 95%, respectively, and increases the percent of a 24-hour period that the gastric pH>3 from 10% to 65% (see table below). This relatively prolonged pharmacodynamic action compared to the short pharmacokinetic half-life (1-2 hours) reflects the sustained inactivation of the H+, K+ATPase.
TABLE 2 GASTRIC ACID PARAMETERS ACIPHEX VERSUS PLACEBO AFTER 7 DAYS OF ONCE DAILY DOSING
*(p<0.01 versus placebo) |
Parameter |
ACIPHEX (20 mg QD) |
Placebo |
Basal Acid Output (mmol/hr) |
0.4* |
2.8 |
Stimulated Acid Output (mmol/hr) |
0.6* |
13.3 |
% Time Gastric pH>3 |
65* |
10 |
Compared to placebo, ACIPHEX, 10 mg, 20 mg, and 40 mg, administered once daily for 7 days significantly decreased intragastric acidity with all doses for each of four meal-related intervals and the 24-hour time period overall. In this study, there were no statistically significant differences between doses; however, there was a significant dose-related decrease in intragastric acidity. The ability of rabeprazole to cause a dose-related decrease in mean intragastric acidity is illustrated below.
TABLE 3 AUC ACIDITY (MMOL•HR/L) ACIPHEX VERSUS PLACEBO ON DAY 7 OF ONCE DAILY DOSING (MEAN±SD)
*(p<0.001 versus placebo) |
AUC interval (hrs)
|
Treatment
|
10 mg RBP
(N=24)
|
20 mg RBP
(N=24)
|
40 mg RBP
(N=24)
|
Placebo
(N=24)
|
08:00 - 13:00
|
19.6±21.5* |
12.9±23* |
7.6±14.7* |
91.1±39.7 |
13:00 - 19:00
|
5.6±9.7* |
8.3±29.8* |
1.3±5.2* |
95.5±48.7 |
19:00 - 22:00
|
0.1±0.1* |
0.1±0.06* |
0.0±0.02* |
11.9±12.5 |
22:00 - 08:00
|
129.2±84* |
109.6±67.2* |
76.9±58.4* |
479.9±165 |
AUC 0-24 hours
|
155.5±90.6* |
130.9±81* |
85.8±64.3* |
678.5±216 |
After administration of 20 mg ACIPHEX once daily for eight days, the mean percent of time that gastric pH>3 or gastric pH>4 after a single dose (Day 1) and multiple doses (Day 8) was significantly greater than placebo (see table below). The decrease in gastric acidity and the increase in gastric pH observed with 20 mg ACIPHEX administered once daily for eight days were compared to the same parameters for placebo, as illustrated below:
TABLE 4 GASTRIC ACID PARAMETERS ACIPHEX ONCE DAILY DOSING VERSUS PLACEBO ON DAY 1 AND DAY 8
a No inferential statistics conducted for this parameter. * (p<0.001 versus placebo)
b Gastric pH was measured every hour over a 24-hour period. |
Parameter |
ACIPHEX 20 mg QD |
Placebo |
Day 1 |
Day 8 |
Day 1 |
Day 8 |
Mean AUC0-24 Acidity |
340.8* |
176.9* |
925.5 |
862.4 |
Median trough pH (23-hr)a
|
3.77 |
3.51 |
1.27 |
1.38 |
% Time Gastric pH>3b
|
54.6* |
68.7* |
19.1 |
21.7 |
% Time Gastric pH>4b
|
44.1* |
60.3* |
7.6 |
11.0 |
Effects on Esophageal Acid Exposure
In patients with gastroesophageal reflux disease (GERD) and moderate to severe esophageal acid exposure, ACIPHEX 20 mg and 40 mg per day decreased 24-hour esophageal acid exposure. After seven days of treatment, the percentage of time that esophageal pH<4 decreased from baselines of 24.7% for 20 mg and 23.7% for 40 mg, to 5.1% and 2.0%, respectively. Normalization of 24-hour intraesophageal acid exposure was correlated to gastric pH>4 for at least 35% of the 24-hour period; this level was achieved in 90% of subjects receiving ACIPHEX 20 mg and in 100% of subjects receiving ACIPHEX 40 mg. With ACIPHEX 20 mg and 40 mg per day, significant effects on gastric and esophageal pH were noted after one day of treatment, and more pronounced after seven days of treatment.
Effects on Serum Gastrin
In patients given daily doses of ACIPHEX for up to eight weeks to treat ulcerative or erosive esophagitis and in patients treated for up to 52 weeks to prevent recurrence of disease the median fasting gastrin level increased in a dose-related manner. The group median values stayed within the normal range.
In a group of subjects treated daily with ACIPHEX 20 mg for 4 weeks a doubling of mean serum gastrin concentrations were observed. Approximately 35% of these treated subjects developed serum gastrin concentrations above the upper limit of normal. In a study of CYP2C19 genotyped subjects in Japan, poor metabolizers developed statistically significantly higher serum gastrin concentrations than extensive metabolizers.
Effects on Enterochromaffin-like (ECL) Cells
Increased serum gastrin secondary to antisecretory agents stimulates proliferation of gastric ECL cells which, over time, may result in ECL cell hyperplasia in rats and mice and gastric carcinoids in rats, especially in females {see Carcinogenesis, Mutagenesis, Impairment of Fertility (13.1)}.
In over 400 patients treated with ACIPHEX (10 or 20 mg/day) for up to one year, the incidence of ECL cell hyperplasia increased with time and dose, which is consistent with the pharmacological action of the proton-pump inhibitor. No patient developed the adenomatoid, dysplastic or neoplastic changes of ECL cells in the gastric mucosa. No patient developed the carcinoid tumors observed in rats.
Endocrine Effects
Studies in humans for up to one year have not revealed clinically significant effects on the endocrine system. In healthy male volunteers treated with ACIPHEX for 13 days, no clinically relevant changes have been detected in the following endocrine parameters examined: 17 β-estradiol, thyroid stimulating hormone, tri-iodothyronine, thyroxine, thyroxine-binding protein, parathyroid hormone, insulin, glucagon, renin, aldosterone, follicle-stimulating hormone, luteotrophic hormone, prolactin, somatotrophic hormone, dehydroepiandrosterone, cortisol-binding globulin, and urinary 6β-hydroxycortisol, serum testosterone and circadian cortisol profile.
Other Effects
In humans treated with ACIPHEX for up to one year, no systemic effects have been observed on the central nervous, lymphoid, hematopoietic, renal, hepatic, cardiovascular, or respiratory systems. No data are available on long-term treatment with ACIPHEX and ocular effects.
Microbiology
The following in vitro data are available but the clinical significance is unknown.
Rabeprazole sodium, amoxicillin and clarithromycin as a three drug regimen has been shown to be active against most strains of Helicobacter pylori in vitro and in clinical infections as described in the CLINICAL STUDIES (14) and INDICATIONS AND USAGE (1) sections.
Helicobacter pylori
Susceptibility testing of H. pylori isolates was performed for amoxicillin and clarithromycin using agar dilution methodology1, and minimum inhibitory concentrations (MICs) were determined.
Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures.
Incidence of Antibiotic-Resistant Organisms Among Clinical Isolates
Pretreatment Resistance:
Clarithromycin pretreatment resistance rate (MIC ≥ 1 μg/mL) to H. pylori was 9% (51/ 560) at baseline in all treatment groups combined. A total of > 99% (558/560) of patients had H. pylori isolates which were considered to be susceptible (MIC ≤ 0.25 μg/mL) to amoxicillin at baseline. Two patients had baseline H. pylori isolates with an amoxicillin MIC of 0.5 μg/mL.
For susceptibility testing information about Helicobacter pylori, see Microbiology section in prescribing information for clarithromycin and amoxicillin.
TABLE 5 CLARITHROMYCIN SUSCEPTIBILITY TEST RESULTS AND CLINICAL/BACTERIOLOGIC OUTCOMESa FOR A THREE DRUG REGIMEN (RABEPRAZOLE 20 MG TWICE DAILY, AMOXICILLIN 1000 MG TWICE DAILY, AND CLARITHROMYCIN 500 MG TWICE DAILY FOR 7 OR 10 DAYS)
a Includes only patients with pretreatment and post-treatment clarithromycin susceptibility test results.
b Susceptible (S) MIC ≤ 0.25 μg/mL, Intermediate (I) MIC = 0.5 μg/mL, Resistant (R) MIC ≥ 1 μg/mL |
Days of RAC Therapy |
Clarithromycin Pretreatment Results |
Total Number |
H. pylori Negative (Eradicated) |
H. pylori Positive (Persistent) Post-Treatment Susceptibility Results |
S b
|
I b
|
R b
|
No MIC |
7 |
Susceptible b
|
129 |
103 |
2 |
0 |
1 |
23 |
7 |
Intermediate b
|
0 |
0 |
0 |
0 |
0 |
0 |
7 |
Resistant b
|
16 |
5 |
2 |
1 |
4 |
4 |
10 |
Susceptible b
|
133 |
111 |
3 |
1 |
2 |
16 |
10 |
Intermediate b
|
0 |
0 |
0 |
0 |
0 |
0 |
10 |
Resistant b
|
9 |
1 |
0 |
0 |
5 |
3 |
Patients with persistent H. pylori infection following rabeprazole, amoxicillin, and clarithromycin therapy will likely have clarithromycin resistant clinical isolates. Therefore, clarithromycin susceptibility testing should be done when possible. If resistance to clarithromycin is demonstrated or susceptibility testing is not possible, alternative antimicrobial therapy should be instituted.
Amoxicillin Susceptibility Test Results and Clinical/Bacteriological Outcomes: In the U.S. multicenter study, a total of >99% (558/560) of patients had H. pylori isolates which were considered to be susceptible (MIC ≤ 0.25 μg/mL) to amoxicillin at baseline. The other 2 patients had baseline H. pylori isolates with an amoxicillin MIC of 0.5 μg/mL, and both isolates were clarithromycin-resistant at baseline; in one case the H. pylori was eradicated. In the 7- and 10-day treatment groups 75% (107/145) and 79% (112/142), respectively, of the patients who had pretreatment amoxicillin susceptible MICs (≤ 0.25 μg/mL) were eradicated of H. pylori. No patients developed amoxicillin-resistant H. pylori during therapy.
Pharmacokinetics
ACIPHEX delayed-release tablets are enteric-coated to allow rabeprazole sodium, which is acid labile, to pass through the stomach relatively intact. After oral administration of 20 mg ACIPHEX, peak plasma concentrations (Cmax) of rabeprazole occur over a range of 2.0 to 5.0 hours (Tmax). The rabeprazole Cmax and AUC are linear over an oral dose range of 10 mg to 40 mg. There is no appreciable accumulation when doses of 10 mg to 40 mg are administered every 24 hours; the pharmacokinetics of rabeprazole is not altered by multiple dosing. The plasma half-life ranges from 1 to 2 hours.
Absorption: Absolute bioavailability for a 20 mg oral tablet of rabeprazole (compared to intravenous administration) is approximately 52%. When rabeprazole is administered with a high fat meal, its Tmax is variable and may delay its absorption up to 4 hours or longer, however, the Cmax and the extent of rabeprazole absorption (AUC) are not significantly altered. Thus rabeprazole may be taken without regard to timing of meals.
Distribution: Rabeprazole is 96.3% bound to human plasma proteins.
Metabolism: Rabeprazole is extensively metabolized. A significant portion of rabeprazole is metabolized via systemic nonenzymatic reduction to a thioether compound. Rabeprazole is also metabolized to sulphone and desmethyl compounds via cytochrome P450 in the liver. The thioether and sulphone are the primary metabolites measured in human plasma. These metabolites were not observed to have significant antisecretory activity. In vitro studies have demonstrated that rabeprazole is metabolized in the liver primarily by cytochromes P450 3A (CYP3A) to a sulphone metabolite and cytochrome P450 2C19 (CYP2C19) to desmethyl rabeprazole. CYP2C19 exhibits a known genetic polymorphism due to its deficiency in some sub-populations (e.g. 3 to 5% of Caucasians and 17 to 20% of Asians). Rabeprazole metabolism is slow in these sub-populations, therefore, they are referred to as poor metabolizers of the drug.
Elimination: Following a single 20 mg oral dose of 14C-labeled rabeprazole, approximately 90% of the drug was eliminated in the urine, primarily as thioether carboxylic acid; its glucuronide, and mercapturic acid metabolites. The remainder of the dose was recovered in the feces. Total recovery of radioactivity was 99.8%. No unchanged rabeprazole was recovered in the urine or feces.
Geriatric: In 20 healthy elderly subjects administered 20 mg rabeprazole once daily for seven days, AUC values approximately doubled and the Cmax increased by 60% compared to values in a parallel younger control group. There was no evidence of drug accumulation after once daily administration. {see USE IN SPECIAL POPULATIONS Geriatric Use (8.5)}.
Pediatric: The pharmacokinetics of rabeprazole was studied in 12 adolescent patients with GERD 12 to 16 years of age, in a multicenter study. Patients received rabeprazole 20 mg once daily for five or seven days. An approximate 40% increase in exposure was noted following 5 to 7 days of dosing compared with the exposure after 1 day dosing. Pharmacokinetic parameters in adolescent patients with GERD 12 to 16 years of age were within the range observed in healthy adult volunteers.
Gender and Race: In analyses adjusted for body mass and height, rabeprazole pharmacokinetics showed no clinically significant differences between male and female subjects. In studies that used different formulations of rabeprazole, AUC0-∞ values for healthy Japanese men were approximately 50-60% greater than values derived from pooled data from healthy men in the United States.
Renal Disease: In 10 patients with stable end-stage renal disease requiring maintenance hemodialysis (creatinine clearance ≤5 mL/min/1.73 m2), no clinically significant differences were observed in the pharmacokinetics of rabeprazole after a single 20 mg oral dose when compared to 10 healthy volunteers. {see DOSAGE AND ADMINISTRATION (2.7)}.
Hepatic Disease: In a single dose study of 10 patients with chronic mild to moderate compensated cirrhosis of the liver who were administered a 20 mg dose of rabeprazole, AUC0-24 was approximately doubled, the elimination half-life was 2- to 3-fold higher, and total body clearance was decreased to less than half compared to values in healthy men.
In a multiple dose study of 12 patients with mild to moderate hepatic impairment administered 20 mg rabeprazole once daily for eight days, AUC0-∞ and Cmax values increased approximately 20% compared to values in healthy age- and gender-matched subjects. These increases were not statistically significant.
No information exists on rabeprazole disposition in patients with severe hepatic impairment. Please refer to the DOSAGE AND ADMINISTRATION section (2.7) for information on dosage adjustment in patients with hepatic impairment.
Combined Administration with Antimicrobials: Sixteen healthy volunteers genotyped as extensive metabolizers with respect to CYP2C19 were given 20 mg rabeprazole sodium, 1000 mg amoxicillin, 500 mg clarithromycin, or all 3 drugs in a four-way crossover study. Each of the four regimens was administered twice daily for 6 days. The AUC and Cmax for clarithromycin and amoxicillin were not different following combined administration compared to values following single administration. However, the rabeprazole AUC and Cmax increased by 11% and 34%, respectively, following combined administration. The AUC and Cmax for 14-hydroxyclarithromycin (active metabolite of clarithromycin) also increased by 42% and 46%, respectively. This increase in exposure to rabeprazole and 14-hydroxyclarithromycin is not expected to produce safety concerns.
NONCLINICAL TOXICOLOGY
Carcinogenesis, Mutagenesis, Impairment of Fertility
In a 88/104-week carcinogenicity study in CD-1 mice, rabeprazole at oral doses up to 100 mg/kg/day did not produce any increased tumor occurrence. The highest tested dose produced a systemic exposure to rabeprazole (AUC) of 1.40 μg•hr/mL which is 1.6 times the human exposure (plasma AUC0-∞ = 0.88 μg•hr/mL) at the recommended dose for GERD (20 mg/day). In a 28-week carcinogenicity study in p53+/- transgenic mice, rabeprazole at oral doses of 20, 60, and 200 mg/kg/day did not cause an increase in the incidence rates of tumors but produced gastric mucosal hyperplasia at all doses. The systemic exposure to rabeprazole at 200 mg/kg/day is about 17-24 times the human exposure at the recommended dose for GERD. In a 104-week carcinogenicity study in Sprague-Dawley rats, males were treated with oral doses of 5, 15, 30 and 60 mg/kg/day and females with 5, 15, 30, 60 and 120 mg/kg/day. Rabeprazole produced gastric enterochromaffin-like (ECL) cell hyperplasia in male and female rats and ECL cell carcinoid tumors in female rats at all doses including the lowest tested dose. The lowest dose (5 mg/kg/day) produced a systemic exposure to rabeprazole (AUC) of about 0.1 μg•hr/mL which is about 0.1 times the human exposure at the recommended dose for GERD. In male rats, no treatment related tumors were observed at doses up to 60 mg/kg/day producing a rabeprazole plasma exposure (AUC) of about 0.2 μg•hr/mL (0.2 times the human exposure at the recommended dose for GERD).
Rabeprazole was positive in the Ames test, the Chinese hamster ovary cell (CHO/HGPRT) forward gene mutation test and the mouse lymphoma cell (L5178Y/TK+/-) forward gene mutation test. Its demethylated-metabolite was also positive in the Ames test. Rabeprazole was negative in the in vitro Chinese hamster lung cell chromosome aberration test, the in vivo mouse micronucleus test, and the in vivo and ex vivo rat hepatocyte unscheduled DNA synthesis (UDS) tests.
Rabeprazole at intravenous doses up to 30 mg/kg/day (plasma AUC of 8.8 μg•hr/mL, about 10 times the human exposure at the recommended dose for GERD) was found to have no effect on fertility and reproductive performance of male and female rats.
|