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Zantac

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Zantac

CLINICAL PHARMACOLOGY

ZANTAC (ranitidine hcl) is a competitive, reversible inhibitor of the action of histamine at the histamine H2-receptors, including receptors on the gastric cells. ZANTAC (ranitidine hcl) does not lower serum Ca++ in hypercalcemic states. ZANTAC (ranitidine hcl) is not an anticholinergic agent.

Pharmacokinetics

Absorption

ZANTAC (ranitidine hcl) is 50% absorbed after oral administration, compared to an intravenous (IV) injection with mean peak levels of 440 to 545 ng/mL occurring 2 to 3 hours after a 150-mg dose. The syrup and EFFERdose formulations are bioequivalent to the tablets. Absorption is not significantly impaired by the administration of food or antacids. Propantheline slightly delays and increases peak blood levels of ranitidine, probably by delaying gastric emptying and transit time. In one study, simultaneous administration of high-potency antacid (150 mmol) in fasting subjects has been reported to decrease the absorption of ZANTAC (ranitidine hcl) .

Distribution

The volume of distribution is about 1.4 L/kg. Serum protein binding averages 15%.

Metabolism

In humans, the N-oxide is the principal metabolite in the urine; however, this amounts to < 4% of the dose. Other metabolites are the S-oxide (1%) and the desmethyl ranitidine (1%). The remainder of the administered dose is found in the stool. Studies in patients with hepatic dysfunction (compensated cirrhosis) indicate that there are minor, but clinically insignificant, alterations in ranitidine half-life, distribution, clearance, and bioavailability.

Excretion

The principal route of excretion is the urine, with approximately 30% of the orally administered dose collected in the urine as unchanged drug in 24 hours. Renal clearance is about 410 mL/min, indicating active tubular excretion. The elimination half-life is 2.5 to 3 hours. Four patients with clinically significant renal function impairment (creatinine clearance 25 to 35 mL/min) administered 50 mg of ranitidine intravenously had an average plasma half-life of 4.8 hours, a ranitidine clearance of 29 mL/min, and a volume of distribution of 1.76 L/kg. In general, these parameters appear to be altered in proportion to creatinine clearance (see DOSAGE AND ADMINISTRATION).

Geriatrics

The plasma half-life is prolonged and total clearance is reduced in the elderly population due to a decrease in renal function. The elimination half-life is 3 to 4 hours. Peak levels average 526 ng/mL following a 150-mg twice-daily dose and occur in about 3 hours (see PRECAUTIONS: Geriatric Use and DOSAGE AND ADMINISTRATION: Dosage Adjustment for Patients With Impaired Renal Function).

Pediatrics

There are no significant differences in the pharmacokinetic parameter values for ranitidine in pediatric patients (from 1 month up to 16 years of age) and healthy adults when correction is made for body weight. The average bioavailability of ranitidine given orally to pediatric patients is 48% which is comparable to the bioavailability of ranitidine in the adult population. All other pharmacokinetic parameter values (t½, Vd, and CL) are similar to those observed with intravenous ranitidine use in pediatric patients. Estimates of Cmax and Tmax are displayed in Table 1.

Table 1: Ranitidine Pharmacokinetics in Pediatric Patients Following Oral Dosing

Population(age) n Dosage Form
(dose)
Cmax
(ng/mL)
Tmax
(hours)
Gastric or duodenal ulcer (3.5 to 16 years) 12 Tablets (1 to 2 mg/kg) 54 to 492 2.0
Otherwise healthy requiring ZANTAC (0.7 to 14 years, Single dose) 10 Syrup (2 mg/kg) 244 1.61
Otherwise healthy requiring ZANTAC (0.7 to 14 years, Multiple dose) 10 Syrup (2 mg/kg) 320 1.66

Plasma clearance measured in 2 neonatal patients (less than 1 month of age) was considerably lower (3 mL/min/kg) than children or adults and is likely due to reduced renal function observed in this population (see PRECAUTIONS: Pediatric Use and DOSAGE AND ADMINISTRATION: Pediatric Use).

Pharmacodynamics

Serum concentrations necessary to inhibit 50% of stimulated gastric acid secretion are estimated to be 36 to 94 ng/mL. Following a single oral dose of 150 mg, serum concentrations of ranitidine are in this range up to 12 hours. However, blood levels bear no consistent relationship to dose or degree of acid inhibition.

In a pharmacodynamic comparison of the EFFERdose with the ZANTAC (ranitidine hcl) Tablets, during the first hour after administration, the EFFERdose tablet formulation gave a significantly higher intragastric pH, by approximately 1 pH unit, compared to the ZANTAC (ranitidine hcl) Tablets.

Antisecretory Activity

1. Effects on Acid Secretion: ZANTAC (ranitidine hcl) inhibits both daytime and nocturnal basal gastric acid secretions as well as gastric acid secretion stimulated by food, betazole, and pentagastrin, as shown in Table 2.

Table 2: Effect of Oral ZANTAC (ranitidine hcl) on Gastric Acid Secretion

  Time After Dose, hours % Inhibition of Gastric Acid Output by Dose, mg
75-80 100 150 200
Basal Up to 4   99 95  
Nocturnal Up to 13 95 96 92  
Betazole Up to 3   97 99  
Pentagastrin Up to 5 58 72 72 80
Meal Up to 3   73 79 95

It appears that basal-, nocturnal-, and betazole-stimulated secretions are most sensitive to inhibition by ZANTAC (ranitidine hcl) , responding almost completely to doses of 100 mg or less, while pentagastrin- and food-stimulated secretions are more difficult to suppress.

2. Effects on Other Gastrointestinal Secretions: Pepsin: Oral ZANTAC (ranitidine hcl) does not affect pepsin secretion. Total pepsin output is reduced in proportion to the decrease in volume of gastric juice.

Intrinsic Factor: Oral ZANTAC (ranitidine hcl) has no significant effect on pentagastrin-stimulated intrinsic factor secretion.

Serum Gastrin: ZANTAC (ranitidine hcl) has little or no effect on fasting or postprandial serum gastrin.

Other Pharmacologic Actions
  1. Gastric bacterial flora—increase in nitrate-reducing organisms, significance not known.
  2. Prolactin levels—no effect in recommended oral or IV dosage, but small, transient, dose-related increases in serum prolactin have been reported after IV bolus injections of 100 mg or more.
  3. Other pituitary hormones—no effect on serum gonadotropins, TSH, or GH. Possible impairment of vasopressin release.
  4. No change in cortisol, aldosterone, androgen, or estrogen levels.
  5. No antiandrogenic action.
  6. No effect on count, motility, or morphology of sperm.

Pediatrics: Oral doses of 6 to 10 mg/kg/day in 2 or 3 divided doses maintain gastric pH > 4 throughout most of the dosing interval.

Clinical Trials

Active Duodenal Ulcer: In a multicenter, double-blind, controlled, US study of endoscopically diagnosed duodenal ulcers, earlier healing was seen in the patients treated with ZANTAC (ranitidine hcl) as shown in Table 3.

Table 3: Duodenal Ulcer Patient Healing Rates

  ZANTAC* Placebo*
Number Entered Healed/ Evaluable Number Entered Healed/ Evaluable
Outpatients
Week 2 195 69/182(38%)† 188 31/164(19%)
Week 4 137/187(73%)† 76/168(45%)
*All patients were permitted antacids as needed for relief of pain.
†P < 0.0001.

In these studies, patients treated with ZANTAC (ranitidine hcl) reported a reduction in both daytime and nocturnal pain, and they also consumed less antacid than the placebo-treated patients.

Table 4: Mean Daily Doses of Antacid

  Ulcer Healed Ulcer Not Healed
ZANTAC 0.06 0.71
Placebo 0.71 1.43

Foreign studies have shown that patients heal equally well with 150 mg twice daily and 300 mg at bedtime (85% versus 84%, respectively) during a usual 4-week course of therapy. If patients require extended therapy of 8 weeks, the healing rate may be higher for 150 mg twice daily as compared to 300 mg at bedtime (92% versus 87%, respectively).

Studies have been limited to short-term treatment of acute duodenal ulcer. Patients whose ulcers healed during therapy had recurrences of ulcers at the usual rates.

Maintenance Therapy in Duodenal Ulcer: Ranitidine has been found to be effective as maintenance therapy for patients following healing of acute duodenal ulcers. In 2 independent, double-blind, multicenter, controlled trials, the number of duodenal ulcers observed was significantly less in patients treated with ZANTAC (ranitidine hcl) (150 mg at bedtime) than in patients treated with placebo over a 12-month period.

Table 5: Duodenal Ulcer Prevalence

Multicenter Trial Double-Blind, Multicenter, Placebo-Controlled Trials
Drug Duodenal Ulcer Prevalence
0-4 Months 0-8 Months 0-12 Months No. of Patients
USA RAN 20%* 24%* 35%* 138
PLC 44% 54% 59% 139
Foreign RAN 12%* 21%* 28%* 174
PLC 56% 64% 68% 165
% = Life table estimate.
* = P < 0.05 (ZANTAC (ranitidine hcl) versus comparator).
RAN = ranitidine (ZANTAC (ranitidine hcl) ).
PLC = placebo.

As with other H2-antagonists, the factors responsible for the significant reduction in the prevalence of duodenal ulcers include prevention of recurrence of ulcers, more rapid healing of ulcers that may occur during maintenance therapy, or both.

Gastric Ulcer: In a multicenter, double-blind, controlled, US study of endoscopically diagnosed gastric ulcers, earlier healing was seen in the patients treated with ZANTAC (ranitidine hcl) as shown in Table 6.

Table 6: Gastric Ulcer Patient Healing Rates

  ZANTAC* Placebo*
Number Entered Healed/ Evaluable Number Entered Healed/ Evaluable
Outpatients
Week 2 92 16/83 94 10/83
(19%) (12%)
Week 6 50/73 35/69
(68%)† (51%)
*All patients were permitted antacids as needed for relief of pain.
†P = 0.009.

In this multicenter trial, significantly more patients treated with ZANTAC (ranitidine hcl) became pain free during therapy.

Maintenance of Healing of Gastric Ulcers: In 2 multicenter, double-blind, randomized, placebo-controlled, 12-month trials conducted in patients whose gastric ulcers had been previously healed, ZANTAC (ranitidine hcl) 150 mg at bedtime was significantly more effective than placebo in maintaining healing of gastric ulcers.

Pathological Hypersecretory Conditions (such as Zollinger-Ellison syndrome): ZANTAC (ranitidine hcl) inhibits gastric acid secretion and reduces occurrence of diarrhea, anorexia, and pain in patients with pathological hypersecretion associated with Zollinger-Ellison syndrome, systemic mastocytosis, and other pathological hypersecretory conditions (e.g., postoperative, “short-gut” syndrome, idiopathic). Use of ZANTAC (ranitidine hcl) was followed by healing of ulcers in 8 of 19 (42%) patients who were intractable to previous therapy.

Gastroesophageal Reflux Disease (GERD): In 2 multicenter, double-blind, placebo-controlled, 6-week trials performed in the United States and Europe, ZANTAC (ranitidine hcl) 150 mg twice daily was more effective than placebo for the relief of heartburn and other symptoms associated with GERD. Ranitidine-treated patients consumed significantly less antacid than did placebo-treated patients.

The US trial indicated that ZANTAC (ranitidine hcl) 150 mg twice daily significantly reduced the frequency of heartburn attacks and severity of heartburn pain within 1 to 2 weeks after starting therapy. The improvement was maintained throughout the 6-week trial period. Moreover, patient response rates demonstrated that the effect on heartburn extends through both the day and night time periods.

In 2 additional US multicenter, double-blind, placebo-controlled, 2-week trials, ZANTAC (ranitidine hcl) 150 mg twice daily was shown to provide relief of heartburn pain within 24 hours of initiating therapy and a reduction in the frequency of severity of heartburn. In these trials, ZANTAC (ranitidine hcl) EFFERdose Tablets were shown to provide heartburn relief within 45 minutes of dosing.

Erosive Esophagitis: In 2 multicenter, double-blind, randomized, placebo-controlled, 12-week trials performed in the United States, ZANTAC (ranitidine hcl) 150 mg 4 times daily was significantly more effective than placebo in healing endoscopically diagnosed erosive esophagitis and in relieving associated heartburn. The erosive esophagitis healing rates were as follows:

Table 7: Erosive Esophagitis Patient Healing Rates

  Healed/Evaluable
Placebo* n = 229 ZANTAC 150 mg 4 times daily* n = 215
Week 4 43/198 (22%) 96/206 (47%)†
Week 8 63/176 (36%) 142/200 (71%)†
Week 12 92/159 (58%) 162/192 (84%)†
*All patients were permitted antacids as needed for relief of pain.
†P < 0.001 versus placebo.

No additional benefit in healing of esophagitis or in relief of heartburn was seen with a ranitidine dose of 300 mg 4 times daily.

Maintenance of Healing of Erosive Esophagitis: In 2 multicenter, double-blind, randomized, placebo-controlled, 48-week trials conducted in patients whose erosive esophagitis had been previously healed, ZANTAC (ranitidine hcl) 150 mg twice daily was significantly more effective than placebo in maintaining healing of erosive esophagitis.

Last reviewed on RxList: 12/6/2010
This monograph has been modified to include the generic and brand name in many instances.

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