Famotidine

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Abstract
Synopsis: Famotidine 1 is a new histamine H 2 -receptor antagonist. On a weight basis, famotidine is 20 times more potent than cimetidine and 7.5 times more potent than ranitidine in inhibiting basal and pentagastrin-stimulated gastric acid secretion in humans. Therapeutic trials have shown that famotidine 20mg twice daily or 40mg at bedtime may be an effective alternative to standard doses of cimetidine for healing gastric ulcers and to standard doses of cimetidine and ranitidine for healing duodenal ulcers. When used prophylactically, a single 20mg dose of famotidine at night decreases the incidence of duodenal ulcer recurrence (versus placebo). However, further study is needed to clarify the comparative efficacy of the H 2 -receptor antagonists, in particular as maintenance therapy for healed peptic ulcer. Preliminary results in a few patients with Zollinger-Ellison syndrome indicate that famotidine, alone or in combination with an anticholinergic agent, gives good control of gastric acid hyperacidity with no evidence of biochemical or haematological toxicity. Famotidine appears to be well tolerated. Unlike cimetidine, it does not have antiandrogenic effects or alter hepatic metabolism of drugs. However, wider clinical experience with famotidine is needed to accurately determine its relative tolerability compared with other antiulcer drugs. Thus, famotidine appears to be a suitable and well tolerated alternative to cimetidine and ranitidine for healing peptic ulcers, but wider clinical experience is needed to assess its relative efficacy and tolerability in the long term maintenance treatment of patients with healed ulcers as well as in patients with Zollinger-Ellison syndrome. Pharmacodynamic Studies: In vitro studies have shown famotidine to be an inhibitor of histamine at H2-receptor sites: it is more potent than cimetidine and ranitidine on a molar basis, and in most in vitro models exhibited classic competitive inhibition. In healthy subjects and patients with peptic ulcer disease, oral famotidine 10 or 20mg effected a greater than 80% inhibition of basal and nocturnal gastric acid secretion and that stimulated by betazole or tetragastrin. In patients with a basal gastric acid output ⩾ 5 mEq/h famotidine 20 or 40mg administered twice daily decreased nocturnal and food-stimulated gastric acid secretion by 41 and 57%, respectively. In healthy subjects, famotidine 40mg administered at bedtime was equipotent as ranitidine 300mg and more potent than cimetidine 800mg at inhibiting nocturnal and pentagastrin-stimulated acid secretion. Famotidine 5mg has also been shown to be both equipotent as and 66% more potent than cimetidine 300mg against pentagastrin-stimulated acid secretion in healthy subjects. Single bedtime doses of famotidine 20 or 40mg and ranitidine 150 or 300mg were not statistically different in their effect on nocturnal and daytime pH and mean acid concentrations in patients with duodenal ulcer. Serum gastrin concentrations of gastric and duodenal ulcer patients were not significantly altered by famotidine administration, but in healthy subjects serum gastrin concentrations were significantly increased 4 hours after administration of famotidine 20 or 40mg, but remained within normal limits. Pepsin output is decreased by famotidine. Serum prolactin secretion is not significantly increased by either acute or repeated administration of usual therapeutic doses of famotidine. Unlike cimetidine, famotidine does not have antiandrogenic effects in animals or humans, and studies to date suggest that famotidine does not affect hepatic metabolism of drugs. Pharmacokinetics: After oral administration, dose-related peak plasma concentrations are achieved within 1 to 3.5 hours. Mean peak plasma concentrations are between 50 and 60 μg/L after a 20mg oral dose. A plasma concentration of 13 μg/L was required for 50% inhibition of tetragastrin-stimulated gastric acid secretion in healthy volunteers. Plasma concentrations are maintained at this level for about 12 hours after a 40mg oral dose. Bioavailability has been reported to be 37 to 45% and is not dose dependent. The apparent volume of distribution of famotidine is 1.1 to 1.4 L/kg, but the tissue distribution of famotidine has not been reported. Protein binding is relatively low (15 to 22%). Famotidine is excreted in the urine, by both glomerular filtration and tubular secretion, and in the faeces. From 25 to 30% of orally administered doses are recovered unchanged in the urine. The elimination half-life of orally administered famotidine is between 2.5 and 4 hours in patients with normal renal function, but increases to approximately 12 hours in patients with a creatinine clearance of less than 30 ml/min. Therapeutic Trials: Both in open and in double-blind comparative trials with cimetidine and ranitidine the endoscopically verified rate of healing of duodenal ulcers after 4 to 6 weeks’ treatment with oral famotidine has ranged between 71 and 92%. At 2, 4 and 8 weeks famotidine 20mg twice daily or 40mg at bedtime was not significantly different in efficacy from ranitidine 150mg twice daily. However, too few patients may have been studied to permit a valid statistical comparison. Famotidine 20mg twice daily was significantly more effective than cimetidine 200mg 4 times daily; 85 and 70% of patients were healed, respectively, at 6 weeks (p < 0.05). However, this is an unusually low healing rate for cimetidine and may be related to the low evening dose (200mg). In Japanese patients famotidine 20mg twice daily was significantly more effective than gefarnate 100mg 3 times daily; 69 and 39% of patients were healed, respectively, at 6 weeks (p < 0.001). Famotidine 20mg orally twice daily healed 83 to 88% of gastric ulcers after 8 weeks of therapy in open studies and double-blind comparisons with cimetidine. In double-blind placebo-controlled studies a famotidine dosage of 40mg at bedtime healed 91 to 97% of patients during the 8 weeks of treatment; the healing rate for placebo was 61 to 66%. In Japanese patients famotidine 20mg twice daily was not significantly different inefficacy from cimetidine 200mg 4 times daily, but healed a significantly greater number of patients than gefarnate 100mg 3 times daily. In the latter study 39% (famotidine) and 10% (gefarnate) of patients were healed at 4 weeks (p < 0.01). Results from 2 long term (1-year) multicentre placebo-controlled studies indicate that famotidine 20mg at bedtime significantly decreases the endoscopically determined rate of relapse of recently healed duodenal ulcers (35.7 and 23.4% relapse rates vs 75.5 and 61% for placebo). In a few patients with Zollinger-Ellison syndrome, famotidine has been shown to be more potent on a molar basis at controlling gastric acid hypersecretion than either cimetidine or ranitidine. In equipotent doses, famotidine had a longer duration of action than either of the other two H2-receptor antagonists. During up to 9 months’ therapy in these patients, famotidine, alone or in combination with an anticholinergic drug, effectively controlled gastric acid hypersecretion with no evidence of biochemical or haematological toxicity. Side Effects: Cumulated Japanese and European side effect data from short term controlled and open studies of the treatment of peptic ulceration, and of patients maintained on treatment for longer periods (up to 10 weeks), reveal that famotidine 20mg twice daily or 40mg at bedtime is well tolerated, side effects having been reported in only 3 to 7% of patients, overall. The most frequently reported side effects are headache, dizziness, constipation and diarrhoea. Dosage and Administration: The recommended oral dose of famotidine for acute duodenal or gastric ulcer is one 40mg tablet or 5ml (40mg) of oral suspension daily at bedtime for 4 to 8 weeks. However, the duration of treatment may be shortened if endoscopy reveals that the ulcer has healed. For the prevention of ulcer recurrence, maintenance therapy with 20mg daily at bedtime is recommended. Patients with Zollinger-Ellison syndrome should be started on a dose of 20mg 6-hourly and dosage adjusted to individual patient needs; doses of up to 480mg daily have been used for up to 1 year. Dosage may be decreased in patients with impaired renal function.

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