ファモチジン
ファモチジンは強力で長時間作用型のヒスタミンH2受容体拮抗薬であり、獣医学において胃酸分泌を抑制し、胃・十二指腸潰瘍、胃炎、食道炎の治療または予防に広く使用されています。 シメチジンと比較して、ファモチジンは著しく強力であり、投与頻度が少なくて済みます。さらに重要なことに、**肝臓のシトクロムP450酵素系を阻害しない**ため、多くの一般的な薬物相互作用を回避できます。 > **臨床のポイント:** 軽度の胃酸抑制や日常的な予防には有効ですが、活動性潰瘍の治療、運動誘発性胃炎の予防、または重度の食道炎の管理には、オメプラゾールなどのプロトンポンプ阻害薬（PPI）が一般的に優れているとされています。さらに、犬や猫に3〜14日間連続して使用すると、薬理学的な耐性（**タキフィラキシー**）が生じやすく、時間の経過とともに胃酸抑制効果が減弱する傾向があります。
作用機序: Famotidine acts as a competitive, reversible antagonist at the **H2 receptors** located on the basolateral membrane of gastric **parietal cells**. By blocking histamine binding, it prevents the activation of adenylate cyclase → decreases intracellular cAMP levels → reduces the activation of the **H+/K+-ATPase proton pump**. This effectively blunts both basal gastric acid secretion and secretion stimulated by food, pentagastrin, or insulin. By decreasing the total volume of gastric juice produced, H2-blockers also indirectly decrease the amount of pepsin secreted. It does not alter gastric emptying time, biliary secretion, or lower esophageal sphincter pressure.
動物種別の用量
- To reduce gastric acid production · 0.2 mg/kg · PO, SC, IM, IV · q24h · See warning about IV use in cats
- To reduce gastric acid production · 0.5 mg/kg · PO, SC, IM, IV · q12-24h
- For esophagitis · 0.5 mg/kg · PO · q12h
- For acute reflex esophagitis · 0.55-1.1 mg/kg · PO · q24h (or q12h if severe) · 2-3 weeks
- Adjunctive treatment of GI effects associated with chronic progressive renal disease · 1 mg/kg · PO · q24h
- Adjunctive treatment of GI effects associated with chronic progressive renal disease · 0.5-1 mg/kg · PO · q12-24h
- All uses (ulcers, gastritis, oesophagitis, hypersecretory conditions) · 0.5-1.0 mg/kg · PO · q12-24h · Effect on gastric pH diminishes with time when given daily.
- For stress induced ulcers · 0.25-0.5 mg/kg · PO, IV · q24h
- In combination with antibiotics for Helicobacter treatment · 0.25-0.5 mg/kg · PO, IV · q24h
- As an adjunct in ulcer treatment · 0.23 mg/kg or 0.35 mg/kg · IV · q8h (for 0.23 mg/kg) or q12h (for 0.35 mg/kg) · ARCI UCGFS Class 5 Drug
投与経路
禁忌
- Known hypersensitivity to H2 blockers
- Known hypersensitivity to famotidine or other H2 receptor antagonists
有害事象
- Bradycardia (if infused too rapidly IV)
- GI effects (anorexia, vomiting, diarrhea)
- Headache
- Dry mouth or skin
- Intravascular hemolysis (rare, anecdotally reported in cats given IV)
- Transient increase in serum gastrin (returns to baseline by 14 days)
- Generally has a very good safety profile with fewer side effects than cimetidine
薬物相互作用
- Azole Antifungals (ketoconazole, itraconazole, fluconazole) · Famotidine raises gastric pH, which may decrease the absorption of these agents. Administer the azole one hour prior to famotidine.
- Cefpodoxime, Cefuroxime · Famotidine may decrease the absorption of these cephalosporins. Taking with food may alleviate this effect.
- Iron Salts (Oral) · Famotidine may decrease the absorption of oral iron. Administer iron at least one hour prior to famotidine.
モニタリング
- Clinical efficacy (decrease in symptomatology, endoscopic examination, resolution of blood in feces)
- Adverse effects
- Clinical signs of GI ulceration or bleeding (vomiting, melena, anorexia)
- Gastric pH (if clinically indicated and feasible)
過量投与
Famotidine has a wide margin of safety. The minimum acute lethal dose in dogs is reported to be >2 grams/kg for oral doses and approximately 300 mg/kg for intravenous doses. IV doses in dogs ranging from 5-200 mg/kg caused vomiting, restlessness, mucous membrane pallor, and redness of the mouth and ears. Higher doses caused hypotension, tachycardia, and collapse. Most overdoses require only monitoring. In massive oral overdoses, gut-emptying protocols should be considered and supportive therapy initiated when warranted.
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