Ranitidine
**Ranitidine** is a competitive histamine H2-receptor antagonist and gastrointestinal prokinetic agent used widely in veterinary medicine. **Key Clinical Features:** * **Acid Suppression:** Used for the treatment and prophylaxis of gastric, abomasal, and duodenal ulcers, uremic gastritis, and stress-related or drug-induced erosive gastritis. * **Prokinetic Activity:** Uniquely among H2 blockers, ranitidine stimulates GI motility, making it useful for delayed gastric emptying and stimulating colonic activity in cats (e.g., for megacolon). * **Systemic Mastocytosis:** Helps manage hypersecretory conditions associated with mast cell tumors (which release massive amounts of histamine). > **Clinical Pearl:** Ranitidine is 3 to 13 times more potent than cimetidine on a molar basis. Furthermore, it has significantly fewer drug interactions because it does not appreciably inhibit hepatic cytochrome P450 enzymes, making it a safer choice for patients on multiple medications.
Mecanismo: Ranitidine exerts its effects via two distinct mechanisms: 1. **Gastric Acid Suppression:** It competitively inhibits **histamine** at the **H2 receptors** located on the basolateral membrane of gastric parietal cells. * Histamine blockade → decreased intracellular cAMP → reduced activation of the **H+/K+ ATPase (proton pump)** → significant reduction in both basal and stimulated gastric acid and pepsin secretion. 2. **Prokinetic Effect:** It reversibly inhibits **acetylcholinesterase (AChE)** in the gastrointestinal tract. * AChE inhibition → decreased breakdown of acetylcholine → increased **acetylcholine (ACh)** at muscarinic receptors → stimulation of GI smooth muscle motility and increased lower esophageal sphincter pressure.
Dosificación por especie
- For ulcer disease/esophagitis · 2.5 mg/kg IV q12h or 3.5 mg/kg PO q12h · IV, PO · q12h
- For ulcer disease/esophagitis · 1-2 mg/kg · PO, IV, SC · q12h
- For ulcer disease/esophagitis · 2 mg/kg · PO, IV · q12h
- As a prokinetic agent to stimulate colonic motility · 1-2 mg/kg · PO · q8-12h
- As a prokinetic agent to stimulate colonic motility · 1-2 mg/kg · PO · q12h
- All uses · 2 mg/kg/day · IV · constant infusion · Not an effective acid suppressant in healthy cats.
- All uses · 2.5 mg/kg · IV · Not specified · Not an effective acid suppressant in healthy cats.
- For Helicobacter mustelae · 24 mg/kg · PO · q8-12h · 14 days · Uses Ranitidine bismuth citrate (must be compounded). Given with clarithromycin (12.5 mg/kg PO q8-12h).
- Foals · 1.5 mg/kg IV q8h or 6.6 mg/kg PO q8h · IV, PO · q8h · Often used with omeprazole (4 mg/kg PO q24h).
- Foals · 6.6 mg/kg IV q4h or 0.8-2.2 mg/kg IV four times a day; 5-10 mg/kg PO two to four times a day. · IV, PO · q4h or QID (IV); BID-QID (PO)
Vías de administración
Contraindicaciones
- Hypersensitivity to ranitidine
- No specific contraindications available in the monograph
Efectos adversos
- Vomiting (associated with rapid IV boluses in small animals)
- Pain at the injection site (IM administration)
- Mental confusion (rare, documented in humans)
- Headache (rare, documented in humans)
- Agranulocytosis (rare)
- Transient cardiac arrhythmias (if given too rapidly IV)
- Cardiac arrhythmias (rare, typically with rapid IV)
- Hypotension (rare, typically with rapid IV)
Interacciones farmacológicas
- Acetaminophen · Ranitidine (dose-dependent) may inhibit acetaminophen metabolism.
- Antacids · High doses may decrease the absorption of ranitidine; administer at least 2 hours apart. · moderate
- Ketoconazole · Absorption of ketoconazole may be reduced secondary to increased gastric pH.
- Itraconazole · Absorption of itraconazole may be reduced secondary to increased gastric pH.
- Metoprolol · Ranitidine may increase metoprolol half-life and peak serum levels.
- Nifedipine · Ranitidine may increase nifedipine AUC by 30%.
- Propantheline · Delays the absorption but increases the peak serum level of ranitidine; relative bioavailability may be increased by 23%.
- Vitamin B-12 · Long-term ranitidine use may reduce oral absorption of Vitamin B-12.
- Sucralfate · May affect absorption; advisable to administer sucralfate 2 hours before H2 blockers · minor
- Digoxin · Ranitidine may reduce absorption or effect; stagger oral doses by 2 hours · moderate
- Metoclopramide · Ranitidine may reduce absorption or effect; stagger oral doses by 2 hours · moderate
Monitorización
- Clinical efficacy (resolution of clinical signs, improved appetite, absence of blood in feces/vomitus)
- Endoscopic examination (if indicated for ulcer healing)
- Serum ALT values (consider monitoring during high-dose, chronic IV therapy)
- Resolution of gastrointestinal clinical signs
- Heart rate and blood pressure (during IV administration)
Sobredosis
Clinical experience with ranitidine overdosage is limited, but the drug has a wide margin of safety. * **Signs of Toxicity:** In laboratory animals, massive doses (225 mg/kg/day) have caused muscular tremors, vomiting, and rapid respirations. Single doses of 1 gram/kg in rodents were not fatal. * **Treatment:** Handle using standard protocols for oral drug ingestions (e.g., gastric decontamination if recent and appropriate). Treat clinical signs symptomatically and supportively. Hemodialysis and peritoneal dialysis can effectively remove ranitidine from the body.
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