Captopril
Captopril is a first-generation, sulfhydryl-containing **Angiotensin-Converting Enzyme (ACE) inhibitor**. Originally related to a peptide isolated from the venom of the South American pit viper (*Bothrops jararaca*), it holds historical significance as the first available ACE inhibitor. While effective as a vasodilator for the treatment of congestive heart failure (CHF) and hypertension, its use in veterinary medicine has been largely supplanted by newer agents like **enalapril** and **benazepril**. This shift is primarily due to captopril's shorter half-life (often requiring TID dosing) and a higher incidence of adverse effects, particularly gastrointestinal upset in dogs. **Clinical Pearl:** Unlike enalapril, captopril is an active drug and does not require hepatic biotransformation to an active metabolite to exert its effects.
Mechanism: Captopril acts as a competitive inhibitor of **angiotensin-converting enzyme (ACE)**, for which it has a much higher affinity than the natural substrate, angiotensin-I. * **Angiotensin I** → (blocked by ACE) → **Angiotensin II** (a potent vasoconstrictor). * The reduction in Angiotensin II leads to **vasodilation**, decreasing total peripheral resistance, pulmonary vascular resistance, and blood pressure. * Decreased Angiotensin II also reduces **aldosterone** secretion, leading to decreased sodium and water retention, while increasing plasma renin activity. * Cardiovascular benefits in CHF include increased cardiac output, stroke volume, and exercise tolerance with little to no change in heart rate.
Dosing by species
- CHF / Hypertension · 1/4 to 1/2 of a 12.5 mg tablet PO q8-12h · PO · q8-12h
- Dilative, restrictive or hypertrophic cardiomyopathy · 0.55-1.54 mg/kg PO q8-12h · PO · q8-12h
- CHF / Hypertension · 0.5-2 mg/kg PO three times daily · PO · TID
- CHF / Hypertension · 0.5-2 mg/kg PO q8-12h · PO · q8-12h
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Hypersensitivity to ACE inhibitors
Adverse effects
- Hypotension
- Renal failure
- Hyperkalemia
- Vomiting
- Diarrhea
- Skin rashes (reported in humans, not dogs)
- Neutropenia/agranulocytosis (rare, reported in humans)
Drug interactions
- Antacids · Reduced oral absorption of captopril; separate dosing by at least two hours.
- Cimetidine · Concomitant use has caused neurologic dysfunction in human patients.
- Digoxin · Digoxin levels may increase 15-30%; monitor serum digoxin levels.
- Diuretics · Concomitant use may cause hypotension; titrate dosages carefully.
- NSAIDs · May reduce the clinical efficacy of captopril when used as an antihypertensive agent.
- Potassium or Potassium-Sparing Diuretics (e.g., spironolactone) · Increased risk of developing hyperkalemia.
- Probenecid · Can decrease renal excretion of captopril, possibly enhancing clinical and toxic effects.
- Vasodilators (e.g., prazosin, hydralazine, nitrates) · Concomitant use may cause additive hypotension; titrate dosages carefully.
Monitoring
- Clinical signs of CHF (respiratory rate/effort, exercise tolerance)
- Blood pressure (especially if treating hypertension or if signs of hypotension arise)
- Serum electrolytes (monitor for hyperkalemia)
- Renal panel (Creatinine, BUN)
- Urine protein
- CBC with differential (periodic)
Overdose
The primary concern in an overdose situation is **hypotension**. * **Treatment:** Supportive treatment with volume expansion using normal saline is recommended to correct blood pressure. * **Toxicity thresholds:** Dogs given 1.5 g/kg orally developed emesis and decreased blood pressure. Dogs receiving doses greater than 6.6 mg/kg q8h may develop renal failure.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.