Captopril

ACE inhibitor (Angiotensin-Converting Enzyme Inhibitor) Renin-Angiotensin-Aldosterone System (RAAS) Inhibitor

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 0.5-2 mg/kg q8h to q12h Duration: Long-term, as needed
Notes: Start at low end of dose range and titrate upward based on response. Monitor blood pressure and renal function. Administer on an empty stomach (1 hour before or 2 hours after feeding) to maximize absorption.
Cat PO 0.5-2 mg/kg q12h to q24h Duration: Long-term, as needed
Notes: Start at low end of dose range. Cats may be more sensitive to hypotension. Monitor renal function and potassium levels.
Horse PO 0.5-1 mg/kg q12h Duration: Not established
Notes: Not commonly used; limited data. Use with caution and monitor blood pressure.
Cattle PO Not established Not established Duration: Not established
Notes: Not indicated for use in cattle.
Small Ruminants PO Not established Not established Duration: Not established
Notes: Not indicated for use in sheep or goats.
Rabbit PO 0.5-1 mg/kg q12h Duration: Not established
Notes: Limited evidence; use with caution. Monitor for hypotension.
Bird/Poultry PO Not established Not established Duration: Not established
Notes: Not indicated for use in birds.
Exotic/Other PO Not established Not established Duration: Not established
Notes: Not indicated for use in exotic species.

Clinical Indications & Species Uses

General Indications
  • Treatment of congestive heart failure
  • Management of systemic hypertension
  • Reduction of proteinuria in chronic kidney disease
Dog (Canine)
  • Congestive heart failure (CHF) due to mitral regurgitation or dilated cardiomyopathy
  • Systemic hypertension (including renal and idiopathic)
  • Protein-losing nephropathy (as adjunctive therapy to reduce proteinuria)
Cat (Feline)
  • Congestive heart failure (CHF) due to hypertrophic cardiomyopathy or other causes
  • Systemic hypertension (especially associated with chronic kidney disease)
  • Protein-losing nephropathy (as adjunctive therapy)

Pharmacology & Mechanism of Action

Drug Class: ACE inhibitor (Angiotensin-Converting Enzyme Inhibitor) | Pharmacological Group: Renin-Angiotensin-Aldosterone System (RAAS) Inhibitor

Mechanism of Action: Captopril is a competitive inhibitor of angiotensin-converting enzyme (ACE), which catalyzes the conversion of angiotensin I to the potent vasoconstrictor angiotensin II. By inhibiting ACE, captopril reduces circulating levels of angiotensin II, leading to vasodilation, decreased aldosterone secretion, and reduced sodium and water retention. It also decreases degradation of bradykinin, which contributes to vasodilatory effects. In veterinary patients, these actions result in reduced systemic vascular resistance, lowered blood pressure, and decreased cardiac workload, making it useful in heart failure and hypertension.

Pharmacodynamics: Captopril produces dose-dependent reductions in blood pressure and systemic vascular resistance. It decreases afterload and preload, improves cardiac output in failing hearts, and reduces myocardial oxygen demand. It also reduces glomerular filtration pressure by dilating efferent arterioles, which can be renoprotective in chronic kidney disease but may cause acute renal dysfunction in volume-depleted patients. The drug's effects on aldosterone lead to increased renal sodium excretion and potassium retention. Clinical effects are typically seen within 1 hour of oral administration and may last 4-6 hours.

⚡ Pharmacokinetics Summary

Absorption: Captopril is rapidly absorbed from the gastrointestinal tract after oral administration. In dogs, oral bioavailability is approximately 60-75%, but food can reduce absorption by up to 30-40%. Peak plasma concentrations occur within 0.5-1.5 hours. In cats, absorption is also rapid, with peak levels at 1-2 hours.
Distribution: Captopril is widely distributed throughout the body. It crosses the placenta and is distributed into milk. It is approximately 25-30% bound to plasma proteins. The volume of distribution is relatively small, and the drug does not penetrate the central nervous system significantly.
Metabolism: Captopril undergoes extensive metabolism, primarily in the liver, to form disulfide conjugates and other metabolites. Some of these metabolites retain ACE inhibitory activity. The drug also undergoes some metabolism in the gastrointestinal tract and blood.
Excretion: Captopril and its metabolites are primarily excreted by the kidneys via tubular secretion and glomerular filtration. In dogs, approximately 50-60% of an oral dose is excreted in urine within 24 hours. In patients with renal impairment, elimination is prolonged, and dose adjustments may be necessary.
Half-Life: The elimination half-life of captopril in dogs is approximately 1.5-2 hours, but it may be prolonged in renal impairment. In cats, the half-life is similar, around 1.5-2.5 hours.
Bioavailability: Oral bioavailability in dogs is approximately 60-75%, but it is reduced by food. In cats, bioavailability is approximately 50-70%.
Protein Binding: Captopril is approximately 25-30% bound to plasma proteins.

Available Formulations & Strengths

Oral Tablet 12.5 mg, 25 mg, 50 mg, 100 mg (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to captopril or other ACE inhibitors
  • History of angioedema related to ACE inhibitor therapy
  • Severe renal impairment (unless used under close monitoring)
  • Hyperkalemia
  • Pregnancy (especially in the second and third trimesters) due to risk of fetal renal damage
  • Concurrent use with aliskiren (in humans; not relevant in veterinary medicine but caution advised)
Warnings & Clinical Precautions:
  • Use with caution in patients with renal insufficiency; monitor renal function and electrolytes (especially potassium) before and during therapy.
  • May cause hypotension, especially in volume-depleted patients or those on diuretics; adjust diuretic dose if needed.
  • In patients with heart failure, monitor for worsening renal function and signs of azotemia.
  • Use with caution in animals with autoimmune diseases or collagen vascular disorders due to risk of neutropenia/agranulocytosis.
  • Do not use in pregnant animals unless the benefits outweigh risks; ACE inhibitors can cause fetal harm.
  • In lactating animals, safety has not been established; use with caution.
  • Administer on an empty stomach to improve absorption; food reduces bioavailability.
  • Monitor blood pressure and renal parameters periodically during therapy.
  • In cats, use with caution due to potential for reduced renal function and hyperkalemia.

Adverse Effects & Reactions

Common:

  • Hypotension
  • Lethargy
  • Anorexia
  • Vomiting
  • Diarrhea
  • Cough (rare in animals but possible)

Serious / Severe:

  • Acute renal failure (especially in volume-depleted patients)
  • Hyperkalemia
  • Angioedema (rare but potentially fatal)
  • Neutropenia/agranulocytosis (rare)
  • Hepatotoxicity (rare)

Rare:

  • Proteinuria
  • Skin rash
  • Taste disturbances (in humans; not observed in animals)
  • Pancytopenia
  • Stevens-Johnson syndrome

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Potassium-sparing diuretics (e.g., spironolactone, triamterene) Increased risk of hyperkalemia due to additive potassium retention. High
Potassium supplements Increased risk of hyperkalemia. High
Nonsteroidal anti-inflammatory drugs (NSAIDs) Reduced antihypertensive effect and increased risk of renal dysfunction, especially in volume-depleted patients. Moderate
Diuretics (e.g., furosemide) Additive hypotensive effect; may cause excessive blood pressure reduction and renal impairment. Moderate
Other antihypertensive agents (e.g., beta-blockers, calcium channel blockers) Additive hypotensive effects; monitor blood pressure closely. Moderate
Lithium ACE inhibitors may increase lithium levels, leading to toxicity. High
Anesthetics May enhance hypotensive effects; monitor during anesthesia. Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Severe hypotension
  • Shock
  • Renal failure
  • Hyperkalemia
  • Bradycardia
  • Dizziness (in humans; not applicable)
  • Electrolyte imbalances

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce emesis if recent ingestion and animal is stable. Administer activated charcoal to reduce absorption. Provide intravenous fluids to maintain blood pressure and renal perfusion. Monitor blood pressure, renal function, and electrolytes. Administer vasopressors (e.g., dopamine, norepinephrine) if hypotension is severe. Correct hyperkalemia with calcium gluconate, insulin/glucose, or sodium bicarbonate as needed. In severe cases, hemodialysis may be considered.

Food Animal Withdrawal Times

Captopril is not approved for use in food-producing animals. Withdrawal times are not established. Use in food animals is prohibited or requires a very long withdrawal period (e.g., >30 days) under extra-label use regulations. Consult regulatory authorities.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25°C or 68-77°F), with excursions permitted between 15-30°C (59-86°F).

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from moisture and light. Keep container tightly closed. Do not crush tablets unless directed; may be compounded into suspensions by a pharmacist.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use. No veterinary-approved formulations exist.

Extra-Label (Off-Label) Use: In the US, captopril is not FDA-approved for veterinary use, but it can be prescribed legally under the Animal Medicinal Drug Use Clarification Act (AMDUCA) for extra-label use in animals, provided a valid veterinarian-client-patient relationship exists. For food animals, extra-label use requires a withdrawal time established by the veterinarian, and the drug must not be used in an extra-label manner that results in violative residues.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Captopril is a short-acting ACE inhibitor that is used in veterinary medicine primarily for the management of congestive heart failure and systemic hypertension in dogs and cats. It is often used when enalapril or benazepril (longer-acting ACE inhibitors) are not available or when a short-acting agent is desired for initial therapy or dose titration. However, due to its short duration of action and need for multiple daily dosing, it is less convenient than newer ACE inhibitors. Captopril should be administered on an empty stomach to maximize absorption. Monitoring should include blood pressure, renal function (BUN, creatinine), and serum potassium levels, especially during the first weeks of therapy. In patients with heart failure, concurrent use of diuretics may require dose adjustment to avoid hypotension. Captopril is contraindicated in pregnancy and should be used with caution in animals with renal insufficiency. Overall, it is a valuable alternative when other ACE inhibitors are not tolerated or available.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)