Aminophylline — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	10 mg/kg	q8h	Duration: As needed for clinical response Notes: Use immediate-release tablets. Monitor serum theophylline levels if possible (target 5-20 µg/mL).
Dog	IV	5-10 mg/kg	q6-8h	Duration: For acute bronchospasm Notes: Administer slowly over 30 minutes. Not for IM or SC due to irritation.
Cat	PO	5 mg/kg	q12h	Duration: As needed for clinical response Notes: Cats metabolize theophylline slowly; use lower dose. Monitor for signs of toxicity.
Horse	PO	5-10 mg/kg	q12h	Duration: For RAO/COPD Notes: Use sustained-release formulations if available. Monitor for excitement and tachycardia.
Horse	IV	5 mg/kg	q12h	Duration: For acute exacerbation Notes: Administer slowly. Not for IM or SC.
Cattle	PO	5-10 mg/kg	q12h	Duration: As needed Notes: Off-label use; observe withdrawal times.
Cattle	IV	5 mg/kg	q12h	Duration: For acute respiratory distress Notes: Administer slowly. Not for IM or SC.
Small Ruminants (Sheep/Goats)	PO	5-10 mg/kg	q12h	Duration: As needed Notes: Off-label use; monitor for toxicity.
Rabbit	PO	5-10 mg/kg	q12h	Duration: As needed Notes: Off-label use; may cause GI upset.
Birds (e.g., Psittacines)	PO	5-10 mg/kg	q12h	Duration: As needed Notes: Off-label use; limited data.

Clinical Indications & Species Uses

General Indications

Bronchodilator for reversible airway obstruction
Adjunct in congestive heart failure (pulmonary edema)
Respiratory stimulant in neonates

Dog (Canine)

Bronchodilation in chronic bronchitis
Tracheal collapse
Heartworm disease-associated respiratory signs
Adjunct in asthma
Apnea of prematurity (off-label)

Cat (Feline)

Feline bronchial disease (asthma)
Chronic bronchitis

Horse (Equine)

Chronic obstructive pulmonary disease (COPD) / Recurrent airway obstruction (RAO)
Heaves
Exercise-induced pulmonary hemorrhage (adjunct)

Cattle (Bovine)

Bovine respiratory disease complex (adjunct)
Pulmonary edema (adjunct)

Rabbit & Small Mammals

Respiratory disease (e.g., Pasteurella-induced bronchoconstriction) (off-label)

Pharmacology & Mechanism of Action

Drug Class: Methylxanthine bronchodilator | Pharmacological Group: Xanthine derivatives

Mechanism of Action: Aminophylline is a complex of theophylline and ethylenediamine, which enhances the solubility of theophylline. Theophylline acts as a non-selective phosphodiesterase inhibitor, increasing intracellular cyclic AMP (cAMP) and cyclic GMP (cGMP) levels. This leads to bronchodilation by relaxing bronchial smooth muscle, inhibition of mediator release from mast cells, and anti-inflammatory effects. Additionally, it antagonizes adenosine receptors, stimulates central respiratory drive, enhances mucociliary clearance, and improves diaphragmatic contractility.

Pharmacodynamics: Theophylline produces bronchodilation in patients with reversible airway obstruction. It also has positive inotropic and chronotropic effects on the heart, increases renal blood flow and glomerular filtration rate, and may cause mild diuresis. In animals, it can reduce pulmonary vascular resistance and improve right ventricular function. The drug also exhibits anti-inflammatory properties, including inhibition of leukocyte activation and cytokine production.

⚡ Pharmacokinetics Summary

Absorption: Aminophylline is well absorbed after oral administration, with theophylline being released. Absorption may be affected by food, with peak concentrations occurring 1-2 hours after oral administration in dogs and cats. In horses, oral absorption is slower and more variable.

Distribution: Theophylline distributes widely into body tissues and fluids, including cerebrospinal fluid, breast milk, and placenta. It is approximately 40-60% protein-bound in plasma, with binding varying by species. Volume of distribution is approximately 0.5-0.7 L/kg in dogs and cats.

Metabolism: Theophylline is extensively metabolized in the liver, primarily by cytochrome P450 enzymes (CYP1A2 in humans, but species-specific). In dogs, metabolism involves N-demethylation and oxidation. In cats, metabolism is slower, leading to a longer half-life. In horses, metabolism is also hepatic.

Excretion: Theophylline and its metabolites are excreted primarily in the urine. In dogs, about 10-20% is excreted unchanged; in cats, a higher proportion may be excreted unchanged. Renal excretion is influenced by urine pH, with alkaline urine increasing excretion.

Half-Life: Dog: 5-8 hours; Cat: 7-11 hours; Horse: 10-15 hours; Cattle: 4-6 hours (estimated); Rabbit: 2-4 hours (estimated).

Bioavailability: Oral bioavailability is nearly 100% for immediate-release formulations in dogs and cats. In horses, oral bioavailability is approximately 70-80%.

Protein Binding: Approximately 40-60% in dogs and cats; lower in horses (about 30%).

Available Formulations & Strengths

Oral Tablet 100 mg, 200 mg (PO)

Oral Solution 105 mg/5 mL (equivalent to 90 mg theophylline) (PO)

Injectable Solution 25 mg/mL (IV)

Sustained-Release Tablet 300 mg, 450 mg (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to aminophylline or theophylline
Concurrent use with other xanthines (e.g., caffeine, theobromine)
Severe cardiac arrhythmias
Hyperthyroidism (uncontrolled)
Peptic ulcer disease
Seizure disorders (unless appropriately managed)

Warnings & Clinical Precautions:

Use with caution in animals with hepatic or renal impairment, as dose adjustment may be necessary.
Monitor serum theophylline concentrations in animals on long-term therapy or with concurrent diseases.
In cats, use lower doses due to slower metabolism.
In horses, avoid rapid IV administration to prevent cardiovascular collapse.
May cause excitement or CNS stimulation in some animals.
Use with caution in animals with cardiac disease, as it can cause tachycardia and arrhythmias.
In food animals, observe withdrawal times.
Do not crush sustained-release tablets.

Adverse Effects & Reactions

Common:

Gastrointestinal upset (vomiting, diarrhea)
Restlessness
Tachycardia
Polyuria
Polydipsia

Serious / Severe:

Cardiac arrhythmias
Seizures
Hypotension
Respiratory arrest
Hyperthermia

Rare:

Allergic reactions
Hepatotoxicity
Hypokalemia
Hyperglycemia

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Fluoroquinolones (e.g., enrofloxacin)	Decreased clearance of theophylline, increasing risk of toxicity.	High
Cimetidine	Inhibits hepatic metabolism of theophylline, increasing serum levels.	High
Erythromycin	Decreases theophylline clearance, increasing risk of toxicity.	High
Phenobarbital	Induces hepatic enzymes, increasing theophylline metabolism and reducing efficacy.	Moderate
Beta-agonists (e.g., albuterol)	Additive bronchodilation but increased risk of cardiovascular effects.	Moderate
Corticosteroids	May increase risk of hypokalemia and hyperglycemia.	Mild
Halothane	Increased risk of cardiac arrhythmias.	High

Overdose & Toxicity Management

Signs of Toxicity:

Vomiting
Tachycardia
Hyperactivity
Tremors
Seizures
Cardiac arrhythmias
Hypotension
Hyperthermia

Emergency Treatment Protocol: Induce emesis if recent oral ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide supportive care: IV fluids, oxygen, and monitoring of cardiac and respiratory function. Control seizures with diazepam or barbiturates. Treat arrhythmias with appropriate antiarrhythmics (e.g., lidocaine for ventricular arrhythmias). Correct electrolyte imbalances, especially hypokalemia. In severe cases, consider hemodialysis or hemoperfusion.

Food Animal Withdrawal Times

🥩 Meat: 7 days🥛 Milk: 3 days

Withdrawal times are estimates; consult regulatory guidelines and veterinary supervision. For cattle, meat withdrawal is 7 days and milk withdrawal is 3 days. For other species, withdrawal times may vary; not approved for use in food animals in some countries.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (20-25°C) in a tight, light-resistant container.

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

Handling & Special Conditions: Protect from moisture. Do not freeze injectable solutions.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use.

Extra-Label (Off-Label) Use: In the US, aminophylline is not FDA-approved for veterinary species, but it can be used legally under the Animal Medicinal Drug Use Clarification Act (AMDUCA) with a valid veterinarian-client-patient relationship. Extra-label use in food animals requires a withdrawal period and may be prohibited in some cases.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Aminophylline is a useful bronchodilator for managing respiratory conditions in companion animals, particularly dogs and cats. It is important to individualize dosing based on species and clinical response, and to monitor for adverse effects. Serum theophylline monitoring is recommended for animals on chronic therapy or those with concurrent diseases. In horses, it is used for RAO but may have limited efficacy compared to other bronchodilators. In food animals, use is off-label and requires careful consideration of withdrawal times. Always consider the underlying cause of respiratory disease and use aminophylline as an adjunct to other therapies (e.g., corticosteroids, antibiotics) as appropriate.

References & Literature

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