Chemistry - A peripheral vasodilating agent, isoxsuprine occurs as an odorless, bitter-tasting, white, crystalline powder with a melting point of about 200°C. It is slightly soluble in water and sparingly soluble in alcohol.
Storage/Stability/Compatibility - Tablets should be stored in tight containers at room temperature (15-30°C).
Pharmacology - Isoxsuprine causes direct vascular smooth muscle relaxation primarily in skeletal muscle. While it stimulates beta-adrenergic receptors it is believed that this action is not required for vasodilitation to occur. In horses with navicular disease, it has been demonstrated that isoxsuprine will raise distal limb temperatures significantly. Isoxsuprine will also relax uterine smooth muscle and may have positive inotropic and chronotropic effects on the heart. At high doses, isoxsuprine can decrease blood viscosity and reduce platelet aggregation.
Uses/Indications - Isoxsuprine is used in veterinary medicine principally for the treatment of navicular disease in horses. It has been used in humans for the treatment of cerebral vascular insufficiency, dysmenorrhea, and premature labor, but efficacies are unproven for these indications.
Pharmacokinetics - In humans, isoxsuprine is almost completely absorbed from the GI tract, but in one study that looked at the cardiovascular and pharmacokinetic effects of isoxsuprine in horses (Mathews and et 1986), bioavailability was low after oral administration, probably due to a high first-pass effect. After oral dosing of 0.6 mg/kg, the drug was non-detectable in the plasma and no cardiac changes were detected. This study did not evaluate cardiovascular effects in horses with navicular disease, nor did it attempt to measure changes in distal limb blood flow. After IV administration in horses, the elimination half-life is between 2.5 - 3 hours.
Contraindications/Precautions - Isoxsuprine should not be administered to animals immediately post-partum or in the presence of arterial bleeding.
Adverse Effects/Warnings - After parenteral administration, horses may show symptoms of CNS stimulation (uneasiness, hyperexcitability, nose-rubbing) or sweating. Adverse effects are unlikely after oral administration, but hypotension, tachycardia, and GI effects are possible.
Overdosage - Serious toxicity is unlikely in horses after an inadvertent oral overdose, but symptoms listed in the Adverse Effects section could be seen. Treat symptoms if necessary. CNS hyperexcitability could be treated with diazepam, and hypotension with fluids.
Drug Interactions - No clinically significant drug interactions have been reported for this agent.
For treatment of navicular disease:
a) 0.6 - 0.66 mg/kg bid PO X 21 days; then once daily for 14 days; then once every other day for 7 days. (Note: 0.66 mg/kg is fifteen 20 mg tabs for a 1000 lb. horse)
For refractory cases: regimen may be repeated at 1.32 mg/kg. If no improvement seen after 6 week course then discontinue. (Forney and Allen 1984)
Monitoring Parameters -
1) Clinical efficacy
2) Adverse effects (tachycardia, GI disturbances, CNS stimulation)
Client Information - To be maximally effective, doses must be given routinely as directed. Tablets may be crushed and made into a slurry/suspension/paste by adding corn syrup, cherry syrup, etc., just before administration.
Dosage Forms/Preparations/FDA Approval Status/Withholding Times -
Veterinary-Approved Products: None
Isoxsuprine HCl Tablets 10 mg, 20 mg; Vasodilan® (Mead Johnson), Voxsuprine® (Major), Generic; (Rx)
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