Tamsulosin

Overview

Tamsulosin is an alpha-1 adrenergic blocking agent that is similar to prazosin and phenoxybenzamine but has a longer duration of action. In dogs and cats, it may be useful as a lower urinary tract relaxant, although published clinical experience remains limited.

Tamsulosin may be used to facilitate passage of nephroliths and ureteroliths and to manage functional urinary obstruction, detrusor-urethral dyssynergia, and postobstructive urethral spasm. The drug achieves higher and more prolonged concentrations in prostatic and urethral tissues than in plasma.

Mechanism of Action (MOA): Tamsulosin blocks sympathetic stimulation of alpha-1 adrenoceptors and is relatively selective for the alpha-1A receptor subtype found primarily in the prostate, prostatic urethra, bladder neck, and prostatic capsule. Blockade of these receptors relaxes smooth muscle in the bladder neck and prostate, improving urine flow and facilitating urinary calculus passage.

Indications

Tamsulosin may be used in dogs and cats as a lower urinary tract relaxant to facilitate urine flow and urinary stone passage.

• Ureteroliths and nephroliths: Facilitation of urinary stone passage.
• Functional urinary obstruction: Management of lower urinary tract obstruction caused by functional urethral resistance.
• Detrusor-urethral dyssynergia: Treatment of detrusor-urethral dyssynergia.
• Postobstructive urethral spasm (cats): Management of urethral spasm following relief of urethral obstruction.
• Benign prostatic hypertrophy: Adjunctive management of benign prostatic hypertrophy.

Dosage (Reference)

Dog

Tamsulosin has been used as an extra-label ureteral and urethral relaxant in dogs, although safety, efficacy, and optimal dosing have not been established.

Cat

An anecdotal dosage has been reported for use as a ureteral or urethral relaxant in cats; however, safety, efficacy, and optimal dosing have not been established.

Warnings & Precautions

Tamsulosin should be used cautiously in dogs and cats because excessive alpha-1 blockade may result in hypotensive effects.

• Hypersensitivity: Contraindicated in patients with known hypersensitivity to tamsulosin.
• Sustained-release formulation: Capsules contain sustained-release beads and should not be crushed or compounded into an oral suspension because rapid absorption may result in hypotension.
• Sulfonamide sensitivity: Hypersensitivity reactions have been reported in some humans with sulfonamide allergies; significance in veterinary patients is unknown.
• Priapism: Has been reported in humans; significance in veterinary patients is unknown.
• Breeding animals: May impair or alter ejaculation and is presumed to have this effect in breeding males.
• Pregnancy and lactation: Safety has not been established in animals. Use only when the maternal benefits outweigh the potential risks to offspring.
• Cataract and glaucoma surgery: Intraoperative floppy iris syndrome has been reported in people taking this drug at the time of cataract surgery, and starting it is not recommended in a patient for whom cataract or glaucoma surgery is planned; its significance in animals is not known.

Drug Interactions

Tamsulosin is metabolized by CYP2D and CYP3A enzyme systems and may interact with drugs that alter its metabolism or enhance its hypotensive effects.

• Alpha-1 adrenergic receptor agonists (e.g., phenylephrine): May decrease the effects of each other.
• Alpha-1 adrenergic blockers (e.g., prazosin, phenoxybenzamine): Concurrent use may exacerbate hypotensive effects.
• Cimetidine: May increase tamsulosin concentrations.
• Clarithromycin and erythromycin: May increase plasma concentrations of tamsulosin.
• Hypotensive agents (e.g., amlodipine, atenolol, enalapril, telmisartan): May have additive hypotensive effects.
• Itraconazole: Concurrent use increases the risk of tamsulosin toxicity, and in people the combination is contraindicated.
• Ketoconazole: May increase plasma concentrations of tamsulosin, and in people concurrent use is not recommended.
• Phosphodiesterase type 5 (PDE5) inhibitors (e.g., sildenafil, tadalafil): May have additive hypotensive effects.
• Terbinafine: May increase plasma concentrations of tamsulosin.
• Warfarin: Interaction data are inconclusive; use concurrently with caution.

Side Effects & Overdose

Side Effects

The adverse effect profile of tamsulosin in dogs and cats has not been well documented. Reported and expected effects are primarily related to its blood pressure-lowering properties.

• Hypotension: Hypotensive effects may occur, particularly at higher doses and may be more pronounced in older dogs. 
• Lethargy or sedation: May occur following excessive exposure or overdose.
• Tachycardia: Increased heart rate has been reported with overdose.
• Vomiting: Gastrointestinal upset may occur following overdose.

Overdose

Severe toxicity appears to be uncommon, but overdose may result in clinically significant cardiovascular effects.

• Management: Hypotension is usually responsive to intravenous fluid therapy.
• Severe cases: Vasopressors may be required if marked hypotension develops.

Key Notes

Practical clinical points that may help optimize the use of tamsulosin in dogs and cats:

• Prostatic and urethral tissue retention: Tamsulosin concentrations remain higher and persist longer in prostatic and urethral tissues than in plasma.
• Rapid absorption in dogs: Following oral administration of a nonsustained-release formulation, peak plasma concentrations occur approximately 0.5 hours after dosing.
• Protein binding: Tamsulosin is highly protein bound (approximately 90% in dogs).
• Canine metabolism: In dogs, CYP3A4 is responsible for metabolism of tamsulosin to the M1 and AM-1 metabolites.
• Limited feline pharmacokinetic data: Pharmacokinetic studies in cats were not located.
• Delayed maximal effect: Clinical improvement may take several days to reach peak effect.
• Monitoring: Record a baseline blood pressure and repeat it if signs of hypotension appear; also monitor urine flow, residual urine volume and heart rate.