SPASTYNIN

Indications

• Overactive bladder syndrome
• Urinary urgency
• Urinary frequency
• Urinary incontinence

Dosage

Children: Refer to the BNF for Children for specific dosing guidance.

Adults: Refer to the BNF for specific dosing guidance.

Mechanism of action

Oxybutynin acts to relax the bladder by competitively inhibiting the muscarinic receptors (types 1, 2, and 3) in the bladder smooth muscle. This inhibition reduces the effects of acetylcholine, resulting in increased bladder capacity and a decrease in urinary urgency and frequency. It also delays the initial desire to void.

Pharmacodynamics

Oxybutynin exhibits antispasmodic effects on the bladder, alleviating symptoms associated with overactive bladder. Its anticholinergic activity can lead to side effects such as confusion, hallucinations, and drowsiness. Caution is advised when operating heavy machinery until the individual's response to the medication is understood. Angioedema, though rare, may occur and requires immediate discontinuation of the drug.

Pharmacokinetics

Oxybutynin is well absorbed from the gastrointestinal tract and undergoes extensive first-pass metabolism, primarily to its active metabolite, N-desethyloxybutynin. It has a half-life that allows for once or twice-daily dosing, depending on the formulation. The drug is primarily excreted in urine, both as unchanged drug and metabolites.

Adverse effects

• Angioedema
• Hallucinations
• Confusion
• Agitation
• Drowsiness

Interactions

• oxybutynin + antiepileptics: Severe (increases risk of overheating and dehydration)
• oxybutynin + zonisamide: Unknown (increases risk of overheating and dehydration)
• clozapine + oxybutynin: Unknown (additive effect)
• cobicistat + oxybutynin: Unknown (increases exposure)
• idelalisib + oxybutynin: Unknown (increases exposure)
• clarithromycin + oxybutynin: Unknown (increases exposure)
• antipsychotics, second generation + oxybutynin: Unknown (additive effect)

Precautions

• Monitor for symptoms of angioedema and discontinue if suspected
• Be cautious with activities requiring alertness until response to oxybutynin is evaluated
• Dose adjustments may be necessary

Pregnancy

Oxybutynin should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. Consult healthcare providers for guidance.

Breast-feeding

It is not known whether oxybutynin is excreted in human milk. Caution is advised when administering to nursing women.

Storage

Store at room temperature, away from moisture and heat. Keep out of reach of children.

Formulations

• Tablets
• Extended-release tablets
• Syrup
• Transdermal patch

Indications

• Urinary frequency
• Urinary urgency
• Urinary incontinence
• Neurogenic bladder instability
• Nocturnal enuresis associated with overactive bladder

Dosage

Adults: Initially 5 mg 2–3 times a day, increased if necessary up to 5 mg 4 times a day. For modified-release tablets, start at 5 mg once daily, with adjustments in steps of 5 mg weekly, maximum 20 mg per day. For transdermal patches, apply 1 patch twice weekly to clean

Mechanism of action

Oxybutynin acts as a competitive antagonist of the muscarinic receptors, specifically M1, M2, and M3 subtypes. By blocking acetylcholine's action at these receptors, it reduces involuntary contractions of the bladder detrusor muscle, thus increasing bladder capacity and decreasing urinary urgency and frequency.

Pharmacodynamics

Oxybutynin exhibits dose-dependent effects on bladder function. By reducing detrusor muscle contractions, it leads to an increase in bladder storage capacity and a decrease in the frequency of involuntary urination. Its efficacy in controlling overactive bladder symptoms is typically observed within a few weeks of treatment initiation. Additionally, oxybutynin has central nervous system effects, which may contribute to its overall efficacy and side effect profile.

Pharmacokinetics

Oxybutynin is well-absorbed after oral administration, with peak plasma concentrations occurring within 1 to 2 hours for immediate-release formulations. It undergoes extensive first-pass metabolism in the liver, primarily via cytochrome P450 3A4. The elimination half-life is roughly 2 hours, but this may be extended in elderly patients or those with hepatic impairment. Oxybutynin is excreted mainly in the urine as metabolites, with less than 1% of the unchanged drug found in urine.

Contra-indications

• Hypersensitivity to oxybutynin or any of its components
• Acute porphyrias
• Uncontrolled narrow-angle glaucoma
• Severe gastrointestinal obstruction
• Myasthenia gravis

Adverse effects

• Dry mouth
• Constipation
• Blurred vision
• Urinary retention
• Dizziness
• Somnolence
• Nausea
• Vomiting

Interactions

• Anticholinergic agents may increase the effects of oxybutynin
• CYP3A4 inhibitors may increase levels of oxybutynin
• CYP3A4 inducers may decrease levels of oxybutynin
• Concurrent use with other medications that cause anticholinergic effects may increase the risk of adverse effects

Precautions

• Use with caution in patients with bladder outlet obstruction
• Caution in patients with a history of seizures or epilepsy
• Use with caution in the elderly due to increased risk of side effects
• Monitor for signs of urinary retention

Pregnancy

Oxybutynin should only be used during pregnancy if the potential benefit justifies the potential risk to the fetus. Limited data is available regarding its safety during pregnancy.

Breast-feeding

Oxybutynin is excreted in breast milk. Caution should be exercised when oxybutynin is administered to nursing mothers.

Storage

Store in a cool, dry place away from direct sunlight. Keep out of reach of children.

Formulations

• Oxybutynin hydrochloride 2.5 mg tablets
• Oxybutynin hydrochloride 5 mg tablets
• Oxybutynin hydrochloride 10 mg modified-release tablets
• Oxybutynin hydrochloride oral solution
• Oxybutynin transdermal patch

Mechanism of action

Oxybutynin acts to relax the bladder by inhibiting the muscarinic action of acetylcholine on smooth muscle, and not skeletal muscle. The active of oxybutynin is metabolite is N-desethyloxybutynin. It competitively inhibits the postganglionic type 1, 2 and 3 muscarinic receptors. The above actions lead to increased urine capacity in the bladder, decreasing urinary urgency and frequency. In addition, oxybutynin delays the initial desire to void. RESULTS OF CYSTOMETRIC STUDIES SHOWED THAT THE DRUG INCR BLADDER CAPACITY @ ONSET OF FIRST CONTRACTION & FIRST DESIRE TO VOID, AS WELL AS @ END OF CYSTOMETRY. /CHLORIDE/

Pharmacodynamics

Oxybutynin exerts antispasmodic actions on the bladder, relieving the uncomfortable symptoms of overactive bladder, including urinary urgency and frequency. These actions occur through the inhibition of muscarinic receptors. **A note on angioedema and anticholinergic effects** Symptoms of angioedema may occur with oxybutynin therapy. If angioedema is suspected, discontinue oxybutynin immediately and provide appropriate medical treatment. In addition, anticholinergic effects may occur with the administration of this drug. Some symptoms may include hallucinations, confusion, agitation, and drowsiness. It is advisable to avoid operating heavy machinery before the response to oxybutynin has been monitored. Dose adjustments may be required.