Oxybutynin

Mechanism

Mechanism of Action

Selective reversible competitive antagonist of the muscarinic receptors M₁, M₂ and M₃, with greater selectivity for the M₁ and M₃ receptors present in the detrusor of the bladder and in the salivary glands. In addition to its direct competitive antimuscarinic activity, it has a moderate direct spasmolytic effect on smooth muscle and local local anesthetic properties (membrane stabilization), completely independent of cholinergic innervation. The M₃ blockade in the detrusor muscle decreases the frequency of involuntary contractions, increases the cystometric capacity of the urinary bladder and delays the urge to urinate.

Pharmacokinetics

Key Pharmacokinetics

• Routes: Oral (immediate or prolonged release tablets), transdermal (continuous release patches for 3-4 days, topical gel).
• Metabolism: Extraordinarily extensive first-pass oxidative hepatic mediated by the CYP3A4 isoenzyme. Its main active metabolite, N-desethyloxybutynin, has high affinity for salivary gland receptors, being responsible for severe xerostomia. The transdermal route avoids the first step and significantly reduces this adverse effect.
• Elimination half-life: 2 to 3 hours for immediate release, biologically prolonged with delayed formulations.
• Excretion: Renal, mostly in the form of inactive conjugated metabolites.

Indicators and dose

Approved and Off-label Indications

Approved: Symptomatic treatment of overactive bladder in adults, characterized by urinary urgency, daytime and nighttime frequency or urgency urinary incontinence; Detrusor instability of neurogenic etiology (eg, due to multiple sclerosis or spinal cord injury).

Off-label: Refractory focal or generalized primary or idiopathic essential hyperhidrosis.

Dosage and Adjustments

Overactive Bladder (Oral - Immediate Release): 5 mg orally two or three times a day. Maximum dose of 20 mg/day.

Overactive Bladder (Oral - Extended Release): 5 to 10 mg once daily orally, with weekly increments of 5 mg until reaching the usual maintenance dose of 10-15 mg once daily (maximum 30 mg/day).

Renal/hepatic adjustment: No preliminary quantitative guidelines have been established; It is advisable to start with minimum effective doses in severe insufficiency.

Security

Contraindications

Absolute: Untreated symptomatic urinary retention, uncontrolled or decompensated narrow angle glaucoma, myasthenia gravis, gastric retention or other severe obstructive pathology of the digestive gastrointestinal tract.

Relative: Moderate to severe ulcerative colitis, Parkinson's disease or Alzheimer's dementia.

Adverse Effects (ADR)

Common: Severe intolerable xerostomia (main cause of treatment abandonment), refractory constipation, blurred vision due to ocular accommodation disorders, dizziness, daytime sleepiness, dry skin, mild headache.

Serious: Anticholinergic delirium and acute or chronic cognitive impairment (due to its high penetration into the CNS mediated by M₁ blockade), orthostatic syncope, acute obstructive urinary retention requiring bladder catheterization.

Interactions

Potent CYP3A4 inhibitors (e.g., clarithromycin, itraconazole, ritonavir): They multiply serum oxybutynin levels, increasing the frequency of severe anticholinergic ADRs.

Anticholinesterase drugs (e.g., donepezil, neostigmine): They mutually antagonize therapeutic effects.

Pregnancy and Breastfeeding

FDA Classification: Category B. No defined teratogenic effects have been documented in preclinical animal models. It is unknown if it is excreted in significant quantities in breast milk; Its chronic use can reduce milk excretion due to glandular blockage.