The durability of intravesical oxybutynin solutions over time

Intravesical oxybutynin is prescribed for patients who do not tolerate the side effects of oral intake. Standard oral tablets are broken up, dissolved and instilled. For convenience some families create a large volume of solutions to use for weeks. We investigated the stability of oxybutynin concentration in these solutions and the effect of varying solvents and solutes.

Solutions of oxybutynin were created with a starting concentration of 125 mug/ml. We varied the method of pill preparation (crushed vs passive dissolution), type of solvent (tap water vs normal saline) and the presence of the commonly instilled antibiotic, gentamicin sulfate (0.48 mg/ml). Solutions were aged for 1 to 4 weeks. Concentrations of oxybutynin were measured by gas chromatography.

Tap water as a solvent had the greatest decrease in concentration, declining below 50% by week 2 and reaching 25% by week 4. The mean concentration of the other solutions decreased less, maintaining 80% of the starting concentration at week 4. The use of normal saline as a solvent and the addition of gentamicin sulfate helped slow the decrease. Crushing vs passive dissolution showed no effect.

Homemade oxybutynin solutions degrade with time. Tap water solvent alone should be avoided. Solutions and additives that lower pH and are more acidic help oxybutynin solubility and stability. Storing oxybutynin solutions for later intravesical use appears feasible up to 4 weeks. The clinical effect of the 20% degradation remains to be demonstrated.