Pharmacokinetics, Efficacy, and Safety of Intravesical Formulation of Oxybutynin in Patients with Detrusor Overactivity

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Abstract
To determine the pharmacokinetics of oxybutynin and its main active metabolite, N-desethyloxybutynin, after multiple dosage (5 mg/30 ml three times daily) of intravesical oxybutynin formulation. Furthermore, to determine the efficacy and safety of intravesical oxybutynin in the symptomatic relief of urge incontinence or urgency in adult patients with detrusor hyperreflexia or instability. Nine patients were randomly allocated to treatment with a special bladder instillation formulation of oxybutynin or placebo for two 14-day treatment periods in a double-blind, cross-over manner. The third, open study period was designed for pharmacokinetic purposes with all patients on the active drug. The pharmacokinetics was depicted by AUC0-24, Cmax, Cmin, and t(max), The efficacy was evaluated from the data collected from urinary voiding diaries and cystometries. The safety was measured by recording adverse events on questionnaires. Patients who were willing to continue with the intravesical oxybutynin treatment entered the 1-year extension part of the study. Oxybutynin was absorbed from the bladder with a geometric mean Cmax of 9.4 ng/ml and AUC0-24 of 92 ng x h/ml. For N-desethyloxybutynin, the geometric mean Cmax was 14.4 ng/ml and AUC0-24 186 ng x h/ml. Elimination of the drug was protracted, as there were detectable serum concentrations of both oxybutynin and N-desethyloxybutynin even 24 hours post-dose. The mean number of toilet visits/day decreased from the baseline value of 6.9 to 5.7 during oxybutynin treatment, whereas during the placebo period the value increased to 7.4 (p = 0.022). It remained at the same decreased level during the one-year follow-up period. Oxybutynin is readily absorbed from the bladder after intravesical administration. The serum concentrations of oxybutynin after single 5 mg intravesical doses are at least as high as those reported after oral drug intake, but the parent drug/metabolite ratio is much higher after intravesical administration. The elimination of oxybutynin as well as its metabolite is prolonged after intravesical administration compared with that reported after oral drug intake. The mean number of daily toilet visits decreased significantly in the oxybutynin group.

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