Purpose: To unravel why intravesical oxybutynin is more effective and causes significantly fewer systemic side effects than oral oxybutynin in the treatment of neurogenic bladder dysfunction, we tested the hypothesis that the absorption and metabolism of oxybutynin are changed after intravesical instillation.
Materials and methods: A high-performance liquid chromatography assay was developed for both oxybutynin and its active metabolite, N-desethyl-oxybutynin. Plasma concentrations were quantified after intravesical (n = 11) and oral (n = 5) administration of oxybutynin in children under steady-state conditions. Pharmacokinetic parameters were calculated.
Results: Oral administration of oxybutynin (0.2 mg./kg./dose) resulted in peak plasma concentrations for N-desethyl-oxybutynin which were 7.4 +/- 1.3 times higher than corresponding values for oxybutynin (n = 5). Also the AUC (area under the plasma concentration time curve) values were higher for N-desethyl-oxybutynin compared with those of oxybutynin, the ratio being 10.8 +/- 1.0 (n = 5). Intravesical instillation (0.2 mg./kg./dose), on the other hand, resulted in reduced metabolite generation and peak plasma concentrations for N-desethyl-oxybutynin which were in the same range as those for oxybutynin, the ratio being 1.2 +/- 0.1 (n = 11). The ratio for the AUC values for N-desethyl-oxybutynin and oxybutynin was 2.1 +/- 0.2 (n = 11).
Conclusions: The significantly lower AUC ratio of the N-desethyl metabolite over the mother compound, due to a reduced first pass metabolism, may explain the clinically relevant reduction of side effects that characterizes intravesical compared with oral oxybutynin therapy.