Antimuscarinic activity resides predominantly in the R-isomer. A metabolite, desethyloxybutynin, has pharmacological activity similar to that of oxybutynin in in vitro studies.

Pharmacokinetics

Absorption
Following the first dose of DITROPAN XL® (oxybutynin chloride), oxybutynin plasma concentrations rise for 4 to 6 hours; thereafter steady concentrations are maintained for up to 24 hours, minimizing fluctuations between peak and trough concentrations associated with oxybutynin.

The relative bioavailabilities of R- and S-oxybutynin from DITROPAN XL® are 156% and 187%, respectively, compared with oxybutynin. The mean pharmacokinetic parameters for R- and S-oxybutynin are summarized in Table 1. The plasma concentration-time profiles for R- and S-oxybutynin are similar in shape; Figure 1 shows the profile for R-oxybutynin.

Table 1 Mean (SD) R- and S-Oxybutynin Pharmacokinetic Parameters Following a Single Dose of DITROPAN XL® 10 mg (n=43) Parameters (units) R-Oxybutynin S-Oxybutynin
Cmax (ng/mL) 1.0 (0.6) 1.8 (1.0)
Tmax (h) 12.7 (5.4) 11.8 (5.3)
t1/2 (h) 13.2 (6.2) 12.4 (6.1)
AUC (0–48) (ng∙h/mL) 18.4 (10.3) 34.2 (16.9)
AUCinf (ng∙h/mL) 21.3 (12.2) 39.5 (21.2)
Figure 1. Mean R-oxybutynin plasma concentrations following a single dose of DITROPAN XL® 10 mg and oxybutynin 5 mg administered every 8 hours (n=23 for each treatment). 

Steady state oxybutynin plasma concentrations are achieved by Day 3 of repeated DITROPAN XL® dosing, with no observed drug accumulation or change in oxybutynin and desethyloxybutynin pharmacokinetic parameters.

DITROPAN XL® steady state pharmacokinetics were studied in 19 children aged 5–15 years with detrusor overactivity associated with a neurological condition (e.g., spina bifida). The children were on DITROPAN XL® total daily dose ranging from 5 to 20 mg (0.10 to 0.77 mg/kg). Sparse sampling technique was used to obtain serum samples. When all available data are normalized to an equivalent of 5 mg per day DITROPAN XL®, the mean pharmacokinetic parameters derived for R- and S-oxybutynin and R- and S-desethyloxybutynin are summarized in Table 2. The plasma-time concentration profiles for R- and S-oxybutynin are similar in shape; Figure 2 shows the profile for R-oxybutynin when all available data are normalized to an equivalent of 5 mg per day.

Table 2 Mean ± SD R- and S-Oxybutynin and R- and S-Desethyloxybutynin Pharmacokinetic Parameters in Children Aged 5–15 Following Administration of 5 to 20 mg DITROPAN XL® Once Daily (n=19), All Available Data Normalized to an Equivalent of DITROPAN XL® 5 mg Once Daily  R-Oxybutynin S-Oxybutynin R- Desethyloxybutynin S- Desethyloxybutynin
Cmax (ng/mL) 0.7 ± 0.4 1.3 ± 0.8 7.8 ± 3.7 4.2 ± 2.3
Tmax (h) 5.0 5.0 5.0 5.0
AUC(ng∙h/mL) 12.8 ± 7.0 23.7 ± 14.4 125.1 ± 66.7 73.6 ± 47.7
Figure 2. Mean steady state (± SD) R-oxybutynin plasma concentrations following administration of 5 to 20 mg DITROPAN XL® once daily in children aged 5–15. Plot represents all available data normalized to an equivalent of DITROPAN XL® 5 mg once daily. 

Food Effects
The rate and extent of absorption and metabolism of oxybutynin are similar under fed and fasted conditio