Oxybutynin (Page 3 of 5)

12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

Oxybutynin relaxes bladder smooth muscle. Oxybutynin chloride exerts a direct antispasmodic effect on smooth muscle and inhibits the muscarinic action of acetylcholine on smooth muscle. No blocking effects occur at skeletal neuromuscular junctions or autonomic ganglia (antinicotinic effects).

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

12.2 Pharmacodynamics

In patients with conditions characterized by involuntary bladder contractions, cystometric studies have demonstrated that oxybutynin increases bladder (vesical) capacity, diminishes the frequency of uninhibited contractions of the detrusor muscle, and delays the initial desire to void.

12.3 Pharmacokinetics

Absorption

Following the first dose of oxybutynin chloride extended-release tablets,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 oxybutynin chloride extended-release tabletsare 156% and 187%, respectively, compared with oxybutynin. The mean pharmacokinetic parameters for R -and S-oxybutynin are summarized in Table 2. The plasma concentration-time profiles for R-and S-oxybutynin are similar in shape; Figure 1 shows the profile for R-oxybutynin.

Table 2 Mean (SD) R-and S-Oxybutynin Pharmacokinetic Parameters Following a Single Dose of Oxybutynin Chloride Extended-Release Tablets 10 mg (n=43)
Parameters (units) R-Oxybutynin S-Oxybutynin
Cmax (ng/mL) 1 (0.6) 1.8 (1)
Tmax (h) 12.7 (5.4) 11.8 (5.3)
t1/2 (h) 13.2 (6.2) 12.4 (6.1)
AUC(0 to 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 oxybutynin chloride extended-release tablets 10 mg and oxybutynin 5 mg administered every 8 hours (n=23 for each treatment).

Oxybutynin chloride Extended Release tablets
(click image for full-size original)

Steady state oxybutynin plasma concentrations are achieved by Day 3 of repeated oxybutynin chloride extended-release dosing, with no observed drug accumulation or change in oxybutynin and desethyloxybutynin pharmacokinetic parameters.

Oxybutynin chloride extended-release tablets steady state pharmacokinetics were studied in 19 children aged 5 to 15 years with detrusor overactivity associated with a neurological condition (e.g., spina bifida). The children were on oxybutynin chloride extended-release tablets 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 of oxybutynin chloride extended-release tablets, the mean pharmacokinetic parameters derived for R-and S-oxybutynin and R-and S-desethyloxybutynin are summarized in Table 3. 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 3 Mean ± SD R-and S-Oxybutynin and R-and S-Desethyloxybutynin Pharmacokinetic Parameters in Children Aged 5 to 15 Following Administration of 5 to 20 mg Oxybutynin Chloride Extended-Release Tablets Once Daily (n=19), All Available Data Normalized to an Equivalent of Oxybutynin Chloride Extended-Release Tablets 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 5 5 5
AUC (ng•h/mL) 12.8 ± 7 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 oxybutynin chloride extended-release tablets once daily in children aged 5 to 15. Plot represents all available data normalized to an equivalent of oxybutynin chloride extended-release tablets 5 mg once daily.

Oxybutynin chloride Extended Release tablets
(click image for full-size original)

Food Effects

The rate and extent of absorption and metabolism of oxybutynin are similar under fed and fasted conditions.

Distribution

Oxybutynin is widely distributed in body tissues following systemic absorption. The volume of distribution is 193 L after intravenous administration of 5 mg oxybutynin chloride. Both enantiomers of oxybutynin are highly bound (>99%) to plasma proteins. Both enantiomers of N-desethyloxybutynin are also highly bound (>97%) to plasma proteins. The major binding protein is alpha-1 acid glycoprotein.

Metabolism

Oxybutynin is metabolized primarily by the cytochrome P450 enzyme systems, particularly CYP3A4 found mostly in the liver and gut wall. Its metabolic products include phenylcyclohexylglycolic acid, which is pharmacologically inactive, and desethyloxybutynin, which is pharmacologically active. Following oxybutynin chloride extended-release tablets administration, plasma concentrations of R-and S-desethyloxybutynin are 73% and 92%, respectively, of concentrations observed with oxybutynin.

Excretion

Oxybutynin is extensively metabolized by the liver, with less than 0.1% of the administered dose excreted unchanged in the urine. Also, less than 0.1% of the administered dose is excreted as the metabolite desethyloxybutynin.

Dose Proportionality

Pharmacokinetic parameters of oxybutynin and desethyloxybutynin (Cmax and AUC) following administration of 5 to 20 mg of oxybutynin chloride extended-release tablets are dose proportional.

Use in Specific Populations

Pediatric

The pharmacokinetics of oxybutynin chloride extended-release tablets were evaluated in 19 children aged 5 to15 years with detrusor overactivity associated with a neurological condition (e.g., spina bifida). The pharmacokinetics of oxybutynin chloride extended-release tablets in these pediatric patients were consistent with those reported for adults (see Tables 2 and 3, and Figures 1 and 2 above).

Gender

There are no significant differences in the pharmacokinetics of oxybutynin in healthy male and female volunteers following administration of oxybutynin chloride extended-release tablets .

Race

Available data suggest that there are no significant differences in the pharmacokinetics of oxybutynin based on race in healthy volunteers following administration of oxybutynin chloride extended-release tablets .

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