Oxybutynin Chloride Extended Release (Page 3 of 5)

System Components and Performance

Oxybutynin chloride extended-release tablets use osmotic pressure to deliver oxybutynin chloride at a controlled rate over approximately 24 hours. The system, which resembles a conventional tablet in appearance, comprises an osmotically active core surrounded by a semipermeable membrane. The unitary tablet core is composed of the drug and excipients (including the osmotically active components). There is a precision-laser drilled orifice in the semipermeable membrane on the side of the tablet. In an aqueous environment, such as the gastrointestinal tract, water permeates through the membrane into the tablet core, causing the drug to go into suspension and the osmotic components to expand. This expansion pushes the drug out through the orifice. The semipermeable membrane controls the rate at which water permeates into the tablet core, which in turn controls the rate of drug delivery. The controlled rate of drug delivery into the gastrointestinal lumen is thus independent of pH or gastrointestinal motility. The function of Oxybutynin chloride extended-release tablets depends on the existence of an osmotic gradient between the contents of the core and the fluid in the gastrointestinal tract. Since the osmotic gradient remains constant, drug delivery remains essentially constant. The biologically inert components of the tablet remain intact during gastrointestinal transit and are eliminated in the feces as an insoluble shell.

USP Drug Release Test 3.

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 tablets are 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(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 Oxybutynin chloride extended-release tablets 10 mg and oxybutynin 5 mg administered every 8 hours (n=23 for each treatment).

figure 1
(click image for full-size original)

Steady state oxybutynin plasma concentrations are achieved by Day 3 of repeated Oxybutynin chloride extended-release tablets 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)

5.0

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 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.

figure 2
(click image for full-size original)

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