CLINICAL PHARMACOLOGY Oxybutynin chloride exerts a direct antispasmodic effect on smooth muscle and inhibits the muscarinic action of acetylcholine on smooth muscle. Oxybutynin chloride exhibits only one-fifth of the anticholinergic activity of atropine on the rabbit detrusor muscle, but four to ten times the antispasmodic activity. No blocking effects occur at skeletal neuro-muscular junctions or autonomic ganglia (antinicotinic effects). Oxybutynin chloride relaxes bladder smooth muscle. 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. Oxybutynin thus decreases urgency and the frequency of both incontinent episodes and voluntary urination. 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. | ||||
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