Effect of OROS controlled-release delivery on the pharmacokinetics and pharmacodynamics of oxybutynin chloride

Aims : Dry mouth is a common side-effect seen with immediate-release oxybutynin (IR-Oxy). Ditropan XL [(Oxy-XL), a controlled-release formulation of oxybutynin chloride, is a once-daily oral dosage form that incorporates the OROS technology. Dry mouth as the pharmacodynamic measure was compared between Oxy-XL and IR-Oxy administration. The steady state stereospecific pharmacokinetics were also established for the two formulations and the kinetic-dynamic relationship of oxybutynin was examined.

METHODS

This was a randomized, repeated-dose, double-blind, two-treatment, two-period, crossover study. After a baseline assessment day, volunteers were randomly assigned to one of two treatment sequences and received 4 days of each treatment with a washout period of 7 days between treatments. The treatments were: 1) Oxy-XL 10 mg in the morning and placebo 8 h later, and 2) IR-Oxy 5 mg in the morning and again 8 h later. Volunteers assessed dry mouth severity subjectively using a 100 mm visual analogue scale, VAS (Baseline, treatment days 1 and 4) and objectively by collecting saliva (Baseline and treatment day 4) before dosing and every hour after the morning dose for approximately 16 h. Several blood samples were collected during each treatment, with frequent sampling on day 4 to analyse for plasma R- and S-oxybutynin and R- and S-desethyloxybutynin concentrations.

RESULTS

Relatively constant plasma concentrations of oxybutynin and its metabolite were seen over 24 h following Oxy-XL administration with the degree of fluctuation being much lower (P = 0.001; 66% to 81% reduction for the various analytes) than IR-Oxy. Compared with IR-Oxy, Oxy-XL yielded higher (131% and 158% for the R- and S-isomer, respectively) oxybutynin and lower (62% and 78% for the R- and S-isomer, respectively) desethyloxybutynin bioavailability, suggesting reduced first-pass metabolism. Saliva output (area under the effect curve) was significantly higher [P = 0.001; 37% (95% confidence interval: 24, 51%)] with Oxy-XL than with IR-Oxy and, accordingly, dry mouth severity (VAS) integrated over the day was significantly lower with Oxy-XL. The decrease in saliva output and the consequent increase in dry mouth severity correlated with the metabolite R-desethyloxybutynin concentration, and no apparent relationship was observed with the R-oxybutynin concentration. This suggests that the metabolite may contribute to the dry mouth. Therefore, the reduction in metabolite exposure with Oxy-XL may be a possible explanation for the observed decrease in dry mouth severity with OXY-XL compared with IR-Oxy.

CONCLUSIONS

Oxy-XL maintains relatively constant plasma drug and metabolite concentrations and minimizes first-pass metabolism of oxybutynin. The metabolite appears to contribute to dry mouth associated with oxybutynin, and following Oxy-XL metabolite exposure is reduced compared with IR-Oxy. Consequently less dry mouth was observed with Oxy-XL as compared with IR-Oxy.

Future trends in the treatment of urinary incontinence

Anticholinergic drugs act on efferent nerves to counteract overactive bladder (OAB) after it occurs. To prevent the occurrence of OAB, therapies should be directed at blocking the afferent nerves that control the bladder. Tachykinin-receptor antagonists hold great promise in this regard, since they can be administered orally and do not increase the risk of urinary retention that occurs with anticholinergics. Advanced drug delivery systems, such as controlled-release oral oxybutynin (oxybutynin-XL) can reduce the incidence of anticholinergic side effects. In a similar manner intravesical therapy for OAB is site specific, and thus also reduces the occurrence of adverse events. Moreover, the difficulties of intravesical therapy may now be overcome with advanced delivery techniques such as an implantable, long-acting drug-delivery pump. Another intravesical therapy that has met with great acceptance and success is the administration of chili pepper extracts, especially resiniferitoxin, which may be effective for up to 3 months with one application. Finally, gene therapy holds great promise for OAB, because it is possible to access all of the genitourinary organs via endoscopy and other minimally invasive techniques that are ideally suited for gene therapy.

Recent clinical studies of new pharmacologic agents and their efficacy in the treatment of incontinence

Two agents for the control of overactive bladder-tolterodine (TOL) and extended-release oxybutynin (Oxy-XL)-have been evaluated in a number of studies for their efficacy in urge incontinence. Studies have demonstrated that TOL achieved a 20% reduction in the frequency of voiding and a 45% reduction in urge incontinent episodes. Efficacy was comparable between TOL and immediate-release oxybutinin (Oxy-IR), the standard anticholinergic comparator. There is a delay of some weeks in achieving relief with TOL, but thereafter there is a continued decrease in the total number of both micturitions and incontinent episodes in 24 hours. Trials demonstrated that there were no safety concerns at all with TOL. In particular, there was a lower incidence of dry mouth with TOL than with Oxy-IR. Dose-ranging studies established that TOL produced the lowest incidence of side effects while maintaining efficacy. In a long-term, community-use study of Oxy-XL, there was a very low incidence of central nervous system side effects, including mental acuity and memory. Among elderly nursing home patients, Oxy-XL achieved a 90% reduction in weekly urge incontinence episodes and an 86% decrease in pad use. Oxy-XL was shown to cause a significantly lower reduction in salivary output than Oxy-IR and TOL. In a recent head-to-head comparison study, there were significant differences found between Oxy-XL and TOL. Other studies have shown that the administration of Oxy-XL results in a significantly lower production of the metabolites responsible for anticholinergic side effect, particularly dry mouth, than with the standard release form, owing largely to the elimination of a first-pass effect. A long-acting form of TOL resulted in a 53% reduction in incontinent episodes. Both these anticholinergic agents have been shown to have excellent efficacy and tolerability. But the future of OAB therapy lies in targeting other mechanisms responsible for incontinence.