Oxybutynin chloride: alterations in drug delivery and improved therapeutic index

Oxybutynin chloride topical gel: a new formulation of an established antimuscarinic therapy for overactive bladder

BACKGROUND

Oxybutynin, a cholinergic-muscarinic receptor antagonist, is established as a safe and effective pharmacological treatment for patients with overactive bladder syndrome (OAB). Oxybutynin is available in multiple immediate- and extended-release oral and two transdermal formulations. Oxybutynin chloride topical gel (OTG) (Gelnique), Watson Pharmaceuticals, Corona, CA, USA) was approved in January 2009 by the US FDA. OTG was designed to provide consistent plasma oxybutynin levels with daily application, favorably altering the circulating N-desethyloxybutynin metabolite:oxybutynin ratio, and to utilize a biocompatible delivery system, thus minimizing both the anticholinergic adverse effects of oral formulations and the application-site skin reactions associated with other available forms of transdermal delivery.

OBJECTIVE/METHODS

This review summarizes the pharmacological properties and the clinical efficacy and safety profile of OTG based on the published literature and unpublished data provided by the manufacturer upon request.

RESULTS/CONCLUSION

OTG represents an efficacious, safe, and convenient alternative to other oxybutynin formulations and other oral anticholinergic medications for the treatment of OAB. Future studies and broad clinical experience should confirm the promising early experience observed with this formulation.

Drug-induced changes in P450 enzyme expression at the gene expression level: a new dimension to the analysis of drug-drug interactions

Drug-drug interactions (DDIs) caused by direct chemical inhibition of key drug-metabolizing cytochrome P450 enzymes by a co-administered drug have been well documented and well understood. However, many other well-documented DDIs cannot be so readily explained. Recent investigations into drug and other xenobiotic-mediated expression changes of P450 genes have broadened our understanding of drug metabolism and DDI. In order to gain additional information on DDI, we have integrated existing information on drugs that are substrates, inhibitors, or inducers of important drug-metabolizing P450s with new data on drug-mediated expression changes of the same set of cytochrome P450s from a large-scale microarray gene expression database of drug-treated rat tissues. Existing information on substrates and inhibitors has been updated and reorganized into drug-cytochrome P450 matrices in order to facilitate comparative analysis of new information on inducers and suppressors. When examined at the gene expression level, a total of 119 currently marketed drugs from 265 examined were found to be cytochrome P450 inducers, and 83 were found to be suppressors. The value of this new information is illustrated with a more detailed examination of the DDI between PPARalpha agonists and HMG-CoA reductase inhibitors. This paper proposes that the well-documented, but poorly understood, increase in incidence of rhabdomyolysis when a PPARalpha agonist is co-administered with a HMG-CoA reductase inhibitor is at least in part the result of PPARalpha-induced general suppression of drug metabolism enzymes in liver. The authors believe this type of information will provide insights to other poorly understood DDI questions and stimulate further laboratory and clinical investigations on xenobiotic-mediated induction and suppression of drug metabolism.

Effect of controlled-release delivery on the pharmacokinetics of oxybutynin at different dosages: severity-dependent treatment of the overactive bladder

OBJECTIVE

To assess the pharmacokinetics of a controlled-release formulation of oxybutynin (OROS-O, ALZA Corp., Mountain View, CA) at different dosages, compared with immediate-release oxybutynin (IR-O), and to determine the pharmacodynamic properties in the severity-dependent reduction of urge urinary incontinence (UUI).

PATIENTS AND METHODS

In all, 105 patients were enrolled in this multicentre, randomized, double-blind study. Individual dose titration was used to assess the minimum effective, maximum tolerated or maximum allowed dose of either OROS-O or IR-O. Blood samples were collected during maintenance therapy with frequent sampling to analyse for R-oxybutynin and R-desethyloxybutynin concentrations. Pre-dose plasma levels before dosing were obtained as the minimum concentrations achieved at steady state during the dosing regimen. In parallel, UUI episodes were assessed at baseline and during maintenance therapy at the final dose level.

RESULTS

For both IR-O and OROS-O, initial R-oxybutynin plasma concentrations increased in a dose-dependent fashion. For comparable dosages pre-dose plasma levels were higher for OROS-O than IR-O. After one dose of IR-O plasma concentrations peaked at approximately 27-fold after 1 h and decreased to baseline levels within 4-8 h. In contrast, plasma concentrations of R-oxybutynin remained constant for up to 24 h after taking OROS-O at all doses. The overall percentage reduction in weekly UUI episodes was 84% (to 4.8 episodes) and 88% (to 3.1 episodes) for OROS-O and IR-O, respectively (P < 0.05). Patients who titrated to final dose levels of 10, 15 and 20 mg OROS-O differed in their weekly UUI episodes at baseline (14.5, 30.3 and 42.0). Because of individual dose titration, UUI episodes/week were profoundly reduced at all applied doses (1.9-2.0, respectively). Fewer patients reported moderate to severe dry mouth with OROS-O than with IR-O (25% vs 46%, P < 0.05).

CONCLUSION

The pharmacokinetics of OROS-O are proportional to dose, with minimal fluctuations between peak and trough concentrations, as associated with IR-O. In clinical practice OROS-O may thus facilitate the highly effective severity-dependent treatment of UUI with flexible dose adaptations based on the patients' needs.