Quantitative characterization of therapeutic index: Application of mixed‐effects modeling to evaluate oxybutynin dose–efficacy and dose–side effect relationships

Describing a therapeutic index for a drug is important for evaluating safe and effective dosage regimens. Therapeutic index can be evaluated as the relative position of the dose‐efficacy and the dose‐side effect curves. Oxybutynin XL (Ditropan XL), a once‐daily oral controlled‐release formulation for oxybutynin chloride, is being developed. Oxybutynin XL efficacy and side‐effect data obtained from two parallel‐group, randomized, controlled clinical trials were modeled to evaluate the therapeutic index.