Current OAB agents are similar, but not identical

Antimuscarinic agents are the primary pharmacologic therapies for overactive bladder, and their ranks continue to grow. The agents in this class-oxybutynin (Ditropan), tolterodine (Detrol), trospium (Sanctura), solifenacin (Vesicare), and darifenacin (Enablex)-are similar in efficacy and side effects, but not identical.

"There are no huge differences in efficacy, though all of these drugs are dose dependent," said Victor Nitti, MD, of the New York University School of Medicine.

An eighth antimuscarinic, fesoterodine, is expected to receive FDA approval by early 2008, Dr. Nitti said. It shares many similarities with other agents in the class.

"Observed differences in efficacy are based largely on patient expectations," he explained. "There is no one drug that shows superior efficacy in real-world patients."

But similarity does not make these drugs identical or interchangeable, Dr. Nitti added. Patients with neurogenic detrusor overactivity (NDO) should probably begin therapy with oral oxybutynin, trospium, or tolterodine, all of which have proven efficacy in NDO.

Darifenacin offers dose titration, favorable central nervous system data in the elderly, and no effect on heart rate or QT interval, but it carries higher rates of constipation and dry mouth.

Oxybutynin ER offers the widest dose titration and is the only agent approved for high-dose administration. But it has been shown to have an effect on cognitive function in healthy elderly patients.

Oxybutynin TDS (Oxytrol), a transdermal patch, has the lowest incidence of antimuscarinic side effects, but is available in a single dose and causes adverse skin reactions in 10% to 20% of patients.

Tolterodine, the most-often prescribed antimuscarinic, has a long safety record, a favorable CNS profile, and data on use in male patients. But doses cannot be titrated, and the agent slightly increases QT interval at supertherapeutic doses.

Trospium is not metabolized in the liver and has fewer drug-drug interactions, is less likely to cross the blood-brain barrier, and shows a higher urine concentration. On the minus side, trospium does not allow dose titration, is dosed twice daily, and increases the heart rate slightly. A once-daily formulation has been submitted for review to the FDA.

Solifenacin allows for dose titration and has a lower incidence of dry mouth. It also shows a mild increase in QT interval at supertherapeutic doses.