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Discovered by XenoPort, arbaclofen placarbil (AP) is a patented, oral new chemical entity that utilizes naturally-occurring, high-capacity nutrient transporters in the gastrointestinal tract to generate active, efficient absorption into the body. Once absorbed, AP is rapidly converted to the R-isomer of baclofen, a generic drug that is racemic (a mixture of the R- and S-isomer). Baclofen is a selective GABA-B agonist. XenoPort currently holds all rights to this product candidate.

AP was the subject of a successful Phase 2 clinical trial in spinal cord injury patients with spasticity. AP was also the subject of a Phase 3 clinical trial in multiple sclerosis (MS) patients with spasticity. The trial was unsuccessful in demonstrating that AP provided statistically significant improvement relative to placebo in the co-primary endpoints of the Phase 3 study. At this time, XenoPort has terminated further development of AP as a treatment for spasticity in patients with MS. As resources permit, XenoPort may pursue AP as a potential treatment for spasticity in spinal cord injury patients.


The pathophysiology of spasticity is unknown, but it is believed to result from an imbalance of inhibitory and excitatory functioning within the central nervous system. Patients with spasticity may experience abnormal increases in muscle tone that are associated with loss of range of motion, increased muscle stretch reflexes, weakness and problems with coordination. Common complications of spasticity include joint and muscle contracture, pain and difficulty performing activities of daily living.

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The U.S. National Institutes of Health provides a Web site of many current and past clinical trials. To view information about XenoPort’s clinical trials, please go to www.clinicaltrials.gov.