A Study to Evaluate the Safety and Tolerability of Arbaclofen Placarbil (XP19986) in Subjects With Acute Back Spasms

Inclusion Criteria:

1. Acute moderate to severe muscle spasms in the lumbar region, as indicated by a minimum Visual Analog Scale pain severity score of 4.0 cm, beginning either:

   • within four days prior to screening for subjects who do not require a 24-hour washout

   Or

   • within three days for subjects who require a 24-hour washout
2. Willing to discontinue all analgesics (e.g. NSAIDS, COX-2 inhibitors, acetaminophen), aspirin >81 mg/day, short-acting muscle relaxants (i.e. carisoprodol, Soma®), and herbal remedies for pain at least 24 hours prior to first dose and to refrain from use during the study (cardio-protective doses of aspirin ≤ 81 mg /day are allowed).

Exclusion Criteria:

1. Clinically significant abnormal neurological history or examination at screening (excluding back spasm), including lumbar radicular symptoms, spinal stenosis, foot drop, herniated nucleus pulposus, or other structural defects
2. Subjects with back spasm related to major trauma to the region
3. Subjects with muscle spasms due to a work-related injury or subjects involved in any injury-related litigation

4. Subjects using any of the following medications at screening:

   • Opioids, both short- and long-acting including but not limited to: morphine, fentanyl patch, oxycodone, tramadol)
   • benzodiazepines, such as valium and lorazepam
   • cyclobenzaprine containing drugs (e.g., Flexeril, Amrix)
   • carisoprodol (e.g., Soma®) within 24 hours of screening