Arbaclofen Placarbil for the Treatment of Spasticity in Subjects With Multiple Sclerosis (MS)

Primary Study Inclusion:

1. Subjects who have successfully completed study XP-B-089.

2. If a subject is on disease modifying MS treatment, the dosage, frequency, and route of administration must have remained stable for the duration of study XP-B-089.

3. Willing to continue to refrain from using for the duration of the study drugs for the treatment of spasticity or likely to affect spasticity including, but not limited to, baclofen, tizanidine, diazepam, clonazepam, metaxalone, dantrolene, cyclobenzaprine, carisoprodol, clonidine, vigabatrin, valproic acid, and cannabis (refer to Appendix 2, Prohibited Concomitant Medications).

Primary Study Exclusion:

1. Scheduled elective surgery or other procedures requiring general anesthesia during the study.

2. Subjects who, in the opinion of the Investigator, would be non-compliant with the study visit schedule, procedures, or medication administration.

3. Subjects who have an unstable or clinically significant medical or psychiatric illness that, in the opinion of the Investigator, could confound the collection of meaningful safety data or pose a risk to the subject.

4. Treatment with botulinum toxin products, phenol, or alcohol injections during or subsequent to the previous study XP-B-089, or plans to receive these treatments during the time period of study XP-B-091

Addendum Study Inclusion:

1. Has multiple sclerosis (MS) based on Poser or McDonald Criteria (all subtypes of MS will be accepted, including relapsing remitting, primary or secondary progressive, if disease is stable per exclusion criteria).

2. Expanded Disability Status Scale (EDSS) rating between 3.0-8.0 inclusive

3. Spasticity Disability Rating of 2 or higher at Screening

4. If a subject is on disease modifying MS treatment, the dosage, frequency, and route of administration must be stable for at least 30 days before screening and is expected to be stable throughout the study.

5. Willing to discontinue and refrain from using for the duration of the study drugs for the treatment of spasticity or likely to affect spasticity (including, but not limited to, baclofen tizanidine, diazepam, clonazepam, metaxalone, dantrolene, cyclobenzaprine, carisoprodol, clonidine, vigabatrin, valproic acid and cannabis).

Addendum Study Exclusion:

1. Spasticity due to neurological disorder other than MS or other conditions that may confound the assessment of spasticity.

2. Subject has clinically evident muscle contractures resulting in irreversible spasticity in lower extremities.

3. Subjects who have suffered an acute relapse of MS (as determined by the Investigator) within 90 days prior to Screening, or have had more than 1 relapse within the year prior to Screening.

4. Subjects receiving concomitant medication from more than one of the following three drug classes: (Antiepileptic drugs, Tricyclic anti-depressants and Opioids)

5. At the start, subjects on the following medications, at doses above the specified limits, are excluded if they cannot maintain a level within these limits:

• Gabapentin ≤ 1800 mg per day or pregabalin ≤ 150 mg per day

• Amitriptyline ≤ 75 mg per day or nortriptyline ≤ 75 mg per day

• Opioids ≤ 30 mg morphine equivalents per day.

6. Botulinum toxin, phenol, or alcohol injections within 6 months of screening.