Safety and Efficacy Study of ELND005 as an Adjunctive Maintenance Treatment in Bipolar I Disorder

Inclusion Criteria: * Meets the DSM-IV-TR criteria for BPD I by the SCID, prior to the Screening Visit. * Has a history in the last 3 years of ≥ 1 manic or mixed episodes of sufficient severity that required hospitalization and/or treatment with a mood stabilizer or antipsychotic, or confirmed by a family member or medical records to ensure the episode fulfills the DSM-IV-TR criteria. * Has experienced a mood episode of any polarity within 4 months prior to the Screening Visit and responded to StOC therapy. * Is euthymic at the Screening Visit (ie, score of ≤ 12 on the MADRS and a score of ≤ 12 on the Y-MRS). * Is receiving maintenance treatment for his or her BPD I with either LTG or VPA; on stable doses for past 4 weeks and therapeutic drug levels (total VPA 50--125 µg/mL and LTG 10-50 µmol/L or as deemed appropriate by the investigator). Dose adjustments made for tolerability reasons will be acceptable. A study patient must meet the following additional criteria to be eligible for randomization in the Double-blind Randomization Phase of this study: \- Maintained in a stable euthymic state during Phase 1, defined as Y-MRS and MADRS scores of ≤ 12, with the following exceptions: a maximum of 2 nonconsecutive excursions will be allowed throughout Phase 1. Excursions are defined as Y-MRS or MADRS scores \>12 but ≤ 16. Exclusion Criteria: * Woman of childbearing potential who is unwilling or unable to use an acceptable method of birth control or is using a prohibited contraceptive method. * Is found to be actively suicidal on the C-SSRS (answer of "yes" to question 4 or 5 \[current or over the last 30 days\]) or a score of ≥4 on the MADRS item 10 at the Screening Visit. * Has suboptimally treated thyroid disease as evidenced by thyroid-stimulating hormone (TSH) \>3 mIU/L at the Screening Visit. * Has received electroconvulsive therapy (ECT) during the current episode or within 6 months prior to the Screening Visit. * Has an estimated glomerular filtration rate \<40 mL/min/1.73 m2 according to the Modification of Diet in Renal Disease formula. A study patient who meets ANY of the above and ANY of the criteria below will not be eligible for enrollment in the Double-blind Randomization Phase of this study: * Has current signs or symptoms of psychosis. * Has become actively suicidal as defined by C-SSRS answer of "yes" to question 4 or 5 (current or over the last 30 days) and/or has a score of ≥4 on MADRS item 10.