A Clinical Trial to Determine the Long-term Safety and Tolerability of an Investigational Drug in People With Major Depressive Episode Associated With Bipolar I Disorder (Bipolar I Depression).

Inclusion Criteria: * Subject provides written informed consent and is willing and able to comply with the protocol in the opinion of the Investigator. * Subject has completed 6 weeks of double-blind treatment and all scheduled assessments from Visit 6/EOT (Day 42) of the lead-in study of SEP-4199 CR. * Subject is medically appropriate for long-term open-label treatment with SEP-4199 CR in the opinion of the Investigator. * Female subjects of childbearing potential must agree to use effective and reliable contraception throughout the study and for at least 30 days after the last dose of study drug has been taken. In the Investigator's judgment, the subject will adhere to this requirement. * Male subjects agree to avoid fathering a child and to use effective methods of birth control throughout the study and until at least 90 days after the last study drug administration. Exclusion Criteria: * Subject is at high risk of non-compliance in the opinion of the Investigator. * Subject plans to initiate treatment with a prohibited psychotropic medication during the study. * Subject plans to initiate treatment with transcranial magnetic stimulation (TMS), electroconvulsive therapy (ECT), vagus nerve stimulation (VNS), or deep brain stimulation (DBS) during the study. * Subject experienced a moderate or severe hyperprolactinemia-related AESI in the lead-in study of SEP-4199 CR. * Subject will require treatment with a drug that is associated with increases in QTc interval. * Subject had any of the following at Visit 6/EOT (Day 42) of the lead-in study of SEP-4199 CR based on machine reading: * increase in QTcF interval of ≥ 30 msec AND a QTcF interval ≥ 480 msec, from baseline of the lead-in study of SEP-4199 CR AND a QTCF interval ≥ 480msec * increase in QTcF interval ≥ 60 msec, from baseline of the lead-in study of SEP-4199 CR * QTcF interval ≥ 500 msec * treatment-emergent clinically significant ECG abnormality. * Subject is considered by the Investigator to be at imminent risk of suicide or injury to self or others, has a MADRS item 10 (suicidal ideation) score ≥ 4, or answers "yes" to "suicidal ideation" item 4 (active suicidal ideation with some intent to act, without specific plan) or item 5 (active suicidal ideation with specific plan and intent) on the C-SSRS assessment at Visit 6/EOT (Day 42) of the lead-in study of SEP-4199 CR. * Female subject of childbearing potential, has a positive urine pregnancy test at Visit 6/EOT (Day 42) of the lead-in study of SEP-4199 CR or plans to become pregnant during the current study. * Subject tests positive for any drug of abuse or cannabis at Visit 6/EOT (Day 42) of the lead-in study of SEP-4199 CR