Key Inclusion Criteria:  Provided written informed consent to participate in Study AXS-05-301 Has treatment-resistant depression, defined as an inadequate response to 2 or 3 prior ADTs Body mass index (BMI) between 18 and 40 kg/m2, inclusive  Key Exclusion Criteria:  Suicide risk Treatment with any investigational drug within 6 months History of electroconvulsive therapy (ECT), vagus nerve stimulation, transcranial magnetic stimulation, or any experimental central nervous system treatment during the current episode or in the past 6 months Pregnant, breastfeeding, or planning to become pregnant or breastfeed during the study