This is a multicenter, randomized, double-blind, placebo-controlled, parallel, multiple dose study designed to evaluate the safety and tolerability of the co-formulation of ALKS 33 with buprenorphine (ALKS 5461) in subjects with Major Depressive Disorder (MDD) who are inadequately/partially responding to current treatment with a stable dose of a serotonin-selective reuptake inhibitor (SSRI) or serotonin-norepinephrine reuptake inhibitor (SNRI).