BACKGROUND: Kappa-opioid antagonism may possess antidepressant properties. We assessed, in a proof-of-concept pilot trial among patients with major depressive disorder with inadequate response to antidepressants, the efficacy of adjunctive CERC-501 (formerly LY2456302), a kappa-selective opioid receptor antagonist. METHODS: In a Sequential Parallel Comparison Design study, patients were pre-randomized to: a) 10 mg/d of CERC-501 for 6 days, b) 20 mg/d of CERC-501 for 6 days, c) placebo for 3 days followed by 10 mg/d of CERC-501 for 3 days, d) placebo for 3 days followed by 20 mg/d of CERC-501 for 3 days, or e) placebo for 6 days. RESULTS: The study was terminated early by the National Institute of Mental Health due to slow enrollment (N = 8). The weighted mean difference of changes (drug vs placebo) in the 6-item Hamilton Depression Rating Scale (HAMD-6) (primary outcome measure) (1.28), Montgomery-Asberg Depression Rating Scale (MADRS) (2.33), Perceived Stress Scale (1.01), Symptoms of Depression Questionnaire (9.17), Positive Affect Scale (PAS) (6.39), Symptom Questionnaire (SQ) Depression scale (2.94), SQ Anger-Hostility scale (1.67), and Patient-Reported Outcomes Measurement Information System Satisfaction with Participation in Discretionary Social Activities (4.67) scores were all numerically but not statistically greater for CERC-501 than for placebo. CONCLUSIONS: Although the small sample size limits the ability to draw conclusions, results suggest that CERC-501 may have antidepressant effects. Additional studies are necessary to further explore these effects of CERC-501.