In this randomized, double-blind, multi-centre, placebo controlled, exploratory, adaptive design study, the antidepressant and plasma cytokine lowering effects of the GW856553 will be investigated in adult subjects diagnosed with MDD. Subjects will receive oral doses of GW856553 or placebo for six weeks. Safety, tolerability, pharmacokinetics and pharmacodynamics, defined as biomarkers in blood and clinical symptoms, will be assessed. The primary endpoint is the change from baseline associated with GW856553 versus placebo at Week 6 in the Bech (6-item HAMD-17) score. Interim analyses of the primary endpoint will be performed throughout the study to potentially adapt the study design by changing the randomization ratio and/ or reducing the total number of subjects to be randomized into the study. Exploratory analyses will be performed by associating changes in cytokine levels and selected clinical symptoms; PK/PD modelling will also be used to identify the most sensitive clinical and biological markers.