According to mounting evidence, inflammation and phosphodiesterase (PDE) pathways may play a role in the pathogenesis of psychiatric diseases such as MDD. PTX is a phosphodiesterase inhibitor and has anti-inflammatory and antioxidant effects. Therefore, it has been hypothesized that MDD patients taking combined administration of the Citalopram, a Selective Serotonin Reuptake Inhibitor (SSRI), and PTX would show a higher improvement in depression symptoms. The relationship between the Hamilton Depression Rating Scale-17 items (Ham-D-17) score and various biological markers and their potential role in the therapeutic outcome of MDD will be assessed.