BDNF and major depressive disorder: This is consistent with previous studies reporting a correlation between plasma BDNF levels and rapid response of ketamine antidepressant, suggesting that BDNF is a potential response biomarker (Duman et al., 2012; Haile et al., 2014) and suggests that early change of plasma mBDNF can predict antidepressant response in patients with SSRI-resistant depression.