BDNF and treatment resistant depression: Pharmacogenetic testing (e.g.,cytochrome P450 (CYP450), BDNF, and FKBP5 polymorphisms) [62, 63, 64, 65], neurobiologicalbiomarkers (e.g., C-reactive protein (CRP), interleukin-6 (IL-6), and and tumornecrosis factor-alpha (TNF-α)) [47], and neuroimaging tools (e.g.,functional magnetic resonance imaging (fMRI) and electroencephalography (EEG))[66] are promising for identifying TRD subtypes and predicting treatmentresponse.