Venlafaxine and O-desmethylvenlafaxine serum levels are positively associated with antidepressant response in elder depressed out-patients

Background: Therapeutic Drug Monitoring (TDM) represents one of the most promising tools in clinical practice to optimise antidepressant treatment. Nevertheless, little is still known regarding the relationship between clinical efficacy and serum concentration of venlafaxine (VEN). The aim of our study was to investigate the association between serum concentration of venlafaxine + O-desmethylvenlafaxine (SCVO) and antidepressant response (AR). Methods: 52 depressed outpatients treated with VEN were recruited and followed in a naturalistic setting for three months. Hamilton Depression Rating Scale-21 was administered at baseline, at month 1 and at month 3 to assess AR. SCVO was measured at steady state. Linear regression analysis and nonlinear least-squares regression were used to estimate association between SCVO and AR. Results: Our results showed an association between AR and SCVO that follows a bell-shaped quadratic function with a progressive increase of AR within the therapeutic reference range of SCVO (i.e. 100–400 ng/mL) and a subsequent decrease of AR at higher serum levels. Discussion: This study strongly suggests that TDM could represent a more appropriate tool than the oral dosage to optimise the treatment with VEN. Specifically, highest efficacy might be achieved by titrating patients at SCVO levels around 400 ng/mL.