Dextromethorphan/quinidine pharmacotherapy in patients with treatment resistant depression: A proof of concept clinical trial

Background: At least one-third of patients with major depressive disorder (MDD) have treatment-resistant depression (TRD), defined as lack of response to two or more adequate antidepressant trials. For these patients, novel antidepressant treatments are urgently needed. Methods: The current study is a phase Ha open label clinical trial examining the efficacy and tolerability of a combination of dextromethorphan (DM) and the CYP2D6 enzyme inhibitor quinidine (Q) in patients with TRD. Dextromethorphan acts as an antagonist at the glutamate N-methyl-d-aspartate (NMDA) receptor, in addition to other pharmacodynamics properties that include activity at sigma-1 receptors. Twenty patients with unipolar TRD who completed informed consent and met all eligibility criteria we enrolled in an open-label study of DM/Q up to 45/10 mg by mouth administered every 12 h over the course of a 10-week period, and constitute the intention to treat (ITT) sample. Six patients discontinued prior to study completion. Results: There was no treatment-emergent suicidal ideation, psychotomimetic or dissociative symptoms. Montgomery-Asberg Depression Rating Scale (MADRS) score was reduced from baseline to the 10-week primary outcome (mean change: -13.0 +/- 11.5, t(19) = 5.0, p < 0.001), as was QIDS-SR score (mean change: -5.9 +/- 6.6, t(19) = 4.0, p < 0.001). The response and remission rates in the ITT sample were 45% and 35%, respectively. Limitations: Open-label, proof-of-concept design. Conclusions: Herein we report acceptable tolerability and preliminary efficacy of DM/Q up to 45/10 mg administered every 12 h in patients with TRD. Future larger placebo controlled randomized trials in this population are warranted.