GRIN2B and treatment resistant depression: Subsequently, MK-0657, another selective GluN2B antagonist was tested in a small, randomized, double-blind, placebo-controlled, crossover pilot study, and an oral formulation of MK-0657 showed antidepressant action in patients with TRD (5 patients) as early as day 5, although no improvement was observed when assessed using MADRS as a primary efficacy measure [8].