This experimental medicine study will examine the effects of a brief period (seven days) of
'add on' ebselen (SPI-105) treatment in patients with resistant depression to see if ebselen
produces changes in emotional responses consistent with a potential clinical antidepressant
effect. The investigators will also seek to confirm ebselen's mode of action on IMPase by
measuring changes in a brain chemical called inositol, using a magnetic imaging method. Half
of the participants will receive ebselen and the other half placebo.