The randomised, double blind and placebo-controlled study will enroll up to 300 patients, and will be conducted at 40 sites across the US.
In the study, Serdaxin will be administered as an oral, extended release tablet.
Patient enrolment will be conducted through the first half of 2011, with preliminary data expected in late 2011.
Rexahn president Rick Soni said the data from Phase IIa study in MDD suggest that Serdaxin has the potential to improve symptoms of depression without the side effects commonly associated with currently marketed antidepressants.
"We look forward to further assessing Serdaxin in patients with MDD," Soni said.