The purpose of this randomized, double-blind, placebo-controlled study is to assess the safety and efficacy of PRX-00023 in patients with major depressive disorder.

Patients with major depressive disorder who meet all of the eligibility criteria will be randomly assigned to receive twice daily doses of either placebo or PRX-00023. Study drug will be taken for up to approximately 9 weeks. Patients will have periodic office visits for the purpose of monitoring safety and tolerability, as well as efficacy assessments.

Key Inclusion Criteria:

• Diagnosis of major depressive disorder

Key Exclusion Criteria:

• Females who are pregnant or nursing
• Electroconvulsive therapy within previous year
• Type 1 diabetes or uncontrolled type 2 diabetes
• HIV, Hepatitis B or Hepatitis C
• Use of illegal drugs, history of drug abuse, and/or alcohol dependence
• Clinically significant abnormal lab results

Other protocol-defined eligibility criteria may apply.