Efficacy and Safety Study of Low-Dose Ondansetron For Adjunctive Therapy In Adult Patients With Obsessive-Compulsive Disorder

This is a multi-center, randomized, double-blind, placebo-controlled, parallel-group study. A sufficient number of patients will be screened to obtain approximately 150 patients randomized to the double-blind portion of the study. This study will consist of at least 6 weeks of retrospectively documented SRI treatment (prior to screening), 6 weeks of prospective SRI treatment after screening (run-in) but prior to randomization, and 12 weeks of double-blind treatment with study drug (ondansetron 0.5 mg twice a day, ondansetron 0.75 mg twice a day or placebo twice a day). All patients will have been maintained on a single SRI at the same dose throughout the retrospective 6-week period and will continue the same SRI at the same dose throughout the run-in and double-blind treatment periods. The 12-week randomized, double-blind, placebo-controlled, parallel-group study will be the "core period" for purposes of efficacy and short-term safety assessment. Patients completing all 12 weeks of the core period will be offered an opportunity to participate in an "extension period" where they will continue to receive treatment for up to 30 months following the core period. Treatment assignment in the extension period will be as follows: responders will continue on the same double-blind treatment to which they were assigned in the core period (ondansetron 0.5 mg twice a day, ondansetron 0.75 mg twice a day, or placebo twice a day); non-responders will be reassigned to continuing treatment according to their prior treatment assignment in the core period (those who received placebo will be assigned to ondansetron 0.5 mg twice a day, those who received ondansetron 0.5 mg twice a day will be assigned to ondansetron 0.75 mg twice a day, and those who received ondansetron 0.75 mg twice a day will continue receiving ondansetron 0.75 mg twice a day). Patients will receive treatment under double-blind conditions (double-blind phase of the extension study) until the core study is completed and the safety and efficacy of an ondansetron dose has been confirmed If after the core study data analysis, no treatment differences are found between ondansetron and placebo in the primary efficacy variable, the extension study will be terminated. If the core study results are positive for the primary efficacy endpoint and safety endpoints, then patients participating in the double-blind phase of the extension study will be offered an opportunity to continue treatment under open-label conditions with the dose deemed efficacious and safe (open-label phase of the extension study). A Data Review Committee will be responsible for reviewing the core study analysis results, providing a dose recommendation for the open-label phase of the extension study, and for communicating the results and their recommendation to the participating institutional review boards/ethics committees (IRBs/ECs) and investigators. Patients may continue receiving treatment for up to 30 months or until the sponsor provides an alternative.