• The primary efficacy endpoint is the change in the PANSS total score from baseline to the end of the double-blind treatment period (day 28). [ Time Frame: 4 weeks ] [ Designated as safety issue: No ]
  

This study will utilize a randomized, double-blind, placebo-controlled, comparator-referenced, multicenter, parallel-group adaptive design with placebo, risperidone (4 mg/day), and up to 7 treatment arms of vabicaserin (50, 100, 150, 200, 300, 400 and 600 mg/day) over the course of the study. Eligible subjects will enter a screening period of 4 to 10 days. Subjects will be equally randomized into 1 of 5 treatment arms: placebo, risperidone, or 1 of 3 doses of vabicaserin. After 10 subjects have been randomized to each treatment arm, an interim analysis will be conducted and the data made available to members of the data monitoring committee (DMC). A statistical algorithm will use observed real-time data on the total PANSS score between baseline and 4 weeks of treatment and other associated observations to adapt the treatment allocations of new subjects. At this point, new dose groups of vabicaserin may be introduced. After the first interim analysis, weekly analyses will be conducted using the statistical algorithm, and results will be provided to members of the DMC. The randomization scheme may be modified at the end of each analysis until enrollment is complete. The goal will be to find the minimally effective dose that yields the desired effect and to define the dose-response curve. Eligible subjects will receive test article for 4 weeks, followed by tapering of test article for an additional week. All subjects will return for a follow-up visit at the end of the taper phase. Subjects will be hospitalized during the screening, baseline, and double-blind treatment phases. Hospitalization is encouraged during the taper phase; however, at the discretion of the investigator, the subject may be discharged from the hospital at any point after completing visit procedures at study day 28.