Dyskinesia in Parkinson's Disease (Study P04501)

The purpose of the study is to assess the efficacy and safety of a range of doses of SCH 420814 (preladenant) when used together with a stable dose of L-dopa/dopa decarboxylase inhibitor to treat Parkinson's disease. In this study, we will be comparing 3 doses (1 mg, 2 mg, and 5 mg taken twice a day) of preladenant with placebo (sugar pill). Following an Interim Analysis (temporary hold for new enrollment-ongoing subjects will continue on treatment) to review drug safety, a new dose group of 10 mg (taken twice a day) may be added. Approximately 160 participants will be randomized in this study in approximately 22 study centers worldwide for the first part of this study. Following the Interim Analysis, 40 new participants may be added, for a total of 200 participants. The study is double blind, which means neither you nor your study doctor will know whether you are receiving the study medication or placebo.