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.