This study will investigate the safety, efficacy and pharmacokinetics of R3421 in patients with moderate to severe chronic plaque psoriasis. Patients will be randomized to one of 3 treatment groups to receive once daily oral treatment with a)R3421 20mg, b)R3421 120mg, or c)placebo. The anticipated time on study treatment is <3 months, and the target sample size is <100 individuals.