This multicenter, randomised, double-dummy, Fumaderm® and placebo-controlled, parallel-group study will compare the efficacy and safety of 500 mg of FP187 (250 mg twice daily) compared to 720 mg Fumaderm® (240 mg three times daily) over 20 weeks of treatment. After an initial wash-out non-drug treatment phase of 1 to 6 weeks, all patients will receive allocated Study treatment up-titrated to the relevant dose level (i.e., 500 mg daily FP187, 720 mg daily Fumaderm®, or placebo). The up-titration to full dose will last 4 weeks for FP187 and 9 weeks for Fumaderm®. After 20 weeks of treatment, all patients will be asked to enter a separate open label treatment protocol expected to continue for up to 5 years.

Inclusion Criteria:

• adult patients, >=18 years of age;
• clinical diagnosis of stable moderate to severe plaque psoriasis for at least 6 months;
• clinical diagnosis of plaque psoriasis with an affected body surface area of no less than 10% and least 10 on the PASI scale and on the sPGA score at least as moderate;
• Besides psoriasis, patient is in good general health
• Patients with a DLQI score of at least 10

Exclusion Criteria:

• Pustular forms of psoriasis, erythrodermic or guttate psoriasis;
• Known immunosuppressive diseases;
• Presence of another serious or progressive disease including skin malignancies;
• Active skin disease;
• Use of topical medical treatment or UVB treatment during the 2 weeks preceding randomization;
• Use of systemic anti-psoriatic treatment preceding randomization: methotrexate, cyclosporine, steroids or PUVA (psoralen + UVA treatment) treatment within 4 weeks; biological treatment within 12 weeks; Stelara within 20 weeks; acitretin within 6 months;
• Treatment with Fumaderm® or other Dimethyl Fumarate containing products within 12 weeks prior randomization;
• Treatment with drugs influencing the course of psoriasis (e.g., antimalarial drugs, lithium) within 4 weeks prior to randomization;
• Treatment with retinoids, other immunosuppressive treatment, cytostatics or drugs with known harmful effects on the kidneys within 3 months prior to randomization;
• On-going stomach or intestinal problems;
• Aspartate transaminase (AST), Alanine transaminase (ALT) > 2 x upper normal limit (ULN), or Gamma-glutamyltransferase (γ-GT) > 2.5 x ULN;
• Creatinine Clearance < 60 ml/min;
• Leucopenia, eosinophilia or lymphopenia;
• Protein in the urine test;