Inclusion Criteria:

Subjects must fulfill all of the following criteria before randomization at Study Day 1:

• Written informed consent, given prior to any study-related procedure not part of the subject’s normal medical care, with the understanding that consent may be withdrawn by the subject at any time without prejudice to his or her future medical care.
• Between the ages 18 and 75 years, inclusive.

• Female subjects must be neither pregnant nor breast-feeding, and must lack childbearing potential, as defined by either:

  • Being post-menopausal (i.e. at least 12 months past last menses) or surgically sterile, or
  • Using an effective form of contraception (i.e. condoms, oral contraceptives or IUD).

[Confirmation that the subject is not pregnant must be established by a negative urinary hCG test within 7 days before SD1. A pregnancy test is not required if the subject is post-menopausal or surgically sterile.]

• An outpatient status at the time of enrollment.
• Plaque psoriasis for at least 12 months.
• Plaque psoriasis covering at least 10% of total body surface area and a PASI score of 12.0 or more.
• Candidate for phototherapy or systemic therapy.
• Static Physician’s Global Assessment (sPGA) of 3 or more.

Exclusion Criteria:

To be eligible for inclusion in this study, the subjects must not meet any of the following criteria:

• Use of more than one NSAID or having a change in NSAID regimen during the 28 days before SD1. Note for guidance: subjects with psoriatic arthritis who are not expected to be controlled with one stable dosing NSAID regimen throughout the entire duration of the study should not be included.
• Previous systemic treatment with biologics, including interferon, and/or cytokines/anti cytokines (e.g., anti-TNF-alpha, anti-CD4, IL-10, IL-1ra, anti-CD11a, etc.) within 3 months before SD1.
• Participation in any other investigational study or experimental therapeutic procedure considered to interfere with the study within 3 months before SD1 (exceptions to be discussed with Sponsor).
• Treatment with any systemic corticosteroids or intra-articular corticosteroid injection during the 28 days before SD1.
• Any previous treatment with chlorambucil or cyclophosphamide.
• Treated with cyclosporine or methotrexate or oral retinoids within 28 days prior to SD1.
• Treatment with topical therapies (vitamin D derivatives, corticosteroids) within 14 days before SD1.
• Phototherapy within 28 days before SD1.
• Use of tanning booths within 14 days before SD1.
• Abnormal liver function, defined by a total bilirubin>/= 1.2 times the upper limit of normal values, aspartate aminotransferase, alanine aminotransferase or alkaline phosphatase level>/= 1.5 times the upper limit of normal values.

• Inadequate bone marrow reserve, defined as:

  • Leukocytes</= 3.5 x 10 to the9th/L, or
  • Thrombocytes</=100 x 10to the9th/L, or
  • Hemoglobin</=5.5 mmol/L (8.9 g/dL).
• Abnormal renal function, defined by serum creatinine >150 micromol/L.
• Seropositivity for human immunodeficiency virus (HIV), hepatitis B virus (HBV), or hepatitis C virus (HCV).
• Planned major surgery within the treatment period of the study.
• History of cancer in the preceding 5 years (except adequately treated basal cell carcinoma of the skin or squamous cell carcinoma in situ of the skin).
• History of active tuberculosis, current active tuberculosis or current treatment or prophylactic therapy for tuberculosis.
• Active severe infection (or non-severe infection at the discretion of the Investigator).
• Opportunistic infection in the 3 months preceding SD1.
• Clinically significant and serious abnormalities on electrocardiography or chest X ray, (at the discretion of the Investigator).
• Other serious concomitant disorders incompatible with the study. In particular, subjects with congestive heart failure, prior or current history of blood dyscrasia or central nervous system demyelinating disorders should not be included in the study.
• History of or current drug (including narcotics) abuse, or current active problems with alcohol abuse.
• Requirement for immunization, allergy desensitization or vaccination during the entire study period (it is recommended that these are scheduled at least 14 days prior to SD1 or >3 months after the last injection of study drug).
• Guttate, erythrodermic or pustular psoriasis as sole or predominant form of psoriasis.
• Evidence of skin conditions other than psoriasis (e.g., eczema) that would interfere with psoriasis disease assessments.
• Clinically significant psoriasis flare during screening or at the time of enrollment necessitating immediate relief (at the Investigator’s discretion).
• Live or killed virus or bacteria vaccines (within 14 days before SD1).
• Wheelchair-bound or bedridden status.
• Previous use of onercept.

In order to participate in this trial a subject must meet all of the inclusion and exclusion criteria specified above. Requests for protocol exceptions/exemptions must come from a participating, fully initiated site at which a prospective patient has consented to undergo screening by the Trial Director. There is no program in place to allow drug for a single patient IND, or for an expanded access protocol. This statement holds true for both children and adults.