To establish the safety of alefacept when administered to adolescent subjects with moderate to severe psoriasis.

A planned interim analysis will be conducted to evaluate safety and pharmacokinetics following the Week 4 evaluation of approximately the first 12 subjects. A dose adjustment may be implemented based on these results. A preliminary review of the Week 4 pharmacokinetics of the first 6 subjects will occur to monitor exposure. If a dose adjustment is required, pharmacokinetics will be repeated for the first 12 subjects enrolled at the new dose (subjects 13-24).

Inclusion Criteria:

• Subject has moderate to severe chronic plaque psoriasis involving at least 10% or greater body surface area
• Subject is a candidate for systemic treatment or phototherapy
• Subject is in good health and alefacept is not contraindicated
• Subject must have absolute total CD4+ lymphocyte counts within the normal range at screening
• Female subjects of child bearing potential have a negative pregnancy test prior to first dose of alefacept and agree to practice effective contraception during the study
• Subject must have predosing laboratory findings without clinically significant abnormal values for hematocrit, hemoglobin, platelets, white blood count and differential, serum creatinine, bilirubin, ALT, AST and prothrombin.
• Subject must have completed all standard childhood immunizations at least 12 weeks prior to the first dose
• Subject meets medication washout requirements and agrees to follow medication restrictions during the study
• Subject agrees to comply with the study requirements and agrees to come to the clinic for required study visits

Exclusion Criteria:

• Subject has a primary dermatological diagnosis of psoriasis other than plaque psoriasis
• Subject has a known hypersensitivity to alefacept or any excipient of the study medication
• Subject has had a serious local infection or systemic infection within 12 weeks prior to the first dose of study drug
• Subject has a fever (body temperature ≥ 38°C [or > 37°C for sites in Latvia]) or symptomatic viral or bacterial infection (including upper respiratory tract infection) within 1 week prior to the first dose of study drug
• Subject is known to be positive for HIV antibodies
• Subject has a history of chronic serious infection including hepatic disease or has positive result to serology test for hepatitis A antibody IgM, hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) or human immunodeficiency virus (HIV) antibody or, in Europe, tubercle bacillus (TB) at Screening
• Subject has a history or evidence of tuberculosis based on serology or a positive PPD skin test at Screening
• Subject has had treatment with any immunosuppressant agent within 12 weeks, any antibody or immuno-globulin within 24 weeks, or any investigational drug or approved therapy for investigational use within 8 weeks prior to the first dose of study drug
• Subject has had more than six herpes simplex virus (HSV) breakouts per year or is currently having an outbreak or has had an outbreak within the last 24 weeks
• Subject has a history of malignancy (other than non-melanoma skin cancers)
• Subject has a chronic condition which is not well controlled
• Subject is pregnant or nursing
• Subject has a history of severe allergic or anaphylactic reactions