Safety and Pharmacokinetics (PK) of Alefacept in Adolescent Subjects With Moderate to Severe Psoriasis

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