Background: There is a current lack of safe and effective psoriasis therapies that provide patients with lasting remissions after treatment is discontinued. Alefacept, a novel and selective biological agent, has demonstrated durable efficacy in patients with chronic plaque psoriasis, and its efficacy has been correlated with reductions in memory-effector T cells.

Objective: To demonstrate the efficacy and safety of both one and two 12-week courses of alefacept 7.5 mg given once weekly as an intravenous (IV) bolus injection in patients with chronic plaque psoriasis.

Methods: Multicentre (51 centres in the USA and Canada), randomized, double-blind, parallel-group study comparing once-weekly alefacept 7.5 mg IV or placebo for two 12-week treatment courses. Each course had a 12-week follow-up phase. Patients were eligible for enrollment if they were > or =16 years of age, had chronic plaque psoriasis for > or =12 months involving > or =10% body surface area, and had CD4+ T-cell counts at or above the lower limit of normal.

Results: 553 patients received treatment in Course 1, and 449 were treated in Course 2. The cohorts were well balanced for demographic and baseline disease characteristics. During the treatment and follow-up period of Course 1, 28% of patients treated with alefacept achieved > or =75% reduction in Psoriasis Area and Severity Index (PASI), compared with 8% of placebo-treated patients (P < 0.001). Patients achieving > or =75% reduction in PASI following a single 12-week course of alefacept maintained > or =50% reduction in PASI for a median of over 7 months. Among patients who received a second course of alefacept therapy, 71% achieved > or =50% reduction in PASI, and 40% achieved > or =75% reduction in PASI over two treatment courses. One or two 12-week courses of alefacept were similarly well tolerated.

Conclusions: Treatment with alefacept 7.5 mg IV provided highly significant improvements in all measures of psoriasis disease activity compared with placebo. A second course of alefacept provided additional benefit.