• Proportion of participants who receive at least 2 cycles of treatment and who show predefined levels of improvement in primary efficacy parameter at 6 months [ Time Frame: 6 months ] [ Designated as safety issue: No ]
  

• Proportion of participants who receive at least 2 cycles of treatment and who show predefined levels of improvement in the primary efficacy parameter at 12 and 24 months [ Time Frame: 12-24 months ] [ Designated as safety issue: No ]
  
• Proportion of participants who receive at least 2 cycles of treatment and who are deemed responders in the ACR20, ACR50, and ACR70 at 6, 12, and 24 months [ Time Frame: 6-24 months ] [ Designated as safety issue: No ]
  
• Proportion of participants who receive at least 2 cycles of treatment and who have at least 20% improvement in ACR response, plus at least 20% improvement in 3 of 5 other assessments and scores. [ Time Frame: 24 months ] [ Designated as safety issue: No ]
  

Psoriatic arthritis is a form of inflammatory arthritis that affects approximately 7% of people who have psoriasis. Treatment typically include drugs such as methotrexate, azathioprine, and etanercept, which suppress the immune system in a nonspecific fashion in an attempt to control the immune responses causing the disease. In some severe cases of psoriatic arthritis, these drugs cannot adequately control the disease, often requiring patients to undergo continuous treatment to prevent or combat disease activity. hOKT3gamma1 (Ala-Ala) is a genetically engineered monoclonal antibody directed against the CD3 antigen on T cells. hOKT3gamma1 (Ala-Ala) specifically targets immune cells that are actively involved in destructive immune responses, such as those that cause psoriatic arthritis. In a small pilot study of eight people with psoriatic arthritis who received a 2-week course of hOKT3gamma1 (Ala-Ala), the drug appeared safe and caused no serious side effects. This study will test the safety and efficacy of hOKT3gamma1 (Ala-Ala) in alleviating symptoms in psoriatic arthritis patients.

This study will last 2 years. Individuals with psoriatic arthritis who are receiving methotrexate or azathioprine therapy and have active disease are eligible to participate. Participants will be randomly assigned to receive hOKT3gamma1 (Ala-Ala) or placebo. Participants will receive a 5-day treatment with the drug or placebo every month for the first 4 months of the study. There will be 5 study visits over 2 years to assess the safety and effectiveness of hOKT3gamma1 (Ala-Ala) and to evaluate laboratory measures related to the underlying immune problems that cause psoriatic arthritis.