Inclusion Criteria:

• Signed informed consent form
• Histologic diagnosis of adenocarcinoma of the prostate
• Metastatic prostate cancer (positive bone scan or measurable disease)
• Total testosterone of greater than 50 ng/dL, except for patients with prior orchiectomy, where testosterone does not need to be measured.
• Patients who are receiving an antiandrogen as part of primary androgen ablation must demonstrate disease progression following discontinuation of antiandrogen and completion of a washout period (specified below), and then observe disease progression. Flutamide must be discontinued at least 4 weeks before beginning screening. Bicalutamide (Casodex), nilutamide, aminoglutethimide, ketoconazole, diethylstilbestrol, megestrol acetate (Megace), and Proscar (finasteride) must be discontinued at least 6 weeks before beginning screening.
• Patients must stop using any herbal product known to decrease PSA levels (eg., saw palmetto and PC-SPES) or any systemic or topical corticosteroid at least 4 weeks prior to screening. Progressive disease must be documented after discontinuation of these products.
• Progressive disease after androgen deprivation (or hormone therapy). For patients with measurable disease, progression will be defined by Response Evaluation Criteria in Solid Tumors (RECIST) criteria. For patients with progression in, or without any measurable disease, a positive bone scan and elevated PSA will be required.
• Patients receiving bisphosphate therapy must have been on stable doses for at least 4 weeks with stable symptoms prior to enrollment.
• No prior chemotherapy or immunotherapy (tumor vaccine, cytokine, or growth factor given to control prostate cancer).
• Prior radiation therapy completed at least 4 weeks prior to enrollment. No prior radiopharmaceuticals (strontium, samarium) within 8 weeks prior to enrollment.
• Life expectancy of 12 weeks or greater.
• ECOG performance status of 0 or 1.
• Patients who are sexually active with a partner who could become pregnant are to use an effective form of barrier contraception.
• Must meet screening laboratory values.

Exclusion Criteria:

• Bone pain due to metastatic bone disease severe enough to require routine narcotic analgesic use.
• History of severe hypersensitivity reactions to drugs formulated with polysorbate 80.
• Other prior malignancy, except for adequately treated basal or squamous cell skin cancer or superficial bladder cancer, or any other cancer from which the patient has been disease-free for 5 years or greater.
• Patients with active autoimmune disease or a history of autoimmune disease that required systemic steroids or immunosuppressive medications, except for patients with vitiligo.
• Prior therapy with any anti-cytotoxic T-lymphocyte antigen 4 (anti-CTLA-4) antibody.
• Active infection requiring therapy, and positive HIV-1, HIV-2, human T-cell leukemia virus type 1 (HTLV-1), hepatitis B surface antigen, or hepatitis C positive RNA.
• Underlying medical condition that, in the investigator's opinion, will make the administration of study drug hazardous or obscure the interpretation of adverse events.
• Concurrent medical condition requiring the use of systemic or topical corticosteroids; systemic or topical corticosteroids must be discontinued at least 4 weeks prior to enrollment. The use of inhaled corticosteroids is acceptable.
• Use of other investigational drugs (drugs not marketed for any indication) within 30 days prior to treatment.