The purpose of this protocol is perform comprehensive immune monitoring studies in patients with castration-resistant prostate cancer receiving Sipuleucel-T in an effort to better understand the mechanism of action of this treatment.

The primary objectives of this study are to:

1. Establish the phenotype and frequency of circulating immune cell compartments in patients undergoing treatment with Sipuleucel-T.
2. Determine the induction and the quality of prostate antigen-specific T cell immunity in patients undergoing treatment with Sipuleucel-T.
3. Correlate whole-blood RNA transcript-based signatures with clinical outcomes in patients treated with Sipuleucel-T.
4. Evaluate the cytokine and chemokine milieu in the peripheral blood pre- and post-treatment with Sipuleucel-T.

Inclusion Criteria:

• Age > 18 years of age
• Written informed consent obtained
• Patients with castration-resistant prostate cancer who are initiating Sipuleucel-T as standard therapy
• No prior systemic chemotherapy for metastatic prostate cancer
• Hemoglobin > 9 mg/dl

Exclusion Criteria:

• Patients unable to understand the research protocol and/or provide informed consent

Patients will be recruited from those patients who receive their care at Mount Sinai Medical Center, or a participating site