The purpose of this study is to evaluate 18F-sodium fluoride positron-emission tomography / computed tomography (18F-NaF PET/CT) imaging as a method for determining treatment response in metastatic bone lesions in patients who are receiving enzalutamide for castration-resistant prostate cancer.

Inclusion Criteria:

• Histologically or cytologically confirmed adenocarcinoma of the prostate without neuroendocrine differentiation, or signet cell or small cell features;
• Presence of bone metastatic disease as assessed by at least two lesions on whole body metastable technetium-methylene diphosphonate (99mTc-MDP) bone scintigraphy;
• Throughout the study, ongoing androgen deprivation therapy with a luteinizing hormone-releasing hormone (LHRH) analogue or prior bilateral orchiectomy (medical or surgical castration);
• Testosterone ≤ 1.73 nmol/L (≤ 50 ng/dL) at screening;
• Progressive disease on androgen deprivation therapy at screening defined as a minimum of two sequentially rising prostate-specific antigen (PSA) values (PSA1 < PSA2 < PSA3);
• The screening PSA (PSA3) must be ≥ 2 μg/L (≥ 2 ng/mL).

Exclusion Criteria:

• Prior enzalutamide, abiraterone acetate, aminoglutethimide, ketoconazole, radium Ra 223 dichloride or other bone-targeting radionuclides, or cytotoxic chemotherapy in the CRPC setting for the treatment of prostate cancer or participation in a clinical trial of an investigational agent that inhibits the androgen receptor or androgen synthesis (unless treatment was placebo);
• Treatment with hormonal therapy (eg, androgen receptor inhibitors, 5-alpha reductase inhibitors) or biologic therapy for prostate cancer (other than LHRH analogue therapy) within 4 weeks before enrollment;
• Initiation of new treatment with denosumab, bisphosphonates, or systemic corticosteroids for treatment of prostate cancer within 4 weeks before enrollment;
• Use of an investigational agent within 4 weeks before the screening visit;
• Radiation therapy to bone within 4 weeks before enrollment;
• Use of opiate analgesics for prostate cancer pain within 4 weeks before enrollment;
• Screening 99mTc-MDP bone scintigraphy showing a superscan;
• Visceral (eg, lung, liver) metastatic disease. Adenopathy is allowed;
• Current or previously treated brain metastasis or active leptomeningeal disease;
• History of seizure any time in the past for any reason or any condition that may predispose to seizures.

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.