In this observational study, blood samples for pharmacokinetic (PK) testing will be collected from subjects with metastatic prostate cancer during their treatment with docetaxel. Plasma levels of docetaxel will be determined, and the subjects docetaxel exposure levels, determined as an area under the curve (AUC), will be retrospectively correlated with reports of toxicity, tumor response, quality of life, time to disease progression and overall survival to provide guidance on what the appropriate target range for docetaxel exposure should be for metastatic prostate cancer subjects receiving docetaxel therapy for their disease.

Inclusion Criteria:

• Histologically or cytologically confirmed diagnosis of adenocarcinoma of the prostate.
• Male subjects 18 years of age or older.
• About to start a new line of treatment with docetaxel (75 mg/m2) in combination with prednisone.
• All subjects must be informed of the investigational nature of this study and be willing to provide written informed consent in accordance with Institutional guidelines and good clinical practices (GCP) indicating that they understand the purpose of and procedures required for the study and are willing to participate in the study prior to the beginning of specific study procedures.
• Prior surgical castration or concurrent use of an agent for chemical castration with a serum testosterone level < 50 ng/dL.
• Subjects with hormone naïve metastatic prostate cancer, must have high-volume disease, defined as extra-nodal visceral disease or bone metastases with at least 4 bone lesions (one being outside of the vertebral column or pelvis).
• Subjects with hormone naïve high-volume metastatic prostate adenocarcinoma must have been on androgen deprivation therapy (including luteinizing hormone-releasing hormone (LHRH) agonist therapy, LHRH antagonist therapy, or surgical castration) for less than 120 days prior to starting docetaxel therapy.
• Eastern Cooperative Oncology Group (ECOG) performance status 0-2.
• For subjects with castrate resistant prostate cancer (CRPC), at least four weeks elapsed between withdrawal of anti-androgens (Bicalutamide, Flutamide or Nilutamide) and initiation of docetaxel therapy.
• For subjects with CRPC, at least four weeks elapsed between last administration of Abiraterone (Zytiga®) or Enzalutamide (Xtandi®) and initiation of docetaxel therapy.
• At least four weeks elapsed between prior surgery or prior radiotherapy and initiation of docetaxel therapy.
• Radiograph-documented evidence of soft tissue or bony metastatic disease.
• Must have adequate hematologic, hepatic and renal function as defined below:
• Hematologic (minimal values): Absolute neutrophil count ≥ 1,500/mm3; Hemoglobin ≥ 10.0 g/dl; Platelet count ≥ 75,000/mm3
• Hepatic Function: Total Bilirubin ≤ 1.5 x institutional upper limit of normal (ULN); asparate transaminase (AST) and alanine transaminase (ALT) < 2 x institutional ULN
• Suitable venous access and healthy enough (as determined by the treating physician) to provide whole blood sample.

Exclusion Criteria:

• Any condition / concomitant disease not allowing chemotherapy with docetaxel, prednisone or required premedication for the treatment regimen.
• Serious concurrent disorders (active infection requiring intravenous antibiotics, unstable angina, uncompensated congestive heart failure (CHF), or hepatic failure) that, in the opinion of the investigator, would prevent the use of docetaxel and/or compromise the subject's ability to provide whole blood samples for participation in the study.
• Concurrent use of any non-FDA approved (i.e. investigational or experimental) anticancer agent(s) or within four (4) weeks of enrolling on the study.
• Pre-existing neuropathy ≥ grade 2 per the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE) version 4.
• Individuals with known seropositivity for human immunodeficiency virus (HIV), hepatitis C virus, hepatitis B surface antigen, or syphilis.
• Unwilling or unable to follow protocol requirements or to provide informed consent.

Metastatic prostate cancer patients, either newly diagnosed or castrate resistant, who are about to start treatment with a 3-weekly docetaxel treatment regimen (starting dose of 75 mg/m2) within the University of Pittsburgh Medical Center (UPMC) Cancer Center network.