This study is a companion to the SYNERGY-201 clinical trial (NCT06228053), which investigates SX-682 and enzalutamide in individuals with prostate cancer. Individuals must be participating in SYNERGY-201 in order to participate in this study. The purpose of this companion study is to learn more about biomarkers, particularly a biomarker called CXCR2, and investigate if CXCR2 can predict who will receive benefit from the SYNERGY-201 drug combination. This study will also investigate how CXCR2 and other biomarkers change over time when participants receive the SYNERGY-201 drug combination. CXCR2 is of particular interest because the SYNERGY-201 drug, SX-682, inhibits CXCR2. After participants provide consent, blood samples will be collected for research purposes at three SYNERGY-201 visits (Baseline, Cycle 3 Day 1 and End of Study Drug). Up to 20 participants will also receive tumor biopsies at the Baseline and SYNERGY-201 Cycle 3 Day 1 visits. Clinical and study data collected as part of SYNERGY-201 will also be used for this study.

Inclusion Criteria:

• Willing and able to provide written informed consent for this study and HIPAA authorization for the release of personal health information.
• Age >18
• Participating in the SYNGERY-201 clinical trial.

Exclusion Criteria:

• History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study
• History or current evidence of any condition, therapy, or laboratory abnormality that might interfere with the subject's participation for the full duration of the study
• History or current evidence of any condition, therapy, or laboratory abnormality that is not in the best interest of the subject to participate (in the opinion of the treating investigator)

The study population includes men with metastatic castration resistant prostate cancer (mCRPC) who are participating in the SYNERGY-201 clinical trial as defined in the eligibility criteria listed below.