This study will test a study drug called BPX-601, a CAR-T cell product manufactured from the patient's own T-cells, to see if it can help treat advanced prostate cancer. BPX-601 is a drug that is only used in clinical studies.

The study is looking at:

• What side effects BPX-601 might cause
• What is the best dose of BPX-601
• How well BPX-601 may work to destroy prostate cancer

Key Inclusion Criteria:

1. Histologically or cytologically confirmed diagnosis of hormone-refractory adenocarcinoma of the prostate without pure small cell carcinoma
2. Metastatic, Castration-Resistant Prostate Cancer (mCRPC) with PSA value at screening ≥4 ng/mL that has progressed within 6 months prior to screening, as defined in the protocol
3. Has received ≥2 lines prior systemic therapy approved in the metastatic and/or castration-resistant setting [in addition to Androgen Deprivation Therapy (ADT)] including at least 1 second-generation anti-androgen therapy (eg, abiraterone, enzalutamide, apalutamide, or darolutamide)
4. Has had either orchiectomy OR is on Luteinizing Hormone-Releasing Hormone (LHRH) agonist or antagonist therapy with serum testosterone <50 ng/dL AND agrees to stay on LHRH agonist or antagonist therapy during the study

Key Exclusion Criteria:

1. Structurally unstable bone lesions suggesting impending fracture
2. Clinical or radiographic evidence of deep vein thrombosis, pulmonary embolism, or other known thromboembolic event that has not been definitively treated. Participants with prior history of coagulopathy must be on low-molecular weight heparin prophylaxis or prophylactic dose of oral anticoagulant and asymptomatic within 4 weeks of the planned BPX-601 infusion
3. Inadequate renal function defined by creatinine clearance <60 mL/min calculated by 24-hour urine collection or using the Cockcroft-Gault formula
4. Inadequate hepatic function defined by Aspartate aminotransferase (AST) and/or Alanine aminotransferase (ALT) >2.5 × ULN or >5 × ULN, if liver metastases, and/or total bilirubin >1.5 × ULN, as described in the protocol
5. Inadequate bone marrow function defined by Absolute Neutrophil Count (ANC) <1.5 × 10^9/L or platelet count <150 × 10^9/L. At least 7 days must have passed since the last dose of filgrastim (or 14 days since the last dose of pegfilgrastim) and at least 7 days must have passed since the last platelet transfusion at the time of ANC or platelet count.

Note: Other protocol-defined Inclusion/Exclusion criteria apply

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing.

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf