• Participants must have histologically confirmed prostate cancer that is metastatic and one of the following: CRPC with PSMA-low disease defined by whole-body SUVmean ≤ 10 determined by standard-of-care 68Ga-PSMA-11 PET/CT imaging with at least 5 metastatic lesions OR NEPC pathology based on tumor biopsy. This may include morphology consistent with small cell carcinoma or mixed adenocarcinoma/NE features and/or IHC staining for NE markers. Participants should have at least 5 metastatic lesions.

• Must have either already had standard of care 68Ga-PSMA PET/CT scan or be planned for 68Ga-PSMA PET/CT scan before or after the planned 18F-fluciclovine-PET/CT.

• Age ≥18 years. Since no dosing or adverse event data are currently available on the use of 18F-fluciclovine in participants \<18 years of age, and most prostate cancer occurs in the adult population, children are excluded from this study but will be eligible for future pediatric trials.

• ECOG performance status ≤2 (Karnofsky ≥60%, see Appendix A)

• Ability and willingness to comply with the study procedures.

• The effects of 18F-fluciclovine on the developing human fetus are unknown. For this reason and because radiopharmaceuticals may be teratogenic, men must agree to use adequate contraception (barrier method of birth control; abstinence) prior to study entry and for 24 hours after the PET/CT scan is completed.

• Ability to understand and the willingness to sign a written informed consent document