A Study in Patients With Metastatic Castration-Resistant Prostate Cancer

The goal of this clinical trial is to find out if FX-111 is safe enough to permit further studies in adult male participants with metastatic castration-resistant prostate cancer (mCRPC). It will also study the drug's pharmacokinetics (how the body breaks down FX-111) and how well FX-111 treats mCRPC. The main questions it aims to answer are: What are the side effects of FX-111? Does FX-111 work to reduce or prevent progression of mCRPC? The study doctor will oversee participants' treatment with FX-111 and ask about any side effects. Participants will take FX-111 every day by mouth and will have regular physical and laboratory examinations to check health and tumor status.

Inclusion Criteria: * Confirmed adenocarcinoma of the prostate. * PSA levels ≥ 2 ng/mL at screening visit. * Progressing PSA, defined as two consecutive increases in the most recent PSA measurements taken at least 1 week apart. * Progressed on Androgen Deprivation Therapy (ADT) and at least one prior potent Androgen Receptor (AR) pathway inhibitor given in castration-sensitive prostate cancer setting or approved for castration-resistant prostate cancer (eg, apalutamide, darolutamide, abiraterone, enzalutamide). * Ongoing primary ADT with gonadotropin-releasing hormone agonist or antagonist in the absence of bilateral orchiectomy. * Acceptable physical functioning and laboratory measurements, per the study protocol. * Discontinued prior therapies within protocol-specified timeframes. * Commit to use of highly-effective contraception while on study and for 90 days after. * Willing and able to adhere to the study visit schedule and other protocol defined requirements. Exclusion Criteria: * Predominance of small cell carcinoma of the prostate/neuroendocrine prostate cancer in most recent tumor biopsy. * Participants with brain metastases that require ongoing treatment with radiation or high-dose steroids. * Not recovered from side effects of prior surgery or cancer treatments. * Evidence of active viral, bacterial, or fungal infection requiring treatment with antivirals, antibiotics, or anti-fungal medications. * Prior treatment with AR degraders and molecules with an AR ligand such as AR Regulated Induced Proximity Targeting Chimera (RIPTAC). * Blood clots ≤ 4 weeks prior to start of treatment. * Concurrent malignancy requiring treatment or history of prior malignancy active within 2 years prior to the first dose of study drug. Patients may be eligible if the malignancy is clinically stable or has been treated with curative intent. * Any evidence of severe or uncontrolled systemic diseases. * Any condition that, in the opinion of the Investigator, would interfere with evaluation of the investigational product or interpretation of the patient's safety or study results.