The aim of the study is to assess the clinical utility of 18F-fluoro-deoxyglucose Positron Emission Tomography (PET)/Computed Tomography (CT) and Whole Body Magnetic Resonance Imaging (MRI) versus conventional bone scan and prostate-specific antigen (PSA) measurements in response prediction to treatment with Enzalutamide in castration-resistant prostate cancer patients.

The study will assess how these 2 imaging modalities perform compared to traditional serial PSA measurements and bone scan in assessing metastatic tumour load, progressive disease and response to treatment with Enzalutamide in castration-resistant prostate cancer patients.

In addition measurements of serially collected circulating tumour cell (CTC) samples, cell-free tumour DNA and RNA will be performed in order to evaluate their predictive value in terms of response measurement.

Castration-resistant prostate cancer patients eligible for 2nd line hormonal treatment will undergo treatment with Enzalutamide (XTANDI). Subjects will receive 1dd 160 mg Enzalutamide orally continuously until progressive disease occurs.

All subjects will undergo 18F-FDG PET/CT scans at baseline, 2 weeks, 2 and 6, 9 and 12 months after starting androgen receptor-directed treatment. All subjects will undergo Whole Body MRI at baseline, 6, 9 and 12 months. Bone scans will be performed at baseline, 3 months, 6 and 12 months. PSA will be measured at baseline and every 4 weeks thereafter until at 12 months. CTC counts and characteristics will be measured at baseline and during Enzalutamide treatment.

Inclusion Criteria:

• Male aged 18 years or older;
• Histologically confirmed adenocarcinoma of the prostate without neuroendocrine differentiation or small cell features;
• Ongoing androgen deprivation therapy with a GnRH analogue or bilateral orchiectomy;
• Three consecutive rises of PSA, 1 week apart, resulting in two 50% increases over the nadir, with PSA of at least > 5 ng/mL but preferably >10 ng/mL;
• Progressive disease as defined by rising PSA levels plus by evidence of progressive and measurable soft tissue or bone disease by 18F-FDG PET/CT, Whole Body MRI or both;
• Castrate serum levels of testosterone < 50 ng/dL or < 1.7 nmol/L;
• Anti-androgen withdrawal for at least 6 weeks for bicalutamide, nilutamide or flutamide for at least 6 weeks;
• No prior treatment with cytotoxic chemotherapy;
• Eastern Cooperative Oncology Group (ECOG) score 0-2;
• A life expectancy of at least 12 months;
• Written informed consent.

Exclusion Criteria:

• Severe concurrent disease, infection, or co-morbidity that, in the judgment of the Investigator, would make the patient inappropriate for enrolment;
• Known or suspected brain metastasis or active leptomeningeal disease;
• History of another malignancy within the previous 5 years other than curatively treated non melanomatous skin cancer;
• Total bilirubin, alanine aminotransferase (ALT) or aspartate aminotransferase (AST) 2.5 times the upper limit of normal at the Screening visit;
• Creatinine > 177 µmol/L (2 mg/dL) at the Screening visit;
• Hemoglobin <6 mmol/L, White blood cells < 4.0 x10^9/L, platelets < 100 x 10^9/L;
• History of seizure or any condition that may predispose to seizure. Also, history of loss of consciousness or transient ischemic attack within 12 months of enrolment (Day 1 visit);
• Contra-indication for MRI (e.g. pacemaker).