To evaluate (18)F-labeled-fluorodeoxyglucose ((18)F-FDG-) and (18)F-labeled-sodium fluoride ((18)F-NaF-) positron emission tomography/computed tomography (PET/CT) as biomarkers in metastatic castrate-resistant prostate cancer (mCRPC). Nine men (53-75 years) in a phase 1 trial of abiraterone and cabozantinib had (18)F-FDG-PET/CT, (18)F-NaF-PET/CT and standard imaging ((99m)Tc-labeled-methylene-diphosphonate ((99m)Tc-MDP) bone scan and abdominal/pelvic CT) at baseline and after 8 weeks of therapy. Baseline disease was classified as widespread (18)F-FDG-avid, oligometastatic (18)F-FDG-avid (1 site), or non-(18)F-FDG-avid. Metabolic response was classified using European Organisation for Research and Treatment of Cancer (EORTC) criteria. Treatment response using Response Evaluation Criteria in Solid Tumors (RECIST) 1.1, Prostate Cancer Working Group 2 (PCWG2) guidelines and days on trial (DOT) were recorded. All men were followed for 1 year or until progression. Four men had (18)F-FDG-avid disease: two with widespread (DOT 53 and 76) and two with oligometastatic disease (DOT 231 and still on trial after 742+ days). Five men had non-(18)F-FDG-avid disease; three remained stable or improved (2 still on trial while one discontinued for non-oncologic reasons; DOT 225-563+), and 2 progressed (DOT 285 and 532). Despite the small sample size, Kaplan-Meier analysis showed a significant difference in progression free survival (PFS) between men with widespread (18)F-FDG-avid, oligometastatic (18)F-FDG-avid and non-(18)F-FDG-avid disease (p < 0.01). All men had (18)F-NaF-avid disease. Neither (18)F-NaF-avid disease extent nor intensity was predictive of treatment response. (18)F-FDG-PET/CT may be superior to (18)F-NaF-PET/CT and standard imaging in men with mCRPC on abiraterone and cabozantinib. (18)F-FDG-PET/CT may have potential to stratify men into 3 groups (widespread vs. oligometastatic (18)F-FDG-avid vs. non-(18)F-FDG-avid mCRPC) to tailor therapy. Further evaluation is warranted.
Keywords: FDG; NaF; PET/CT; bone scan; prostate cancer.