uPAR PET/CT for Prognostication and Response Assessment in Patients with Metastatic Castration-Resistant Prostate Cancer Undergoing Radium-223 Therapy: A Prospective Phase II Study

The aim of this Phase II study was to investigate the potential for response assessment and prognostication of positron emission tomography (PET) using the ligand (68)Ga-NOTA-AE105 targeting the urokinase-type plasminogen activator receptor (uPAR) in patients receiving Radium-223-dichloride therapy ((223)RaCl(2)). A combined whole-body uPAR PET and computed tomography (CT) was performed before initiation of (223)RaCl(2) and after two cycles of therapy. Standardized uptake value (SUV) in selected bone metastases was measured and the lesion with the highest SUV(max) was considered the index lesion. Clinical outcomes were overall survival (OS), radiographic progression free survival (rPFS) and occurrence of symptomatic skeletal event (SSE). A total of 17 patients were included and 14 patients completed both baseline and follow-up uPAR-PET/CT. Baseline SUV(max) of the index lesion was associated with OS; hazard ratio 2.51 (95% CI: 1.01–6.28, p = 0.05) per unit increase in SUV(max). No association between changes in SUV(max) from baseline to follow-up and OS, progression during therapy, or rPFS was found. Baseline SUV(max) was a significant predictor of SSE with receiver operating characteristics (ROC) area under the curve (AUC) = 0.81 (95% CI: 0.58–1.00, p = 0.034). A cut-off for tumor SUV(max) could be established with an odds ratio of 14.0 (95% CI: 1.14–172.6, p = 0.023) for occurrence of SSE within 12 months. Although based on a small number of patients, uPAR-PET SUV(max) in bone metastases was predictive for OS and risk of SSE in mCRPC patients receiving (223)RaCl(2). However, a relatively low uptake of the uPAR ligand in bone metastases impedes visual evaluation and requires another modality for lesion delineation.