Impact of ⁶⁸Ga-PSMA PET/CT in patients with metastatic castration-resistant prostate cancer (mCRPC) treated with enzalutamide.

48 Background: 68 Ga prostate specific membrane antigen positron emission tomography/computed tomography ( 6 8 Ga-PSMA PET/CT) is a highly sensitive diagnostic tool to detect prostate metastatic sites even at low levels of Prostate Specific Antigen (PSA). We evaluated the impact of 68 Ga-PSMA PET/CT in patients treated with enzalutamide as first-line therapy for mCRPC. Methods: In an observational prospective study, 67 consecutive mCRPC patients were treated with enzalutamide 160 mg once daily in first-line for mCRPC. 68 Ga-PSMA PET/CT was performed at baseline, after 3 months, during follow-up and at PSA/clinical progression. Patients were evaluated on a monthly basis for serological PSA response and safety. We measured at baseline the sum of metabolic total volume (SMTV), mean standardized uptake volume (SSUVmean), maximum standardized uptake volume (SSUVmax) and total lesion activity (STLA), which is the product of SMTV and SSUVmean, for a maximum of 20 lesions. These parameters together with baseline PSA level, Eastern Cooperative Oncology Group performance status (ECOG PS), Gleason Score (GS) and age were analyzed by univariate and multivariate Cox regression models as potential predictors of progression-free survival (PFS) and overall survival (OS). Results: At the moment of the present analysis, 58 mCRPC patients were considered fully evaluable. The median age was 75 years (range, 47-91), ECOG PS was 0 in 47 cases (81%), 1-2 in 11 (19%), GS was <8 in 22 (38%), 8-10 in 36 (62%), the median baseline PSA was 2.66 µg/L (range 0.09-197). We observed a median SMTV of 5.73 cm 3 , median SSUVmax of 44.85, median SSUVmean of 25.80 and median STLA of 59.66. At the median follow-up of 52 months, median PFS was 28.9 months (95% CI 16.3-43.6), median OS was not reached (95% CI 36.8-not reached). In univariate analysis, SSUVmax and STLA were significant for PFS (p=0.015 and p=0.001, respectively) and OS (p=0.026 and p=0.019, respectively), while SSUVmean was significant only for OS (p=0.028). On multivariate analysis, STLA only remained significant for PFS (p<0.001) and OS (p=0.026). Conclusions: The assessment of mCRPC by 68 Ga-PSMA PET/CT before starting enzalutamide was associated with longer median PFS and OS compared to prior studies using standard imaging only. STLA, expression of both volume and intensity of 68 Ga-PSMA uptake, appeared the strongest parameter able to predict PFS and OS.