Ga-68-PSMA PET/CT for Response Assessment and Outcome Prediction in Metastatic Prostate Cancer Patients Treated with Taxane-Based Chemotherapy

We aimed to evaluate the role of PET targeting the prostate-specific membrane antigen (PSMA) for response assessment in metastatic prostate cancer (PCa) patients treated with taxane-based chemotherapy (docetaxel or cabazitaxel) and its predictive value on patient outcome. Methods: We retrospectively evaluated 37 patients with metastatic hormone-sensitive PCa or metastatic castration-resistant PCa (mCRPC) who underwent Ga-68-PSMA-11 PET/CT at baseline and after the last cycle of taxane-based chemotherapy (docetaxel or cabazitaxel) without treatment modification between scans. Biochemical response (BR) was defined as an undetectable or at least 50% decreased level of prostate-specific antigen, compared with baseline. Associations between BR and different PET parameters were tested. A cutoff of at least a 30% decrease in PSMA total tumor volume (PSMA-TV) was used to define a PSMA response (PSMA-R) versus a PSMA nonresponse (PSMA-NR). Correlations between PSMA PET/CT response and BR were evaluated using the phi-coefficient. Associations between PET response and overall survival (OS) was tested using Cox regression and the Kaplan-Meier method. Results: Our cohort comprised 8 (22%) metastatic hormone-sensitive PCa and 29 (78%) mCRPC patients. Twenty-one patients received docetaxel treatment, and 16 received cabazitaxel (median, 6 cycles; interquartile range, 5-8 cycles). BR was found in 18 of 37 patients. Using PSMA total tumor volume, PSMA PET/CT response was concordant with BR in 35 of 37 patients (phi = 0.89, P < 0.0001). Eighteen of 37 patients had PSMA-R (6, complete response; 12, partial response), and 19 had PSMA-NR (17, progressive disease; 2, stable disease). After a median follow-up of 23 mo, there was a statistically significant longer OS for PSMA-R than for PSMA-NR (median OS not reached vs. 12 mo, respectively; hazard ratio, 0.10; 95% CI, 0.03-0.39; P=0.001) for the entire population. Among the mCRPC subgroup, differences in OS were also observed (median, 22 vs. 12 mo, respectively; hazard ratio, 0.22; 95% CI, 0.06-0.82; P=0.023), with a 12-mo OS rate of 100% for PSMA-R and 52% for PSMA-NR (P=0.011). Conclusion: This retrospective analysis suggests that Ga-68-PSMA-11 PET/CT is a promising imaging modality for assessing response to taxane-based chemotherapy in metastatic PCa. Changes in PSMA expression might be used as a predictive biomarker for OS to help tailor individual therapy and select eligible patients for clinical trials.