35 Background: Imaging-based response to systemic therapies in metastatic prostate cancer is still challenging. The aim of this study was to retrospectively investigate the association between Prostate-Specific Membrane Antigen (PSMA) expression changes on 68Ga PSMA PET/CT and the response to treatment following the start of taxane-based chemotherapies (docetaxel or cabazitaxel) in both metastatic hormonosensitive (mHSPC) and castration-resistant prostate cancer (mCRPC) patients. Methods: We retrospectively reviewed all 68Ga-PSMA-11 PET/CT scans performed at our institution from November 2014 to June 2020. We included mHSPC and mCRPC patients with a baseline 68Ga-PSMA PET/CT performed < 2 months before, and a second 68Ga-PSMA PET/CT performed < 3 months after docetaxel or cabazitaxel (with no modification in therapy between scans). The biological data was used to distinguish PSA-non-responders (increasing PSA or decreasing PSA by < 50% from pretherapy level) and PSA-responders (decreasing PSA > 50% following the start of therapy or the peak thereafter). The PSMA PET/CT response was assessed visually by two independent nuclear medicine physicians blinded to clinical data and patient outcomes. Patients were classified as PSMA-non-responders (≥1 new PSMA-expressing metastases, majority of PSMA-expressing metastases increasing in intensity, or stable PSMA-expression of the disease) or PSMA-responders (complete disappearance of pathologic PSMA uptake, or majority of PSMA-expressing metastases decreasing in intensity). Majority of lesions was defined as > 50% of registered lesions. Descriptive statistics and measures of associations (two-sided Fisher’s exact test and Phi coefficient) were calculated. Results: A total of 37 patients were included (8 mHSPC and 29 mCRPC), 21 patients under docetaxel and 16 cabazitaxel. All patients received at least 3 cycles [median 6 (range 3-10)]. The mean time (±standard deviation) between the first 68Ga-PSMA PET/CT and the start of therapy was 22±16 days. The mean time between the the last cycleand the second 68Ga-PSMA PET/CT was 36±27 days. PSA-response and PSMA-response were concordant in 95% of cases with 18 patients PSA/PSMA-non-responders and 17 PSA/PSMA-responders (Phi = 0.90, p = 0.0001). Two discordant patients presented with PSMA-response but PSA-non-response. No isolated PSMA flare was observed, as PSA-response was always associated with a decrease in PSMA-expression on the second PET/CT. Conclusions: This retrospective study suggests PSMA expression changes on PET/CT at least after 3 cycles of chemotherapy (docetaxel or cabazitaxel) are strongly associated with PSA response. No flare phenomenon was found at this evaluation. Prospective studies are needed to better define the potential role of PSMA-PET/CT in response assessment.
650TiP PARADIGM: Plasma analysis for response assessment and to direct the management of metastatic prostate cancer (mPCa)
