1787P Prostate specific membrane antigen positron emission tomography (PSMA PET)-directed clinical outcomes in metastatic hormone-sensitive prostate cancer (mHSPC): Implications for the STAMPEDE2 trial design

The potential for PSMA PET-directed treatment decisions to improve outcomes in mHSPC remains unknown. We explored first response and progression/relapse patterns in newly diagnosed PSMA PET-detected mHSPC and correlated PET-directed staging with clinical outcomes. 184 patients with baseline PSMA PET were retrospectively reviewed. Patients were sub-grouped into oligometastatic disease (OMD) and polymetastatic disease (PMD) utilising the STAMPEDE2 trial definition. Data on patient demographics, treatment, and imaging findings were collected. Treatment response was assessed using PSA nadir and radiological imaging at six months from PSA nadir. Disease progression/relapse was assessed using PCWG3, RECIST 1.1 and PERCIST 1.0 criteria. Kaplan Meier method estimated median time to radiographic progression-free survival (rPFS). Median age was 69 years. All patients had androgen deprivation therapy. 161/184 (88%) had additional systemic therapies. 99/184 (54%) had OMD and 85/184 (46%) had PMD. Prostate radiotherapy was given in 76/99 (77%) OMD patients and 47/85 (55%) PMD patients. Median time to PSA nadir in OMD and PMD was 10 months (range 1 to 29) and 8 months (range 1 to 36), respectively. Response images were available in 35/99 (35%) OMD patients and 39/85 (46%) PMD patients. Median time to radiological response in OMD and PMD was 10 months (range 2 to 58) and 9 months (range 2 to 36), respectively. Radiological progression in OMD most frequently occurred with new sites of disease (73%). Median rPFS was 53 months (95% CI 40, undefined) in OMD patients and 35 months (95% CI 22, 43) in PMD patients.Table: 1787PRadiological response and progression/relapse in OMD and PMDOMD (N= 99)PMD (N= 85)Radiological responsen= 35 (35%)n= 39 (46%)Complete Response12 (34%)7 (18%)Partial Response21 (60%)29 (74%)Stable Disease1 (3%)2 (5%)Not evaluable1 (3%)1 (3%)Site of Radiological progression/relapse compared with baseline diseasen= 22 (22%)n= 37 (43%)Same sites5 (23%)18 (49%)Different sites10 (45%)9 (24%)Mixed sites6 (27%)10 (27%)Not evaluable1 (5%)0 (0%) Open table in a new tab PSMA PET-guided outcomes were similar to series with conventional staging, suggesting PET-directed therapy choices might not significantly influence time to progression. This will be further investigated in the forthcoming STAMPEDE2 trial.