Results of a 50 patient single-center phase II prospective trial of Lutetium-177 PSMA-617 theranostics in metastatic castrate-resistant prostate cancer.

228 Background: Lutetium-177 ( 177 Lu)-PSMA-617 (LuPSMA) is a radiolabelled small molecule that binds with high affinity to prostate specific membrane antigen (PSMA) enabling targeted delivery of beta-radiation. We have previously reported favourable activity with low toxicity in a 30 patient study in men with metastatic castrate-resistant prostate cancer (mCRPC) and now report updated results including a twenty patient extension cohort. Methods: In this phase II trial, 50 pts with PSMA-avid mCRPC who had progressed after standard therapies received up to 4 cycles of LuPSMA every 6 weeks. The primary endpoints were PSA response (PCWG2) and toxicity (CTCAE v4). Other endpoints included imaging response, PSA PFS and OS. Cut-off for analysis 5 Oct 2018. Results: 76 men were screened to identify 50 patients eligible for treatment. Median PSA doubling time was 2.6 months. The majority of patients had received prior docetaxel (84%), cabazitaxel (48%), and abiraterone and/or enzalutamide (90%). The mean administered radioactivity was 7.5 GBq/cycle. PSA decline ≥ 50% was achieved in 32 of 50 patients (64%, 95% CI 50-77%), including 22 patients (44%, 95% CI 30-59%) with a PSA decline ≥ 80%. 27 patients had measurable soft tissue at baseline and 56% of these patients had a partial or complete response by RECIST 1.1. The most common toxicities attributed to LuPSMA were transient G1-2 dry mouth in 68%, G1-2 nausea in 48%, and G1-2 fatigue in 36%. G3-4 toxicities attributed to LuPSMA were infrequent with thrombocytopenia in 10% and anaemia in 10%. Median PSA PFS was 6.9 months (95% CI 6.0-8.7) and median OS was 13.3 months (95% CI 10.5-18.0). Upon subsequent progression, further LuPSMA was administered to 14 patients (median 2 cycles commencing 359 days from enrolment); PSA ≥ 50% response occurred in 9 patients (64%). Conclusions: This expanded 50 patient cohort confirms high response rates and low toxicity with LuPSMA in men who had progressed after standard therapies. In patients who subsequently progressed and were administered further LuPSMA, high response rates were also observed. These results have provided the basis for randomised controlled trials currently underway. Clinical trial information: ACTRN12615000912583.