Poor Outcomes for Patients with Metastatic Castration-resistant Prostate Cancer with Low Prostate-specific Membrane Antigen (PSMA) Expression Deemed Ineligible for ¹⁷⁷Lu-labelled PSMA Radioligand Therapy

BACKGROUND: Prostate-specific membrane antigen (PSMA) is overexpressed in metastatic castration-resistant prostate cancer (mCRPC) and represents a target for imaging and therapy. We undertook a prospective trial of 177Lu-PSMA-617 radioligand therapy in men with high PSMA expression who progressed after standard therapies. OBJECTIVE: To determine outcomes for men screened for the trial but not treated because of low PSMA expression. DESIGN, SETTING, AND PARTICIPANTS: Patients screened with 68Ga-PSMA-11 and 18F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography in a prospective trial. Patients ineligible for enrolment with low PSMA expression or FDG-positive PSMA-negative (discordant FDG-avid) disease were assessed. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: Subsequent treatments received were recorded. Kaplan-Meier analysis was used to determine overall survival from date of screening. RESULTS AND LIMITATIONS: Sixteen patients (24%) had low PSMA expression (n=8) or discordant FDG-avid disease (n=8). The median prostate-specific antigen doubling-time was 2.1mo. Eleven patients had Gleason ≥8 disease. All patients had previously progressed after docetaxel, 44% after cabazitaxel, and 94% after abiraterone and/or enzalutamide. Nine patients had subsequent systemic antitumour treatment. Fifteen patients died, with median OS of 2.5mo (95% confidence interval 1.7-5.0). Study limitations include uncertainty for imaging thresholds that define low PSMA expression. It is also possible that theranostic therapy could have improved survival in this cohort. CONCLUSIONS: Low PSMA expression or discordant FDG-avid disease in patients with mCRPC who progress after conventional therapies identifies a group with poor prognosis and short survival. PATIENT SUMMARY: The 177Lu-PSMA-617 radioligand may be an effective therapy for patients with advanced prostate cancer who progress after standard therapies. In this report we looked at outcomes for patients who were not eligible for this novel therapy on the basis of low prostate-specific membrane antigen uptake on screening positron emission tomography scans. We found that their outcomes were poor, with short survival.