Exploring the Flare Phenomenon in Patients with Castration-Resistant Prostate Cancer: Enzalutamide-Induced PSMA Upregulation Observed on PSMA PET

Androgen receptor-targeting agents, particularly enzalutamide, show promise in enhancing prostate cancer diagnostic and therapeutic strategies by modulating prostate-specific membrane antigen (PSMA). <b>Methods:</b> A retrospective clinical cohort study investigated 9 men with metastatic castration-resistant prostate cancer on enzalutamide. PSMA PET/CT scans were obtained before and after enzalutamide initiation to assess PSMA expression changes. Lesions and organs at risk were evaluated visually and semiquantitatively. The flare phenomenon was characterized by a significant increase (≥20%) in the SUV_max of existing lesions or the appearance of new PSMA-positive lesions. <b>Results:</b> Exposure to enzalutamide led to a significant PSMA expression increase in 56% of assessed lesions (<i>n</i> = 42), with new lesions detected in 1 patient (11%). PSMA expression in organs at risk remained largely unaffected, indicating a tumor-specific response. <b>Conclusion:</b> Enzalutamide induces PSMA upregulation in metastatic castration-resistant prostate cancer, potentially enhancing diagnostic and therapeutic strategies. Further exploration of the flare phenomenon's clinical implications is warranted.