Augmented alpha-beta PSMA radioligand therapy in metastatic prostate cancer: Analysis of prognostic factors for survival.

e17025 Background: PSMA-targeted radioligand therapy (PRLT) using the alpha-emitter Ac-225 PSMA-617 is effective in metastatic prostate cancer progressing after beta-emitter-based PRLT using Lu-177 PSMA-617, and in the presence of diffuse bone marrow metastases. Xerostomia is a dose-limiting factor in PRLT using Ac-225. Augmented Alpha-Beta PRLT is a concept of concomitantly administering Lu-177 and Ac-225 PSMA in lower activities, as a trade-off between efficacy and the dose-limiting toxicity, i.e., xerostomia. The objective of this study was to compare overall survival (OS), progression-free survival (PFS), and incidence of xerostomia among metastatic prostate cancer patients who received Ac-225 PSMA-617 augmentation in the initial phase of Lu-177 PSMA-617 RLT compared to those who received it in the late phase. Initial phase is defined as first two cycles of PSMA radioligand therapy (PRLT) vs late phase as after two cycles. We also examined prognostic factors associated with OS and PFS. Methods: Kaplan Meier method was used to examine survival curves and log-rank tests were conducted to examine differences in survival by groups. Cox Proportional Hazard (CPH) model was used to examine the impact of bone marrow and visceral metastasis and age on OS and PFS. Progression was defined as an increase of ≥ 25% in prostate-specific antigen (PSA) levels. Results: 68 patients who received Ac-225 PSMA-617 by itself, or Ac-225 PSMA-617 augmentation to Lu-PSMA-617 were included in the analysis. Out of these, 15 received early augmentation, 50 received late augmentation, and 3 received Ac-225 PSMA-617 alone. Of these, 48 experienced cancer progression and 21 died due to cancer progression. Median OS was 47 months and PFS was 11 months. Those who received early augmentation had significantly better PFS (median PFS not reached during study period) compared to those who received late augmentation (median PFS: 9 months) (p = 0.02). Early augmentation was associated with lower likelihood of xerostomia (p = 0.09). CPH model suggested that visceral or bone marrow metastasis were associated with significantly worse prognosis for PFS (p = 0.04). Conclusions: In patients progressing after Lu-177 PSMA radioligand therapy and those with diffuse bone-marrow metastases, an earlier augmentation with Ac-225 PSMA-617 is associated with better PFS and lower side effects such as xerostomia. Having visceral or bone marrow metastasis is a poor prognostic indicator for PFS in augmented alpha-beta PSMA radioligand therapy.