PREDICTORS OF OVERALL AND DISEASE FREE SURVIVAL IN METASTATIC CASTRATION-RESISTANT PROSTATE CANCER PATIENTS RECEIVING ²²⁵Ac-PSMA-617 RADIOLIGAND THERAPY

Abstract

Background: Metastatic prostate carcinoma over-expresses the prostate specific membrane antigen (PSMA) making this antigen a suitable target for radioligand therapy of the disease. Here we report on our experience in a series of 73 castrate resistant prostate carcinoma patients treated with ²²⁵Ac-PSMA-617, identifying variables predictive for overall and progression free survival following ²²⁵Ac-PSMA-617 treatment. Methods: ²²⁵Ac-PSMA-617 was administered to patients with mCRPC who had exhausted available therapy options for their disease. Full blood count, glomerular filtration rate and liver function test were obtained at baseline and on follow-up for evaluation of toxicity. ⁶⁸Ga-PSMA PET/CT was obtained at baseline, prior to every treatment cycle and on follow-up for patients’ selection for treatment, to determine the activity of treatment agent to be adminsitered and for response assessment. Serial (prostate specific antigen) PSA was obatianed for PSA response assessment. Results: Seventy-three men (mean age=69 years, range:45-85) with mCRPC were treated with 210 cycles of ²²⁵Ac-PSMA-617. In 70% of patients, a PSA decline of ≥50% was obtained while 83% of patients had any PSA decline. In 29% of patients, all lesions on ⁶⁸Ga-PSMA-PET resolved in response to treatment. During follow-up, 23 patients experienced disease progression while 13 patients died from their disease. The estimated median progression free survival (PFS) and overall survival (OS) were 15.2 months (95% CI: 13.1 – 17.4) and 18 months (95% CI: 16.2 – 19.9) respectively. On univariate analyses factors such as baseline PSA, any PSA decline, PSA decline ≥50%, prior chemotherapy, prior radiation therapy, baseline hemoglobin level were associated with longer PFS and OS (all p<0.05). On multivariate analyses, only prior Lu-PSMA therapy and a PSA decline ≥50% remained significant in their association with a longer PFS while only PSA decline ≥50% remained significantly associated with OS. Xerostomia was seen in 85% of patients, none was severe enough to warrant discontinuing treatment. Anaemia was seen in 27 patients, no patients with grade IV bone marrow toxicity was seen. Renal failure grade III-IV was seen in five patients with baseline renal impairement. Conclusion: In this study, a PSA decline of > 50% proved significantly associated with OS and PFS in multivariate analysis following treatment with ²²⁵Ac-PSMA-617. Furthermore, previous ¹⁷⁷Lu-PSMA treatment was negatively associated with PFS in both uni- and multivariate analysis.