Outcomes in men with metastatic castration-resistant prostate cancer who received sipuleucel-T and no immediate subsequent therapy: experience at Dana Farber and in the PROCEED Registry.

Background: Sipuleucel-T has demonstrated survival benefit in phase 3 trials but is utilized in few men with metastatic castration-resistant prostate cancer (mCRPC) in part due to low rates of PSA and objective response. Given the requirement to develop immune-mediated antitumor activity as vaccine-based therapy, sipuleucel-T may have delayed clinical activity. We explored this in a cohort of men from PROCEED (NCT01306890), an FDA-requested outcomes registry, and in a separate institutional cohort of mCRPC patients treated with sipuleucel-T at Dana-Farber Cancer Institute (DFCI).

Methods: Men with mCRPC who received 3 infusions of sipuleucel-T and did not initiate a new mCRPC directed therapy for ≥6 months after completion of sipuleucel-T were included. All patients had rising PSA before starting sipuleucel-T and available post-treatment PSA measurements. Clinical outcomes of interest included: PSA response rate, time to subsequent mCRPC directed therapy, and overall survival (OS).

Results: Of 1902 men with mCRPC treated in PROCEED and 255 patients treated consecutively with sipuleucel-T between 4/2010 and 4/2017 at DFCI, 171 and 28 patients were included, respectively. In the PROCEED sample, PSA response was observed in 34 (19.9%) of patients at a median of 5.5 months (IQR: 3.9-9.5) since the last sipuleucel-T infusion; median time to subsequent mCRPC directed therapy was 10 months (95% CI: 9-11); and median OS was 49 months (95% CI: 43-NR). In the DFCI cohort, PSA response was observed in 4 (14.3%) of patients at a median of 6.3 months (IQR: 4.7-7.0); median time to subsequent mCRPC directed therapy was 9 months (95% CI: 9-11); and median OS was 60 months (95% CI: 51-74).

Conclusions: In this analysis of mCRPC patients treated with sipuleucel-T who did not immediately initiate subsequent therapy using two datasets, delayed PSA response was observed in a subset of patients indicating delayed clinical activity.