Long-term follow-up from STAMP, a phase II trial, evaluating sipuleucel-T and concurrent (CON) vs sequential (SEQ) abiraterone acetate + prednisone in metastatic castration-resistant prostate cancer patients (pts)

190 Background: The optimal sequence and combination of life-extending anticancer therapies in mCRPC pts remains unknown. Sipuleucel-T (sip-T), an autologous cellular immunotherapy approved for the therapy of asymptomatic or minimally symptomatic metastatic castration-resistant prostate cancer (mCRPC) pts, was evaluated in combination with abiraterone acetate and prednisone (abi) in the phase II STAMP trial (NCT01487863), with pts randomly assigned to receive CON sip-T + abi or SEQ sip-T followed by abi. The combination was well-tolerated and did not alter the immune response parameters that correlate with overall survival (OS) (Small Clin Can Res 2015). Here, we present long-term follow-up of clinical outcomes, including OS. Methods: mCRPC pts were randomized 1:1 to CON or SEQ therapy with sip-T and abi. Abi began 1 day after (CON) or at wk 10 (SEQ) after the first sip-T infusion and continued for 26 wk of therapy, after which continued abi therapy was permitted. Long-term clinical outcomes included OS, disease-specific death (DSS), progressive disease (PD), time to first anticancer intervention (tACI), and safety. Results: 69 pts were enrolled (35 CON; 34 SEQ). Median OS was 34.0 mo (95% CI, 24.4-not estimable [NE]; 30.0 mo CON; 34.2 mo SEQ; p = 0.921), and median time to DSS was not reached (CON vs SEQ; p = 0.733). Median time to PD was 17.3 mo (95% CI, 9.7–NE; 17.7 mo CON vs 13.9 mo SEQ; p = 0.914; consistent with higher rates of abi discontinuation due to PD in SEQ [26.5% vs 14.3% in CON]). tACI was similar between arms at 15.4 mo (95% CI, 11.0–19.9). No new safety signals were observed with the combination, and no discernable difference in clinical outcomes was observed with CON or SEQ treatments. Conclusions: Long-term follow-up data confirm that sip-T + CON or SEQ abi is well-tolerated, with no new safety signals. No clear differences were observed in clinical outcomes between arms, although the study was not powered to detect these differences. Future and more appropriately powered studies on the effect of sip-T + continuous abi for responding pts may provide further insights on the benefit of combination therapy. Clinical trial information: NCT01487863.