Randomized Phase II Trial of Sipuleucel-T with or without Radium-223 in Men with Bone-metastatic Castration-resistant Prostate Cancer

PURPOSE

To investigate whether radium-223 increases peripheral immune responses to sipuleucel-T in men with bone-predominant, minimally symptomatic metastatic castration-resistant prostate cancer (mCRPC).

PATIENTS AND METHODS

A total of 32 patients were randomized 1:1 in this open-label, phase II multicenter trial. Patients in the control arm received three sipuleucel-T treatments, 2 weeks apart. Those in the combination arm received six doses of radium-223 monthly, with sipuleucel-T intercalated between the second and fourth doses of radium-223. The primary endpoint was a comparison of peripheral antigen PA2024-specific T-cell responses (measured by proliferation index). Secondary endpoints were progression-free survival (PFS), overall survival (OS), and PSA responses.

RESULTS

We enrolled 32 patients, followed for a median of 1.6 years. Six weeks after the first sipuleucel-T dose, participants in the control arm had a 3.2-fold greater change in PA2024-specific T-cell responses compared with those who received combination treatment (P = 0.036). Patients in the combination arm were more likely to have a >50% PSA decline [5 (31%) vs. 0 patients; P = 0.04], and also demonstrated longer PFS [39 vs. 12 weeks; HR, 0.32; 95% confidence interval (CI), 0.14-0.76] and OS (not reached vs. 2.6 years; HR, 0.32; 95% CI, 0.08-1.23).

CONCLUSIONS

Our data raise the possibility of greater clinical activity with the combination of sipuleucel-T and radium-223 in men with asymptomatic bone mCRPC, despite the paradoxically lower immune responses observed. Additional study to confirm these findings in a larger trial is warranted.

Results of the randomized phase II study of sipuleucel-T (Sip-T) +/- Radium-223 (Ra-223) in men with bone-metastatic castration resistant prostate cancer

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Background: It has been suggested that immune modulation can be augmented by radiation, possibly by enhancing tumor-antigen display. SipT-induced antigen-specific immune responses in mCRPC patients correlate with survival. We hypothesized that the combination of Ra223 and SipT would enhance SipT-related immune response and improve outcomes compared to SipT alone. Methods: Patients with asymptomatic, bone-predominant mCRPC, without visceral mets >1.0 cm, were randomized (1:1) to SipT alone or with 6 doses of Ra223 (NCT02463799). Men in the SipT+Ra223 arm started SipT between the 2nd and 3rd dose of Ra223. The primary immunologic endpoint was PA2024-specific T-cell proliferation 6 wks after the first SipT infusion. Secondary immune endpoints were PA2024-specific ELISPOT response, PAP-specific proliferation and ELISPOT, humoral responses against both antigens, and antigen spread. Clinical endpoints were radiographic PFS, PSA response (≥50% decline), AlkPhos response (≥30% decline), and safety. Results: 32 men were randomized, 16 per arm. Baseline characteristics in SipT+Ra223 and SipT arms were similar: age (median 71 vs. 70 yrs), Gleason (8-10: 69% vs. 69%), baseline PSA (med 25 vs. 33 ng/mL), AlkPhos (med 89 vs. 92 U/L) and ECOG score (≥1: 31% vs. 19%). There was no significant difference in prior use of abi/enza (38% vs. 44%), or chemo (0% vs. 25%). At 6 weeks, absolute PA2024-specific T-cell proliferation was 2.1-fold higher in the Sip-T arm compared to the SipT+Ra223 arm (35.6 vs. 16.6; P=0.03) and remained higher through week 26. Relative to baseline, the 6-week PA2024-specific T-cell proliferation change was 3.6 times greater in the Sip-T arm compared to the SipT+Ra223 arm ( P=0.007) and remained higher through week 14. There were no significant differences in antigen spread or humoral responses. Median radiographic PFS was longer in the SipT+Ra223 arm (9.3 vs. 3.2 months; HR 0.26, 95% CI 0.11–0.61; P=0.007). PSA and AlkPhos responses were better in the SipT+Ra223 arm (PSA50: 5/15=33% vs. 0/14=0%; P=0.04; AlkPhos30: 9/15=60% vs. 1/15=7%; P=0.01). There was no difference in SREs (13% vs. 7%). Conclusions: SipT+Ra223 was associated with improved clinical outcomes and a higher rate of PSA responses compared to SipT alone, although surprisingly, the SipT arm demonstrated higher peripheral PA2024-specific T-cell proliferation. Since neither agent reliably induces PSA responses alone, these data suggest a synergistic effect of the combination. Larger randomized studies of this combination are planned. Clinical trial information: NCT02463799 .

Randomized phase II study of sipuleucel-T (SipT) with or without radium-223 (Ra223) in men with asymptomatic bone-metastatic castrate-resistant prostate cancer (mCRPC)

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Background: SipT-induced antigen-specific immune responses in mCRPC patients correlate with survival. Due to the immunomodulatory effects of radiopharmaceutical agents (e.g. enhancing tumor-antigen display), we hypothesized that combined use of Ra223 and SipT would augment SipT-related immune response and improve outcomes compared to SipT alone. Methods: Patients with asymptomatic mCRPC and bone-predominant mets, without visceral mets >1.0 cm, were randomized (1:1) to standard SipT alone or combined with 6 doses of Ra223 (NCT02463799). Men in the SipT+Ra223 arm received SipT between the 2nd and 3rd dose of Ra223. Clinical endpoints were radiographic/clinical PFS, PSA response (≥50% decline), AlkPhos response (≥30% decline), and safety. Immunologic endpoints were PA2024-specific T-cell proliferation 6 wks after the first SipT infusion, PA2024-specific ELISPOT response, PAP-specific proliferation and ELISPOT, humoral responses against both antigens, and antigen spread. Results: 32 men were randomized, 16 per arm. Baseline characteristics in SipT and SipT+Ra223 arms were matched with respect to age (median 70 vs 71 yrs), Gleason (8-10: 69% vs 69%), PSA (median 82 vs 72 ng/mL), AlkPhos (median 125 vs 125 U/L) and ECOG scores (≥1: 19% vs 31%). After median follow up of 5.3 (range 2.8–26.6) mo, median PFS was longer in the SipT+Ra223 arm (10.7 vs 3.1 mo; HR 0.35, 95% CI 0.15–0.81; P=0.02). Outcomes were also better in the SipT+Ra223 arm with respect to PSA responses (5/15=33% vs 0/14=0%; P=0.04) and AlkPhos responses (9/15=60% vs 1/15=7%; P=0.01). No safety concerns were observed with the combination (grade 3 AEs shown in the Table). Conclusions: SipT combined with Ra223 was associated with improved clinical outcomes compared to SipT alone. Since neither agent reliably induces PSA responses alone, these data suggest a synergistic effect of the combination. Immunologic endpoints will be presented at the meeting. Larger randomized studies of this combination are warranted. Clinical trial information: NCT02463799. [Table: see text]