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.

This article is featured in Highlights of This Issue, p. 1583

Translational Relevance

Radiotherapy, such as radium-223, is thought to augment the activity of the immune system and may have an impact on the response to immune therapies, such as sipuleucel-T. In this study, we report the results of a randomized phase II trial that evaluated sipuleucel-T with or without radium-223 for men with metastatic castration-resistant prostate cancer. This trial demonstrated that radium-223 in combination with sipuleucel-T was associated with paradoxically lower peripheral T-cell immune responses to the target antigen, PA2024, compared with patients who received sipuleucel-T alone. Other T-cell markers of immune response were not different between the arms. Interestingly, there seemed to be a clinical benefit to the combination with an improvement in the progression-free survival. On the basis of these findings, a larger trial is being planned.

Sipuleucel-T is an FDA-approved autologous cell-based immunotherapy for use in patients with asymptomatic or minimally symptomatic metastatic castration-resistant prostate cancer (mCRPC; refs. 1–3). Although the seminal study of sipuleucel-T demonstrated an overall survival (OS) benefit of 4.1 months compared with placebo, PSA responses were observed in only 2.6% of patients and there was no difference in progression-free survival (PFS) compared with placebo (1). African American patients and those with low baseline PSA values seem to be more likely to benefit from sipuleucel-T (4, 5). Antigen-specific humoral and T-cell responses also correlate with a survival benefit to sipuleucel-T, suggesting that a stronger antitumor immune response may lead to better clinical outcomes (1, 6).

Radiotherapy is known to augment the activity of the immune system (7). Radiation-induced cell death stimulates tumor-specific immune responses by enhanced display of tumor-associated antigens and upregulation of tumor-suppressive proteins and inflammatory cytokines (8). Radium-223 is an α-emitting radioisotope and bone-seeking calcium mimetic, which selectively targets areas with increased bone turnover (9). It is FDA approved for the treatment of men with mCRPC with symptomatic bone metastases, without visceral disease or bulky nodal disease, based on the pivotal phase III trial demonstrating a survival benefit (10). Similar to sipuleucel-T, treatment with radium-223 rarely results in a PSA response.

On the basis of the potential immunomodulatory effects of radiotherapy, we hypothesized that the combined use of radium-223 and sipuleucel-T would enhance sipuleucel-T–induced immune responses and improve clinical outcomes in men with minimally symptomatic, bone-predominant mCRPC.

Study design and patients

This was an open-label, randomized phase II, investigator-initiated study conducted at four sites in the United States (NCT02463799). All patients provided written informed consent. Key eligibility criteria included age ≥18 years with histologically documented prostatic adenocarcinoma, PSA level of ≥ 2.0 ng/mL, at least one sclerotic bone metastasis, and progressive castration-resistant prostate cancer as demonstrated by two consecutive rises in PSA or new lesions on bone or CT scan. Patients were required to have asymptomatic or minimally symptomatic disease, an Eastern Cooperative Oncology Group (ECOG) performance status of ≤1, adequate bone marrow and hepatorenal function, and no narcotics for cancer-related pain. Prior abiraterone and enzalutamide were permitted but not required. Concurrent osteoclast-inhibitory therapies were permitted, but not required. Patients were excluded whether they had any lung or liver metastases >1 cm in long-axis diameter, lymphadenopathy >3 cm in short-axis diameter, known brain metastasis, previous chemotherapy for mCRPC, were on steroids within 2 weeks of randomization or received prior α- or β-emitting radiopharmaceutical drugs. Detailed inclusion and exclusion criteria are available in the full protocol (Supplementary Data). This study was approved by the Institutional Review Board at each participating center and was conducted in accordance with the Declaration of Helsinki; patients at all sites provided written informed consent before enrollment.

Treatment

Eligible patients were randomized (1:1) to receive sipuleucel-T alone or sipuleucel-T with radium-223. In the sipuleucel-T alone arm, patients received three infusions of commercial sipuleucel-T, separated by 2 weeks, according to the standard administration. In the combination arm, patients received six doses of radium-223, each 4 weeks apart, according to standard dose and schedule. Intercalated between the second and third doses of radium-223, sipuleucel-T was initiated and given every 2 weeks for three doses. Thus, the first dose of sipuleucel-T was administered in between the second and third doses of radium-223, the second dose of sipuleucel-T was given concurrently with the third dose of radium-223, and the third dose of sipuleucel-T was given in between the third and fourth doses of radium-223 (see trial schema; Fig. 1). The start of the next systemic therapy was not prespecified and could begin any time after the completion of clinical trial therapy. Participants were censored at the start of the next therapy if they did not reach a progression event by that time.

Figure 1.

Clinical trial schema.

Figure 1.

Clinical trial schema.

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Endpoints

It has previously been shown that sipuleucel-T generates PA2024- and PAP-specific immune responses in most patients, and these generally peak at 6 weeks after treatment initiation (6). PA2024 is the target antigen of sipuleucel-T and represents a recombinant fusion protein of PAP and GM-CSF. The primary endpoint of the trial was to compare peripheral PA2024-specific T-cell proliferation responses using a 3H-thymidine incorporation assay. Baseline was considered just prior to the first dose of sipuleucel-T. The fold change in proliferation was compared at week 6 after the first sipuleucel-T infusion. This was reported as the stimulation index (SI), defined as 3H-thymidine incorporation in the presence of PA2024 antigen divided by 3H-thymidine incorporation with media alone (6). Secondary immune endpoints include peripheral PA2024- and PAP-specific T-cell proliferation responses at 10, 14, and 26 weeks after the first infusion of sipuleucel-T; PA2024 and PAP-specific antibody response to sipuleucel-T, which are the most strongly correlated with OS (6); and sipuleucel-T–induced antigen spread (i.e., humoral responses to nontarget antigens; ref. 11).

Key secondary clinical endpoints included PFS (defined as the time from sipuleucel-T initiation to clinical or radiographic progression or death), and OS (defined as the interval from randomization to death from any cause). PSA 50% response rates (PSA50) were also explored. To compare PFS outcomes in the two arms, baseline was considered to be the first dose of sipuleucel-T (which was administered 6 weeks after the start of radium-223 in the combination arm). A sensitivity analysis was done using date of randomization as the baseline. Radiologic assessment was investigator assessed, with progression being defined based on CT and bone scans according to PCWG2 criteria (12, 13).

Statistical analysis

The study was designed to test the null hypothesis of no difference in immune response to sipuleucel-T based on PA2024-stimulated T-cell proliferation at 6 weeks, comparing sipuleucel-T alone and sipuleucel-T plus radium-223. T-cell proliferation was compared between groups using Wilcoxon test. We sought to detect a 3.6-fold difference between the arms with 80% power in mean SI of PA2024-specific 3H-thymidine uptake at week 6 after the first infusion of sipuleucel-T using a one-sided test at the 0.05 level, yielding a sample size of at least 30 total patients (i.e., at least 15 patients per arm). The immune response population was defined as subjects who received at least one infusion of sipuleucel-T and at least two infusions of radium-223, in the combination arm. Additional immune response parameters comparing fold change from baseline and absolute differences were compared using Wilcoxon tests.

PSA50 response rates between the groups were compared using Fisher exact test. PFS and OS estimates were calculated using the Kaplan–Meier method, and were compared using the log-rank test. Univariate Cox proportional hazards models were used to estimate HRs and 95% confidence intervals (CIs). These statistical tests were two-sided, and statistical significance was set at P ≤ 0.05, unless otherwise specified.

Patients

A total of 32 participants were enrolled and randomized from May 2017 through November 2018. The consort diagram is shown in Fig. 2. Sixteen patients were randomized to receive sipuleucel-T alone and 16 were randomized to sipuleucel-T plus radium-223. Baseline demographic and clinical characteristics are presented in Table 1. Median age of study participants was 71.6 and 70.3 years, respectively. Race, Gleason score, baseline laboratory studies, and ECOG functional status were balanced between the arms. Four participants (25%) in each group were African American. The median PSA at study entry was 33 and 25 ng/mL, and the median alkaline phosphatase was 89 and 92 U/L, respectively. No participant in the combination arm and four participants (25%) in the sipuleucel-T arm received prior chemotherapy for metastatic hormone-sensitive prostate cancer. Six participants (38%) in the combination arm and seven participants (44%) in the sipuleucel-T arm received prior abiraterone or enzalutamide. Ten participants (63%) in the combination arm and 11 participants (69%) in the sipuleucel-T arm were on bone-protective agents at baseline. All patients completed the full treatment course, except for one patient in the sipuleucel-T arm who only received two of the three doses, and two patients in the combination arm who received all sipuleucel-T infusions but only received three and four doses of radium-223, respectively. There was no difference between the groups with regards to the cumulative CD54 upregulation of the final product parameters or the cumulative total nucleated cell count in the final product parameters.

Figure 2.

CONSORT diagram.

Figure 2.

CONSORT diagram.

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Table 1.

Baseline characteristics of patients.

Arm 1Arm 2
Ra-223 + Sipuleucel-T (N = 16)Sipuleucel-T (N = 16)
Age, mean (range) 71.6 (64–88) 70.3 (57–86) 
Gleason sum 
 6 1 (6%) 1 (6%) 
 7 4 (25%) 4 (25% 
 8–10 11 (69%) 11 (69%) 
Race 
 White 11 (69%) 10 (63%) 
 African American 4 (25%) 4 (25%) 
 Other 1 (6%) 2 (12%) 
Ethnicity 
 Hispanic (% yes) 0 (0%) 1 (7%) 
ECOG performance status 
 0 11 (69%) 13 (81%) 
 1 5 (31%) 3 (19%) 
Prior treatment 
 Prior RP (% yes) 10 (63%) 6 (38%) 
 Prior RT (% yes) 9 (56%) 8 (50%) 
 Prior abiraterone (% yes) 0 (0%) 4 (25%) 
 Prior enzalutamide (% yes) 6 (38%) 3 (19%) 
 Prior docetaxel chemotherapy for mHSPC (% yes) 0 (0%) 4 (25%) 
Baseline PSA ng/mL, median (IQR) 25 (9.2–110.1) 33 (4.6–71) 
Baseline alkaline phosphatase U/L, median (IQR) 89 (79–112) 92 (80–114) 
On bone protective agent (% yes) 10 (63%) 11 (69%) 
Arm 1Arm 2
Ra-223 + Sipuleucel-T (N = 16)Sipuleucel-T (N = 16)
Age, mean (range) 71.6 (64–88) 70.3 (57–86) 
Gleason sum 
 6 1 (6%) 1 (6%) 
 7 4 (25%) 4 (25% 
 8–10 11 (69%) 11 (69%) 
Race 
 White 11 (69%) 10 (63%) 
 African American 4 (25%) 4 (25%) 
 Other 1 (6%) 2 (12%) 
Ethnicity 
 Hispanic (% yes) 0 (0%) 1 (7%) 
ECOG performance status 
 0 11 (69%) 13 (81%) 
 1 5 (31%) 3 (19%) 
Prior treatment 
 Prior RP (% yes) 10 (63%) 6 (38%) 
 Prior RT (% yes) 9 (56%) 8 (50%) 
 Prior abiraterone (% yes) 0 (0%) 4 (25%) 
 Prior enzalutamide (% yes) 6 (38%) 3 (19%) 
 Prior docetaxel chemotherapy for mHSPC (% yes) 0 (0%) 4 (25%) 
Baseline PSA ng/mL, median (IQR) 25 (9.2–110.1) 33 (4.6–71) 
Baseline alkaline phosphatase U/L, median (IQR) 89 (79–112) 92 (80–114) 
On bone protective agent (% yes) 10 (63%) 11 (69%) 

Abbreviations: IQR, interquartile range; mHSPC, metastatic hormone-sensitive prostate cancer; PSA, prostate-specific antigen; RP, radical prostatectomy; RT, radiation.

Immune responses

Cellular responses

Our primary endpoint was not met. At week 6, the fold change in PA2024-specific proliferation index was 3.2-fold higher in the sipuleucel-T arm compared with the combination arm [median, 22.4; interquartile range (IQR), 14.2 vs. 7.0; IQR, 16.1; P = 0.036; Fig. 2A], an unexpected result for our primary endpoint. The fold change in PA2024-specific proliferation was generally higher in the sipuleucel-T alone arm through week 26 when compared with the combination arm (Fig. 3A). The 6-week change in PAP-specific proliferation was numerically 2-fold higher in the sipuleucel-T alone arm compared with the combination arm but this was not statistically significant between arms (0.4, IQR 2.2 vs. 0.2, IQR 0.7; P = 0.27; Fig. 3B); there was little difference between the arms at later time points with respect to this parameter (Fig. 3B). There were no significant differences in the fold change of antigen-specific T cells, detected by IFN-γ ELISPOT assays, against PA2024 or PAP at week 6 or later (Fig. 3C and D). Individual changes in each of these over time is shown in the Supplementary Data.

Figure 3.

Immunologic endpoints (at week 6, N = 12 in combination arm with paired data, and N = 11 in sipuleucel-T alone arm with paired data). A, Fold change in PA2024 proliferation index compared with baseline. B, Fold change in PAP proliferation index compared with baseline. C, Fold change in PA2024-specific T cells by ELISPOT. D, Fold change in PAP-specific T cells by ELISPOT.

Figure 3.

Immunologic endpoints (at week 6, N = 12 in combination arm with paired data, and N = 11 in sipuleucel-T alone arm with paired data). A, Fold change in PA2024 proliferation index compared with baseline. B, Fold change in PAP proliferation index compared with baseline. C, Fold change in PA2024-specific T cells by ELISPOT. D, Fold change in PAP-specific T cells by ELISPOT.

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Humoral responses

Although there was an increase in the absolute antibody responses to PA2024 and PAP in both arms, there was no significant difference between the arms when comparing absolute antibody levels and no significant difference in the change in humoral responses (relative to baseline) to PAP or PA2024 between the arms (Supplementary Data). There was no indication of broad differential antigen spread responses between the groups. More specifically, there was no difference in the fold change of antibody response to the nontarget antigens: PSA, KRAS, ERAS, or KLK2. Of note, the sipuleucel-T group had a significantly higher change in antibody response to LGALS3 at week 6, week 10, and week 14 compared with the baseline (Supplementary Data).

Cytokine analyses

There were no significant differences in the percentage change in IFNγ, IL6, or TNFα at any of the time points examined (Supplementary Data). Of the other cytokines tested (IL10, IL12p70, IL13, IL1β, IL2, IL4, and IL8), the relative changes compared to baseline were not different between the arms. With regard to absolute levels, the sipuleucel-T group had higher IL13 levels at week 10 (3,936 pg/mL in sipuleucel-T arm, IQR 5,094 compared with 1,388 pg/mL in the combination arm, IQR 2,960; P = 0.019) and week 26 (33,488 pg/mL in sipuleucel-T arm, IQR 16,078 compared with 675 pg/mL in the combination arm, IQR 2,381; P = 0.036). There was no difference between the absolute levels of any of the other cytokines when comparing the two arms.

Clinical outcomes

There were 29 patients evaluable for PSA response (three subjects never had posttreatment PSA checks before coming off study). Five patients in the combination arm had a PSA decline of at least 50% (PSA50), of which four were confirmed with a repeat PSA value 4 weeks later (Fig. 4); there were no PSA50 responses in the monotherapy arm [5/15 (33%) vs. 0/14 (0%); P = 0.04].

Figure 4.

Waterfall plot showing best PSA response rate.

Figure 4.

Waterfall plot showing best PSA response rate.

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After a median of 1.6 years of follow-up, there were 24 radiographic progression events and one clinical progression event. The median PFS was 39 weeks in the combination arm compared with 12 weeks in the sipuleucel-T alone arm (HR, 0.32; 95% CI, 0.14–0.76; P < 0.01; Fig. 5A). In the sensitivity analysis, using the date of registration as the baseline, the median PFS was 44 weeks in the combination versus 12 weeks in the sipuleucel-T alone arm (HR, 0.24; 95% CI, 0.10–0.56). There was also a trend toward improved OS in the combination arm, with a median survival of 2.6 years in the sipuleucel-T alone arm whereas the median survival was not reached in the combination arm (HR, 0.32; 95% CI, 0.08–1.23; P = 0.08; Fig. 5B). Subsequent therapies used for treatment are shown in the Supplementary Data.

Figure 5.

Clinical endpoints. A, PFS of the radium-223 + sipuleucel-T (red) arm compared with sipuleucel-T alone (teal). B, OS comparing the two arms.

Figure 5.

Clinical endpoints. A, PFS of the radium-223 + sipuleucel-T (red) arm compared with sipuleucel-T alone (teal). B, OS comparing the two arms.

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Safety

The treatment-emergent adverse events observed in this trial are consistent with the known side effects of each of the therapies, and there did not appear to be any additive toxicities (Table 2). Pain was the most common symptom reported. Chills, dizziness, and lightheadedness were more common in the sipuleucel-T arm. These were related to infusion of the immunotherapy product and were transient as expected. Nausea, diarrhea, and marrow suppression (thrombocytopenia and leukopenia) were more common in the combination arm and are likely attributed to radium-223 as anticipated. There were two skeletal-related events in the combination arm, and one in the sipuleucel-T arm—all pathologic fractures. No grade 4/5 events were observed.

Table 2.

Adverse events if they occurred in ≥ 3 patients or were grade 3. There were no grade 4/5 events.

Sip-T + Rad-223Sip-T
Any gradeGrade 3Any gradeGrade 3
Constitutional 
 Pain 10 (63%) 2 (13%) 12 (75%) — 
 Chills 2 (13%) — 6 (38%) — 
 Fatigue 5 (31%) — 3 (19%) — 
 Flu like symptoms 4 (25%) — 3 (19%) — 
 Fever 2 (13%) — 1 (6%) — 
 Dizziness/lightheaded 1 (6%) — 3 (19%) — 
 Fall 1 (6%) — 2 (13%) — 
Hematologic 
 Leukopenia 2 (13%) — — — 
 Anemia 5 (31%) 1 (6%) 5 (31%) 1 (6%) 
 Thrombocytopenia 1 (6%) — — — 
Gastrointestinal     
 Nausea 5 (31%) 1 (6%) 4 (25%) — 
 Diarrhea 7 (44%) 1 (6%) — — 
 Constipation 2 (13%) — 5 (31%) — 
 Vomiting 2 (13%) — 2 (13%) — 
 Abdominal pain 1 (6%) 1 (6%) 2 (13%) — 
Cardiac     
 Hypertension 2 (13%) 1 (6%) — — 
Other     
 Headache 3 (19%) — — — 
 Edema 1 (6%) — 2 (13%) — 
 Insomnia 2 (13%) — 1 (6%) — 
 Catheter-related infection — — 1 (6%) 1 (6%) 
 Urinary retention — — 1 (6%) 1 (6%) 
Skeletal-related event 2 (13%)  1 (6%)  
Sip-T + Rad-223Sip-T
Any gradeGrade 3Any gradeGrade 3
Constitutional 
 Pain 10 (63%) 2 (13%) 12 (75%) — 
 Chills 2 (13%) — 6 (38%) — 
 Fatigue 5 (31%) — 3 (19%) — 
 Flu like symptoms 4 (25%) — 3 (19%) — 
 Fever 2 (13%) — 1 (6%) — 
 Dizziness/lightheaded 1 (6%) — 3 (19%) — 
 Fall 1 (6%) — 2 (13%) — 
Hematologic 
 Leukopenia 2 (13%) — — — 
 Anemia 5 (31%) 1 (6%) 5 (31%) 1 (6%) 
 Thrombocytopenia 1 (6%) — — — 
Gastrointestinal     
 Nausea 5 (31%) 1 (6%) 4 (25%) — 
 Diarrhea 7 (44%) 1 (6%) — — 
 Constipation 2 (13%) — 5 (31%) — 
 Vomiting 2 (13%) — 2 (13%) — 
 Abdominal pain 1 (6%) 1 (6%) 2 (13%) — 
Cardiac     
 Hypertension 2 (13%) 1 (6%) — — 
Other     
 Headache 3 (19%) — — — 
 Edema 1 (6%) — 2 (13%) — 
 Insomnia 2 (13%) — 1 (6%) — 
 Catheter-related infection — — 1 (6%) 1 (6%) 
 Urinary retention — — 1 (6%) 1 (6%) 
Skeletal-related event 2 (13%)  1 (6%)  

In this randomized phase II trial, we found (unexpectedly) that participants in the sipuleucel-T alone arm had an increased antigen-specific peripheral immune response compared with the combination arm, with a higher change in the T-cell proliferation index against the sipuleucel-T target antigen PA2024. There were no significant differences in the other secondary immune parameters including antibody response to PA2024 or PAP, and antigen spread to secondary nontarget antigens that have been associated with improved outcomes in sipuleucel-T in prior studies (11). LGALS8 was higher in the sipuleucel-T group in our study; however, it has not previously been associated with differences in outcomes in patients treated with sipuleucel-T (11, 14). We also observed an increase in IL13 levels in the sipuleucel-T alone arm compared with the combination arm; the significance of this finding is not clear. Radium-223 has not been associated with any change in T cells producing cytokines such as IFNγ, TNFα, or IL13 (15). Overall, we did not detect a signal of improved immunologic activity using radium-223 plus sipuleucel-T over sipuleucel-T alone. Furthermore, we did not examine long-term immune consequences beyond approximately 26 weeks.

Despite the lack of augmented peripheral cellular and humoral responses in the combination-therapy arm, significant clinical activity was observed for those treated with sipuleucel-T plus radium-223. Five patients (33%) in the combination arm achieved a PSA50 response, compared with none in the sipuleucel-T alone arm. This clinical signal is encouraging, as PSA responses are only seen in approximately 10%–15% of patients treated with radium-223 and approximately 2% of patients treated with sipuleucel-T, although this could be within what is expected from radium-223 alone (1, 10, 16, 17). In addition, the median PFS was significantly longer in the combination-therapy arm, with a trend toward an improvement in OS as well in the combination arm, which might not be surprising given that both agents are approved on the basis of their individual clinical benefits. This suggests possible additive clinical effects with the use of combination radioimmunotherapy, but does not prove synergy.

Previous research has shown that antigen spread, evident by humoral responses to PSA and LGALS2, has been associated with improved OS in patients treated with sipuleucel-T in prospective trials (11). This was not evident in our study. One hypothesis is that the radioimmunotherapy combination may have increased the number of tumor-infiltrating CD8+ lymphocytes while subsequently resulting in a compensatory decrease in circulating antigen-specific T lymphocytes, although this notion is speculative. It is also possible that radium-223 may inhibit B- and T-cell proliferation and function. Although a prior small study did not show a negative impact of radium-223 on lymphocyte function (18), that study used different time points to assess immune parameters and did not examine longer term T-cell effects. Thus, radium-223 could have been myelosuppressive, leading to dampened immune responses overall; however, there was no evidence of this detected within the product parameters for sipuleucel-T. Another hypothesis is that genomic alterations could be playing a role, especially if these were not balanced across arms. It has been previously reported that patients with DNA damage repair mutations may have better responses to radium-223, which could theoretically contribute to these results (19). In addition, anecdotal reports have suggested striking clinical responses to sipuleucel-T in some patients with mismatch-repair gene mutations and microsatellite instability (20). Our study was conducted at a time when most patients were not undergoing tumoral DNA sequencing, and hence somatic (and germline) genomic analysis were not part of the trial.

The notion that radiotherapy may enhance the clinical effects of sipuleucel-T is supported by a recently published randomized trial (21). In that study, patients with asymptomatic mCRPC were randomized to receive either sipuleucel-T (n = 24) alone or sipuleucel-T 1 week after receiving external-beam radiotherapy to a metastatic site (n = 25). To this end, the median PFS in subjects receiving the combined-modality approach versus those getting sipuleucel-T alone was modestly enhanced (3.7 vs. 2.5 months; P = 0.06). Interestingly, and broadly consistent with our findings, T-cell responses to PA2024 (measured by IFNγ ELISPOT assays) were observed in both arms, but were more robust in the monotherapy compared with the combination arm (P = 0.03); other humoral and cellular responses were similar across arms. However, the long-term effects of this difference in immune response on patient outcomes is unknown. In addition, not part of this study, but also to be considered, is how the timing of the two agents would affect outcomes and if concurrent versus sequential treatment of these two agents would result in different outcomes.

In our study, there were no adverse safety signals in the group of patients treated with the combination of sipuleucel-T plus radium-223. The main toxicities observed in the combination arm were consistent with the known individual side effects of each agent, and their prevalence or intensity did not appear to be augmented compared with prior published studies of each monotherapy.

There are several limitations to our study. First, this study was focused on immune-related endpoints associated with sipuleucel-T and these immunologic correlates were not specific to radium-223 treatment. To this end, the immune effects of radium-223 are understudied and incompletely understood, and might possibly be lymphodepleting. One small study of patients treated with radium-223 showed that after a single dose, treated patients had a decrease in PD-1–expressing memory CD8+ T cells (15). However, that study was not powered for clinical endpoints. Furthermore, due to the small sample size of our study, all conclusions with respect to clinical efficacy should be interpreted with caution and require prospective validation. Importantly, the synergy of this combination could not be proven here. In addition, the use of the first sipuleucel-T treatment as the baseline time point for PFS estimates is unconventional and represents a departure from the norm (i.e., rather than time of randomization as the baseline, as was done here for OS estimates); the reason for doing this relates to the fact that imaging assessments would not have been concordant in the two arms (and would have been off by 6 weeks) had we used randomization as the baseline time point for PFS analysis. However, the sensitivity analysis yielded similar results. In addition, progression events were determined by the investigators and we cannot rule out unconscious bias on the part of the investigators in determining events. The therapies received prior to and after the study also differed; notably, more patients in the sipuleucel-T arm received prior docetaxel. The number of participants who received subsequent abiraterone or enzalutamide after study completion was also numerically higher for the combination arm, and these differences may be contributing to the difference in OS seen. Furthermore, the lack of germline and somatic mutational data limit conclusions about the potential role of DNA damage response alterations and outcomes, and there may have been imbalances in DNA repairome abnormalities across the two arms.

In conclusion, this hypothesis-generating randomized trial raises the possibility of an additive clinical effect of the combination of sipuleucel-T and radium-223, while peripheral antigen-specific immune responses were paradoxically greater in the sipuleucel-T alone arm than the combination arm. The radioimmunotherapy combination was feasible to execute, and there were no unexpected safety signals implying synergistic toxicity. A larger randomized study testing this combination, with a focus on long-term clinical outcomes, is being planned to confirm these findings.

C.H. Marshall reported personal fees from Dendreon and Bayer during the conduct of the study, as well as personal fees from McGraw Hill Publishing Company and Dava Oncology outside the submitted work. P.T. Tran reported grants and personal fees from RefleXion Medical; personal fees from Noxopharm, Janssen-Taris Biomedical, Myovant, AstraZeneca; grants from Bayer Healthcare and Astellas Pharm outside the submitted work; in addition, P.T. Tran had a patent for Compounds and Methods of Use in Ablative Radiotherapy. Patent#: 9114158 licensed and with royalties paid from Natsar Pharm. D.Y. Song reported grants from Bayer outside the submitted work. P. Barata reported other from EMD Serono, Caris, Bayer, AstraZeneca, Pfizer, and Clovis and grants from Blue Earth Diagnostics outside the submitted work. C.G. Drake reported personal fees from Dendreon outside the submitted work; and currently is an employee of Janssen R&D, Springhouse PA. A.J. Armstrong reported grants and personal fees from Janssen, Pfizer, Astellas, AstraZeneca, and Merck; personal fees from BMS, Novartis, Constellation, and Genentech outside the submitted work. O. Sartor reported grants and personal fees from Advanced Accelerator Applications, AstraZeneca, Bayer, Constellation, Dendreon, Janssen, Progenics, and Sanofi; personal fees from Astellas, Blue Earth Diagnostics, Inc., Bavarian Nordic, Bristol Myers Squibb, Clarity Pharmaceuticals, Clovis, EMD Serono, Fusion, Isotopen Technologien Meunchen, Myovant, Myriad, Noria Therapeutics, Inc., Novartis, Noxopharm, POINT Biopharma, Pfizer, Tenebio, Telix, and Theragnostics; grants from Invitae, Merck, SOTIO outside the submitted work. No disclosures were reported by the other authors.

C.H. Marshall: Data curation, formal analysis, investigation, writing-original draft, writing-review and editing. W. Fu: Data curation, formal analysis, methodology, writing-review and editing. H. Wang: Conceptualization, data curation, formal analysis, methodology, writing-review and editing. J.C. Park: Conceptualization, writing-review and editing. T.L. DeWeese: Conceptualization, writing-review and editing. P.T. Tran: Investigation, writing-review and editing. D.Y. Song: Investigation, writing-review and editing. S. King: Data curation, investigation, project administration. M. Afful: Data curation, project administration. J. Hurrelbrink: Data curation, project administration. C. Manogue: Data curation, project administration. P. Cotogno: Data curation, investigation, project administration. N.P. Moldawer: Data curation, project administration. P.C. Barata: Investigation, writing-review and editing. C.G. Drake: Conceptualization, investigation, methodology, writing-review and editing. E.M. Posadas: Investigation, writing-review and editing. A.J. Armstrong: Investigation, writing-review and editing. O. Sartor: Investigation, writing-review and editing. E.S. Antonarakis: Conceptualization, resources, data curation, supervision, investigation, methodology, writing-original draft, writing-review and editing.

This study was funded by Dendreon Corporation. Radium-223 was provided free of charge by Bayer Pharmaceuticals. Support was also received from the NIH Cancer Center support grant P30CA006973.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

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