Immunotherapy for Prostate Cancer: A Current Systematic Review and Patient Centric Perspectives

Prostate cancer is the most commonly diagnosed cancer in men worldwide, making up 21% of all cancer cases. With 345,000 deaths per year owing to the disease, there is an urgent need to optimize prostate cancer care. This systematic review collated and synthesized findings of completed Phase III clinical trials administering immunotherapy; a current clinical trial index (2022) of all ongoing Phase I–III clinical trial records was also formulated. A total of four Phase III clinical trials with 3588 participants were included administering DCVAC, ipilimumab, personalized peptide vaccine, and the PROSTVAC vaccine. In this original research article, promising results were seen for ipilimumab intervention, with improved overall survival trends. A total of 68 ongoing trial records pooling in 7923 participants were included, spanning completion until June 2028. Immunotherapy is an emerging option for patients with prostate cancer, with immune checkpoint inhibitors and adjuvant therapies forming a large part of the emerging landscape. With various ongoing trials, the characteristics and premises of the prospective findings will be key in improving outcomes in the future.


Brief Overview
Prostate cancer (PC) is the most commonly diagnosed cancer in men and is the second most commonly occurring disease among males in the United States (US) [1]. In 2022 alone, 268,490 new cases of PC occurred in the US [1]. PC makes up around 21% of all cancer cases in males [1]. The disease leads to 345,000 deaths per year; it is the second most common cancer-causing death in the US following lung and bronchus cancer. PC is often termed a 'cold tumor' given its immunosuppressive microenvironment [2]. The tumor-infiltrating lymphocytes inhibit T effecter cell activity, thereby contributing to the progression of PC. Biopsy specimens have depicted that the infiltrating lymphocytes skew towards T helper 17 and T regulatory phenotypes that suppress the body's antitumor immune and autoreactive T cell responses [2,3]. There is a growing need to design therapies that can boost immunity with effector T cells and antigen-presenting cells [4]. The subgroup of antigen-presenting cells, dendritic cells, are notable CD8+ T cells that can be used in activating and killing tumors [5]. Studies show associations of positive prognosis with dendritic cell tumor infiltration [6]. Androgen deprivation therapy (ADT) has also led to the mitigation of T cell

Rationale
Immunotherapy remains to be a momentous area of prostate cancer care, and is an appealing treatment paradigm in optimizing the management of the disease. Despite obtaining success against other cancer types, prostate cancer has so far shown mixed findings with immunotherapy. With the first-ever prostate cancer vaccine approved in 2010, patients with advanced prostate cancer were provided with a viable treatment modality to improve outcomes of disease. However, the spileucel-T vaccine has only partially improved survival outcomes. Thereby, the purpose of this study is to provide readers with an updated view of trials specifically in Phase III of testing, since these trials test if the novel immunotherapy is better than standard treatment. On the other hand, Phase I trials test only the safety of new therapies, while Phase II trials tend to assess efficacy of the new treatment among patients with prostate cancer. This systematic review will include current literature comprising Phase III trials that are key in navigating the direction of patient care. At present, there are only three FDA-approved immunotherapy options for adult male patients with prostate cancer. These include sipuleucel-T, which is a vaccine made with patients' immune cells that have been stimulated to target the prostatic acid phosphatase (PAP) protein. This is approved for only the subset of patients with advanced prostate cancer. The other two options comprise immunomodulating therapies, including dostarlimab and pembrolizumab. Both of these are immune checkpoint inhibitors that target the PD-1/PD-L1 pathways; these are approved among the subset of patients with DNA mismatch repair deficiency (dMMR), microsatellite instability (MSI-H), or high mutational burden (TMB-H). Notably, the FDA has approved six drugs since 2017 which have histology-agnostic indications of interest in metastatic castration-resistant PC [24]. These include pembrolizumab (tumors with dMMR/high MSI), dostarlimab (dMMR tumors), entrectinib and lartotrectinib (tumors with neurotrophic tyrosine receptor kinase fusions), and trametinib combined with dabrafenib (tumors with BRAF V600E mutations) [24].

Aims and Objectives
While three immunotherapies are approved for prostate cancer and are being administered among patients that fulfil the criteria of administration, the aims and objectives of this systematic review are to collate evidence for patients with any stage/grade of prostate cancer, being intervened either with immunotherapy alone or in combination compared with control/standard care. There are three primary outcomes of interest; these include progression-free survival (PFS), overall survival (OS), and response rate (RR). We will firstly collate and synthesize findings of all completed Phase III clinical trials administering immunotherapy to patients with prostate cancer. Secondly, we will present a current clinical trial index (2022) of all Phase I-III clinical trial records that are ongoing in the field.

Literature Search
To obtain completed Phase III clinical trials, a systematic search was conducted in PubMed/MEDLINE, Embase, Scopus, and CINAHL adhering to PRISMA Statement 2020 guidelines. The search was conducted from inception until 20 November 2022. An additional search was conducted in Elsevier, BMJ, JAMA, NEJM, and The Lancet to locate relevant literature; this methodology is referred to as handsearching and is utilized to identify any additional randomized, controlled trials administering immunotherapy to patients with prostate cancer. To identify ongoing prostate cancer immunotherapy clinical trials in Phase I-III, a systematic search was conducted in ClinicalTrials.Gov and the World Health Organization's International Clinical Trial Registry Platform (ICTRP); both engines were searched until 20 November 2022. A combination of the following keywords was applied across the databases and search engines: immunotherapy, prostate, cancer, neoplasm, carcinoma, clinical, and trial. The search string is attached in the Supplementary Materials. Gray literature was not included in this study. The PICO framework for this systematic review is as follows: Participants: Adult patients with prostate cancer; Intervention: Any form of immunotherapy; Comparator: Standard care (chemotherapy, radiotherapy, surgery) or placebo; Outcome: Any form of survival, progression, responder rate, adverse events, or other treatment outcomes.

Eligibility Criteria
This study is divided into two parts. The first is a systematic assessment of Phase III completed clinical trials. The second is Phase I-III ongoing clinical trial records, presented systematically as an index for readers.
Clinical trials were the only study and record type that were considered for this study. No language restrictions were placed. All non-English-language studies were translated into English using Google Translate. Cohorts, case controls, case reports, brief reports, systematic reviews, and meta-analytical studies were omitted.
The participants were male adults, with prostate cancer at a local, metastatic, or any stage of progression, being intervened with immunotherapy alone or in combination with standard-of-care therapies, with outcomes of survival, progression-free disease, adverse events, or other key indicators or prognosis of treatment.

Study Selection
The title and abstract screening in addition to the full-text screening was led by two mid-career researchers (Z.S. and A.S.) independently. Any disagreements were resolved through discussion with a third researcher (I.C.-O). The data extraction was performed by all researchers and was rechecked independently by Z.S. in the shared spreadsheets, which were first tested and adapted on sample studies. The studies' bibliographic data was stored in EndNote X9 (Clarivate Analytics). The reference management software employed in this study was Mendeley (Elsevier, Amsterdam, The Netherlands).

Data Extraction
The data for completed clinical trials were extracted as number, author and year, title, journal, phase, design, inclusion criteria, intervention, primary outcome measures, follow-up, sample size, efficacy outcomes, and remarks.
For ongoing trials, the data were extracted in two parts. The first part comprised NCT number, status, conditions, interventions, and outcome measures. The second part The bias among the completed clinical trials was assessed using Version 2 of the Cochrane risk-of-bias tool for randomized trials (RoB 2). The RoB 2.0 assessment comprises the following five domains of bias: randomization process, deviations from intended interventions, missing outcome data, measurement of the outcome, and selection of the reported result. Domain-level judgments about risk of bias were classified as low risk of bias, some concerns, and high risk of bias. The traffic light plot of bias assessment and the weighted summary plot of the overall type of bias are illustrated in Section 3.3: risk of bias synthesis.

Protocol Registration and Role of Funding
The protocol of this systematic review was registered with Open Science Framework (OSF): osf.io/4vs7w. No funding was obtained.

Results
During the identification of studies via databases, a total of 3808 studies were identified, of which 467 duplicates were removed. A total of 3341 studies were screened with titles and abstracts, after which 3135 met the exclusion criteria. Finally, 206 full-text studies were assessed, of which four Phase III clinical trials were included in this systematic review. During the identification phase of clinical trial records, a total of 318 were identified from websites. Of these, 208 records were sought for retrieval and assessed for eligibility. Of them, 68 records were included in this systematic review. The PRISMA flowchart depicting the study selection process is illustrated in Figure 1.

Phase III Clinical Trials
Four Phase III trials of immunotherapy for prostate cancer were included [25][26][27][28]. A total of 3588 participants were enrolled across all trials. The designs were randomized and controlled with standard-of-care approaches in all of the included studies. The choice of interventions comprised autologous dendritic cell-based immunotherapy (DCVAC),

Phase III Clinical Trials
Four Phase III trials of immunotherapy for prostate cancer were included [25][26][27][28]. A total of 3588 participants were enrolled across all trials. The designs were randomized and controlled with standard-of-care approaches in all of the included studies. The choice of interventions comprised autologous dendritic cell-based immunotherapy (DCVAC), intravenous ipilimumab therapy, personalized peptide vaccination, and PROSTVAC (a vaccine). Individual trial findings are further described below and are tabulated in Table 1. Vogelzang and colleagues identified the efficacy and safety of autologous dendritic cell-based immunotherapy (DCVAC) among metastatic castration-resistant prostate cancer with a castration period of over 4 months [25]. DCVAC was an add-on and maintenance given every 3-4 weeks for up to 15 doses. The primary outcome measure was overall survival. The Phase III double-blind, parallel-group, placebo-controlled, randomized trial enrolled 1182 participants with a follow-up period of 58 months. The trial did not meet its outcome measures given that there were no differences in median OS between DCVAC and placebo groups reported at 23.9 months and 24.3 months, respectively. With an HR of 1.04, there was no notable difference in the likelihood of death in either group.
Fizazi et al. conducted a final analysis of their Phase III trial, which administered ipilimumab intravenously post bone-directed radiotherapy or among non-progressing prostate cancer patients [26]. With a total of 799 patients enrolled, the patients were followed for 2.4 years, with a single primary outcome of overall survival. The overall survival rates were higher in the ipilimumab group compared to placebo at 2 (25.2% and 16.6%) and 5 years (1.9% and 2.7%), respectively. One caveat is that 1.8% of patients in the ipilimumab group and 0.3% in the placebo group died due to study drug toxicity.
Noguchi et al. (2021) conducted a randomized, double-blind, placebo-controlled trial of personalized peptide vaccination for castration-resistant prostate cancer after receiving docetaxel [27]. The 310 patients enrolled in the trial were HLA-A24 positive and were stratified as aged less than and more than 75 years. The primary outcome measure of increasing overall survival in the vaccine group was not met, with 16.1 months with intervention and 16.9 months in the standard care group.
Gulley et al. (2019) conducted a Phase III trial of PROSTVAC, a vaccine designed to enable the immune system to recognize and attack prostate cancer cells [28]. PROSTVAC (250 ug, lyophilized) was combined with GM-CSF in one arm; PROSTVAC was given alone in the second arm; the third arm received a placebo. The primary outcome measure was overall survival, with follow-ups made until 25 weeks. None of the active treatment arms yielded an effect on median overall survival rates, with 34.4 months for the first arm, 33.2 months for the second arm, and 34.3 months for the placebo.

Risk-of-Bias Synthesis
On noting the bias arising from the randomization process, all four RCTs had low concerns. The risk of bias due to deviation from the intended intervention was low in all of the included studies. On assessing bias due to missing outcome data, two RCTs had some concerns, whereas two had low concerns. When noting bias in the measurement of the outcome, all RCTs had low concerns. For bias in the selection of the reported result, three RCTs had low concerns whereas one study had some concerns. Overall, three RCTs had low concerns for risk of bias while one RCT had some concerns (Figure 2).           concerns. The risk of bias due to deviation from the intended intervention was low in all of the included studies. On assessing bias due to missing outcome data, two RCTs had some concerns, whereas two had low concerns. When noting bias in the measurement of the outcome, all RCTs had low concerns. For bias in the selection of the reported result, three RCTs had low concerns whereas one study had some concerns. Overall, three RCTs had low concerns for risk of bias while one RCT had some concerns ( Figure 2).

Figure 2.
Risk-of-bias assessment of RCTs using the ROB-2 tool. Traffic light plot of study-by-study bias assessment. Weighted summary plot of the overall type of bias encountered in all studies [25][26][27][28].

Discussion
In this systematic review, a total of four Phase III trials administering immunotherapy to patients with prostate cancer were included. A total of 3588 participants were polled across these trials being administered DCVAC, ipilimumab, personalized peptide vaccine, and the PROSTVAC vaccine. Thus far, promising results of overall survival were seen with ipilimumab therapy (25.2% overall survival in the intervention group compared to 16.6% in placebo) [26]. A total of 68 ongoing trials were tabulated and thereby Figure 2. Risk-of-bias assessment of RCTs using the ROB-2 tool. Traffic light plot of study-by-study bias assessment. Weighted summary plot of the overall type of bias encountered in all studies [25][26][27][28].

Discussion
In this systematic review, a total of four Phase III trials administering immunotherapy to patients with prostate cancer were included. A total of 3588 participants were polled across these trials being administered DCVAC, ipilimumab, personalized peptide vaccine, and the PROSTVAC vaccine. Thus far, promising results of overall survival were seen with ipilimumab therapy (25.2% overall survival in the intervention group compared to 16.6% in placebo) [26]. A total of 68 ongoing trials were tabulated and thereby discussed. These trials were currently pooling 7923 participants worldwide, spanning completion until June 2028.
The past decade has led to the development of immune checkpoint inhibitors (ICIs) for prostate cancer [29,30]. While numerous Phase III clinical trials have provided mixed prognostic findings, ICIs-including pembrolizumab, approved by the FDA in 2017-have been utilized in clinical trials, but have only prevented DNA repair in less than 5% of men with advanced prostate cancer [31][32][33]. Therapeutic cancer vaccines, including sipuleucel-T, PROSTVAC, and personalized peptide vaccines, have not led to significant survival differences in patient populations [34][35][36]. Newer trials combining vaccines and other agents, the immune response, and ICIs may be able to downgrade the tumor defenses against T cells [37][38][39][40]. Sipuleucel-T did, however, lead to differences in T cells that were thrice activated in vaccinated patients as compared to placebo groups; therefore, the vaccine may prime patients' immune response [41][42][43][44]. The PROSTVAC (PSA-TRICOM) vaccine was another variant utilizing the poxvirus to deliver genes to spur molecular production of T cells and improve the targeting of PSA [28,[45][46][47]. However, the Phase III trial's findings in 2019 were unfavorable in infiltrating the tumor, despite generating an immune response [28]. PROSTVAC is currently being tested in men with locally advanced prostate cancer along with PD1 inhibitors [48][49][50][51][52]. A small cohort of a clinical trial in progress has revealed that two out of six participants showed PSA level reductions by more than 90% and one of six participants showed no evidence of disease during the 5 years [53]. The evidence suggests that combination immunotherapy increased CD4+ T cell density in the invasive margin with similar trends noted in the intratumoral and benign compartments [53]. The CD8+ T cell density also increased in the benign and invasive margins. T regulatory cells were present in low frequencies in the tumor immune microenvironment, and the Ki67 tumor cells dropped after treatment, suggesting that combination may control tumor growth [53]. The neoadjuvant PROSTVAC and nivolumab may lead to increased infiltration of immune cells [54][55][56]. The combination is being tested to control prostate cancer growth [53].
Other combinations of vaccines including the mRNA variant are being tested with ICIs, including cemiplimab, which is currently approved for skin cancer [57][58][59][60]. mRNA vaccines are also being combined with androgen receptors and with pembrolizumab [61][62][63]. Other trials have combined PROSTVAC with ipilimumab, a monoclonal antibody that targets CTLA-4, which is a protein located on regulatory T cells and can deactivate other T cells [45,47,61,64]. Experimental testing has also steered efforts in adding a third modality of a cytokine, interleukin-15, to target immune signaling to target natural killer cells [65][66][67][68]. The QuEST1 study showed that the triple-hit approach of BNVax (a therapeutic poxviral vaccine targeting brachyury), anti-PD-L1 monoclonal antibodies, and interleukin-15 superagonist complexes have eradicated traces of bone detectability of bony metastasis in two patients with metastatic disease [69]. The tripartite therapy is experimental and the QuEST1 study interrogated the safety and efficacy of immunotherapy combinations for CRPC [69].
The combination of ICIs and vaccines is not the only modality of current immunotherapy paradigms. CAR-T cell therapy is also being deployed in the early clinical trial setting; it comprises T cell extraction from the patient and engineering to target specific cancer cells and reverse administering them to the individual [70][71][72][73]. The modality has been successful in cancers of hematological origin [74][75][76]. CAR-T cell therapy is being tested in prostate cancer research centers [77][78][79][80]; a recent report identifies 13 patients being treated with engineered CAR-T cells to target prostate-specific membrane antigens (PSMA) [78]. While PSMA is rarely found in many tissues, it is located near 80% of prostate cancer cells and increases in prevalence as cancer progresses. Three of the 13 participants had a 30% reduction in PSA levels; however, five patients experienced cytokine release syndrome, which is an inflammatory reaction to treatment; one patient died [78]. Another trial was halted due to the neurotoxic side effects of CAR-T cell therapy [81][82][83]. This has led to the consideration of selectively injecting CAR-T cells into the tumor directly as compared to system administration, which has led to mostly adverse outcomes [84,85].
Another treatment modality is the bi-specific T cell engagement (BiTE), which are monoclonal antibodies with two hooks [86][87][88]. One hook is for the protein outside the tumor cells whereas the second hook is for the T cell surface receptor, CD3; BiTE brings the two cells together. BiTE is currently under investigation with acapatamab (AMG 160), with response rates to Phase I trials approaching 33% [89]. The modality is being combined with PD1 blockers and hormone therapy. However, a common adverse event is cytokine storm syndrome, which is the double-edged sword of immunotherapeutic treatment [90][91][92]. Newer formulations of molecules with lower affinity for CD3 may help in overcoming the cytokine storm among patients [93,94].
For patients to receive beneficial immunotherapies, the patient groups must be segregated based on the immunogenicity of individual diseases. The consensus is that prostate cancer may respond to immunotherapy approaches once the patient populations are personalized; this has been noticed in skin, kidney, and breast cancers, but has not been the present reality of prostate cancer. Immunotherapy is also believed to only work among a small group of men whose tumors fit narrow inclusion criteria based on molecular and pathological factors. However, once an effective combination is tested in combination regimens, the therapy can reach a larger scale, insofar as adverse events, including neurotoxicity and cytokine storm-like responses, have hindered scalability. Another caveat is that immunological interventions have largely been administered to patients with advanced disease only; however, with the progression of the disease, the T cells decrease in count. It may be worthwhile to deploy immunotherapy at an earlier stage of the disease or immediately after radiation or surgical interventions. Radiotherapy may also act as a primer of the immune system, thereby allowing immune responses to be more effective. The consideration of administering immunotherapy before ADT is also existent. Immunotherapy may lower testosterone levels, allow T cells to circulate in the prostate gland, release inflammatory cytokines, and reduce the need for hormone therapy altogether. The decision can be reviewed if immunotherapy does not work; the first choice of hormone therapy typically leads to fatigue, weight gain, and muscle loss. One Phase II trial of pembrolizumab and enzalutamide (androgen-receptor blocker) presented exceptional responses in five out of 20 participants, despite body metastasis present in two of the responders to treatment [95]. Therefore, immunotherapy approaches must ideally target bone tumors as well.

Limitations
Our study has certain limitations that future studies must address. Firstly, given the nature of this study, the number of completed clinical trials is relatively small. Secondly, our criteria were to only include clinically relevant Phase III trials to make them useful for the patient population. To address this, we included all ongoing trials being conducted in the arena of prostate cancer and immunotherapy. Lastly, we utilized Google Translate during the study selection process to screen and include studies; this was in lieu of using interpreters specialized in medical research.

Future Directions
Emerging evidence points towards cytokines and chemokines as key players of the pleiotropic actions of PC-such as angiogenesis, growth, endothelial mesenchymal transition, leukocyte infiltration, and hormone escape for advanced cases. As a result, the chemokine system and immune cells are key targets to be scaled in suppressing tumorigenic environments while serving as potential immunotherapy for prostate cancer [96]. There has been sanguinity towards prostate cancer immunotherapy based on small-scale clinical trials. The recent development of CAR-T therapy has also revolutionized the treatment of resistant malignancies, with many studies underway utilizing this technology in treating solid tumors [97]. It is yet to be determined if immunotherapies either alone or in combination can lead to remission in patients with advanced prostate cancer. There is cautious optimism about the path ahead.

Conclusions
Completed trials using immunotherapy with vaccines and immune checkpoint inhibitors have so far been unable to make a breakthrough in the treatment of patients with advanced prostate cancer. Proof-of-concept studies, however, have shown success among select responders by inducing immunologic responses. Immunotherapy is an emerging option for treating patients with prostate cancer. Various obstacles have been noted with current immunotherapies, including mRNA vaccines, CAR-T cell therapy, and PD-1 blockers. Overall, ICIs, and neo-and adjuvant therapies form a large part of the emerging landscape. The timing of commencing immunotherapy has also led to baffling findings. With 68 ongoing trials of immunotherapy and prostate cancer, the characteristics and premises of the prospective findings will be key in improving outcomes in the near future.