1. Introduction

Prostate cancer (PCa) is the most commonly diagnosed malignancy among men and the second most common cause of cancer-associated death in industrialized nations ^[1]. The standard primary treatments for localized PCa are radical prostatectomy or radiation therapy (RT) with or without androgen-deprivation therapy (ADT), while the primary treatment modality for PCa patients in the advanced setting is ADT. Despite initial therapeutic responses with ADT, the majority of patients are destined to progress to metastatic castration-resistant PCa (mCRPC) ^[2]. Therefore, novel therapeutic approaches with durable response rates for advanced disease are warranted.

Current agents approved by the US Food and Drug Administration (FDA) that have shown efficacy in terms of overall survival (OS) in patients with mCRPC include docetaxel, cabazitaxel, radium-223, sipuleucel-T, and androgen receptor axis-targeted agents including abiraterone and enzalutamide ^{[3][4][5][6][7][8]}. Since the approval of sipuleucel-T in 2010 ^[4], there have been significant advancements and innovations in the field of immunotherapy. However, with the exception of sipuleucel-T, no other immunotherapeutic treatment has been approved for the treatment of mCRPC, owing to limited response rates and modest clinical efficacy. Nevertheless, with the introduction of next-generation genomic diagnostic platforms and advancements in our understanding of the molecular pathophysiology, we are now observing a considerable shift in the paradigm of immunotherapy for the treatment of PCa.

This review will highlight the re-emergence of immunotherapeutic approaches in the treatment of PCa, mainly focusing on immune checkpoint inhibitors (ICIs), which have shown the potential to revolutionize the treatment of both localized and advanced PCa.

2. Pathophysiology of Prostate Cancer

The pathogenesis of PCa is a slow, ongoing progression that involves small developing cancers that gradually evolve into different clonal entities with various clinical outcomes ^[9][10][11]. Previous studies have shown that chronic inflammation is frequent in the prostates of elderly men and that this change is associated with an increased risk of PCa development ^[12][13]. The underlying mechanisms of chronic prostate inflammation and its clinical relevance to the development of PCa are unclear. Nevertheless, clinical evidence shows that chronic inflammation is a potential risk factor for disease progression and poor clinical outcomes ^{[14][15][16][17]}.

Cellular pathways that modulate the proliferation, migration, and survival of cancer cells are upregulated through cytokines and signaling molecules, including tumor necrosis factor-alpha (TNF-α), transforming growth factor-beta (TGF-β), C-C motif chemokine ligand 2 (CCL-2), and interleukins (IL-2, IL-6, IL-8, and IL-10) ^[17]. T cell activity and the associated autoimmune reaction induce epithelial and stromal cell proliferation. Infiltrating B cells are associated with increased antibody production, and recent studies have reported that specific subsets, namely regulatory B cells, may also play a significant role in tumor progression ^[18][19][20]. Moreover, increased B cell infiltrates have been observed in PCa ^[21]. Although T and B cells are the immune cell types that are predominantly observed in PCa, various ILs and inflammatory cell cytokines that dwell in the tumor stroma, such as granulocytes, macrophages, natural killer (NK) cells, myeloid-derived suppressor cells (MDSCs), and monocytes, additionally play an essential role in promoting the autocrine or paracrine proliferation of PCa cells ^{[22][23][24][25]}.

Along with the aforementioned immune cells, androgen receptors (ARs) are a crucial factor in the tumorigenesis of PCa. The AR is a ligand-dependent transcription factor that is densely populated in thymic epithelial cells (TECs) and is essential for the development and function of male accessory sex organs ^[26][27]. ARs have a crucial role in the proliferation of PCa cells. The downregulation of gene expression required for regular TEC activity results in decreased thymocyte proliferation and an increase in apoptosis, which ultimately leads to thymic involution ^[27]. AR signaling directly affects the activity of circulating T cells by increasing the transcription of the protein tyrosine phosphatase non-receptor type 1 (PTPN 1). This downregulates the Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling pathway and subsequently suppress T helper type 1 (Th1) cell differentiation ^[28]. Moreover, androgen cessation can inversely affect the involution of the thymus by upregulating thymocyte proliferation and differentiation and can subsequently stimulate T cell infiltration ^[29][30]. Evidence from numerous studies has shown that PCa cells consist of a highly reactive stroma, which is featured by upregulated T cell infiltrates densely populated with regulatory T cells that have immunosuppressive tendencies ^[31][32].

As integral members of the innate immune system, myeloid cells can be stratified into two individual groups. The first group consists of mononuclear cells such as dendritic cells, macrophages, and monocytes, and the second consists of polymorphonuclear cells, which include basophils, eosinophils, mast cells, and neutrophils. Many studies have shown a tumor-promoting function of terminally differentiated myeloid-derived cells, as they can promote cancer cell proliferation and migration, tumor angiogenesis, and immune suppression ^{[33][34][35][36][37]}. In the PCa microenvironment, the cessation of androgen induces CCL2 upregulation and activation of the STAT 3 pathway, which initiates the recruitment of macrophages and induces polarization of M2 macrophages^[38][39][40]. Activated M1 macrophages have the ability to promote a Th1 response and can exert an efficient antigen-presenting function with the capability of terminating tumors. However, by initiating the proliferation of epithelial and stromal cells, neovascularization, and suppression of the immune system, the anti-inflammatory characteristics of M2 macrophages stimulate the Th2 response and tissue remodeling, all of which creates favorable changes in the microenvironment for disease progression ^{[41][50][51][52][53][54][55][56][57][58][59][60][61]}.

3. Immunotherapy Resistance of Prostate Cancer

Various clinical trials of PCa immunotherapy have been targeted at metastatic disease, with specific advances being made for novel therapies for CRPC. Patients with metastatic PCa have a dysfunctional and compromised immune system ^[42]. The immunogenicity of the cancer cells is the main determining factor in the immunotherapeutic approach to tumor eradication. Cancer cells are integrated into the self-immune system and, therefore, do not express foreign antigens. However, cancer cells exhibit tumor self-antigens, which can be immunogenic. Tumor neoantigens are the result of somatic mutations that are amassed in dividing cancer cells and are clinically correlated with the degree of mutation ^[43]. The load of tumor mutation has been associated with clinical outcomes of immunotherapy; however, tumor immunogenicity is influenced by numerous components that are regulated by the cancer cells ^{[44][45][46][47][48][49]}.

Patients with metastatic PCa are known to have disrupted cellular immunity as well as a tumor microenvironment with increased immunosuppressive qualities. The compromised immune system of patients with PCa is characterized by a reduction in NK cell activity and renewal and inhibited expression of CD3 in NK and T cells, which may ultimately result in the reduction of T cell receptors and NK cell-activating receptors [50–52]^[50][51][52]. Moreover, patients with mCRPC also have a reduced number of total T cells ^[53] and an increased number of myeloid suppressor cells and regulatory T cells in the tumor microenvironment and circulation [54–56]^[54][55][56]. Another potential explanation for the resistance and tolerance to immunotherapy in PCa is the slow disease progression [57,58]^[57][58]. The low mutational tumor burden in patients with PCa may contribute to de novo immunotherapy resistance (Figure 1) ^[59]. This view, however, is still under debate, as a recent genomic analysis indicated that patients with PCa had a higher tumor mutation burden than patients with renal cell carcinoma ^[60].

Further research is warranted to elucidate the exact pathological mechanism underlying the resistance of patients with PCa to immunotherapy to pave the way for an effective immunotherapeutic regimen for metastatic PCa.

Figure 1. Factors contributing to resistance to immunotherapy. Several potential tumor-related, host-related, and environmental factors may elicit resistance to immunotherapies.

4. Immune Checkpoint Inhibitors for the Treatment of Prostate Cancer

The development of ICIs has been one of the most dynamic advances during the last

decade. To date, sipuleucel-T, a cancer vaccine based on autologous antigen-presenting cell (APC)

immunotherapy, is the only immunotherapeutic agent that is FDA-approved for the treatment

of PCa [4]. ICIs, which are antibodies inhibiting the immune checkpoint receptors, cytotoxic T

lymphocyte-associated protein 4 (CTLA-4), and programmed death 1/programmed death ligand 1

(PD-1/PD-L1), have shown potential therapeutic benefits by producing antitumor effects and long-term

survival benefits in a broad spectrum of malignancies (Figure 2) [61–63]. However, numerous clinical

trials in patients with mCRPC utilizing CTLA-4 or PD-1/PD-L1 inhibitors have been unsatisfactory,

with limited survival benefit when administered as a monotherapy in unselected patients [64–68].

The sole exception is the FDA-approved usage of pembrolizumab for all malignancies with mismatch

repair (MMR) deficiency or a microsatellite instability (MSI)-high status [69,70]. To overcome the

therapeutic limitation, combination regimens consisting of ICIs with other modalities, or in a specific

subset of patients who would most likely benefit, have resulted in better treatment ecacies in patients

with a specific subtype of mCRPC.

Figure 2. Molecular mechanisms of CTLA4 and PD-1 attenuation of T cell activation. Schematic

diagram of the molecular interactions and downstream signaling induced by ligation of CTLA4 and

PD-1 by their respective ligands. APC: antigen-presenting cell, CAR: chimeric antigen receptor, CTLA-4:

cytotoxic T lymphocyte-associated protein 4, PD-1: programmed death 1, PL-L1: programed death

ligand 1, TCR: T cell receptor.

4.1. ICI Monotherapy

Ipilimumab, a monoclonal antibody blocking the immune properties of CTLA-4, was the first

eective ICI to show potential ecacy for melanoma [71]. The therapeutic success of ipilimumab and

its FDA-approval in 2011 triggered an interest in its use in PCa. An open-label phase I/II multicenter

clinical trial (NCT00323882) investigated ipilimumab with or without RT in patients with mCRPC [72].

After stratifying the patients into two individual dose escalation arms, the study endpoints included

adverse events (AE), prostate-specific antigen (PSA) decline, and tumor response. A select number of

patients in both study groups exhibited antitumor activity as measured by PSA response. Eight (16%)

patients in the combination arm (10 mg/kg) showed a PSA decline >50%, while one (2.0%) patient

showed a complete response (CR) lasting 11 months. Six (12.0%) patients had stable disease which

lasted for 2.8 to 6.1 months [72]. Encouraging clinical evidence of antitumor activity in previous studies incited further examination of ipilimumab in several phase III clinical trials. In the CA184-043

phase III trial (NCT00861614), patients with mCRPC who had at least one bone metastasis following

taxane-based chemotherapy were allocated to either ipilimumab or placebo following palliative RT

every three weeks [67]. Although the study barely missed the primary endpoint in terms of OS between

the ipilimumab group and the placebo group (HR 0.85, CI 0.72–1.00; p = 0.053), progression-free

survival (PFS) was significantly superior in the ipilimumab group. Moreover, subset analysis revealed

that ipilimumab was the most therapeutically ecacious in the subgroup of patients who exhibited

favorable prognostic characteristics, including normal serum alkaline phosphatase levels, normal

hemoglobin concentration, and no visceral metastases. The OS outcomes in this subgroup of patients

were superior with ipilimumab compared to placebo [67]. In a recent randomized, double-blind

phase III trial (NCT01057810), ipilimumab monotherapy was compared to a placebo in patients with

asymptomatic or minimally symptomatic chemotherapy-naïve mCRPC [65]. The median OS was

28.7 months in the ipilimumab group and 29.7 months in the placebo group (p = 0.367). Median PFS

was 5.6 months in the ipilimumab group and 3.8 months in the placebo group (HR = 0.67; 95.87% CI

0.55–0.81). This trial concluded that ipilimumab failed to prolong OS in patients with mCRPC.

However, after observing a longer median PFS in the ipilimumab group compared to the placebo

group (5.6 months vs. 3.8 months) and a decline in serum PSA values, the investigators concluded

that these findings suggest the presence of antitumor activity in a specific subgroup of patients [65].

Indeed, future studies are required to determine how to measure such antitumor activity, preferably

with biomarkers, to identify the subgroup of patients who would most benefit from ipilimumab.

Several studies focused on PD-1/PD-L1 inhibition in patients with metastatic PCa. In a phase Ib

trial of nivolumab (NCT00730639), 296 patients with non-small cell lung cancer, advanced melanoma,

renal cancer carcinoma, or CRPC received nivolumab every two weeks [68]. Favorable objective

response rates (ORRs) were shown in patients with melanoma, non-small cell lung cancer, and renal

cell carcinoma. However, there were no responses observed in 17 patients with CRPC. PCa samples

collected from the patients yielded negative results for PD-L1 expression [68]. Martin et al. focused

on the deficiency of PD-L1 expression in PCa and emphasized another challenge in the ongoing

battle of PCa treatment [73]. Contrary to the phase I nivolumab trial, the non-randomized phase Ib

KEYNOTE-028 (NCT02054806) clinical trial only included patients with PD-L1 expression in 1% of

tumor or stroma cells [66]. Patients received pembrolizumab 10mg/kg every two weeks for up to two

years, with the primary endpoint being ORR. Preliminary results showed an ORR of 17.4%, with 34.8%

of patients exhibiting stable disease. The median period of response was 13.5 months [66]. Similar to

various solid tumor types, more studies would be needed in order to elucidate the prognostic impact

of PD-L1 expression as a biomarker of response to ICIs in patients with PCa.

In general, PCa has a relatively low expression level of PD-L1 [73]. Bishop et al. investigated

whether the targets of immunotherapy, namely PD-1, PD-L1/2, and CTLA-4, are overexpressed in

patients with mCRPC who were resistant to enzalutamide. The results revealed that tumor resistance to

enzalutamide is associated with an upregulation in the expression of PD-L1 [74]. Evidence of antitumor

activity was documented in a phase II study in which the anti-PD-1 antibody pembrolizumab was added

to patients receiving enzalutamide treatment who had progressive disease [75]. Three (30%) patients

experienced a rapid decline in serum PSA level to below 0.2 ng/mL. Two patients with measurable

disease at enrolment both showed a partial response. One of the partial responders was revealed

to be MSI-positive, which validates the observations of previous reports of MSI as a prognosticator

of response to anti-PD-1 antibodies [76]. Although only one patient in the aforementioned trial of

pembrolizumab had PCa, this ICI is now approved for the treatment of patients with MSI-positive

tumors. Table 1 summarizes clinical trials evaluating the clinical ecacies of ICI monotherapies.

4.2. Combination Immunotherapy Regimens

Several studies have investigated the ecacy of anti-PD-1 inhibitors in combination with

various pharmacotherapeutic regimens for the treatment of advanced PCa. The two most promising

combination immunotherapeutics for the treatment ofmCRPCare the combined usage of two individual

ICIs or one ICI with the augmentation of enzalutamide, a novel androgen receptor-axis-targeted agent

approved for mCRPC. The phase II CHECKMATE-650 (NCT02985957) clinical trial administered a

combination of the PD-1 inhibitor nivolumab with the anti-CTLA4 antibody ipilimumab as a secondor

third-line therapy for patients with asymptomatic or minimally symptomatic mCRPC [77]. In this

clinical trial, the study population was stratified into two arms based on the previous administration

of taxane-based chemotherapy. A total of 78 patients were enrolled, with a minimum observational

period of six months. A 10% ORR was observed in the combined ICI arm, which included patients

who previously received chemotherapy. However, a superior ORR of 26% was observed in patients

who were chemotherapy-naïve [77].

In the setting of PCa in which ICI monotherapy has shown limited success, the combination

of nivolumab and ipilimumab showed promising clinical results. Further investigations regarding

this combination would likely explore alternative dosages and administration intervals of both ICIs.

Ongoing studies are determining whether this combination would be eective for a dierent subgroup

of patients. In 2012, an open-labeled phase I dose-escalation trial (NCT01510288) evaluated ipilimumab

in combination with the granulocyte-macrophage colony-stimulating factor-transduced cell-based

allogeneic prostate cancer vaccine (GVAX) [78]. Patients with chemotherapy-naïve mCRPC enrolled in

the study showed promising antitumor activity, with 25% of the patients exhibiting a PSA decline of

greater than 50% and overall good tolerance to the agent [78]. The prospective phase II STARVE-PC trial

(NCT02601014) enrolled patients with metastatic PCa with androgen receptor splice variant 7 (AR-V7)

positive phenotypes [79]. Patients were administered nivolumab 3 mg/kg with ipilimumab 1 mg/kg

every 3 weeks for four doses, and maintenance therapy was performed with nivolumab 3 mg/kg every

2 weeks. For the determination of DNA repair deficiency, targeted next-generation sequencing was

utilized. The outcomes were generally more favorable in the patient population with AR-V7-positive

PCa with DNA repair deficiency. The currently ongoing single-arm phase II NEPTUNES clinical trial

(NCT03061539) is investigating the ecacy of ICI combination therapy in patients with higher tumor mutation burden (TMB) due to a defective DNA damage response (dDDR) or DNA mismatch repair

gene mutations (dMMR) and in patients with high tumor-infiltrating lymphocytes [80].

Improvements in our molecular biology knowledge have shown that the AR signaling pathway is

a critical element in CRPC progression and is, therefore, a rational target for the treatment of advanced

disease and CRPC. Novel combination regimens involving PD-1 inhibitors with enzalutamide have

shown promising results in this clinical field [81]. KEYNOTE-365 (NCT02861573) was a phase Ib/II

umbrella study that tested various combination therapies in patients with CRPC [82]. The study cohort

consisted of 69 patients with mCRPC who have progressed on chemo-naïve abiraterone and were

treated with pembrolizumab and enzalutamide. Primary endpoints were safety and PSA response

rate. The study results revealed an ORR of 20% and a PSA response rate of 33% [82]. A phase II trial

(NCT02312557) enrolled 58 patients with chemotherapy-naïve mCRPC whose disease had progressed

during enzalutamide and received four doses of 200mg IV pembrolizumab every three weeks while

still on enzalutamide [75]. The primary endpoint was a PSA response greater than 50% (PSA50). Five of

28 (18%) patients achieved PSA50, while three of 12 patients (25%) with measurable disease at baseline

had objective responses documented by radiographic imaging [75]. With promising results in the

aforementioned clinical trials, KEYNOTE-641 (NCT03834493), a randomized double-blind phase III

trial, has started accrual of patients with mCRPC as of July 2019 to evaluate the ecacy and therapeutic

safety of the pembrolizumab and enzalutamide combination vs. enzalutamide plus placebo [83].

Table 2 summarizes clinical trials evaluating the clinical ecacies of combination immunotherapies.

4.3. The Utilization of Genomic Selection for Checkpoint Inhibitor Immunotherapy

Another strategy for the use of ICIs in the treatment of PCa focuses on specifically tailored

patient selection. This approach of patient selection first started with colon cancer patients, as these

patients also exhibit limited treatment response to ICI monotherapy [76]. Investigators noted that

patients with dMMR variants of colon cancer have more somatic mutations compared to those with

MMR-proficient tumors and higher immune infiltration [76]. A breakthrough study demonstrated

the clinical importance of MMR gene mutations and analyzed clinical response to pembrolizumab

monotherapy in cancers with and without dMMR variants. Patients with dMMR cancers exhibited an

ORR of 40% and a 12-month PFS rate of 78%, while those with MMR-proficient colorectal cancer showed no objective responses and a 12-month PFS rate of 11% [84]. Based on this finding, pembrolizumab was

FDA-approved in 2017 for the treatment of unresectable or metastatic solid organ malignancies of any

histologic origin exhibiting dMMR or MSI-high expression [85]. This FDA-approval of pembrolizumab

was the first cancer treatment to be approved based on a biomarker instead of the primary site of

origin [85].

According to a clinical genomic analysis reported by Robinson et al., 3% to 5% of advanced

PCa expresses MMR gene mutations, which supports the notion that this disease spectrum could be

susceptible to pembrolizumab [11]. To date, retrospective studies have analyzed the therapeutic ecacy

of PD-1 inhibitors in patients with mCRPC with dMMR mutations. Antonarakis et al. utilized the Johns

Hopkins somatic genomic database for pathogenic loss of function MMR mutations in 13 patients with

a deleterious mutation in the MMR gene. Results showed that six (46%) patients had MSH6 mutations,

while two of four (50%) patients who had received PD-1 inhibitors showed a clinically meaningful

PSA50 [70]. Interestingly, this subset of patients was also observed to be sensitive to standard ADT,

exhibiting a 90% PSA response rate of 85% and a median PSA PFS of 55 months [70]. In another

study, 1551 tumors from 1346 patients with PCa were reviewed. Among 1033 patients who were

eligible for the study, 32 (3.1%) patients with mCRPC exhibited MSI-high or dMMR tumors. Among

these patients, 11 (34.4%) had been treated with a single-agent anti-PD-1 or PD-L1 treatment. It was

observed that six (54.5%) patients achieved PSA50, and four (36.4%) patients showed a radiographic

response [69]. Although the MSI-high/dMMR phenotype is uncommon and the subset of this PCa

population is small, the biomarker-specific approval of pembrolizumab and the clinical evidence of

therapeutic ecacy for this specific subgroup of patients with PCa supports the prospect of tumor

sequencing for MMR deficiency in all patients with mCRPC, as recommended in the 2019 version of

the National Comprehensive Cancer Network Guidelines [86].

Along with the MSI-high/dMMR genetic expression, clinical trials are ongoing to discover other

molecular subtypes that may respond to ICIs. In the open-label phase II KEYNOTE-199 (NCT02787005)

trial, the clinical ecacy of pembrolizumab for patients with chemotherapy-resistant mCRPC was

assessed in three parallel cohorts [64]. Pembrolizumab monotherapy showed a 12% ORR in patients

with somatic ATM or BRCA1/2 mutations, which was higher than the ORR of 4% for patients without

the aforementioned mutations [64]. A comparable trend was observed in the CHECKMATE-650

trial, in which a superior response rate was observed in patients with homologous repair-deficient

mCRPC [77]. The ImmunoProst (NCT03040791) study is a multicenter, single-arm, open-label phase

II trial of nivolumab in patients with mCRPC who have progressed to a previous taxane-based

chemotherapy regimen. The study will analyze germline and somatic DNA repair defects in that

patient cohort [87]. Another multicenter phase II trial (NCT03248570) is investigating the eects of

pembrolizumab in patients with mCRPC stratified according to the presence of defects in DNA damage

repair [88].

There is accumulating clinical evidence that a subgroup of patients with CDK12 inactivation

may benefit from immunotherapy. Due to numerous focal tandem duplications and increased gene

fusion activity, along with an upregulation of neoantigens and T cell infiltration, solid malignancies

with CDK12 biallelic mutations have been suggested to harbor a distinctive immune signature [89].

This observation infers the existence of another potential genomic subtype of PCa that could be

susceptible to ICIs [90]. Various studies estimate that 4.7% of patients with mCRPC harbor this

mutation, making PCa the tumor population with the greatest distribution of biallelic inactivation of

CDK12 among all solid tumors [89,91,92]. Wu et al. reported the ecacy of PD-1 inhibitor treatment in

four patients with mCRPC harboring CDK12 mutations and showed that two (50%) patients showed

antitumor activity in terms of PSA decline [89]. A recently conducted multicenter trial in patients with

advanced PCa with loss of function CDK12 mutations evaluated the ecacies of the poly (ADP-ribose)

polymerase and PD-1 inhibitors. Three of eight (38%) patients with CDK12-mutated mCRPC had a

significant PSA decline, with a median PFS of 6.6 months [93]. The ongoing IMPACT (NCT03570619) trial is investigating whether this subtype of mCRPC is more susceptible to a combination of ICIs

consisting of ipilimumab and nivolumab followed by nivolumab monotherapy [94].

With advances in next-generation tumor DNA sequencing, new subsets of patients with PCa

are being identified through precision medicine. POLE mutations, or mutations in the exonuclease

domain of the proofreading DNA polymerase epsilon enzyme, have been observed in patients with

PCa. These mutation phenotypes were observed despite an MSI-stable signature [95]. Although

POLE mutations are reported to aect only 0.1% of patients with mCRPC, this gene mutation may

have potential therapeutic ecacy with anti-PD1 antibodies due to the vast quantity of predicted

mutation-associated neoantigens [96].

Despite recent advances and achievements of checkpoint inhibition and other modalities of

immunotherapy, inconsistencies continue to occur between treatment and response across the dierent

tumor spectra. PCa develops complex genotype mutations that significantly aect the outcome of

immunotherapy [97]. The classification and identification of patients who will successfully respond

to immunotherapy is of paramount importance and is dependent on the future development of

biomarkers. It is crucial to establish reliable biomarkers that can aid the initial decision to determine

which individual group of patients will be more responsive to a specific mode of immunotherapy.

Clinical trials focusing on genomic and DNA transcription data are ongoing to unravel immune

signatures to aid the prediction of therapeutic ecacy and response [97–99].

4.4. Adoptive Cellular Therapy in the Treatment of Prostate Cancer

Adoptive cellular therapy (ACT) is an emerging treatment strategy in immunotherapy, in which

the autologous T cells of the patient are extracted and genetically manipulated prior to reinfusion.

Various approaches for ACT, such as engineered T cell receptors, tumor-infiltrating lymphocytes,

and chimeric antigen receptors (CAR) T cells, have been developed for the treatment of PCa patients

with metastases or in advanced disease settings [100]. Of the aforementioned engineered T cells, CAR-T

cells have shown the most therapeutic potential. These cells are genetically constructed to express

a specific CAR gene ex vivo to target antigens utilizing an antibody derived single-chain variable

fragment (scFv) [101]. After infusion, CAR-T cells activate an inflammatory reaction that results in the

cytolysis of the antigen-expressing tumor cell [102].

In patients with PCa, CAR-T cells are used to target prostate-specific membrane antigen (PSMA),

which is a type II transmembrane glycoprotein that is specifically upregulated in PCa cells [103].

In a phase I clinical trial conducted by Junghans et al., “designer” CAR-T cells targeting PSMA were

continuously infused with a low dose of IL-2 [104]. Three of the five enrolled patients met the 20%

engraftment target of CAR-T cells, and there were no anti PSMA toxicities, while two of the three

engraftment successful patients exhibited a PSA50 response [104]. In another phase I trial, the safety

and dosage of modified autologous PSMA targeted T cells was analyzed. The study concluded that a

dosage up of 3  107 cells/kg could be safely administered, with persistence of T cell activity lasting

for up to 2 weeks [105]. Interestingly, one patient experienced a long-term response over 16 months,

which suggests clinical activity [105].Similar to other cellular-based immunotherapies, CAR-T cell therapy elicits limited therapeutic

responses in the treatment of solid tumors, which includes PCa [101]. The major obstacle in CAR-T

cell therapy is the tumor microenvironment, which has immunosuppressive properties. To overcome

this hindrance, a PSMA directed/TGF- insensitive CAR-T cell is undergoing a phase I clinical trial

(NCT03089203) [106]. The distinctive TGF- insensitive features of this cell product have the potential

to show therapeutic ecacy in the immunosuppressive PCa microenvironment caused by high levels

of TGF- [106]. With the occurrence of neoantigens with resistant clones, further clinical studies and

research focusing on next-generation CAR-T cells with multiple scFv targeting multiple tumor-specific

antigens are warranted.

5. Conclusions

Considerable progress has been made in the immunotherapeutic approach to the treatment of

PCa. However, further research is still warranted to fully understand the immunological mechanisms

involved in the PCa microenvironment. For now, the treatment benefits of ICIs show promise and

potential. Along with standard monotherapeutic strategies utilizing novel ICIs, the ecacy of the

combined usage of two individual ICIs and the administration of one ICI with standard therapies

still needs to be elucidated. In the present clinical setting, these regimens will likely be used in some

capacity and show therapeutic benefit in a select group of patients with PCa. In the near future,

there will be other classes of immunotherapeutic agents, which may become an appropriate treatment

for patients with mCRPC. For now, the classification of patients and administration modalities of novel

immunotherapeutic agents remain unsolved. Moreover, studies are warranted to investigate how the

modification in the sequencing of prior agents could aect the ecacy of the novel agents.