Immunotherapy for castration-resistant prostate cancer: has its time arrived?

Abstract

Introduction:

Within the last decade, multiple innovative immune platforms have been developed and tested in patients with metastatic castration resistance prostate cancer (mCRPC) with only one demonstrating a survival benefit. The advent of immunogenomics along with the availability of diverse checkpoint inhibitors provides inroads in treating these patients, in many cases with significant clinical impact but unfortunately not in all patients. How to exploit these novel platforms remains an area of increased interest especially in the setting of new agents that can affect the tumor microenvironment and potentially render a “cold” tumor to become “hot”.

Areas Covered:

This review highlights the current changes and challenges in this field and how to best use our current knowledge for better trial designs in patients with mCRPC.

Expert Opinion:

Recent understanding of the inhibitory milieu within the tumor microenvironment has fostered the use of combinatorial strategies that target not only tumor cells but capitalize on controlling inhibitory cell populations and cytokines that induce a hostile setting for immune cells. Immunogenomics and genomic interrogation of prostate cancers have opened a vista as to how patients’ tumors that can respond to immune agents that previously were thought have minimal antitumor activity.

1. Introduction

There have been significant additions to our knowledge of the landscape of metastatic castration resistant prostate cancer (mCRPC), especially as it pertains to the genomic profile of the disease and now being able to fit a treatment to the mutation. This has opened up an unprecedented clinical trial space now exploring novel drugs that are target-directed not to mention target-effective.

Prostate cancer has remained a challenge for the immunologist. While the first solid tumor to have an approved immune therapy with a survival benefit [1], prostate cancer still is a conundrum as to its limitations as a target for immune therapies. Despite naked DNA and conjugate carbohydrate vaccines and viral platforms with and without prime boosts, no one immune approach has shown significant impact on tumor regression. This has also been true regarding the impact of immune checkpoint inhibitors, for example, the anti-CTLA-4 drug, ipilimumab (Yervoy®) which despite two phase III trials, one in the post chemotherapy [2], the other in the pre-chemotherapy setting [3], did not show survival benefit as per its pre-specified endpoint. However, there appeared to be continued benefit in a small number of patients at the “tail” of the Kaplan-Meier curves. Explanations for lack of immune responsiveness to checkpoint inhibitors for example, have include a lack of a hypermutated state, lack of expression of immune infiltrates within the tumor, ie, “cold” microenvironment; lack of PD1/PD-L1 on the tumor cells, in addition to a paucity of genomic mutations that could render then responsive to therapies. To date, only a handful of tumors are considered “hot” based on the significant presence of cytokines, chemokines, and tumor infiltrating lymphocytes (TILS) and include bladder and kidney cancers, head and neck cancers, hepatocellular carcinoma, melanoma and non-small cell lung cancers. This does not exclude those cancers which have genomic alterations for which agnostic use of checkpoint inhibitors have been approved.

Time has disproven many of the negative theories about prostate’s lack of responsiveness to immune therapies based on new clinical trial data that are not only using novel agents but unique platforms in combination with biologic agents. A review of the new prostate landscape is now in order.

2. Areas covered

2.1. Prostate cancer is an immune desert.

This has been the observation made by many in that unlike colorectal tumors where an immunoscore can be derived that evaluates the immune infiltrates of the cancer; prostate has never shown a significant population of cells within the actual tumor or tumor/stromal interface. In addition, the immune desert showed upregulation of the glycolytic and aldose reductase pathways to create a metabolically hostile microenvironment in which T cell function is profoundly suppressed.[4] T cells infiltrating the tumor have repressed mitochondrial activity and biogenesis that leads to loss of metabolic sufficiency, a state that cannot be rescued by PD-1 blockade therapy alone.[5] Recent findings by Giunchi et al [6] acknowledge that metabolic alterations play a likely role in prostate cancer and that these observations can lead to the discovery of unique signatures that can be leveraged for the development of novel therapeutics. The authors suggest that there is more than sufficient evidence to establish a relationship between metabolic pathways and oncogenes, in particular, multiple metabolic enzymes have been found to be regulated by oncogenes(Figure 1). As such, direct or indirect blockade of metabolic pathways an impact on cancer cell survival [6–8]. The authors provide further support in their own observations that showed that oncogene-associated metabolic signatures in prostate cancer were supported by the idea that PI3K activation results in a glycolytic phenotype, whereas MYC can induces aberrant lipid metabolism resulting in variable heterogeneity.[6] Patterns of metabolic flux along with inhibitory pathways and may contribute to a “hostile” tumor microenvironment. Not to be ignored are inhibitory cell populations including macrophages and fibroblasts, the latter being the most abundant with the microenvironment and which metabolically sustain cell survival, growth, immune and inflammatory cellular recruitment and promote cancer cell “stemness” along with EMT. As noted by Chiarugi, et al [9], interrupting the crosstalk between both fibroblasts and other cells within the environment could lead to ultimately “drugging” specific stromal cell populations that could lead to a less hostile and more infiltration rich environment.[9] Similar observations regarding changes within the metabolic signature within the cancer microenvironment have been made by others.[10]

Figure 1:

Pathways and enzymatic regulation in fatty acid synthesis and cholesterol metabolism and biosynthesis that can be used as tools that are predictive of a tumor’s behavior and may be considered as potential therapeutic targets. Reproduced from European Urology Oncology, 2, Giunchi F, Fiorentino M, Loda M, The metabolic landscape of prostate cancer, 28–36., Copyright 2019, with permission from Elsevier [6]

The adenosine (ADO) pathway has recently come to be recognized as a potential key “checkpoint” that may be amenable to pathway inhibitors.[11–13] In a manner similar that that seen with the PD1/PD-L1 axis, adenosine can-be co-opted by tumors in order to enhance and promote growth in addition to impair immunity. A number of studies have identified the cellular changes within the TME that occur in response to changes in ATP, the latter waxing and waning as a result of cell death, hypoxia, apoptosis and chronic inflammation.[13–15] Intratumoral adenosine levels can reach high concentrations. High expression within the TME of ectonucleotidases such as CD38 and CD73[15], both of which have been overexpressed on the surface of tumor cells and even cancer-associated fibroblasts, stromal cells, myeloid-derived suppressor cells and tumor-associated macrophages [15–18] have been shown to be associated with poor clinical outcome. Mechanistically, it is thought that adenosine in the TME facilitates the ability of tumor cells to escape immune surveillance and detection via common immune cellular infiltrates including macrophages, T cells dendritic and NK cells immunosuppressive cells types that can serve as a shield while concurrently fostering a complementary functioning group of cells that are inhibitory (Figure 2). This includes Tregs and myeloid-derived suppressor cells (MDSCs). Adenosine has “pleiotropic” functions, facilitating neovasculature and re-channeling neighboring fibroblast function to provide ancillary support. In other words, ADO has a multiplicity of functions that affect tumor cells to survive, proliferate, migrate and overrun neighboring tissues.

Figure 2:

Illustration of how adenosine is generated within the tumor environment with subsequent suppression of a diversity of immune subsets. Arrows correspond to increased activation or expression; T bars indicate inhibition or decreased activity. Reproduced with permission from [12] under the Creative Commons Attribution License.

2.2. Can the tumor microenvironment be made more hospitable, ie, can a cold tumor become hot?

Several key cell types and soluble factors come to light when considering the active cell types within the intratumoral milieu. These include Treg, Th17, inhibitory macrophages, IL-6, and even RANK-Ligand, the latter produced by activated T cells and osteocytes. Rank-Ligand can also target dendritic cells to promote antigen presentation. [11] The most common goal is trying to improve the intratumoral milieu to make the tumor become the proverbial “hot” or “bland” site. A number of preclinical observations have been offered suggesting that the intratumoral milieu can be “corrected”. [19–22] These have demonstrated that tumors can also disrupt chemokine expression and “deregulate” the immune response. Also noted is that tumors that lack CD8+ T cells also have a reduction in the chemokines necessary for the recruitment of T cells that effect killing. In man, CXCR3 on CD8+ T cells may have decreased production that results in “non-inflamed” or “cold” tumors. It should also be kept in mind that the tumor cells themselves can produce CXCL9 and CXCL10. Observations in pancreatic cancer also confirms the role of these chemokines in contributing to an immunosuppressive environment.

Other theories for the cold environment include lack of or loss of tumor antigens, poor or defective recruitment and maintenance of antigen presenting cells, lack of co-T cell stimulation and activation following antigen processing and presentation as well as suppressive effects that limit the ability of T cells to migrate and invaded the tumor microenvironment.[19] To date, agents exist that could one-by-one address these immunosuppressive characteristics and may offer a means of correcting in a meaningful way the obstacles that prevent immune therapies from working. These include de-methylating agents [19,22] that can reverse the epigenetic silencing of Th1-type chemokines in tumor cells and permit a more favorable milieu for immunotherapy. Other approaches that can “warm up” the microenvironment can include APO inhibitors, concomitant use of radiation with immune agents, including antibodies against immune cytokines, antibody-drug conjugates, vaccines against TGF-B, and a diversity of checkpoint inhibitors. Other strategies to “convert” the cold to hot environment have been successful in other more “immunogenic” tumors even with the less than deemed optimal immune cell infiltrate. An example of this has been demonstrated in melanoma where an approved oncolytic virus talimogene laherparepvec (T-VEC, Imlygic™) [19,23], a genetically engineered herpes virus transduced with a gene for human GM-CSF that is directly injected into an unresectable melanoma has demonstrated real world response rates of up to 88.5% with complete response rates of up to 61.5%.[23] Improvement in the efficacy of this therapy was seen when given in combination with pembrolizumab irrespective of PD-L1 status, CD8+ infiltration or interferon-gamma signature of the tumor infiltrating lymphocytes (24). Other approaches that have shown some evidence of success include the potential knocking out of CXCL1 in tumors leading to increased T cell infiltration and response to immunotherapy. Li, et al [25] have demonstrated in preclinical models that “cold” pancreatic tumor cells be made “hot” by virtue of knocking out CXCL1 thereby resulting in enhanced T-cell infiltration and sensitivity to immunotherapy. This is based on observations that cold pancreatic tumors can make CXCL1 which serves as a recruiter of myeloid derived suppressor cells thereby preventing the necessary anti-tumor infiltrates from entering the intratumoral milieu.

2.2.1. Microbiome versus Microenvironment

A novel view of looking at the internal immune environment comes from work on the urinary microbiome. However, it is important to further define what is meant by each term. “Microbiome” refers to the gut and the residing bacteria, fungi or other microbes and their relationship with the immune system in maintaining health. The “microenvironment” refers to the intratumoral constitution that includes not only tumor cells but immune cells and ancillary cytokines that may contribute to either a suppressive or active anti-tumor environment. Work by Sfanos, et al [26] provide data that prostate cancer is thought to have an “inflammatory” microenvironment with resident innate immune effector cells including macrophages and mast cells with focal infiltrates of neutrophils and other inflammatory cells seen inflamed regions. As the normal prostate transitions to clinical cancer, inflammation potentially acts as a driver of somatic genomic and epigenomic changes, likely predisposing to cancer development with the ultimate findings of loss of basal cells accompanied by proliferative inflammatory atrophy. It remains supposition whether or not the intestinal microbiome may be essential for the potential efficacy of immunotherapeutic agents as it can be postulated that the gastrointestinal microbiome immune modulation is a result of disruption of the intestinal mucosal barrier and that there may be a relationship between oral androgen receptor targeted therapies and the gut albeit unconfirmed. Further work by Massari, et al [27] substantiates a potential relationship between acute and chronic prostate inflammation and their associations with bacteria particularly in the setting of bacterial prostatitis also lays groundwork for a microbiome-microenvironmental relationship.

There are no relevant data published to date on the further characterization of the prostate microbiome except that as noted in preclinical models. It is believed that prostate cancer patients may have a highly interconnected relationship between the microbiota and the microenvironment; there remains continued speculation regarding just how the microbiome can impact on prostate cancer development and susceptibility to treatment.

2.3. Immunogenomics to define susceptibility to immunotherapies.

More recently, several clinical trials have suggested that prostate cancer is sensitive to checkpoint inhibitors in the absence of an agnostic indication, ie, no evidence of BRCA or MSI^hipositivity. Responses have largely been in patients with castrate-resistant metastatic prostate cancer who had these genomic alterantions as have been seen in isolated cases [26].[28] Recent work by Wu, et al [29] revealed the potential importance of profiling patients’ tumors to assess for genomic alterations that may be actionable with the right drug combinations. Prostate cancer has been shown to have DNA repair deficiencies [30–32], including homologous recombination-deficient (HRD) and mismatch repair-deficient (MMRD) changes along with a variety of mutations including ETS fusions and mutations in SPOP along with focal tandem duplications (FTD) that can lead to increased gene fusions with resulting variable gene expressions.[30–32] Among those prostate cancers with FTD, there was an associated CDK12 loss that resulted in gain of genes that were associated with the cell cycle as well as DNA replication. A meticulous analysis of prostate tumors led to the characterization of bi-allelic loss of CDK12 representing a potentially new group of patients with lethal prostate cancer. While these tumors showed genomic instability, nevertheless, of interest if the immunologic phenotype of these tumors resulting from an increased neoantigen burden (Figure 3). More importantly, the findings indicate that these tumors are inherently more immunogenic given the response of these tumors to pembrolizumab. As such, these finding pave the way to examining tissues for novel genomic alterations that may change the inherent immunologic landscape of a patient’s tumor rendering it more susceptible to unique targeted agents. Additional insight into the potential for prostate cancer to respond to checkpoint agents was reported as an update by Antonarakis, et al in the KEYNOTE 144 trial.[33] Pembrolizumab demonstrated anti-tumor activity and disease control in patients who had previously received prior androgen-signaling inhibitors and at least 1 or 2 prior chemotherapies including docetaxel. Three patient groups were studied: group 1, PDL-1+; group 2, PD-L1-negative, and group 3, non-measurable, bone-predominant disease. At the time of reporting, median overall survival was 9.5 months in group 1, 7.9 months in group 2 and 14.1 months in group 3. While still immature, this suggested that neither PD-1 nor PD-L1 expression played a role in treatment responsiveness. [33]

Figure 3:

Immunohistochemistry on fixed tumor specimens using anti-T cell (CD3) antibody. Six cases discussed by Wu, et al including two CDK12 mutant tumors, one MMRD (mismatch repair deficient) tumor, and three tumors that were wildtype for CDK12, MMR (mismatch repair deficient) genes, and HR (homologous recombinant) genes. Reproduced via open access and permission of creative commons. Reproduced from Cell, 173/7, Wu Y-M, Cieslik M, Lonigro R et al, Inactivation of CDK12 Delineates a Distinct Immunogenic Class of Advanced Prostate Cancer, 1170–1782, Copyright 2018, with permission from Elsevier [29].

2.4. Update on Role of Cellular Therapies for Prostate Cancer.

CAR T cells have sufficient rationale for their continued development. Efforts continue to develop CAR T cells that are not only specific for tumor-associated antigens expressed on prostate cancer cells but area able to proliferate, migrate, and home to the site of tumor. Issues remain about the stability of the constructs given differences in the vectors used to genetically engineer the cells as well as how to deliver these cells to the intratumoral milieu. Often observed in preclinical models is that the CAR cells can home to the tumor/stromal interface but cannot go beyond into the intratumoral milieu. While this could be due to inhibitory cytokines and cell populations, including inhibitory fibroblasts and macrophages, success with CARs to date has been limited, begging the question as to how to build a better CAR.[34,35] While this is not the only improvement needed, addressing a hostile tumor microenvironment may be better served by trying to down-regulate inhibitory factors to induce a more favorable milieu to the tumor cell. Efforts are ongoing to address this issue with novel constructs alone and in combination with biologic agents.

A unique cellular approach is currently ongoing in clinical trials and is based on a technology of using a bispecific T cell engager (BiTE®). The BiTE® platform endeavors to recruit T cells to cancer cells by forming a “cytolytic synapse”.[36] The “synapse” forms with the target cell, in this case, a tumor cell, leading to a physical reorganization of the tubular elements within the cell and bringing the centrosome close to the point where T cell signaling occurs on the cell surface membrane. As such, secretory granules are delivered to this point on the membrane. The T cell becomes activated releasing these chemicals and fusing with the cancer cell to allow these enzymes to enter the cell and start apoptotic processes. Upon apoptosis occurring, activated sustained T cells can then target serial cancer cells in the area.[37–39] The goal is do induced local cell T cell proliferation in addition to induction and expansion of multi-clonal memory T cells. A recent report of AMG160 [40] utilized a fully human half-life extended BiTE® that targeted PSMA on tumor cells and CD3 on T cells. In the small cohort of patients studied, this approach was safe and demonstrated specificity for PSMA positive tumor cells.[40] A subsequent phase 1 multicenter first-in-human dose-escalating trial [41] enrolled 16 patients into 5 dosing cohorts with pasotuxizumab as continuous intravenous infusion in order to determine safety and maximum tolerated dose. No anti-drug antibodies were observed; antitumor activity was indicated by declines in serum PSA that was dose-dependent. PSA decreases of ≥50% was seen in 3 patients, with each decline seen in each dosing cohort of 20μg/d, 40μg/d and 80μg/d. One patient showed complete regression of soft-tissue metastases as well as bone disease as validated by PSMA-PET scan; a >90% reduction in PSA was noted with durable improvement in disease-related symptoms.

3. Conclusion

There is no question that immunotherapy remains an important option for patients with mCRPC. Prostate cancer is a heterogeneous disease and metastases are reflective of multiple clonal origins each displaying unique behaviors. Not all metastases respond equally to a particular therapy suggesting once again that combinatorial approaches must be used to combat these diverse behaviors. However, the best way to circumvent the clonality of these metastases remains to be determined. What is new and exciting is that genomic profiling is leading the way to exploring how unique genomic alterations can render prostate cancer more susceptible to checkpoint inhibitors which until now lacked the robustness seen in other solid tumors. The fact that T cells can be redirected to carry chemotherapy or radiation payloads as well as being constructed to bind to other cells to induce tumor killing is one of the greatest accomplishments to date. Are BiTES the wave of the future or is there still room for CAR T cells and checkpoint inhibitors? This question will likely to be answered in the near future. However, despite these promising approaches, enthusiasm is tempered by the fact that we are still controlling the disease of the few who have unique genomic alteration rather than the many who lack these markers of drug susceptibility; the field is in need of other strategies that have complementary functionality with other agents that more directly access to the microenvironment in which the prostate cancer resides. Indeed, it looks as if immunotherapy has arrived in prostate cancer!

4. Expert Opinion

Data from clinical trials suggest that there may be strategies to make mCRPC more susceptible to checkpoint inhibitors and other novel immunotherapeutic platforms. While single agent and ipilimumab and pembrolizumab have shown some durable responses in a small number of patients, it is clear that there are patients who can benefit based on genomic profiling. This does not help with the majority of patients who do not harbor these genetic defects and therefore, approaches that can target the circulating tumor cells as well as the tumor microenvironment is strongly needed. For the vast majority of patients to date, single agent therapy has been shown to be insufficient, hence the need for approaches that can combine agents that can target specific inhibitory pathways (adenosine), or cells (macrophages, myeloid derived suppressor cells) may be appropriate. The technology for immunologic and genomic profiling of individual cancers is rapidly providing additional insight into the in vivo behavior of tumors. As noted, the future has brightened considerably for the application of immune therapies for prostate cancer.

Article highlights.

• Inhibitory cell pathways and cell populations within the tumor microenvironment

• Role of genomic profiling as indicator of susceptibility of checkpoint inhibitors in select patient populations

• Improvements in CAR T cells constructs and the use of BiTES offer new cellular strategies