The Prostate Cancer Immune MicroenvironmentThe microenvironment associated with prostate cancer includes low cytolytic activity of natural killer (NK) cells,5 high secretion of TGF-beta by prostate tissue (which inhibits NK and lymphocyte function),6 and recruitment of T regulatory cells that down-regulate antitumor immunity.7 As such, the prostate cancer microenvironment has been described as an immunosuppressive state. Furthermore, based on the chronicity of the prostate cancer disease spectrum, the immune microenvironment is likely dynamic, with changes over time/clinical states and with treatment exposure.8 For example, there are increased tumor-infiltrating lymphocytes in the prostate bed following androgen deprivation therapy,9 and higher levels of PD-1 ligand and PD-L2 expression on the surface of enzalutamide-treated prostate cancer cells.10 Several aspects make prostate cancer attractive for immunotherapy-based treatment options, including a high-level of tumor-associated antigens such as prostate-specific antigen (PSA), prostate acid phosphatase (PAP), and prostate-specific membrane antigen (PSMA).
Cell-based vaccinesCell-based vaccines consist of whole cells that are modified in order to induce anti-tumor immune responses. Sipuleucel-T is an autologous vaccine processed following peripheral dendritic cell collection via leukapheresis. This is then incubated with GCS-F and PAP protein, followed by reinfusion into the patient (after a 36-44 hour period) in order to generate a PAP-specific CD4+ and CD8+ T cell response.11,12
Sipuleucel-T was FDA approved based on results of the Phase III IMPACT clinical trial.1 This trial enrolled 512 patients with mCRPC who had asymptomatic disease/minimally symptomatic with no visceral metastases, randomizing men to three infusions of sipuleucel-T (n=341) or placebo (n=171). The IMPACT trial noted a 4.1-month improvement in overall survival (OS) for those taking sipuleucel-T compared to placebo and a 22% reduction in risk of death. There was no difference between the groups with regards to objective disease progression or PSA response (secondary endpoints). An assessment of safety profile for patients in this study found that the treatment was overall well tolerated with minimal concern for severe adverse events.13 Furthermore, immunologic assessment showed that patients with high antibody titers against PA2024 benefited the most from treatment, noting longer survival.1 Despite the results and safety profile of IMPACT, reported nearly a decade ago, the use of sipuleucel-T has not been widely adopted primarily due to the lack of cost-effectiveness and the infrastructure required to administer this treatment.
Vector-based vaccinesVector-based vaccines consist of genetically engineered nucleic acids that encode specific tumor-associated antigens transmitted by vectors such as bacterial plasmids or viruses. DNA-based vaccines can be incorporated by host cells and generate an immune response to recruiting antigen-presenting cells. pTVG-HP is a DNA plasmid vector vaccine that encodes PAP protein. pTVG-HP has been tested in the non-metastatic CRPC setting, demonstrating increased PSA doubling time from 6.5 months to 9.3 months after one year of treatment.14
PROSTVAC is a PSA-target pox-virus-based vaccine. PROSTVAC was tested in a Phase II study of 125 patients with minimally symptomatic mCRPC who were randomized to receive the vaccine or placebo.15 Although the study was negative for its primary endpoint of progression-free survival (PFS), OS after 3 years of follow-up was significantly increased by 8.5 months (25.1 vs 16.6 months; HR 0.56; p=0.0061).
PROSTVAC was subsequently tested in a Phase III trial that reported results earlier this year.16 Patients were randomly assigned to PROSTVAC (n = 432), PROSTVAC plus granulocyte-macrophage colony-stimulating (GMCS) factor (n = 432), or placebo (n = 433), stratified by PSA (< 50 ng/mL vs. >= 50 ng/mL) and lactate dehydrogenase (< 200 vs >= 200 U/L). The primary endpoint for this trial was OS, and secondary endpoints were patients alive without events (AWE): radiographic progression, pain progression, chemotherapy initiation, or death at 6 months. Unfortunately, neither active treatment had an effect on median OS: (i) PROSTVAC: 34.4 months, hazard ratio (HR) 1.01, 95% confidence interval (CI) 0.84-1.20 (ii) PROSTVAC plus GMCS factor: 33.2 months, HR 1.02, 95% CI 0.86-1.22 (iii) placebo: 34.3 months. Furthermore, AWE at 6 months was similar between the arms. Based on these results, the authors noted that focus is currently ongoing for combination therapies.
DCVAC/PCa is an autologous dendritic cell vaccine derived from mononuclear cells that are pulsed with killed prostate cancer cells. In a Phase I/II trial, there were 25 men with mCRPC that received DCVAC/PCa plus docetaxel, demonstrating good tolerability and a median OS of 19 months.17 Currently, there is a Phase III (VIABLE) trial of DCVAC/PCa ongoing, which began accrual in 2014, with a target of 1,170 patients and planned completion date in 2020.
Immune Checkpoint inhibitorsCheckpoint inhibitors are antibodies that target molecules, such as cytotoxic T-lymphocyte protein 4 (CTLA-4) or PD-1 and its ligand PD-L1. Among men with mCRPC, ipilimumab was tested in the Phase III for those who had progressed on docetaxel chemotherapy, randomizing 799 patients to ipilimumab or placebo after bone-directed radiotherapy.18 The primary endpoint was OS, with no difference between the groups (ipilimumab 11.2 months vs placebo 10 months; HR 0.85, p=0.053); however, there was a small benefit in PFS favoring ipilimumab (4.0 vs 3.1 months; HR 0.70, p < 0.0001). More recently, Beer et al.19 reported findings of another Phase III trial randomizing 602 patients (2:1) with metastatic chemotherapy-naïve CRPC to ipilimumab vs placebo. Similar to the post-docetaxel patients, there was no difference in OS between the groups (HR 1.11, 95% CI 0.88-1.39), however men receiving ipilimumab had improved PFS (5.6 months vs 3.8; HR 0.67, 95% CI 0.55-0.81) compared to those receiving placebo.
Pembrolizumab has recently moved into the mCRPC arena, receiving FDA approval in a tumor agnostic indication for MSI-high (MSI-H) mutation CRPC patients in 2017. A study from the Memorial Sloan Kettering Cancer Center assessed the prevalence of MSI-H/dMMR prostate cancer among 1,033 patients treated at their institution,20 finding that 32 (3.1%) had MSI-H/dMMR disease. This included 23 patients (2.2%) that had tumors with high MSIsensor scores, and 7 of the 32 MSI-H/dMMR patients (21.9%) with pathogenic germline mutation in a Lynch syndrome-associated gene. Eleven patients with MSI-H/dMMR CRPC received anti-PD-1/PD-L1 therapy and six of these had a greater than 50% decline in PSA levels. Based on these data, experts in the field of advanced prostate cancer feel that every mCRPC patient should be tested for MSI-H status and potential pembrolizumab eligibility.
The KEYNOTE-028 study was a trial of pembrolizumab in advanced solid tumors among patients with PD-1 expression ≥1% of tumor or stromal cells. Among 245 men screened, there were 35 PD-1% (14.3%) and 23 patients who enrolled.21 There were four partial responses, for an objective response rate of 17.4% and 8 of 23 (34.8%) patients had stable disease. Median duration of response was 13.5 months, and median PFS and OS were 3.5 and 7.9 months, respectively. Furthermore, the 6-month PFS and OS rates were 34.8% and 73.4%, respectively. Recently, off-label use of pembrolizumab among a heavily pre-treated population of mCRPC patients has recently been reported. At the 2019 ASCO GU meeting, Tucker and colleagues presented data on 51 patients, 86% of which had received three or more prior lines of therapy. Most patients had previously received abiraterone (88%), docetaxel (86%), enzalutamide (80%), and sipuleucel-T (74%). Among these patients, 16% had a >50% confirmed PSA decline with pembrolizumab, with 8% having >90% PSA decline. Fifty-nine percent of men were treated with some form of concurrent therapy along with pembrolizumab, most commonly enzalutamide (47%).
At the 2019 ASCO GU meeting, results of the Phase II KEYNOTE-650 were also presented. This trial tested the combination of nivolumab plus ipilimumab for men with mCRPC. There were two cohorts for this study: cohort 1 – asymptomatic or minimally symptomatic, who had progressed after at least 1 second generation hormone therapy with no prior chemotherapy, and cohort 2 – progression after chemotherapy. Overall response rates were 26% in cohort 1 and 10% in cohort 2, including two patients in each cohort who had a complete response. Median time to response was approximately two months. PSA response rate was 18% in cohort 1 and 10% in cohort 2.
Future DirectionsUnlike many other tumor sites, to date, there has not been robust data to demonstrate a large role for immunotherapy in patients with mCRPC. However, there are several potential ways to increase immunotherapy response with the goal of improving the outcomes of immunotherapy for prostate cancer: (i) combination therapy, (ii) immune modulation, (iii) biomarkers for improving patient selection.
Options for combination therapies:
1) Combination of immunotherapies: multiple vaccines, vaccine plus an immune checkpoint inhibitor, or an immunocytokine plus an immune checkpoint inhibitor. KEYNOTE-650 combining nivolumab plus ipilimumab is an example of improved efficacy among patients receiving combination therapy.
2) Combinations with therapies to capitalize on immunologic synergy: these studies assess the effect of the addition of other accepted treatments such enzalutamide, poly ADP ribose polymerase PARP inhibitors, radium-223, and docetaxel to immunotherapy regimens.
3) Given the changing microenvironment of prostate cancer across disease states, beginning combination immunotherapy earlier (castration-sensitive) may improve the immunotherapeutic benefit.
There are several options of immunomodulatory agents that target the tumor microenvironment to improve immunotherapy efficacy. Docetaxel has been proven to induce immunogenic modulations, such as increasing expression of ICAM-1, MUC-1, and MHC class 1 molecules.22 Additionally, tasquinimod is an immunomodulatory agent that blocks S100A9, a key regulatory molecule of myeloid cells. In a Phase II trial of 206 asymptomatic chemotherapy-naïve mCRPC patients randomized to tasquinimod vs placebo, men receiving tasquinimod had significantly improved disease progression (7.6 vs 3.3 months, p = 0.0042).23 Unfortunately, a Phase III trial assessing tasquinimod did not improve OS (21.3 for tasquinimod vs 24 months for placebo, HR 1.10, p=0.25), however, there was an improvement in radiographic PFS (7.0 months vs 4.4 months, HR 0.64, p = 0.001).24
Biomarkers continue to be an active area of research, not just for the selection of appropriate patients for immunotherapy, but also for other treatment regimens for advanced prostate cancer (ie. BRCA status for selecting patients for PARP inhibitors).
As follows is a summary of several of the current biomarkers as related to immunotherapy and prostate cancer (adapted from Maia and Hansen8):
As follows is a summary of ongoing, recruiting phase III trial assessing immunotherapy in prostate cancer:
To date, immunotherapy in prostate cancer has been less successful than other cancer types, with only Sipuleucel-T demonstrating an OS advantage of 4.1 months in a Phase III trial. Given the plethora of other treatment options, Sipuleucel-T is uncommonly used. However, with improved combination therapy, immunomodulation and biomarkers in addition to ongoing Phase III trials, there are additional assessments and results in upcoming that may improve the immunotherapy landscape and add to the armamentarium of treatment options for men with advanced prostate cancer.
Published Date: November 2019