There has been a resurgence of interest in developing noncytotoxic immune therapies for patients with either hormone-naive biochemically relapsed post-primary therapy or castrate metastatic prostate cancer. The rationale for developing an immunotherapeutic approach has been based on the overexpression and underglycosylation of a wide variety of altered "self" molecules including prostate-specific antigen (PSA), acid phosphatase (ACP), prostate stem cell antigen (PSCA), and prostate-specific membrane antigen (PSMA), which can serve as targets for immune recognition and attack. In addition, such a strategy could theoretically make use of the patient's immune system to fight the tumor particularly if their disease is of reasonably low volume. A variety of immunotherapeutic approaches have been explored through phase I, II, and now phase III trials demonstrating that immunologic tolerance could be broken, as evidenced by the development of high-titer antibodies and T-cell responses specific for the tumor. What appears to be revolutionizing the immunotherapy field is the combination of vaccines with cytokines or immune modulators, which not only potentiate immune reactivity in vivo but foster dramatic antitumor responses. This review explores the challenges now faced in establishing a role for immune therapies for prostate cancer treatment.
Dr. Susan Slovin, an expert on immunologic treatments for genitourinary malignancies, has crafted a well-written and comprehensive review of the state of the art in immunotherapy for prostate cancer. Her article highlights the differences between passive and active immunotherapy, discusses difficulties in monitoring an antitumor immune response, and reviews ongoing and completed clinical trials of both passive and active immunotherapy. While Dr. Slovin's article strikes an enviable balance between comprehensiveness and brevity, a small number of areas might prove worthy of further elaboration.
Immunotherapy With Monoclonal Antibodies
Dr. Slovin carefully reviews the development of a monoclonal antibody to prostate-specific membrane antigen (PSMA). This antibody was originally developed by the Bander group and evaluated in an unconjugated form (J591). Subsequently, evaluation has progressed through several robust phase II trials, including a number involving radiolabeled constructs (yttrium-90 and lutetium-177). While impressive targeting results have been obtained with J591-based monoclonal antibodies, the rate of objective clinical responses in patients with advanced disease has been somewhat underwhelming to date.
While moving to earlier disease stages is a logical extension of these studies, another potential approach to improve clinical efficacy might be to combine antibody-mediated targeting with active vaccination. Such an approach has shown synergistic efficacy in laboratory studies by the Reilly group, and is currently being extended to the breast cancer arena in an important phase II study initiated by Dr. Leisha Emens. Although dauntingly complex from a regulatory perspective, combinatorial studies using anti-PSMA monoclonal antibodies in combination with one of the active immunotherapy approaches for prostate cancer in the later stages of clinical development such as sipuleucel-T (Provenge) or GVAX might be considered in the near future.
It is also notable that a small number of additional targeted monoclonal antibodies are in early stages of development for prostate cancer. These agents include a fully human monoclonal antibody directed against prostate stem cell antigen (PSCA), being codeveloped by Merck and Agensys, Inc. Since PSCA appears to be relatively overexpressed on prostate metastases, PSCA might prove a particularly attractive target in men with advanced disease.
Another target currently under consideration in the clinic is the pro-inflammatory cytokine interleukin (IL)-6. Preclinical data show that IL-6 promotes the survival and proliferation of prostate cancer cells, and serum IL-6 levels are a marker for poor prognosis in men with advanced disease. The current clinical trial of this agent employs a combinatorial approach, administering mitoxantrone-based chemotherapy along with a chimeric monoclonal antibody developed by Centocor, Inc. An interesting facet of this trial is that the combinatorial approach is being explored early on, perhaps suggesting a wider acceptance of the notion that combination approaches will prove necessary to achieve clinical efficacy with immunotherapy.
Immune Checkpoint Blockade
Dr. Slovin provides an eloquent rationale for approaches combining immune checkpoint blockade with active immunotherapy in prostate cancer, and discusses an ongoing trial in which GVAX immunotherapy is combined with anti-CTLA-4 (ipilimumab) in men with hormone-refractory prostate cancer. It is perhaps notable that a similar combinatorial approach combining anti-CTLA-4 with a viral-based anti-PSA vaccine is currently enrolling patients at the National Cancer Institute. The principal investigator of this trial, Dr. James Gulley, and his colleague Dr. Philip Arlen have conducted a number of combinatorial trials with this vaccine vector, including combination with radiotherapy and antiandrogen administration.
While CTLA-4 blockade is currently the cornerstone of most of the ongoing combinatorial immunotherapy approaches, it is noteworthy that CTLA-4 knockout mice have a fairly significant phenotype, developing multisystem autoimmunity that proves fatal at 18 to 28 days of age. These data suggest that effective blockade of the CTLA-4/B7-1/2 axis might be associated with a measurable incidence of autoimmunity in the clinic; in fact, this has been observed. Thus, a number of additional checkpoints are under study in multiple laboratories, with the eventual goal of translation to combinatorial or single-agent studies in men with prostate cancer.
Perhaps most notable among these is the checkpoint mediated by interactions between PD-1 (programmed death 1) on T cells and B7-H1 (a B7 family member) on either tumor cells or associated antigen-presenting cells.[10,11] Recent data suggest that PD-1 marks exhausted, nonfunctional CD8 T cells in a number of disease states, including chronic infection with lymphocytic choriomeningitis virus (LCMV) and human immunodeficiency virus (HIV). Pathologic data suggest that such an upregulation occurs in cancer, but more importantly that blockade of PD-1 interactions with antibodies directed against either PD-1 or B7-H1 have significant clinical potential, particularly in combination with active immunotherapy.
Another checkpoint mediator worth considering is the molecule LAG-3 (lymphocyte activation gene 3), which we and others have shown as a potential marker of regulatory T cells. Additionally, the T-cell molecule OX40 has recently been demonstrated to have a direct role as a costimulator of CD8 T cells, and agonist antibodies to this molecule were shown to be synergistic with whole-cell based vaccination in a relevant preclinical model.
In summary, while CTLA-4 blockade is certainly the coinhibitory molecule that has advanced the farthest in terms of clinical development, a series of potentially less toxic coinhibitory and costimulatory molecules are progressing through preclinical and clinical development for prostate cancer, and may have a significant impact on combinatorial approaches for this disease in the relatively near future.
The associated review article touches briefly on the difficulties involved in identifying and selecting tumor-associated or tumor-specific antigens to be used for targets in immunotherapy approaches to prostate cancer. Current clinical trials utilize mostly well-defined and relatively prostate-specific targets such as PSMA, PSCA, prostate specific antigen (PSA), and prostatic acid phosphatase (PAP). However, the presence or even relative overexpression of a protein in a prostate cancer cell does not guarantee that epitopes representing that particular protein will be present in class I major histocompatibility complex (MHC) molecules on that cell's surface to an extent that permits CD8 T-cell-mediated lysis.
Indeed, the mechanics of antigen processing and presentation are wonderfully complex and surprisingly poorly understood. Published data indicate that mRNA can provide peptides for MHC presentation in any of the three forward- or backward-reading frames. In addition, splicing at the RNA level has been described, making the universe of potential epitopes that can arise from a single tumor-associated or specific protein large and intimidating. Adding even further to this complexity are recent observations that CD8 T-cell epitopes may even arise from splicing at the protein level.[19,20] Thus, it has become apparent that differential mRNA expression analyses or even complex proteomic approaches might prove inadequate to identify the predominant MHC class I epitopes present on tumor cells.
From a treatment perspective, one potential workaround to this problem of complexity involves the use of immunotherapy vectors based on whole-tumor cells. These cells undergo apoptotic death in the host in vivo, and epitopes are cross-presented on host antigen-presenting cells for immune priming. One clever approach to the identification of potential tumor antigens has been coined "functional genomics" by the Jaffee group. This process involves screening lymphocytes from patients treated with cell-based immunotherapy for epitopes identified separately by overexpression data, and correlating observed in vitro responses with clinical outcome. Using this approach, these investigators recently identified mesothelin as a potential tumor-associated antigen in pancreatic cancer-providing both a novel vaccine target as well as a potential biomarker for future studies. Given the large number of patients involved in phase III GVAX studies for prostate cancer, it is conceivable that such a functional genomic approach for antigen identification could soon be applied to prostate cancer as well.
Direct approaches to class I epitope identification are also feasible, although complex and time-consuming. The Hildebrand group has demonstrated the feasibility of such a direct methodology; transfecting several cell types with secreted human class I MHC molecules, collecting large quantities of secreted peptide/MHC complexes, and then identifying the presented peptides through a combination of acid elution and mass spectrometry. The results of these studies were surprising, suggesting a relative overexpression of RNA-binding proteins and a spectrum of presented antigens different than what would have been empirically predicted. Application of this technology to human prostate cancer cells has yet to be reported, but would most likely serve to identify a number of new potential targets.
Two active immunotherapy approaches for prostate cancer have progressed to phase III trials that are either completed (sipuleucel-T) or ongoing (GVAX). Published data on a completed trial of sipuleucel-T report a significant survival benefit-a notable first for the field. Future immunotherapy approaches for prostate cancer will almost certainly combine active immunotherapy with either antitumor monoclonal antibodies or with coinhibitor blockade to augment efficacy.
As discussed above, the area of specific antigen identification remains relatively open, providing encouragement for the development of more directed targeting approaches. Finally, it is worth noting that a large number of diverse active immunotherapy approaches are in active clinical and preclinical development. These include DNA vectors, attenuated viral vectors, loaded dendritic cells, and novel vectors based on attenuated bacterial vectors. Taken together, these observations confirm Dr. Slovin's assertion that immunotherapy is likely to emerge as a viable treatment option for patients with prostate cancer.
-Charles G. Drake, MD, PhD
Dr. Drake has received honoraria from Dendreon, Cell Genesys, and Bristol-Myers Squibb.
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