The Future of Immunotherapy in Prostate Cancer

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).[1] 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.[2]

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,[3] and is currently being extended to the breast cancer arena in an important phase II study initiated by Dr. Leisha Emens.[4] 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,[5] 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[6] and antiandrogen administration.[7]

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[8]. 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.[9] 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)[12] and human immunodeficiency virus (HIV).[13] 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[14] and others[15] 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.[16]

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.

Antigen Identification

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.[17] Published data indicate that mRNA can provide peptides for MHC presentation in any of the three forward- or backward-reading frames.[18] 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.[21] 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.[22] 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.

Summary

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[23]-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,[24] attenuated viral vectors,[25] loaded dendritic cells,[26] and novel vectors based on attenuated bacterial vectors.[27] 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

Disclosures:

Dr. Drake has received honoraria from Dendreon, Cell Genesys, and Bristol-Myers Squibb.

References:

1. Nanus DM, Milowsky MI, Kostakoglu L, et al: Clinical use of monoclonal antibody HuJ591 therapy: Targeting prostate specific membrane antigen. J Urol 170:S84-S88, 2003.

2. Bander NH, Milowsky MI, Nanus DM, et al: Phase I trial of 177lutetium-labeled J591, a monoclonal antibody to prostate-specific membrane antigen, in patients with androgen-independent prostate cancer. J Clin Oncol 23:4591-4601, 2005.

3. Wolpoe ME, Lutz ER, Ercolini AM, et al: HER-2/neu-specific monoclonal antibodies collaborate with HER-2/neu-targeted granulocyte macrophage colony-stimulating factor secreting whole cell vaccination to augment CD8+ T cell effector function and tumor-free survival in Her-2/neu-transgenic mice. J Immunol 171:2161-2169, 2003.

4. Emens LA, Reilly RT, Jaffee EM: Breast cancer vaccines: maximizing cancer treatment by tapping into host immunity. Endocr Relat Cancer 12:1-17, 2005.

5. Lam JS, Yamashiro J, Shintaku IP, et al: Prostate stem cell antigen is overexpressed in prostate cancer metastases. Clin Cancer Res 11:2591-2596, 2005.

6. Gulley JL, Arlen PM, Bastian A, et al: Combining a recombinant cancer vaccine with standard definitive radiotherapy in patients with localized prostate cancer. Clin Cancer Res 11:3353-3362, 2005.

7. Arlen PM, Gulley JL, Todd N, et al: Antiandrogen, vaccine and combination therapy in patients with nonmetastatic hormone refractory prostate cancer. J Urol 174:539-546, 2005.

8. Chambers CA, Kuhns MS, Egen JG, et al: CTLA-4-mediated inhibition in regulation of T cell responses: Mechanisms and manipulation in tumor immunotherapy. Annu Rev Immunol 19:565-594, 2001.

9. Blansfield JA, Beck KE, Tran K, et al: Cytotoxic T-lymphocyte-associated antigen-4 blockage can induce autoimmune hypophysitis in patients with metastatic melanoma and renal cancer. J Immunother 28:593-598, 2005.

10. Chen L: Co-inhibitory molecules of the B7-CD28 family in the control of T-cell immunity. Nat Rev Immunol 4:336-347, 2004.

11. Greenwald RJ, Freeman GJ, Sharpe AH: The B7 family revisited. Annu Rev Immunol 23:515-548, 2005.

12. Barber DL, Wherry EJ, Masopust D, et al: Restoring function in exhausted CD8 T cells during chronic viral infection. Nature 439:682-687, 2006.

13. Day CL, Kaufmann DE, Kiepiela P, et al: PD-1 expression on HIV-specific T cells is associated with T-cell exhaustion and disease progression. Nature 443:350-354, 2006.

14. Huang CT, Workman CJ, Flies D, et al: Role of LAG-3 in regulatory T cells. Immunity 21:503-513, 2004.

15. Macon-Lemaitre L, Triebel F: The negative regulatory function of the lymphocyte-activation gene-3 co-receptor (CD223) on human T cells. Immunology 115:170-178, 2005.

16. Murata S, Ladle BH, Kim PS, et al: OX40 costimulation synergizes with GM-CSF whole-cell vaccination to overcome established CD8+ T cell tolerance to an endogenous tumor antigen. J Immunol 176:974-983, 2006.

17. Yewdell JW: The seven dirty little secrets of major histocompatibility complex class I antigen processing. Immunol Rev 207:8-18, 2005.

18. Yewdell JW: Immunology. Hide and seek in the peptidome. Science 301:1334-1335, 2003.

19. Warren EH, Vigneron NJ, Gavin MA, et al: An antigen produced by splicing of noncontiguous peptides in the reverse order. Science 313:1444-1447, 2006.

20. Engelhard VH: Creating new peptide antigens by slicing and splicing proteins. Nat Immunol 5:128-129, 2004.

21. Thomas AM, Santarsiero LM, Lutz ER, et al: Mesothelin-specific CD8(+) T cell responses provide evidence of in vivo cross-priming by antigen-presenting cells in vaccinated pancreatic cancer patients. J Exp Med 200:297-306, 2004.

22. Hickman HD, Luis AD, Bardet W, et al: Cutting edge: class I presentation of host peptides following HIV infection. J Immunol 171:22-26, 2003.

23. Small EJ, Schellhammer PF, Higano CS, et al: Placebo-controlled phase III trial of immunologic therapy with sipuleucel-T (APC8015) in patients with metastatic, asymptomatic hormone refractory prostate cancer. J Clin Oncol 24:3089-3094, 2006.

24. Johnson LE, Frye TP, Arnot AR, et al: Safety and immunological efficacy of a prostate cancer plasmid DNA vaccine encoding prostatic acid phosphatase (PAP). Vaccine 24:293-303, 2006.

25. Arlen PM, Kaufman HL, DiPaola RS: Pox viral vaccine approaches. Semin Oncol 32:549-555, 2005.

26. Su Z, Dannull J, Yang BK, et al: Telomerase mRNA-transfected dendritic cells stimulate antigen-specific CD8+ and CD4+ T cell responses in patients with metastatic prostate cancer. J Immunol 174:3798-3807, 2005.

27. Yoshimura K, Jain A, Allen HE, et al: Selective targeting of antitumor immune responses with engineered live-attenuated Listeria monocytogenes. Cancer Res 66:1096-1104, 2006.