PSA-based vaccines for the treatment of prostate cancer

A meta-analysis of epitopes in prostate-specific antigens identifies opportunities and knowledge gaps

BACKGROUND

The Cancer Epitope Database and Analysis Resource (CEDAR) is a newly developed repository of cancer epitope data from peer-reviewed publications, which includes epitope-specific T cell, antibody, and MHC ligand assays. Here we focus on prostate cancer as our first cancer category to demonstrate the capabilities of CEDAR, and to shed light on the advances of epitope-related prostate cancer research.

RESULTS

The meta-analysis focused on a subset of data describing epitopes from 8 prostate-specific (PS) antigens. A total of 460 epitopes were associated with these proteins, 187 T cell, 109B cell, and 271 MHC ligand epitopes. The number of epitopes was not correlated with the length of the protein; however, we found a significant positive correlation between the number of references per specific PS antigen and the number of reported epitopes. Forty-four different class I and 27 class II restrictions were found, with the most epitopes described for HLA-A*02:01 and HLA-DRB1*01:01. Cytokine assays were mostly limited to IFNg assays and a very limited number of tetramer assays were performed. Monoclonal and polyclonal B cell responses were balanced, with the highest number of epitopes studied in ELISA/Western blot assays. Additionally, epitopes were generically described as associated with prostate cancer, with little granularity specifying diseases state. We found that in vivo and tumor recognition assays were sparse, and the number of epitopes with annotated B/T cell receptor information were limited. Potential immunodominant regions were identified by the use of the ImmunomeBrowser tool.

CONCLUSION

CEDAR provides a comprehensive repository of epitopes related to prostate-specific antigens. This inventory of epitope data with its wealth of searchable T cell, B cell and MHC ligand information provides a useful tool for the scientific community. At the same time, we identify significant knowledge gaps that could be addressed by experimental analysis.

Recent Advances in Molecular Mechanisms of Cancer Immunotherapy

Cancer is among the current leading causes of death worldwide, despite the novel advances that have been made toward its treatment, it is still considered a major public health concern. Considering both the serious impact of cancer on public health and the significant side effects and complications of conventional therapeutic options, the current strategies towards targeted cancer therapy must be enhanced to avoid undesired toxicity. Cancer immunotherapy has become preferable among researchers in recent years compared to conventional therapeutic options, such as chemotherapy, surgery, and radiotherapy. The understanding of how to control immune checkpoints, develop therapeutic cancer vaccines, genetically modify immune cells as well as enhance the activation of antitumor immune response led to the development of novel cancer treatments. In this review, we address recent advances in cancer immunotherapy molecular mechanisms. Different immunotherapeutic approaches are critically discussed, focusing on the challenges, potential risks, and prospects involving their use.

Spherical Nucleic Acid Vaccine Structure Markedly Influences Adaptive Immune Responses of Clinically Utilized Prostate Cancer Targets

Cancer vaccines, which activate the immune system against a target antigen, are attractive for prostate cancer, where multiple upregulated protein targets are identified. However, many clinical trials implementing peptides targeting these proteins have yielded suboptimal results. Using spherical nucleic acids (SNAs), we explore how precise architectural control of vaccine components can activate a robust antigen-specific immune response in comparison to clinical formulations of the same targets. The SNA vaccines incorporate peptides for human prostate-specific membrane antigen (PSMA) or T-cell receptor γ alternate reading frame protein (TARP) into an optimized architecture, resulting in high rates of immune activation and cytolytic ability in humanized mice and human peripheral blood mononuclear cells (hPBMCs). Specifically, administered SNAs elevate the production and secretion of cytokines and increase polyfunctional cytotoxic T cells and effector memory. Importantly, T cells raised from immunized mice potently kill targets, including clinically relevant cells expressing the whole PSMA protein. Treatment of hPBMCs increases costimulatory markers and cytolytically active T cells. This work demonstrates the importance of vaccine structure and its ability to reformulate and elevate clinical targets. Moreover, it encourages the field to reinvestigate ineffective peptide targets and repackage them into optimally structured vaccines to harness antigen potency and enhance clinical outcomes.

Trial Watch: Toll-like receptor agonists in cancer immunotherapy

Toll-like receptor (TLR) agonists demonstrate therapeutic promise as immunological adjuvants for anticancer immunotherapy. To date, three TLR agonists have been approved by US regulatory agencies for use in cancer patients. Additionally, the potential of hitherto experimental TLR ligands to mediate clinically useful immunostimulatory effects has been extensively investigated over the past few years. Here, we summarize recent preclinical and clinical advances in the development of TLR agonists for cancer therapy.

The synergy between ionizing radiation and immunotherapy in the treatment of prostate cancer

There has been a surge in the use of immunotherapy for genitourinary malignancies. Immunotherapy is an established treatment for metastatic renal cell carcinoma and nonmuscle invasive bladder cancer, but its potential for treating prostate cancer (PCa) remains under investigation. Despite reported survival benefits, no published Phase III PCa trials using immunotherapy only as a treatment has demonstrated direct antitumor effects by reducing prostate-specific antigen levels. Subsequently, the thought of combining immunotherapy with other treatment modalities has gained traction as a way to achieving optimal results. Based on data from other malignancies, it is hypothesized that radiotherapy and immunotherapy can act synergistically to improve outcomes. We will discuss the clinical potential of combining immune-based treatments with radiotherapy as a treatment for advanced PCa.

Dendritic/tumor fusion cells as cancer vaccines

A promising cancer vaccine involves the fusion of dendritic cells (DCs) with tumor cells such that a broad array of tumor antigens are presented in the context of DC-mediated costimulation and stimulatory cytokines. In diverse animal models, vaccination with DC/tumor fusions results in protection from an otherwise lethal challenge of tumor cells and eradication of established disease. In phase I clinical studies, vaccination with DC/tumor fusions was well tolerated, and induced immunologic responses in the majority of patients and clinical responses in a subset. Vaccine efficacy may be blunted by the immunosuppressive milieu characteristic of patients with malignancy, including the increased presence of regulatory T cells, and inhibitory pathways such as the PD-1/PDL-1 pathway. A current focus of research interest lies in enhancing response to cancer vaccines, by combining vaccination with tumor cytoreduction, regulatory T-cell depletion, and blockade of critical inhibitory pathways.

Therapeutic cancer vaccine fulfills the promise of immunotherapy in prostate cancer

For many years, preclinical and clinical studies have attempted to harness the power of the immune system and focus it on malignant cells in an attempt to improve clinical outcomes for patients with cancer. This paper describes the landmark Phase III trial that led to the first US FDA approval of a therapeutic cancer vaccine. In a randomized trial of 512 patients, those treated with sipuleucel-T survived for 25.8 months compared with those treated with placebo, who survived 21.7 months (HR: 0.78; p = 0.03). There was, however, no change in time to progression, which may relate to the underlying mechanism of this new class of therapeutics.

Vaccines as monotherapy and in combination therapy for prostate cancer

Prostate cancer is the second leading cause of cancer death among men in the United States. Standard-of-care chemotherapy for metastatic castration-resistant prostate cancer is associated with significant but modest survival benefit, indicating a need for alternative and/or additional approaches. The use of therapeutic cancer vaccines for the treatment of prostate cancer represents a novel targeted therapeutic approach. Whereas vaccine strategies are being developed for the treatment of various stages of prostate cancer, this article focuses on novel vaccine strategies for castration-resistant prostate cancer that have been translated into late-stage clinical studies.

Therapeutic vaccines in metastatic castration-resistant prostate cancer: principles in clinical trial design

Although docetaxel was approved for the treatment of metastatic castration-resistant prostate cancer in 2004, additional therapies are still required. Prostate cancer is often slow-growing and expresses many tumor-associated antigens, making it a feasible target for immunotherapy. Several therapeutic cancer vaccines have been developed for prostate cancer, including antigen-presenting-cell-based, vector-based, and whole tumor cell vaccines. Initial trials demonstrated that vaccine approaches have limited toxicity. Clinical trials of targeted biologic therapies have demonstrated that patient selection is vital, and there is preliminary evidence that clinical parameters can be used to encompass metastatic prostate cancer patients who will more probably respond to vaccine treatment. In addition to appropriate patient selection, a successful clinical trial must have an appropriate primary endpoint as well. Three randomized, 'placebo'-controlled studies in metastatic castration-resistant prostate cancer have suggested a clinically significant survival advantage in spite of a lack of improvement in time to progression, implying that overall survival is the ideal endpoint for such trials. Careful examination of data from completed immunotherapy clinical trials in prostate cancer has identified appropriate patient populations and endpoints. Those principles need to be applied to future trial design to properly evaluate prostate cancer vaccines.

Kallikreins on steroids: structure, function, and hormonal regulation of prostate-specific antigen and the extended kallikrein locus

The 15 members of the kallikrein-related serine peptidase (KLK) family have diverse tissue-specific expression profiles and putative proteolytic functions. The kallikrein family is also emerging as a rich source of disease biomarkers with KLK3, commonly known as prostate-specific antigen, being the current serum biomarker for prostate cancer. The kallikrein locus is also notable because it is extraordinarily responsive to steroids and other hormones. Indeed, at least 14 functional hormone response elements have been identified in the kallikrein locus. A more comprehensive understanding of the transcriptional regulation of kallikreins may help the field make more informed hypotheses about the physiological functions of kallikreins and their effectiveness as biomarkers. In this review, we describe the organization of the kallikrein locus and the structure of kallikrein genes and proteins. We also focus on the transcriptional regulation of kallikreins by androgens, progestins, glucocorticoids, mineralocorticoids, estrogens, and other hormones in animal models and human prostate, breast, and reproductive tract tissues. The interaction of the androgen receptor with androgen response elements in the promoter and enhancer of KLK2 and KLK3 is also summarized in detail. There is evidence that all kallikreins are regulated by multiple nuclear receptors. Yet, apart from KLK2 and KLK3, it is not clear whether all kallikreins are direct transcriptional targets. Therefore, we argue that gaining more detailed information about the mechanisms that regulate kallikrein expression should be a priority of future studies and that the kallikrein locus will continue to be an important model in the era of genome-wide analyses.

Functional roles of human kallikrein-related peptidases

Kallikrein-related peptidases constitute a single family of 15 (chymo)trypsin-like proteases (KLK1-15) with pleiotropic physiological roles. Aberrant regulation of KLKs has been associated with diverse diseases such as hypertension, renal dysfunction, skin disorders, inflammation, neurodegeneration, and cancer. Recent studies suggested that coordinated activation and regulation of KLK activity are achieved via a complex network of interactions referred to as the "KLK activome." However, it remains to be validated whether these hypothetical KLK activation cascade pathways are operative in vivo. In addition, KLKs have emerged as versatile signaling molecules. In summary, KLKs represent attractive biomarkers for clinical applications and potential therapeutic targets for common human pathologies.

Prostvac-VF: a vector-based vaccine targeting PSA in prostate cancer

Prostvac is a prostate cancer vaccine regimen consisting of a recombinant vaccinia vector as a primary vaccination, followed by multiple booster vaccinations employing a recombinant fowlpox vector. Both vectors contain the transgenes for prostate-specific antigen (PSA) and multiple T-cell co-stimulatory molecules (TRICOM). The PSA-TRICOM vaccines infect antigen-presenting cells (APCs) and generate proteins that are expressed on the surface of the APCs in an immune context. The interaction of these APCs with T cells initiates a targeted immune response and T cell-mediated tumor cell destruction. Preliminary clinical trials have indicated negligible toxicity, and Phase II trials have suggested a survival benefit after treatment with Prostvac, especially in patients with indolent disease characteristics. Preclinical and clinical data indicate that radiation, hormonal therapy, and chemotherapy may be combined with Prostvac to enhance the vaccine's efficacy. Additional strategies are in development to further enhance the clinical benefits of Prostvac, and a Phase III trial is being planned in metastatic castration-resistant prostate cancer.

Analysis of overall survival in patients with nonmetastatic castration-resistant prostate cancer treated with vaccine, nilutamide, and combination therapy

PURPOSE

We reported previously the first randomized study of any kind in patients with nonmetastatic, castrate-resistant prostate cancer. The study employed vaccine, the hormone nilutamide, and the combined therapy (crossover for each arm) with an endpoint of time to progression. We now report survival analyses at 6.5 years from the initiation of therapy with a median potential follow-up of 4.4 years.

EXPERIMENTAL DESIGN

Forty-two patients were randomized to receive either a poxvirus-based prostate-specific antigen (PSA) vaccine or nilutamide. Patients in either arm who developed increasing PSA without radiographic evidence of metastasis could cross over to receive the combined therapies.

RESULTS

Median survival among all patients was 4.4 years from date of enrollment. Median survival exhibited a trend toward improvement for patients initially randomized to the vaccine arm (median, 5.1 versus 3.4 years; P = 0.13). Starting from the on-study date, the retrospectively determined subset of 12 patients who initially received vaccine and then later received nilutamide suggested improved survival compared with the 8 patients who began with nilutamide and subsequently were treated with vaccine (median, 6.2 versus 3.7 years; P = 0.045). A subgroup analysis of patients randomized to the vaccine arm versus the nilutamide arm showed substantial improvements in survival if at baseline patients had a Gleason score <7 (P = 0.033) and PSA <20 ng/dL (P = 0.013) or who had prior radiation therapy (P = 0.018).

CONCLUSIONS

These data indicate that patients with nonmetastatic castration-resistant prostate cancer (D0.5) who receive vaccine before second-line hormone therapy may potentially result in improved survival compared with patients who received hormone therapy and then vaccine. These data also suggest that patients with more indolent disease may derive greater clinical benefit from vaccine alone or vaccine before second-line hormone therapy compared with hormone therapy alone or hormone therapy followed by vaccine. These findings have potential implications for both the design and endpoint analysis of larger vaccine combination therapy trials.

Cancer immunotherapy targeting tumour-specific antigens: towards a new therapy for minimal residual disease

BACKGROUND

Clinical investigation of cancer immunotherapy has been very active and several approaches have been evaluated in Phase III trials. In particular, the characterisation at the molecular level of tumour-specific antigens, together with expert knowledge from GSK Biologicals in recombinant protein manufacturing and immunological Adjuvant Systems, has led the company to develop Antigen-Specific Cancer Immunotherapeutic (ASCI).

OBJECTIVE/METHODS

This paper reviews the different cancer immunotherapy approaches that have reached Phase III clinical development. A special attention is given to GSK's ASCI approach.

CONCLUSION

Based on encouraging data in a double-blind Phase II trial in non-small-cell lung cancer, the selection of the most suitable adjuvant system in melanoma and the choice of the adequate clinical setting for the clinical development of cancer immunotherapy, the ASCI approach offers the perspective that the long quest towards a new cancer treatment approach is about to succeed.

GSK's antigen-specific cancer immunotherapy programme: pilot results leading to Phase III clinical development

From the first evidence that the immune system could recognize tumors, different types of tumor antigens have been identified and deeply characterized. Several different approaches aimed at targeting these antigens have already been the subject of clinical studies. In this field, the GSK Biologicals' approach relying on recombinant proteins combined with an immunological Adjuvant System in a specific clinical setting, has entertained hopes of developing a new class of well tolerated anti-cancer therapy. This methodology led to promising advances with MAGE-A3 immunotherapy in NSCLC and has the potential to be applied to all tumor types.

Generation of robust cytotoxic T lymphocytes against prostate specific antigen by transduction of dendritic cells using protein and recombinant adeno-associated virus

Prostate cancer is the most common male cancer and there is an urgent need for adjuvant therapy such as immunotherapy. Recombinant adeno-associated virus type 2 (rAAV) vectors are useful for antigen gene-loading of human dendritic cells (DC) and for the rapid generation of cytotoxic T lymphocytes (CTL). In this study, we report a protocol for AAV-loading of DC with the AAV-loading of self-antigen prostate specific antigen (PSA) resulting in generation of CTL. PSA and cytokine expression, Cell surface marker analysis of DC and CTL cells were done using a FACScalibur flow cytometer. Chromium-51 release assay was used to analyze the killing activity of CTL. It was found that AAV-loading of DC with the PSA gene is superior to PSA protein loading of the same antigen for generating effective CTL. AAV/PSA-loading of DC was found to result in: (1) strong, rapid PSA-specific, MHC Class I-restricted CTL, (2) PSA expression in DC, (3) high CD80, CD83, and CD86 expression on DC, (4) high level of IL-12 and low level of IL-10 in DC, (5) T cell populations with significant interferon gamma (IFNgamma) expression, but low IL-4 expression, (6) high proliferation of T cell populations, (7) high CD8:CD4 and CD8:CD56 T cell ratios. The reason for generation of robust CTL is partly explained by the characteristics of DC and CTL described. This protocol may be useful for adoptive immunotherapy against self antigens such as PSA for prostate cancer.