DNA vaccine encoding prostatic acid phosphatase (PAP) elicits long-term T-cell responses in patients with recurrent prostate cancer

Immunotherapy for castration-resistant prostate cancer: Progress and new paradigms

BACKGROUND

The approval of sipuleucel-T in conjunction with data from other immunotherapeutic trials for prostate cancer and other solid tumors demonstrates the potential of harnessing the patients' immune system for long-term survival. Thus, a range of therapeutic approaches are under evaluation. This review describes the rationale for immunotherapy for prostate cancer, summarizes the approaches under evaluation, and discusses sequencing options for immunotherapy in the current treatment paradigm.

DESIGN

References for this review were identified through searches of PubMed with the search terms "prostate cancer," "immune system," "vaccine," "immunotherapy," and "T cells." Articles were also identified through searches of the authors' own files. The final reference list was generated based on originality and relevance.

RESULTS

The immune system can recognize and respond to prostate tumor antigens, effected through tumor-associated antigens and tumor infiltration of immune effector cells. However, evidence also suggests that prostate tumors are adept at escaping immunological recognition, thus hypothesizing multiple therapeutic strategies. Therapeutic approaches could include vaccination and modulation of T-cell function via the blockade of checkpoint receptors such as cytotoxic T-lymphocyte antigen-4 and programmed death 1. In phase III trials, sipuleucel-T improved overall survival for an M1 patient population with castration-resistant prostate cancer and ipilimumab also did so when given after radiotherapy in a subset of better risk patients. In randomized phase II trials, prostate-specific antigen-TRICOM improved overall survival and tasquinimod improved progression-free survival.

CONCLUSION

Although immunotherapy has the potential to affect advanced prostate cancer, additional research is needed to (1) identify predictive or surrogate markers of activity, (2) understand which agents are clinically effective alone or in combination with other therapies, and (3) define the optimal timing for an immunotherapy to achieve maximal benefit.

Beyond sipuleucel-T: immune approaches to treating prostate cancer

OPINION STATEMENT

At present, sipuleucel-T represents the only approved immunotherapy for prostate cancer. Sipuleucel-T is an autologous cellular therapy, which primes autologous antigen-presenting cells against the prostatic acid phosphatase (PAP) antigen. For patients with metastatic castrate-resistant prostate cancer (CRPC) who are asymptomatic or minimally symptomatic, sipuleucel-T monotherapy is one of the standard of care treatment options pre- or postdocetaxel. With the approval of new treatments, including abiraterone and enzatutamide, sequencing and combination of these treatments with sipuleucel-T represent unanswered questions facing the field. Whereas steroids that are coadministered with abiraterone and chemotherapy have long been thought to be immunosuppressive, early results show that concurrent abiraterone and prednisone does not significantly impact the ability to develop immune responses to this treatment. Additional clinical data are needed to elucidate optimal sequencing of therapeutic agents in CRPC. Several novel immunotherapies are currently in development, and enrollment in clinical trials should be considered. These include PROSTVAC-VF, a viral vaccine that encodes PSA and T-cell costimulatory molecules, which is currently undergoing phase III clinical trials. DNA plasmid-based vaccines targeting different antigens, including PAP, also are under investigation. Immune checkpoint blockade with ipilimumab, a monoclonal antibody against CTLA-4, which is approved for metastatic melanoma, also is being evaluated. Whereas this treatment failed to show significant improvement in overall survival in CRPC patients treated with docetaxel, results from a phase III trial in the predocetaxel setting are pending. Conventional therapies for prostate cancer, such as radiation and hormonal therapy, may have immunomodulatory effects. Future areas for research include the sequencing and combination of immunotherapies as well as other conventional therapies.

Therapeutic vaccines for cancer: an overview of clinical trials

The therapeutic potential of host-specific and tumour-specific immune responses is well recognized and, after many years, active immunotherapies directed at inducing or augmenting these responses are entering clinical practice. Antitumour immunization is a complex, multi-component task, and the optimal combinations of antigens, adjuvants, delivery vehicles and routes of administration are not yet identified. Active immunotherapy must also address the immunosuppressive and tolerogenic mechanisms deployed by tumours. This Review provides an overview of new results from clinical studies of therapeutic cancer vaccines directed against tumour-associated antigens and discusses their implications for the use of active immunotherapy.

Real-time immune monitoring to guide plasmid DNA vaccination schedule targeting prostatic acid phosphatase in patients with castration-resistant prostate cancer

PURPOSE

We have previously reported that a DNA vaccine encoding prostatic acid phosphatase (PAP) could elicit PAP-specific T cells in patients with early recurrent prostate cancer. In the current pilot trial, we sought to evaluate whether prolonged immunization with regular booster immunizations, or "personalized" schedules of immunization determined using real-time immune monitoring, could elicit persistent, antigen-specific T cells, and whether treatment was associated with changes in PSA doubling time (PSA DT).

EXPERIMENTAL DESIGN

Sixteen patients with castration-resistant, nonmetastatic prostate cancer received six immunizations at 2-week intervals and then either quarterly (arm 1) or as determined by multiparameter immune monitoring (arm 2).

RESULTS

Patients were on study a median of 16 months; four received 24 vaccinations. Only one event associated with treatment >grade 2 was observed. Six of 16 (38%) remained metastasis-free at 2 years. PAP-specific T cells were elicited in 12 of 16 (75%), predominantly of a Th1 phenotype, which persisted in frequency and phenotype for at least 1 year. IFNγ-secreting T-cell responses measured by ELISPOT were detectable in 5 of 13 individuals at 1 year, and this was not statistically different between study arms. The overall median fold change in PSA DT from pretreatment to posttreatment was 1.6 (range, 0.6-7.0; P = 0.036).

CONCLUSIONS

Repetitive immunization with a plasmid DNA vaccine was safe and elicited Th1-biased antigen-specific T cells that persisted over time. Modifications in the immunization schedule based on real-time immune monitoring did not increase the frequency of patients developing effector and memory T-cell responses with this DNA vaccine.

Xenogeneic therapeutic cancer vaccines as breakers of immune tolerance for clinical application: to use or not to use?

Accumulation of firm evidence that clinically apparent cancer develops only when malignant cells manage to escape immunosurveillance led to the introduction of tumor immunotherapy strategies aiming to reprogramm the cancer-dysbalanced antitumor immunity and restore its capacity to control tumor growth. There are several immunotherapeutical strategies, among which specific active immunotherapy or therapeutic cancer vaccination is one of the most promising. It targets dendritic cells (DCs) which have a unique ability of inducing naive and central memory T cell-mediated immune response in the most efficient manner. DCs can be therapeutically targeted either in vivo/in situ or by ex vivo manipulations followed by their re-injection back into the same patient. The majority of current DC targeting strategies are based on autologous or allogeneic tumor-associated antigens (TAAs) which possess various degrees of inherent tolerogenic potential. Therefore still limited efficacy of various tumor immunotherapy approaches may be attributed, among various other mechanisms, to the insufficient immunogenicity of self-protein-derived TAAs. Based on such an idea, the use of homologous xenogeneic antigens, derived from different species was suggested to overcome the natural immune tolerance to self TAAs. Xenoantigens are supposed to differ sufficiently from self antigens to a degree that renders them immunogenic, but at the same time preserves an optimal homology range with self proteins still allowing xenoantigens to induce cross-reactive T cells. Here we discuss the concept of xenogeneic vaccination, describe the cons and pros of autologous/allogeneic versus xenogeneic therapeutic cancer vaccines, present the results of various pre-clinical and several clinical studies and highlight the future perspectives of integrating xenovaccination into rapidly developing tumor immunotherapy regimens.

Immunotherapy for prostate cancer: lessons from responses to tumor-associated antigens

Prostate cancer (PCa) is the most common cancer in men and the second most common cause of cancer-related death in men. In recent years, novel therapeutic options for PCa have been developed and studied extensively in clinical trials. Sipuleucel-T is the first cell-based immunotherapeutic vaccine for treatment of cancer. This vaccine consists of autologous mononuclear cells stimulated and loaded with an immunostimulatory fusion protein containing the prostate tumor antigen prostate acid posphatase. The choice of antigen might be key for the efficiency of cell-based immunotherapy. Depending on the treatment strategy, target antigens should be immunogenic, abundantly expressed by tumor cells, and preferably functionally important for the tumor to prevent loss of antigen expression. Autoimmune responses have been reported against several antigens expressed in the prostate, indicating that PCa is a suitable target for immunotherapy. In this review, we will discuss PCa antigens that exhibit immunogenic features and/or have been targeted in immunotherapeutic settings with promising results, and we highlight the hurdles and opportunities for cancer immunotherapy.

DNA vaccines encoding altered peptide ligands for SSX2 enhance epitope-specific CD8+ T-cell immune responses

Plasmid DNA serves as a simple and easily modifiable form of antigen delivery for vaccines. The USDA approval of DNA vaccines for several non-human diseases underscores the potential of this type of antigen delivery method as a cost-effective approach for the treatment or prevention of human diseases, including cancer. However, while DNA vaccines have demonstrated safety and immunological effect in early phase clinical trials, they have not consistently elicited robust anti-tumor responses. Hence many recent efforts have sought to increase the immunological efficacy of DNA vaccines, and we have specifically evaluated several target antigens encoded by DNA vaccine as treatments for human prostate cancer. In particular, we have focused on SSX2 as one potential target antigen, given its frequent expression in metastatic prostate cancer. We have previously identified two peptides, p41-49 and p103-111, as HLA-A2-restricted SSX2-specific epitopes. In the present study we sought to determine whether the efficacy of a DNA vaccine could be enhanced by an altered peptide ligand (APL) strategy wherein modifications were made to anchor residues of these epitopes to enhance or ablate their binding to HLA-A2. A DNA vaccine encoding APL modified to increase epitope binding elicited robust peptide-specific CD8+ T cells producing Th1 cytokines specific for each epitope. Ablation of one epitope in a DNA vaccine did not enhance immune responses to the other epitope. These results demonstrate that APL encoded by a DNA vaccine can be used to elicit increased numbers of antigen-specific T cells specific for multiple epitopes simultaneously, and suggest this could be a general approach to improve the immunogenicity of DNA vaccines encoding tumor antigens.

Preclinical and clinical development of DNA vaccines for prostate cancer

Prostate cancer is the most commonly diagnosed cancer in the United States. It is also the second leading cause of cancer-related death in men, making it one of the largest public health concerns today. Prostate cancer is an ideal disease for immunotherapies because of the generally slow progression, the dispensability of the target organ in the patient population, and the availability of several tissue-specific antigens. As such, several therapeutic vaccines have entered clinical trials, with one autologous cellular vaccine (sipuleucel-T) recently gaining Food and Drug Administration approval after demonstrating overall survival benefit in randomized phase III clinical trials. DNA-based vaccines are safe, economical, alternative "off-the-shelf" approaches that have undergone extensive evaluation in preclinical models. In fact, the first vaccine approved in the United States for the treatment of cancer was a DNA vaccine for canine melanoma. Several prostate cancer-specific DNA vaccines have been developed in the last decade and have shown promising results in early phase clinical trials. This review summarizes anticancer human DNA vaccine trials, with a focus on those conducted for prostate cancer. We conclude with an outline of special considerations important for the development and successful translation of DNA vaccines from the laboratory to the clinic.

Messenger RNA vaccine based on recombinant MS2 virus-like particles against prostate cancer

Prostate cancer (PCa) is the most diagnosed cancer in the western male population with high mortality. Recently, alternative approaches based on immunotherapy including mRNA vaccines for PCa have shown therapeutic promise. However, for mRNA vaccine, several disadvantages such as the instability of mRNA, the high cost of gold particles, the limited production scale for mRNA-transfected dendritic cells in vitro, limit their development. Herein, recombinant bacteriophage MS2 virus-like particles (VLPs), which based on the interaction of a 19-nucleotide RNA aptamer and the coat protein of bacteriophage MS2, successfully addressed these questions, in which target mRNA was packaged by MS2 capsid. MS2 VLP-based mRNA vaccines were easily prepared by recombinant protein technology, nontoxic and RNase-resistant. We show the packaged mRNA was translated into protein as early as 12 hr after phagocytosed by macrophages. Moreover, MS2 VLP-based mRNA vaccines induced strong humoral and cellular immune responses, especially antigen-specific cytotoxic T-lymphocyte (CTL) and balanced Th1/Th2 responses without upregulation of CD4(+) regulatory T cells, and protected C57BL/6 mice against PCa completely. As a therapeutic vaccine, MS2 VLP-based mRNA vaccines delayed tumor growth. Our results provide proof of concept on the efficacy and safety of MS2 VLP-based mRNA vaccine, which provides a new delivery approach for mRNA vaccine and implies important clinical value for the prevention and therapy of PCa.

Prostate cancer vaccines

In 2010, the US FDA approved the first therapeutic cancer vaccine for the treatment of castration refractory prostate cancer - sipuleucel-T. Prostate cancer is an ideal model for cancer vaccine development based on the ready demonstration of humoral and cellular immunity to a range of cancer antigens as well as often slow progression which means that patients who are otherwise well may have a radiologically evaluable minor progression, after conventional treatment and can undergo vaccine therapy over sufficient periods of time, so as to allow the generation of a robust antitumor response. The association of prostate cancer with one of the few serum cancer biomarkers in general use has also allowed assessment of response and risk stratification of patients. In this review, we will examine key aspects of the evolution of prostate cancer vaccines, which provides an accurate prototype for other cancers, and the challenges we face.

Immunotherapy and immune evasion in prostate cancer

Metastatic prostate cancer remains to this day a terminal disease. Prostatectomy and radiotherapy are effective for organ-confined diseases, but treatment for locally advanced and metastatic cancer remains challenging. Although advanced prostate cancers treated with androgen deprivation therapy achieves debulking of disease, responses are transient with subsequent development of castration-resistant and metastatic disease. Since prostate cancer is typically a slowly progressing disease, use of immune-based therapies offers an advantage to target advanced tumors and to induce antitumor immunity. This review will discuss the clinical merits of various vaccines and immunotherapies in castrate resistant prostate cancer and challenges to this evolving field of immune-based therapies.

Prostate cancer vaccines: Update on clinical development

Prostate cancer is a common malignancy among elderly men and is essentially incurable once it becomes metastatic. Results from clinical trials testing a panel of specific vaccines in patients with castration-resistant prostate cancer (CRPC) suggest that alternative therapies may one day substitute or support the current gold standard (docetaxel plus prednisone). Here, we summarize the results of germane clinical trials completed during the last 12 y and provide updates on some currently ongoing studies. As it stands, prostate cancer vaccines appear to be safe and capable of generating prostate-specific T lymphocyte responses with potential antitumor activity.

Proposed mechanisms of action for prostate cancer vaccines

Prostate cancer is responsible for the deaths of more than 33,000 American men every year. Once this disease has become metastatic, there is no curative treatment. Alternative therapies to chemotherapy and radical prostatectomy are being increasingly explored. Prostate cancer vaccines--which trigger a tumour-specific cytotoxic-T-lymphocyte-mediated immune attack by the patient's immune system--have been investigated in clinical trials with modest, yet encouraging, results. When developing and administering prostate cancer vaccines, it is critical to consider how vital parameters, such as the stage of disease progression and the nature of adjuvant therapies, could influence treatment outcome. Of particular interest are current and future strategies for diminishing the activity of regulatory T lymphocytes.

Demystifying immunotherapy in prostate cancer: understanding current and future treatment strategies

Immunotherapy has emerged as a viable therapeutic option for patients with prostate cancer. There are multiple potential strategies that use the immune system, including therapeutic cancer vaccines that are designed to stimulate immune cells to target antigens expressed by cancer cells. Sipuleucel-T is a vaccine currently approved for the treatment of minimally symptomatic metastatic prostate cancer, whereas the vaccine PSA-TRICOM and the immune-checkpoint inhibitor ipilimumab are in phase III testing. Although there are no short-term changes in disease progression or available biomarkers to assess response, these agents appear to improve survival. One hypothesis suggests that this apparent paradox can be explained by the growth-moderating effects of these treatments, which do not cause tumor size to diminish, but rather stall or slow their growth rate over time. For this reason, the use of immunotherapy earlier in the disease process is being investigated. Another approach is to block immune-regulatory mechanisms mediated by the molecules cytotoxic T lymphocyte antigen 4 and programmed cell death protein 1. Additional future strategies will combine immunotherapy with other standard therapies, potentially enhancing the latter's clinical impact and thereby improving both time to progression and overall survival due to the combined effects of both treatments. Prospective trials are currently evaluating these hypotheses and will ultimately serve to optimize immunotherapy in the treatment of prostate cancer.

Presence of antigen-specific somatic allelic mutations and splice variants do not predict for immunological response to genetic vaccination

Background

Antigen-specific anti-tumor vaccines have demonstrated clinical efficacy, but immunological and clinical responses appear to be patient-dependent. We hypothesized that naturally-occurring differences in amino acid sequence of a host’s target antigen might predict for immunological outcome from genetic vaccination by presentation of epitopes different from the vaccine.

Methods

Using peripheral blood cells from 33 patients who had been treated with a DNA vaccine encoding prostatic acid phosphatase (PAP), we sequenced the exons encoding PAP and PSA genes from somatic DNA to identify single nucleotide polymorphisms. In addition, mRNA was collected to detect alternative splice variants of PAP.

Results

We detected four synonymous coding mutations of PAP among 33 patients; non-synonymous coding mutations were not identified. Alternative splice variants of PAP were detected in 22/27 patients tested. The presence of detectable splice variants was not predictive of immunological outcome from vaccination. Immune responses to peptides encoded by these splice variants were common (16/27) prior to immunization, but not associated with immune responses elicited with vaccination.

Conclusions

These results suggest that antigen-specific immune responses detectable after treatment with this genetic vaccine are specific for the host-encoded antigen and not due to epitope differences between the vaccine and a particular individual’s somatic coding sequence.

Immunotherapy for castration-resistant prostate cancer

The improved survival with sipuleucel-T, an autologous antigen-presenting cell-based agent, for the treatment of patients with metastatic asymptomatic and minimally symptomatic castration-resistant prostate cancer supports immunotherapy as a valid approach. Also, multiple novel immunotherapeutic approaches are undergoing vigorous investigation. T-lymphocyte checkpoint blockade and poxvirus-based prime-boost approaches are in phase III evaluation. Other immunotherapeutic platforms undergoing early investigation include radioimmunoconjugates and adenovirus-based, DNA-based, and Listeria-based approaches. The development of predictive markers for immune response that translate into improved long-term outcomes is important. This article reviews the emerging data and the unique strengths and weaknesses of these approaches.

Local radiotherapy increases the level of autoantibodies to ribosomal P0 protein but not to heat shock proteins, extracellular matrix molecules and EGFR/ErbB2 receptors in prostate cancer patients

Prostate cancer is a common cancer among men in developed countries. Although hormonotherapy and radiotherapy (RT) represent valid therapies for prostate cancer treatment, novel immunological approaches have been explored. The development of clinical trials employing cancer vaccines has indicated that immune response to tumor antigens can be boosted and that vaccine administration can improve patient survival. Immune response to tumor antigens could also be enhanced after standard therapies. In the present study, we determined the occurrence of antibodies to extracellular matrix (ECM) molecules, heat shock protein (HSP), ribosomal P0 protein, EGFR, ErbB2 and prostate-specific antigen (PSA) in 35 prostate cancer patients prior to and following local RT and hormonotherapy. We demonstrated that immunity to P0, ECM molecules [collagens (C) CI, CIII, CV, fibronectin (FN) and laminin (LM)] and to HSP90 was associated with malignancy in untreated patients. None of the patient sera showed antibodies to EGFR, while 2 and 1 patients showed reactivity to ErbB2 and PSA, respectively. We also demonstrated that 8 months after therapy the IgG serum levels to CI, CIII, FN and HSP90 significantly decreased. Conversely, the level of P0 autoantibodies increased after therapy in 10 patients. Five of the 10 patients with increased levels of P0 autoantibodies were treated with RT plus hormonotherapy. Treatment of patients did not change the levels of antibodies against EGFR, ErbB2 and PSA. Our results indicated that the modification of antibody level to self molecules after standard treatment of prostate cancer patients is influenced by the type of antigen. Ribosomal P0 protein appears to be a high immunogenic antigen and its immunogenicity increases following RT. In addition, 10 patients with increased levels of autoantibodies to P0 showed PSA mean levels lower than the remaining 25 patients at 18 months. This study may contribute to a better understanding of the immunobiological behavior of prostate cancer patients following standard treatment.

Human prostate tumor antigen-specific CD8+ regulatory T cells are inhibited by CTLA-4 or IL-35 blockade

Regulatory T cells play important roles in cancer development and progression by limiting the generation of innate and adaptive anti-tumor immunity. We hypothesized that in addition to natural CD4(+)CD25(+) regulatory T cells (Tregs) and myeloid-derived suppressor cells, tumor Ag-specific Tregs interfere with the detection of anti-tumor immunity after immunotherapy. Using samples from prostate cancer patients immunized with a DNA vaccine encoding prostatic acid phosphatase (PAP) and a trans-vivo delayed-type hypersensitivity (tvDTH) assay, we found that the detection of PAP-specific effector responses after immunization was prevented by the activity of PAP-specific regulatory cells. These regulatory cells were CD8(+)CTLA-4(+), and their suppression was relieved by blockade of CTLA-4, but not IL-10 or TGF-β. Moreover, Ag-specific CD8(+) Tregs were detected prior to immunization in the absence of PAP-specific effector responses. These PAP-specific CD8(+)CTLA-4(+) suppressor T cells expressed IL-35, which was decreased after blockade of CTLA-4, and inhibition of either CTLA-4 or IL-35 reversed PAP-specific suppression of tvDTH response. PAP-specific CD8(+)CTLA-4(+) T cells also suppressed T cell proliferation in an IL-35-dependent, contact-independent fashion. Taken together, these findings suggest a novel population of CD8(+)CTLA-4(+) IL-35-secreting tumor Ag-specific Tregs arise spontaneously in some prostate cancer patients, persist during immunization, and can prevent the detection of Ag-specific effector responses by an IL-35-dependent mechanism.

DNA Vaccines for Prostate Cancer

Delivery of plasmid DNA encoding an antigen of interest has been demonstrated to be an effective means of immunization, capable of eliciting antigen-specific T cells. Plasmid DNA vaccines offer advantages over other anti-tumor vaccine approaches in terms of simplicity, manufacturing, and possibly safety. The primary disadvantage is their poor transfection efficiency and subsequent lower immunogenicity relative to other genetic vaccine approaches. However, multiple preclinical models demonstrate anti-tumor efficacy, and many efforts are underway to improve the immunogenicity and anti-tumor effect of these vaccines. Clinical trials using DNA vaccines as treatments for prostate cancer have begun, and to date have demonstrated safety and immunological effect. This review will focus on DNA vaccines as a specific means of antigen delivery, advantages and disadvantages of this type of immunization, previous experience in preclinical models and human trials specifically conducted for the treatment of prostate cancer, and future directions for the application of DNA vaccines to prostate cancer immunotherapy.

Therapeutic cancer vaccines: the latest advancement in targeted therapy

Therapeutic cancer vaccines represent an emerging therapeutic modality that may play a more prominent role in cancer treatment in the future. Therapeutic cancer vaccines are designed to generate a targeted, immune-mediated antitumor response. There are 2 main types of therapeutic vaccines: patient-specific (generated either from a patient's own cells or tumor) and patient- nonspecific, where a peptide- or vector-based vaccine induces an immune response in vivo against specific tumor-associated antigens. Studies are currently underway to investigate methods to enhance vaccine strategies, including combinations with standard anticancer therapies or immune-modulating agents. Cancer vaccines are usually well tolerated, with minimal toxicity compared with chemotherapy. This review summarizes selected therapeutic cancer vaccines in late clinical development.

Sipuleucel-T immune parameters correlate with survival: an analysis of the randomized phase 3 clinical trials in men with castration-resistant prostate cancer

Purpose

Sipuleucel-T, the first FDA-approved autologous cellular immunotherapy for treatment of advanced prostate cancer, is manufactured by activating peripheral blood mononuclear cells, including antigen presenting cells (APCs), with a fusion protein containing prostatic acid phosphatase. Analysis of data from three phase 3 trials was performed to immunologically characterize this therapy during the course of the three doses, and to relate the immunological responses to overall survival (OS).

Methods

Sipuleucel-T product characteristics [APC numbers, APC activation (CD54 upregulation), and total nucleated cell (TNC) numbers] were assessed in three randomized, controlled phase 3 studies (N = 737). Antigen-specific cellular and humoral responses were assessed in a subset of subjects. The relationships between these parameters and OS were assessed.

Results

APC activation occurred in the first dose preparation [6.2-fold, (4.65, 7.70); median (25th, 75th percentile)] and increased in the second [10.6-fold (7.83, 13.65)] and third [10.5-fold (7.89, 13.65)] dose preparations. Cytokines and chemokines associated with activated APCs were produced during the manufacture of each dose; T-cell activation-associated cytokines were detected in the second and third dose preparations. Antigen-specific T cells were detectable after administration of the first sipuleucel-T dose. Cumulative APC activation, APC number, and TNC number correlated with OS (P < 0.05). Antigen-specific immune responses were observed in 78.8 % of monitored subjects and their presence correlated with OS (P = 0.003).

Conclusion

Sipuleucel-T broadly engages the immune system by activating APCs ex vivo and inducing long-lived immune responses in vivo. These data indicate antigen-specific immune activation as a mechanism by which sipuleucel-T prolongs OS.

Electronic supplementary material

The online version of this article (doi:10.1007/s00262-012-1317-2) contains supplementary material, which is available to authorized users.

Combining immunological and androgen-directed approaches: an emerging concept in prostate cancer immunotherapy

PURPOSE OF REVIEW

The autologous antigen-presenting cell immunotherapy, sipuleucel-T, was the first and remains the only US Food and Drug Administration-approved immunotherapy for prostate cancer. In this article, we will summarize recent clinical data on several additional immune-directed strategies, some of which have now entered phase 3 trials.

RECENT FINDINGS

Multiple studies are now testing sipuleucel-T in different disease settings and/or in combination with conventional and novel hormonal therapies. In addition, a poxviral-based vaccine has shown promise in early-phase clinical studies and has now entered phase 3 testing in men with metastatic prostate cancer. Next, a DNA vaccine has been evaluated in men with biochemically recurrent prostate cancer and has shown early signs of clinical efficacy. Finally, several studies are evaluating the role of immune checkpoint blockade using ipilimumab in pivotal phase 3 trials in prostate cancer patients with advanced disease, as well as in earlier phase studies in combination with androgen ablation.

SUMMARY

The abundance of new treatment options for men with advanced prostate cancer will challenge the role of immunotherapy in these patients. Future progress may rely on optimal combination and sequencing of various immunotherapies with androgen-directed approaches as well as with other standard prostate cancer therapies, an effort which is now just beginning.

A molecular description of acid phosphatase

Acid phosphatase is ubiquitous in distribution in various organisms. Although it catalyzes simple hydrolytic reactions, it is considered as an interesting enzyme in biological systems due to its involvement in different physiological activities. However, earlier reviews on acid phosphatase reveal some fragmentary information and do not give a holistic view on this enzyme. So, the present review summarizes studies on biochemical properties, structure, catalytic mechanism, and applications of acid phosphatase. Recent advancement of acid phosphatase in agricultural and clinical fields is emphasized where it is presented as potent agent for sustainable agricultural practices and diagnostic marker in bone metabolic disorders. Also, its significance in prostate cancer therapies as a therapeutic target has been discussed. At the end, current studies and prospects of immobilized acid phosphatase are included.

Identification of prostatic acid phosphatase (PAP) specific HLA-DR1-restricted T-cell epitopes

BACKGROUND

Prostatic acid phosphatase (PAP) is a prostate cancer tumor antigen and is an immunological target in several active immunotherapy clinical trials for the treatment of prostate cancer. We and others have demonstrated that PAP-specific T-cell responses can be elicited and augmented following antigen-specific immunization in both humans and animal models. We have previously reported that prostate cancer patients immunized with a DNA vaccine encoding PAP (pTVG-HP) developed both CD4+ and CD8+ T-cell responses. PAP-specific, CD4+ T-cell proliferative responses were generated in three out of four HLA-DRB1*0101 patients suggesting the possibility that DR1-restricted epitopes exist.

METHODS

To identify PAP-specific HLA-DRB1*0101 restricted epitopes, we immunized HLA-A2.01/HLA-DRB1*0101 (A2/DR1) transgenic mice with the pTVG-HP DNA vaccine. To map DRB1*0101-restricted epitopes, splenocytes from immunized mice were screened against a library of overlapping 15-residue, PAP-derived peptides using an IFNγ ELISPOT assay.

RESULTS

We identified four HLA-DRB1*0101 epitopes for PAP in A2/DR1 mice (PAP(161-175) , PAP(181-195) , PAP(191-205) , and PAP (351-365) ). T cells specific for one epitope (PAP(181-195) ) were found to be augmented after immunization in a HLA-DRB1*0101+ prostate cancer patient.

CONCLUSIONS

The identification of MHC class II epitopes may provide tools to directly monitor immune responses after vaccination and may be important for the design of future prostate cancer vaccines.

Development of novel immune interventions for prostate cancer

Prostate cancer is the leading cause of cancer-related morbidity and mortality in men in the Western world. Use of traditional and newer therapeutic regimens is constrained in terms of tolerance, efficacy, and cross-resistance. There is a need for newer therapies without overlapping mechanisms of action and toxicities to improve the outcome. Advances in the field of immunology and cancer biology have led to an improved understanding of the interactions between the immune system and tumors, propelling the field of cancer vaccines to the forefront of clinical investigation. Recent US Food and Drug Administration approval of sipuleucel-T, an autologous dendritic cell-based vaccine for the treatment of castration refractory prostate cancer, represents a significant advancement in the field of cancer vaccines. However, the overall survival benefits with sipuleucel-T are modest at best, and the field of cancer vaccine therapy is in a continuous state of evolution and expansion. Further improvements are expected to result from the selection of more appropriate tumor antigens, which circumvent immune tolerance, and from the development of more effective immunization strategies aimed at inducing an effective cytotoxic T-cell response. This review summarizes recent developments in the field of immunotherapy in prostate cancer with a focus on dendritic cell vaccines, virus-based vaccines, DNA-based vaccines, cell-based vaccines, peptide-based vaccine and therapies blocking immune checkpoints to break peripheral immune tolerance.

Immunotherapy in prostate cancer: emerging strategies against a formidable foe

Recent clinical trials have shown therapeutic vaccines to be promising treatment modalities against prostate cancer. Unlike preventive vaccines that teach the immune system to fight off specific microorganisms, therapeutic vaccines stimulate the immune system to recognize and attack certain cancer-associated proteins. Additional strategies are being investigated that combine vaccines and standard therapeutics, including radiation, chemotherapy, targeted therapies, and hormonal therapy, to optimize the vaccines' effects. Recent vaccine late-phase clinical trials have reported evidence of clinical benefit while maintaining excellent quality of life. One such vaccine, sipuleucel-T, was recently FDA-approved for the treatment of metastatic prostate cancer. Another vaccine, PSA-TRICOM, is also showing promise in completed and ongoing randomized multicenter clinical trials in both early- and late-stage prostate cancer. Clinical results available to date indicate that immune-based therapies could play a significant role in the treatment of prostate and other malignancies.

Immunotherapy for prostate cancer: biology and therapeutic approaches

Although prostate cancer was not historically considered to be a particularly immune-responsive cancer, recent clinical trials have demonstrated that immunotherapy for prostate cancer can lead to improvements in overall survival (OS). These studies include randomized controlled trials with sipuleucel-T and another with PROSTVAC-VF, both of which rely on stimulating the immune system to target prostate proteins. This review discusses the most promising developments over the past year in immune-based therapy for prostate cancer and the opportunities that lie ahead. Recent randomized immunotherapy trials in prostate cancer have demonstrated improvements in OS but without the concomitant improvements in progression-free survival. This uncoupling of survival from clinical response poses challenges to clinical management, because conventional measures of objective response cannot be used to identify patients benefiting from treatment. There is a significant need to identify immunologic or clinical surrogates for survival so that clinical benefit can be assessed in a timely manner. Immunotherapy is now an established treatment approach for prostate cancer, with multiple clinical trials demonstrating improvements in OS. Significant challenges to this modality remain, including determining best clinical setting for immunotherapy, identifying patients who benefit, and defining relevant clinical and immunologic end points. Nevertheless, the broader availability of novel immunotherapies will provide opportunities not only to target different components of the immune system but also to combine immunotherapies with other treatments for improved clinical efficacy.

Recent advances in immunotherapy for the treatment of prostate cancer

INTRODUCTION

Prostate cancer vaccines attempt to induce cancer-specific systemic immune responses and represent a new class of targeted therapies, many of which are non-toxic. Several vaccine technologies are in development.

AREAS COVERED

An autologous antigen presenting cell vaccine loaded with prostate acid phosphatase conjugated with GM-CSF, sipuleucel-T confers a survival advantage in men with metastatic castration-resistant prostate cancer (CRPC) and is now FDA approved based on the IMPACT trial. A poxvirus-based vaccine, PROSTVAC-VF TRICOM targeting prostate-specific antigen (PSA), has demonstrated improved survival in a randomized Phase II trial of patients with metastatic CRPC. Novel T lymphocyte checkpoint inhibitors of cytotoxic T lymphocyte antigen 4 and programmed death-1 are also emerging. Recognition of improved survival without an earlier clinical signal of activity by conventional criteria has led to new guidelines to evaluate immunotherapeutic agents. The clinical benefit of combining vaccines with chemotherapy, radiotherapy and other immunotherapeutic and biologic agents is being evaluated in the context of disappointing results of combination GVAX vaccine and docetaxel chemotherapy.

EXPERT OPINION

To build on the success of early phase trials, efforts must be made to optimize vaccine approaches and patient selection.

Characterization of acid phosphatase from the tick Haemaphysalis longicornis

The full-length cDNA encoding acid phosphatase (HL-3) from Haemaphysalis longicornis was obtained by 5' rapid amplification of cDNA ends (RACE). The cDNA contained a 1137 bp open reading frame (ORF) coding for 356 amino acids with a predicted theoretical isoelectric point (pI) of 6.35 and molecular weight of 41.0 kDa. The recombinant protein was expressed in Escherichia coli. The enzyme could hydrolyze para-nitrophenyl phosphate (pNPP) substrate at an optimum pH of 5.0. Real-time RT-PCR analysis showed that the HL-3 transcripts were expressed in various stages of unfed ticks and were significantly induced by blood feeding. Furthermore, the expression of HL-3 in midguts was significantly higher than in other tested tissues of partially fed adult ticks. The transcripts of the HL-3 mRNA in lipopolysaccharide (LPS)-injected ticks were 1.75 times of the PBS-injected control; Theileria sergenti infected larvae expressed 3.86 more times than that of uninfected ones. Western blot analysis showed that rabbit antiserum against the recombinant rHL-3 could recognize a native protein of approximately 41.0 kDa in the lysates from different stages of ticks. Vaccination of rabbits with the rHL-3 conferred partial protective immunity against ticks, resulting in 28% mortality and 10.6% reduction in engorgement weight of adult ticks, respectively. These results suggested that the HL-3 was involved in tick innate immunity and could be used as a potential candidate antigen for the development of anti-tick vaccines.

Sipuleucel-T: immunotherapy for advanced prostate cancer

Prostate cancer continues to be one of the most serious afflictions of men of advanced age, remaining the most commonly diagnosed and second leading cause of cancer-related deaths in American men. The treatment options for patients with incurable metastatic, castrate-resistant disease have long focused on various chemotherapeutic approaches, which provide a slight survival benefit while being associated with potentially significant side effects. However, the recent approval of sipuleucel-T has given patients with advanced disease an additional treatment option that has demonstrated benefit without the side effects associated with chemotherapy. Sipuleucel-T is an antigen-presenting cell-based active immunotherapy that utilizes a patient’s own immune cells, presumably to activate an antigen-specific immune response against tumor cells. This review focuses on the development and implementation of sipuleucel-T as a therapy for prostate cancer. Specifically, we present some of the issues associated with the management of advanced prostate cancer, the research and development that led to the approval of sipuleucel-T, how the approval of sipuleucel-T could change the clinical management of prostate cancer, and current and future areas of investigation that are being pursued with regard to sipuleucel-T and other treatments for advanced prostate cancer.

T cell coinhibition in prostate cancer: new immune evasion pathways and emerging therapeutics

T cell-mediated adaptive immune response is controlled by both positive costimulation and negative coinhibition, generated mainly by the interaction between the B7 family and their receptor CD28 family. Coinhibition is exploited by prostate cancer as an immune evasion pathway. Overexpression of coinhibitory B7x and B7-H3 in prostate cancer correlates with poor disease outcome, whereas tumor-infiltrating immune cells have enhanced expression of PD-L1 and its receptor PD-1. New insights into the complex mechanisms governing B7 expression in the tumor microenvironment have been reported and therapies aimed at overcoming T cell coinhibition with antagonistic monoclonal antibodies are emerging as effective tumor immunotherapies. Therapies that block B7x and B7-H3, either as monotherapies or in synergism with traditional therapies, should be pursued.