Antigen-presenting cells containing multiple costimulatory molecules promote activation and expansion of human antigen-specific memory CD8+ T cells

PROSTVAC® targeted immunotherapy candidate for prostate cancer

Abstract: 

Targeted immunotherapies represent a valid strategy for the treatment of metastatic castrate-resistant prostate cancer. A randomized, double-blind, Phase II clinical trial of PROSTVAC® demonstrated a statistically significant improvement in overall survival and a large, global, Phase III trial with overall survival as the primary end point is ongoing. PROSTVAC immunotherapy contains the transgenes for prostate-specific antigen and three costimulatory molecules (designated TRICOM). Research suggests that PROSTVAC not only targets prostate-specific antigen, but also other tumor antigens via antigen cascade. PROSTVAC is well tolerated and has been safely combined with other cancer therapies, including hormonal therapy, radiotherapy, another immunotherapy and chemotherapy. Even greater benefits of PROSTVAC may be recognized in earlier-stage disease and low-disease burden settings where immunotherapy can trigger a long-lasting immune response.

Cancer vaccines targeting carcinoembryonic antigen: state-of-the-art and future promise

Concurrent with the US FDA's approval of the first therapeutic cancer vaccine, and supported by mounting clinical evidence indicating that targeting carcinoembryonic antigen (CEA) can safely overcome pre-existing tolerance, a multitude of novel CEA cancer vaccines are now in various stages of development. Since cancer-driven immune suppression often limits the efficacy of vaccines, numerous strategies are being examined in both preclinical and clinical settings to overcome immunosuppressive elements, including the combined use of vaccines with certain chemotherapies, immune checkpoint inhibitors, small-molecule targeted therapies and radiation. This review discusses the current state and future direction of therapeutic cancer vaccines targeting CEA, based on advances achieved over the last 5 years.

Poxviral vectors for cancer immunotherapy

INTRODUCTION

Poxviral vaccines have been given to over 1 billion people in the successful global eradication of smallpox. Recombinant poxviruses have been investigated extensively as a novel immunotherapy for cancer, undergoing several iterations to optimize their immunogenicity and efficacy. The current platform expressing multiple costimulatory molecules plus a tumor-associated antigen such as PSA, that is, PSA-TRICOM (PROSTVAC-V/F), is promising and is currently in a Phase III randomized, placebo-controlled clinical trial in metastatic castration-resistant prostate cancer.

AREAS COVERED

This review discusses the clinical development of poxviral-based cancer vaccines, with a particular focus on the rationale for combining vaccines with other treatment modalities, including radiotherapy, chemotherapy, hormonal therapy, other immune-based therapies and molecularly targeted therapy. We also discuss the importance of appropriate patient selection in clinical trial design.

EXPERT OPINION

Preclinical and early clinical studies employing poxviral-vector vaccines have shown promising results with this novel immunologic approach, both alone and combined with other therapies. The challenges of translating the science of immunotherapy to clinical practice include clinical trial design that includes appropriate patient selection, appropriate end points and identification of meaningful surrogate biomarkers.

Clinical evaluation of TRICOM vector therapeutic cancer vaccines

We have developed an "off-the-shelf" vector-based vaccine platform containing transgenes for carcinoma-associated antigens and multiple costimulatory molecules (designated TRICOM). Two TRICOM platforms have been evaluated both preclinically and in clinical trials. PROSTVAC consists of rV, rF-PSA-TRICOM and is being used in prostate cancer therapy trials. PANVAC consists of rV, rF-CEA-MUC1-TRICOM; the expression of the two pan-carcinoma transgenes CEA and MUC-1 renders PANVAC vaccination applicable for therapeutic applications for a range of human carcinomas. Many new paradigms have emerged as a consequence of completed and ongoing TRICOM vaccine trials, including (1) clinical evidence of patient benefit may be delayed, because multiple vaccinations may be necessary to induce a sufficient anti-tumor immune response; (2) survival, and not strict adherence to RECIST criteria or time-to-progression, may be the most appropriate trial endpoint when TRICOM vaccines are used as monotherapy; (3) certain patient populations are more likely to benefit from vaccine therapy as compared to other therapeutics; and (4) TRICOM vaccines combined with standard-of-care therapeutics, either concomitantly or sequentially, are feasible because of the limited toxicity of vaccines.

Targeting costimulatory molecules to improve antitumor immunity

The full activation of T cells necessitates the concomitant activation of two signals, the engagement of T-cell receptor by peptide/major histocompatibility complex II and an additional signal delivered by costimulatory molecules. The best characterized costimulatory molecules belong to B7/CD28 and TNF/TNFR families and play crucial roles in the modulation of immune response and improvement of antitumor immunity. Unfortunately, tumors often generate an immunosuppressive microenvironment, where T-cell response is attenuated by the lack of costimulatory molecules on the surface of cancer cells. Thus, targeting costimulatory pathways represent an attractive therapeutic strategy to enhance the antitumor immunity in several human cancers. Here, latest therapeutic approaches targeting costimulatory molecules will be described.

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.