Future directions in tumor immunotherapy: CTLA4 blockade

Soluble B7-CD28 Family Inhibitory Immune Checkpoint Proteins and Anti-Cancer Immunotherapy

Co-inhibitory B7-CD28 family member proteins negatively regulate T cell responses and are extensively involved in tumor immune evasion. Blockade of classical CTLA-4 (cytotoxic T lymphocyte-associated antigen-4) and PD-1 (programmed cell death protein-1) checkpoint pathways have become the cornerstone of anti-cancer immunotherapy. New inhibitory checkpoint proteins such as B7-H3, B7-H4, and BTLA (B and T lymphocyte attenuator) are being discovered and investigated for their potential in anti-cancer immunotherapy. In addition, soluble forms of these molecules also exist in sera of healthy individuals and elevated levels are found in chronic infections, autoimmune diseases, and cancers. Soluble forms are generated by proteolytic shedding or alternative splicing. Elevated circulating levels of these inhibitory soluble checkpoint molecules in cancer have been correlated with advance stage, metastatic status, and prognosis which underscore their broader involvement in immune regulation. In addition to their potential as biomarker, understanding their mechanism of production, biological activity, and pathological interactions may also pave the way for their clinical use as a therapeutic target. Here we review these aspects of soluble checkpoint molecules and elucidate on their potential for anti-cancer immunotherapy.

Trial watch: Monoclonal antibodies in cancer therapy

During the past 20 years, dozens-if not hundreds-of monoclonal antibodies have been developed and characterized for their capacity to mediate antineoplastic effects, either as they activate/enhance tumor-specific immune responses, either as they interrupt cancer cell-intrinsic signal transduction cascades, either as they specifically delivery toxins to malignant cells or as they block the tumor-stroma interaction. Such an intense research effort has lead to the approval by FDA of no less than 14 distinct molecules for use in humans affected by hematological or solid malignancies. In the inaugural issue of OncoImmunology, we briefly described the scientific rationale behind the use of monoclonal antibodies in cancer therapy and discussed recent, ongoing clinical studies investigating the safety and efficacy of this approach in patients. Here, we summarize the latest developments in this exciting area of clinical research, focusing on high impact studies that have been published during the last 15 months and clinical trials launched in the same period to investigate the therapeutic profile of promising, yet hitherto investigational, monoclonal antibodies.

Autoimmune mediated regulation of ovarian tumor growth

OBJECTIVES

An immune response sufficient to induce organ failure may provide protection and therapy against tumors derived from the targeted organ particularly when removal or ablation of the organ is part of the standard therapy and does not threaten survival. We have previously shown that a targeted immune response directed against the ovarian-specific protein, inhibin-α, causes ovarian failure. Here we determined whether inhibin-α autoimmunity is effective in both prevention and treatment of ovarian tumors.

METHODS

A transgene consisting of the SV40 large tumor transformation antigen under the regulation of an anti-Mullerian hormone promoter (AMH-SV40Tag) was transferred by backcrossing for 12 generations to SJL/J mice producing SJL.AMH-SV40Tag (H-2(s)) females that develop a high incidence of autochthonous granulosa cell tumors. We determined whether immunization of SJL.AMH-SV40Tag female mice with the IA(s)-restricted p215-234 peptide of mouse inhibin-α was capable of preventing and treating these ovarian tumors.

RESULTS

The growth of autochthonous ovarian granulosa cell tumors in SJL.AMH-SV40Tag transgenic mice was significantly inhibited in mice immunized with Inα 215-234. In addition, significant inhibition of tumor growth occurred when mice with established ovarian granulosa cell tumors were therapeutically vaccinated with Inα 215-234.

CONCLUSIONS

Our results indicate that induction of ovarian-specific autoimmunity may serve as an effective way to prevent the emergence of autochthonous ovarian tumors and control the growth of established ovarian malignancies.

Immunotherapy for prostate cancer: recent advances, lessons learned, and areas for further research

A surge of interest in therapeutic cancer vaccines has arisen in the wake of recent clinical trials suggesting that such vaccines can result in statistically significant and clinically meaningful improvements in overall survival-with substantially limited side effects compared with chemotherapy-in patients with metastatic castration-resistant prostate cancer. One of these trials led to the registration of sipuleucel-T, the first therapeutic vaccine to be approved for cancer patients. In this review we highlight emerging patterns from clinical trials that suggest a need for more-appropriate patient populations (i.e., with lower tumor volume and less-aggressive disease) and endpoints (i.e., overall survival) for studies of immunotherapy alone, as well as biologically plausible explanations for these findings. We also explore the rationale for ongoing and planned studies combining therapeutic vaccines with other modalities. Finally, we attempt to put these findings into a practical clinical context and suggest fertile areas for future study. Although our discussion focuses on prostate cancer, the concepts we address most likely have broad applicability to immunotherapy for other cancers as well.

Current perspectives in prostate cancer vaccines

The use of vaccines as a potential therapeutic modality for the treatment of cancer has been extensively studied. Recent advances include identification and characterization of tumor-associated antigens, novel vaccine delivery systems, and the combination of vaccines with immune stimulants and other therapeutic modalities. Immunotherapy as a modality for treatment of prostate cancer has received significant attention. There are several characteristics of prostate cancer that make it an ideal target for immunotherapy. Prostate cancer's relative indolence allows sufficient time to generate immune responses, which may take weeks or months to mount. In addition, prostate cancer-associated antigens direct the immune response to prostate cancer cells, thus sparing normal vital tissue. This review focuses on promising new vaccines and novel perspectives in the treatment of prostate cancer.

Therapeutic cancer vaccines in prostate cancer: the paradox of improved survival without changes in time to progression

Therapeutic cancer vaccines represent a new class of agents in the treatment of cancer. Sipuleucel-T is an antigen-presenting cell-based vaccine that recently demonstrated a significant 4.8-month improvement in overall survival in advanced prostate cancer patients and was well tolerated. The findings of that study have been met with skepticism, primarily because the agent did not change initial disease progression and yet led to longer survival. Although the commonly accepted treatment paradigm suggests that treatments should initially decrease tumor volume, perhaps vaccines work differently. Vaccines may induce delayed responses not seen in the first few months of therapy or they may initiate a dynamic immune response that ultimately slows the tumor growth rate, resulting in longer survival. Subsequent therapies may also combine with the induced immune response, resulting in a combination that is more effective than conventional treatments alone. Also, other treatments may alter tumor-associated antigen expression, enhancing the immune response. Future trials are currently planned to investigate these hypotheses; however, the results of the sipuleucel-T vaccine in prostate cancer should not be dismissed. Results with another vaccine in prostate cancer are similar, perhaps suggesting a class effect. In a broader context, clinicians may need to reconsider how they measure success. Several agents have been approved that produce superior disease progression results, but do not affect overall survival. Given the toxicity and costs of cancer therapies, perhaps studies should put more weight on long-term survival endpoints than on short-term endpoints that may be less consequential.

Promising novel immunotherapies and combinations for prostate cancer

The field of therapeutic cancer vaccines is currently in a state of active preclinical and clinical investigation, and certain novel therapies involving tumor immunotherapy have recently come to the forefront of prostate cancer research. While no therapeutic cancer vaccine has yet been approved by the US FDA, recent findings have demonstrated that new paradigms of combination therapies involving vaccines, employed in clinical trials with appropriate design and end points, may ultimately lead to cancer vaccines being used to treat various malignancies. Several characteristics of prostate cancer make it an ideal target for immunotherapy. Its relative indolence allows sufficient time to generate immune responses, which usually take weeks or months to mount. In addition, prostate cancer-associated antigens direct the immune response to prostate cancer cells, thus sparing normal tissue. This review focuses on the future of promising new vaccines and novel perspectives in the treatment of prostate cancer.

CTLA-4: negative regulator of the immune response and a target for cancer therapy

A novel approach for cancer immunotherapy is to augment T-cell-mediated immunity by blocking inhibitory signals that suppress T-cell function. Cytotoxic T-lymphocyte antigen-4 (CTLA-4) is a key negative regulator of T-cell activation. CTLA-4 blockade using anti-CTLA-4 monoclonal antibodies (mAbs) potentiates the T-cell response against tumors, and preliminary data on these agents demonstrate good efficacy and tolerability in the treatment of patients with metastatic melanoma and other cancers. This paper will review data from studies with anti-CTLA-4 mAbs to date, discuss some of the key clinical considerations emerging from early clinical trials with this therapeutic strategy, and provide an overview of ongoing and planned clinical trials for anti-CTLA-4 mAb therapy in metastatic melanoma and other cancers.

Ctla-4 blockade confers lymphocyte resistance to regulatory T-cells in advanced melanoma: surrogate marker of efficacy of tremelimumab?

PURPOSE

Anti-CTL antigen-4 (CTLA-4) monoclonal antibody (mAb) has led to encouraging antitumor activity associated with immune-related adverse events in patients with heavily pretreated melanoma. However, mechanisms of action and surrogate immunologic markers of efficacy have not been reported thus far.

EXPERIMENTAL DESIGN

We monitored the immune responses of 10 melanoma patients included in a phase II clinical trial, which evaluated the efficacy of a second line of therapy of tremelimumab anti-CTLA-4 mAb in patients with metastatic melanoma. The frequency of blood leukocyte populations in association with T cell and regulatory T cell (Treg) functions were evaluated.

RESULTS

Prior to therapy, patients with advanced melanoma presented with a severe CD4+ and CD8+ T cell lymphopenia associated with blunted T-cell proliferative capacities that could be assigned to Treg. Tremelimumab rapidly restored the effector and memory CD4+ and CD8+ T-cell pool and TCR-dependent T-cell proliferation that became entirely resistant to Treg-mediated suppression. Progression-free survival and overall survival was directly correlated with the acquisition of a biological response defined as the resistance of peripheral lymphocytes to Treg-inhibitory effects (obtained in 7 of 10 patients).

CONCLUSION

CTLA-4 blockade seems to be a valuable strategy to revive reactive memory T cells anergized in the context of stage IV melanoma, and our work suggests that memory T-cell resistance to Treg resulting from anti-CTLA-4 treatment might be a biological activity marker for tremelimumab in patients with melanoma.

Immunotherapy for gastrointestinal cancer: current status and strategies for improving efficacy

BACKGROUND

Despite improvement in conventional strategies for treating gastrointestinal (GI) carcinoma, large numbers of patients still suffer from incurable or progressive disease.

OBJECTIVE

Here we consider the prospects for circumventing limitations and maximising the efficacy of different immunotherapies.

METHODS

We summarise different cancer vaccines and targeted drugs and highlight the scientific rationale of using immunotherapy for targeting GI cancers, in addition to the potential strategies for improving immunotherapeutic efficacy.

RESULTS/CONCLUSION

Many cancer vaccines and antibody-directed therapies have been tested in early phase clinical trials and demonstrated proof of concept and safety. As yet few have been properly evaluated for clinical efficacy; although adoptive transfer of tumour-associated-antigen-specific T cells has shown dramatic clinical responses in some patients. The recognition of a role for T regulatory cells in limiting anti-tumour immunity has provided momentum for developing strategies to over-ride such immunoinhibitory effects. There is some evidence that conventional therapies may work by influencing these negative factors and allowing expression of immune control mechanisms. An important developing area for clinical evaluation is the testing of combined conventional and immunotherapeutic modalities which may provide for synergy; thereby circumventing the limitations of individualised treatments and generating additional clinical benefits.

Paradigm shifts in cancer vaccine therapy

Cancer vaccines constitute a unique therapeutic modality in that they initiate a dynamic process involving the host's immune response. Consequently, (a) repeated doses (vaccinations) over months may be required before patient clinical benefit is observed and (b) there most likely will be a "dynamic balance" between the induction and maintenance of host immune response elements to the vaccinations vs. host/tumor factors that have the potential to diminish those responses. Thus "patient response" in the form of disease stabilization and prolonged survival may be more appropriate to monitor than strictly adhering to "tumor response" in the form of Response Criteria In Solid Tumors (RECIST) criteria. This can be manifested in the form of enhanced patient benefit to subsequent therapies following vaccine therapy. This article will review these phenomena unique to cancer vaccines with emphasis on prostate cancer vaccines as a prototype for vaccine therapy. The unique features of this modality require the consideration of paradigm shifts both in the way cancer vaccine clinical trials are designed and in the way patient benefit is evaluated.

Cancer vaccines: moving beyond current paradigms

The field of cancer vaccines is currently in an active state of preclinical and clinical investigations. Although no therapeutic cancer vaccine has to date been approved by the Food and Drug Administration, several new paradigms are emerging from recent clinical findings both in the use of combination therapy approaches and, perhaps more importantly, in clinical trial design and end point analyses. This article will review recent clinical trials involving several different cancer vaccines from which data are emerging contrasting classic "tumor response" (Response Evaluation Criteria in Solid Tumors) criteria with "patient response" in the manifestation of increased patient survival post-vaccine therapy. Also described are several strategies in which cancer vaccines can be exploited in combination with other agents and therapeutic modalities that are quite unique when compared with "conventional" combination therapies. This is most likely due to the phenomena that (a) cancer vaccines initiate a dynamic immune process that can be exploited in subsequent therapies and (b) both radiation and certain chemotherapeutic agents have been shown to alter the phenotype of tumor cells as to render them more susceptible to T-cell--mediated killing. Consequently, evidence is emerging from several studies in which patient cohorts who first receive a cancer vaccine (as contrasted with control cohorts) benefit clinically from subsequent therapies.