Vaccine Therapy, Oncolytic Viruses, and Gliomas

After years of active research and refinement, vaccine therapy and oncolytic viruses are becoming part of the arsenal in the treatment of gliomas. In contrast to standard treatment with radiation therapy and chemotherapy, vaccines are more specific to the patient and the tumor. The majority of ongoing vaccine trials are investigating peptide, heat shock protein, and dendritic cell vaccines. The immunosuppression triggered by the tumor itself and by its treatment is a major obstacle to vaccine and oncolytic virus therapy. Thus, combination therapy with different agents that affect the immune system will probably be necessary.

Introduction

Despite decades of research in developing sophisticated surgical and radiation techniques and in optimizing chemotherapy agents, the survival of patients with malignant glioma remains limited. Even the best surgical techniques will always be plagued by the infiltrative, insidious nature of gliomas within the central nervous system (CNS). Given at sufficient doses, radiation therapy can eradicate almost all neoplastic cells, but unfortunately, the curative potential of this therapy is limited by a low therapeutic index in the nervous system. The damage to normal tissue within the radiation portal is almost inevitable because the radiation dose required for tumor control is very close to, if not higher than, the toxic dose for neighboring normal tissue.

The efficacy of chemotherapy is limited as well. Given at a high enough dose, chemotherapy can eliminate a significant quantity of cancer cells. However, this is achieved at the cost of severe toxicity to the bone marrow, gastrointestinal tract, and other sites, which often puts patients at significant risk for hemorrhage, infection, and other adverse events, and can even cause death. As research advances, the previous goal of “curing” malignant gliomas by removing all the neoplastic cells surgically or exterminating them with radiation therapy and chemotherapy has been replaced by the goal of “controlling” the cancer.

The development of therapeutic strategies that will diminish glioma cells by a combination of surgery, radiation therapy, and chemotherapy, but that will also awaken the patient’s own defense mechanisms, is now feasible. Over a century ago, the immune system was hypothesized to have a key role in recognizing and eliminating the spontaneous development of neoplastic cells.[1] To be able to grow and proliferate, tumor cells develop the ability to evade recognition by the immune system.[2] Cancer immunoediting is the process of preventing spontaneous tumor formation and limiting the growth of established tumors via effective antitumor immunity.[3]

Glioblastomas (GBMs) trigger an environment of immunosuppression in both the peripheral blood and the tumor, which can in part explain the shortened time to progression and limited survival of patients.[4-16] To complicate matters further, despite the ability of systemically activated T cells to cross the blood-brain barrier, the CNS is still considered a relatively immune-privileged location, which helps brain tumors evade immune detection.[17,18]

Given its potential to manipulate or enhance the immune system machinery to attack and kill tumor cells, immunotherapy is now gaining momentum in the clinic after decades, if not a century, of investigation. Multiple strategies to manipulate or enhance the immune system are being investigated, including checkpoint inhibitors, vaccine therapies, oncolytic viruses, and gene therapy. With the aim of inducing a specific host immune response to tumor-associated antigens (TAAs),[19] vaccine and virus-based immunotherapies[2] are particularly appealing as treatment strategies-compared with surgery, radiation therapy, and chemotherapy-because of high specificity for cancerous tissue and low toxicity to surrounding normal brain tissue.[20] Furthermore, vaccine and oncolytic virus therapies have the potential to generate long-term immunity directed against cancer cells.[21]

Multiple essential steps are necessary to obtain an effective immune response against the tumor, including TAAs; antigen-presenting cells, such as dendritic cells (DCs); and effector lymphocytes (T cells and B cells). Effector T cells comprise CD4+ helper cells, which are responsible for triggering cytokines in charge of stimulating and regulating other immune components, and CD8+ cytotoxic T cells (CTLs), which have the potential to kill cancerous cells. Certain elements of the innate immune system are also essential: natural killer cells, heat shock proteins (HSPs), and toll-like receptors.[21]

In this article, we will discuss some of the vaccination and oncolytic virus strategies being evaluated in the clinic for malignant gliomas (Table). The vaccines reviewed here include the cell-based (DC vaccines) and the non–cell-based (peptide and HSP vaccines).

Rationale for Vaccine Therapies in Gliomas and Clinical Experience

Peptide vaccines

Among the major vaccine strategies in malignant glioma are peptide vaccines, which involve the direct administration of TAAs.[22] Optimally, peptide vaccines are engineered to trigger an immune response against antigens present only in tumor cells and not in normal brain cells; this minimizes the risk of autoimmunity.[2,23] Peptide vaccines are often coadministered with immunostimulatory adjuvants to assist the cross-presentation of the antigen.[20,23,24] In fact, peptide vaccines are similar to standard vaccines for infectious diseases.[25]

One of the most well-known peptide vaccines is rindopepimut, which is directed against epidermal growth factor receptor variant III (EGFRvIII). EGFRvIII is a glioma-specific epitope expressed in 27% to 67% of GBMs.[26,27] Multicenter phase II trials have demonstrated the efficacy and safety of rindopepimut when administered in combination with temozolomide in the adjuvant phase (following concomitant radiation therapy and temozolomide) in patients with newly diagnosed GBM. In two multicenter phase II trials, median overall survival (OS) ranged from 21.8 to 26 months.[28,29] No symptomatic autoimmune reactions were observed in either trial. Almost all patients had mild to moderate injection site reactions (grade 1/2).[28,29] Schuster et al observed additional treatment-related toxicity: fatigue (26%), rash (17%), nausea (12%), pruritus (2%), and headache (8%), as well as grade 3/4 adverse events (limited to one patient each) of elevation of alanine aminotransferase, angioedema, asthenia, elevation in lactate dehydrogenase, fatigue, elevation of gamma-glutamyl transferase, headache, hypokalemia, hypophosphatemia, leukopenia, lymphopenia, toxic epidermal necrolysis, and urticaria.[29] No fatal adverse events were reported.

The suggestion of increased survival and acceptable toxicity led to a phase III, international, randomized, double-blind, controlled study of rindopepimut/granulocyte-macrophage colony-stimulating factor (GM-CSF) with adjuvant temozolomide in patients with newly diagnosed, surgically resected, EGFRvIII-positive GBM (the ACT IV study). Enrollment in ACT IV has now been completed, and the results should be available by the end of 2016 or the beginning of 2017.

Rindopepimut in combination with bevacizumab was investigated in patients with recurrent GBM in a phase II clinical trial (the ReACT trial). Results of ReACT were recently presented at the 2015 Annual Meeting of the Society for Neuro-Oncology.[30] Thirty-six patients were enrolled in the combination arm (rindopepimut and bevacizumab), and 37 patients were enrolled in the control arm (keyhole limpet hemocyanin [KLH] and bevacizumab). Median OS was 11.3 months for the combination arm vs 9.3 months for the control arm (hazard ratio [HR], 0.53; P = .0137); 25% of patients in the combination arm (rindopepimut and bevacizumab) were still alive at 24 months, compared with 0% of those in the control arm.

No serious adverse events attributed to rindopepimut occurred, and no patients discontinued rindopepimut because of adverse events. No increase in cerebral edema was reported. One grade 2 hypersensitivity reaction and frequent grade 1/2 injection site reactions (erythema and pruritus) were observed. An objective radiographic response was noted in 30% of patients in the treatment arm vs 18% of those in the control arm. Furthermore, more patients were able to discontinue corticosteroids and for a longer period in the treatment arm than in the control arm: 9 out of 18 patients were able to discontinue corticosteroids for ≥ 2 months and 6 out of 18 for ≥ 6 months in the treatment arm vs 5 out of 19 and 0 patients, respectively, in the control arm. Despite the small number of patients enrolled, this double-blind randomized trial showed a marked survival benefit, as well as an advantage to the combination of rindopepimut and bevacizumab in terms of progression-free survival (PFS), objective response rate, and corticosteroid requirement when compared with the control group treated with KLH and bevacizumab.[30]

Peptide vaccines are relatively easy to produce and administer[31]; however, the possibility of antigen escape exists.[32] Previous phase II trials of rindopepimut have shown elimination of EGFRvIII from recurrent tumor samples after > 3 months of therapy in 67% to 82% of patients.[28,29]

Because of this, peptide vaccines might require multipeptides. One such vaccine is IMA950, which is composed of 11 human leukocyte antigen (HLA)-binding tumor-associated peptides. Forty-five patients with newly diagnosed GBM were enrolled in a first-in-human phase I clinical trial of IMA950 plus GM-CSF. Adverse events related to the study drug included minor injection site reaction (58%); rash (11%); pruritus (9%); fatigue (7%); neutropenia (4%); and single cases of allergic reaction, anemia, and anaphylaxis. Two grade 3 dose-limiting toxicities were observed: fatigue and anaphylaxis. Of 40 evaluable patients, 36 (90%) were tumor-associated peptide responders, and 20 (50%) were multiple tumor–associated peptide responders. The 6-month PFS rate was 74%. Given the good tolerability and the fact that more than 30% of patients demonstrated a response to multiple tumor–associated peptides, it was felt that further investigation was warranted.[33] A phase I/II clinical trial of IMA950 with intramuscular poly-ICLC in combination with adjuvant temozolomide in HLA-A2–positive patients with newly diagnosed GBM is currently underway.

Heat shock protein vaccines

HSPs are principally implicated in the regulation of protein chaperoning and protein folding[1,34] and, to a lesser degree, in the immune response.[35,36] The goal of an HSP vaccine is the internalization of the HSPs by the antigen-presenting cells[1,34]-by a receptor-mediated endocytosis (eg, CD14, CD91)[35-38]-with subsequent presentation of the peptides on the major histocompatibility complexes (MHCs) in order to generate immunogenicity against those antigens.[36-43]

The polyvalent HSP protein complex-96 vaccine (HSPPC-96; Prophage) was evaluated in patients with gliomas.[40] This vaccine contains patient-specific TAAs conjugated to HSP gp96. In a phase II, open-label, single-arm trial for patients with surgically resectable recurrent GBM, a median OS of 42.6 weeks was observed in the 41 patients who underwent gross total resection and received the vaccine.[44] Injection site reactions were the most frequent adverse events related to vaccination. Only one patient experienced grade 3 fatigue related to vaccination. Otherwise, grade 3/4 serious adverse events were deemed expected as part of a surgical procedure. No deaths attributable to the vaccine were observed. The study group concluded that the HSPPC-96 vaccine was safe and warranted further investigation.[44] As a result, a phase II three-arm trial evaluating HSPPC-96 in combination with bevacizumab vs HSPPC-96 followed by bevacizumab at progression vs bevacizumab alone has been initiated and is recruiting patients.

One of the HSP vaccine’s strengths is also one of its limitations. The potential of the vaccine as a means of extracting an enriched source of TAAs for antigenic presentation is limited by the significant quantity of tissue required, which limits patient eligibility.[45] It is possible that the therapy might be better suited to patients with a recent diagnosis, when maximal tumor resection is the goal for every patient and before the tumor tissue has been degraded by other therapies, such as radiation therapy and chemotherapy.

Dendritic cell vaccines

DC vaccines have been extensively investigated in patients with GBM.[2,46-52] DCs are a subset of immune cells that act as “professional” antigen-presenting cells.[23] They play a significant role in generating CD4 and CD8 immune responses.[2,22] DC vaccines usually require extraction of the patient’s DCs and “loading” of the DCs with glioma cell antigens by fusion with MHC-matched glioma cells, or by pulsing with apoptotic tumor cells, total tumor RNA, tumor lysates, or peptides.[2,53] After the DCs with glioma cell antigens are administered to the patient,[22] they present the glioma antigens, activate cytotoxic CD8+ cells and CD4+ T helper cells, and induce tumor cell death.[2,54-58] The patient’s DCs are usually extracted via leukapheresis, in which high amounts of peripheral blood mononuclear cells are collected.[21]

The DCVax platform is probably the most well known of the DC vaccines. DCVax-L is composed of autologous differentiated DCs that are pulsed with autologous tumor lysate; it is being investigated in patients with GBM.[59] Preliminary data show that for 20 patients with newly diagnosed GBM who were treated with DCVax-L in combination with radiation therapy and temozolomide, the median PFS was approximately 24 months and the median OS was 36 months; 2 patients survived more than 10 years after diagnosis.[60] DCVax-L is currently being investigated in a phase III trial for patients with newly diagnosed GBM, and it is estimated that primary data should be available by the fall of 2016.[61]

ICT-107 is an autologous, six-antigen DC vaccine-loaded with MAGE-1 (HLA-A1), AIM-2 (A1), gp100 (A2), IL-13Rα2 (A2), HER2/neu (A2), and TRP2 (A2)-which was recently investigated in a phase II, double-blind, placebo-controlled trial. The trial randomized 124 patients in a 2:1 fashion. Twenty-four US sites participated in the trial. The vaccine was added to the standard of care of surgery, radiation therapy, and temozolomide. HLA-A2–positive patients had a stronger immune response than HLA-A1–positive patients: 50% vs 34% (P = .0578), respectively. Immune-responding HLA-A2–positive patients treated with ICT-107 had a longer OS than nonresponder HLA-A2–positive patients who received ICT-107: 22.5 vs 15.2 months (P = .0147).

In the intent-to-treat population, patients treated with ICT-107 had a median OS of 18.3 months, compared with 16.7 months for the control group. In HLA-A2–positive patients who were unmethylated for the O⁶-methylguanine-DNA methyltransferase (MGMT) gene promoter, those in the treatment arm had a median OS of 15.8 months, compared with 11.8 months for the control group. For HLA-A2–positive MGMT-methylated patients treated with ICT-107, the median OS was 37.7 months, compared with 23.9 months for HLA-A2–positive MGMT-methylated patients in the control group.[62] A phase III clinical trial is currently underway for patients with HLA-A2–positive GBM who have less than 1 cm³ of residual disease on a magnetic resonance imaging (MRI) scan obtained following standard combined radiation therapy and temozolomide.

Every-2-weeks ultrasound-guided intranodal administration of an α-type-1-polarized DC (αDC1) vaccine loaded with synthetic peptides for glioma-associated antigen (GAA) epitopes was evaluated in a phase I/II trial in combination with twice-weekly intramuscular injection of poly-ICLC in HLA-A2–positive patients with recurrent malignant gliomas. Twenty-two patients with malignant gliomas (13 with GBMs and 9 with anaplastic gliomas) were enrolled in the study. Toxicities were similar to those observed with other vaccine strategies. No grade 3 or 4 toxicities, deaths, or autoimmunity were observed. Grade 1/2 injection site reactions occurred in 82% of patients, and grade 1 flu-like symptoms-including fatigue, myalgia, fever, chills/rigors, and headache-were common in the 24 hours following vaccination. One patient had grade 2 lymphopenia. Median PFS was 4 months for patients with GBM and 13 months for those with anaplastic glioma. Fifty-eight percent of patients demonstrated a positive immune response against at least one of the vaccination-targeted GAAs in their peripheral blood mononuclear cells. Furthermore, peripheral blood samples demonstrated significant upregulation of type 1 cytokines and chemokines, including interferon-α and chemokine (C-X-C motif) ligand 10.[51]

A vaccine composed of DCs pulsed with cytomegalovirus (CMV) phosphoprotein 65 (pp65) RNA has been shown to trigger a significant immune response when coadministered with the tetanus-diphtheria toxoid or basiliximab, an anti-CD25 antibody. Forty-one patients with newly diagnosed GBM were enrolled in a phase I clinical trial of the coadministration of the DC CMV vaccine with basiliximab. The patients underwent leukapheresis after gross total tumor resection. Coadministration of the DC CMV vaccine and basiliximab was initiated on day 21 of the first cycle of adjuvant temozolomide, after combined radiation therapy and temozolomide. No dose-limiting toxicity or adverse events attributable to treatment were observed.[63]

In a small trial of 12 patients (6 patients per arm), the same DC CMV vaccine demonstrated a significant increase in both PFS (HR, 0.845; P = .027) and OS (HR, 0.820; P = .023) when administered with the tetanus-diphtheria toxoid as a preconditioning of the vaccine site, compared with vaccine site preconditioning with mature DCs only.[64] Patients with vaccine site preconditioning with mature DCs only had a median PFS of 10.8 months and a median OS of 18.5 months from the time of diagnosis. In contrast, three of the six patients with tetanus-diphtheria toxoid preconditioning at the vaccine site had not progressed and were alive at the time of survival analysis at more than 36.6 months.[64] A phase II randomized trial of the DC CMV vaccine in combination with unpulsed DC preconditioning prior to vaccination vs tetanus-diphtheria toxoid preconditioning prior to vaccination or basiliximab infusions with tetanus-diphtheria toxoid preconditioning prior to vaccination is presently enrolling patients, with a goal of about 102 patients.

Rationale for Oncolytic Virus Immunotherapies in Gliomas and Clinical Experience

Another approach to triggering an immune response to brain tumors is oncolytic virus immunotherapy. A number of oncolytic viruses are capable of selective tumor cell killing, with a range of inflammatory and immune-stimulatory effects on the tumor itself, the tumor stromal component, and the host immune system at large. The goal of oncolytic virus immunotherapy is to recruit effector adaptive immune responses against TAAs that can produce lasting immunologic control of cancers.

DNX-2401

DNX-2401 (formerly known as Delta-24-RGD) is a novel replication-competent, tumor-selective, oncolytic adenovirus.[65] In the first-in-human phase I clinical trial of DNX-2401, patients with recurrent malignant gliomas were assigned to two arms. Twenty-five patients were enrolled in arm A and received one intratumoral injection of DNX-2401 into biopsy-proven recurrent glioma. In arm B, 12 patients had an initial intratumoral injection via an implanted catheter, followed 14 days later by tumor resection and injection of DNX-2401 into the resection cavity. A maximum tolerated dose of 3 × 10¹⁰ virus particles was identified. A median OS of 11 months was observed, and three patients (12%) had complete responses. Imaging studies showed an increase in enhancement before tumor regression, which was consistent with inflammatory-mediated responses. The three responders had 10- to 1,000-fold increases in interleukin-12p70, which induces T helper 1 responses and cell-mediated immunity.[65] In a symptomatic patient in arm A, histologic analysis of a resected lesion identified macrophages and CD8 inflammatory-mediated responses during the period of increased MRI enhancement. Two additional trials that are evaluating DNX-2401 are currently recruiting patients.

G207

G207, a mutant herpes simplex virus (HSV) type 1, was evaluated in a phase I trial that assessed the safety of the stereotactic intratumoral administration of G207 when given 24 hours before a single radiation dose (5 Gy) in patients with recurrent malignant glioma. Nine patients were treated in the study with good tolerability, and HSV encephalitis did not develop in any patient. The median OS was 7.5 months (95% CI, 3.0–12.7 months), and 6 of 9 patients had stable disease or partial response as best radiographic response.[66]

Oncolytic parvovirus H-1

The oncolytic parvovirus H-1 (H-1PV) is being evaluated in a phase I/IIa trial. It is planned that a total of 18 patients with recurrent GBM will be treated in two groups. Patients in the first group will receive H-1PV by intratumoral injection, followed by administration into the walls of the tumor cavity during tumor resection. In the second group, the virus initially will be injected intravenously and then into the surrounding brain tissue during tumor removal.[67]

PVSRIPO

PVSRIPO is a genetically recombinant, nonpathogenic poliovirus:rhinovirus chimera with a tumor-specific conditional replication phenotype.[68] It consists of the genome of the live-attenuated poliovirus serotype 1 (Sabin) vaccine with its cognate internal ribosome entry site (IRES) element replaced with that of the human rhinovirus type 2. PVSRIPO tumor tropism is mediated by the poliovirus receptor CD155. CD155, an oncofetal cell adhesion molecule ectopically upregulated in ectodermal/neuroectodermal cancers, is broadly expressed on cancerous cells, cancer “stem cell–like” cells, and tumor-associated proliferating vasculature. Infection with PVSRIPO results in swift destruction of tumor cells (Figure). Poliovirus-inherent neuropathogenicity was removed by IRES exchange; this ablated the virus’s ability to propagate in cells of neuronal lineage and to cause poliomyelitis.[68]

The first-in-human PVSRIPO clinical trial was initiated at the Preston Robert Tisch Brain Tumor Center at Duke University Medical Center in early 2012 as a phase I trial in patients with histologically confirmed recurrent World Health Organization grade IV malignant glioma (GBM or gliosarcoma). PVSRIPO was infused intratumorally via convection-enhanced delivery. Only one dose-limiting toxicity was observed: an intracranial hemorrhage at the time of catheter removal. Adverse events observed so far have been related to secondary inflammatory response and prolonged corticosteroid use. Initial worsening on MRI suggestive of inflammatory toxicity seems to improve over time at lower dose levels. A dose level of 5.0 × 10⁷ tissue culture infective dose has been determined as optimal. As of the last update, three patients were disease-free more than 23, 35, and 36 months after PVSRIPO infusion.[69]

Limitations and Challenges

Unfortunately, any oncology therapy is plagued by multiple limitations. For vaccine therapy for malignant gliomas, finding the right target is a significant challenge. GBMs are highly heterogeneous, which limits the clinical usefulness of vaccines directed to one target, as demonstrated by some trials.[47,70] Some vaccines with only one target have shown a survival benefit, which could potentially be improved if it were not dampened by tumor recurrence caused by glioma cells that lack the target and are thus resistant to the immune response triggered by the vaccine.[23,28] The identification of more broadly present targets, as well as the use of polyvalent vaccines or better combination strategies, might overcome this difficulty. Combination strategies to better target the “immune-evasive” tumor cells could be an important avenue, with the potential to combine vaccines or oncolytic viruses with chemotherapies, checkpoint inhibitors, adaptive immunotherapy, or other biologic therapies that can help in the recruitment of natural killer cells or macrophages.[23,71] A better understanding of the interplay between immune response and chemotherapy, radiation therapy, and even surgery will facilitate improved administration of the therapies[23] and determination of the optimal timing of immunotherapy in overall treatment.[72] The generalized use of corticosteroids in the management of intracranial edema in malignant gliomas is also a significant concern in the optimization of immunotherapy strategies.[70] In addition, the cost and technical demands of vaccine preparation remain a limitation, mostly for DCs and autologous tumor cell vaccines.[70,71]

Another challenge is the evaluation of the imaging of malignant gliomas once they have been treated with immunotherapy. The primary goal of the updated response assessment criteria for high-grade gliomas, published in 2010,[73] was to refine the previously used Macdonald criteria.[74] The update had been necessitated by the advent of antiangiogenic therapies and the description of pseudoprogression following radiation and temozolomide therapy.[73] Imaging changes related to immunotherapy were mentioned in the updated response assessment criteria for high-grade glioma,[73] but a need to refine the imaging criteria for these patients was identified. Often, the radiographic assessment of patients treated with immunotherapy, including vaccine therapy and oncolytic viruses, is plagued by an increase in cerebral inflammation, progression of the treated tumor, or even the appearance of new tumors, followed by a delayed tumor shrinkage leading to prolonged survival. The previous description of tumor progression, which was primarily based on imaging, is now blurred, making progression difficult to define. As a result, a report on immunotherapy response assessment in neuro-oncology was recently published.[75] As new vaccines, oncolytic viruses, and combination strategies of immunotherapy agents are developed, imaging criteria specific to each intervention will probably be necessary. Finally, selecting the appropriate therapy for each patient will be crucial.

Conclusion

Multiple vaccine and oncolytic virus strategies have demonstrated safety and shown early signs of efficacy in phase I and II clinical trials, and many agents are now undergoing phase III investigations. While these vaccine and oncolytic virus strategies are evaluated, other immunotherapies (such as checkpoint inhibitors), new therapeutic strategies, and a better understanding of the biology of malignant gliomas will hopefully help in the development of the best treatment approach for each patient.

Financial Disclosure:Dr. Desjardins is co-owner of the intellectual property related to the Oncolytic Poliovirus for Human Tumors; has served on advisory boards for Cavion LLC, EMD Serono, Inc, Genentech, Novella, and PTC Therapeutics, Inc; and has stock options in ISTARI Oncology, Inc. Dr. Vlahovic has received consulting fees from Bristol-Myers Squibb, Genentech, Hospira, and Pfizer. Dr. Friedman is co-owner of the intellectual property related to the Oncolytic Poliovirus for Human Tumors; has received consulting fees from Genentech; and has stock options in Arno Therapeutics, ISTARI Oncology, Inc, and Tactical Therapeutics.

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