Glioblastoma (GBM) is a very lethal tumor and represents the most common primary brain tumor in adults1. The current multimodal therapy of surgery, chemotherapy and radiotherapy is often ineffective. Consequently, higher rates of tumor recurrence and progression are often the rule as opposed to the exception with GBM, and the prognosis is very dismal. Patients expectedly have a median survival of less than 15 months2. Furthermore, by the 5th year of diagnosis, over 90% of patients would have succumbed to the disease3. Novel and effective alternative therapeutic strategies are therefore highly desired.
Immunotherapy is rapidly emerging as a very attractive and novel therapeutic approach for cancer. Cancer-specific immune responses can occur through several non-mutually exclusive strategies which include activation of the immune system with tumor antigens; neutralization of tumor antigens with antibodies; enhancement of immune-stimulatory signaling pathways that are promote cytotoxic T cell activity; or adoptive T cell tumor-targeting mechanisms.
Two pivotal landmark studies resulted in FDA approval of immunotherapy-based treatment for malignant melanoma4 and castration-resistant prostate cancer5. Hodi and colleagues, demonstrated a significant improvement in overall survival(OS) in metastatic melanoma patients treated with ipilimumab, a monoclonal antibody directed against cytotoxic T-lymphocyte antigen 4 (CTLA4)4. By releasing CTLA4 inhibition of cytotoxic T lymphocytes (CTLs), CTLs easily recognized and destroyed melanoma cells. In parallel, Kantoff and colleagues demonstrated a significant improvement in OS for patients with castration-resistant prostate cancer who were treated with a dendritic cell vaccine called sipuleucel-T5. The above studies concretized the role and integration of immunotherapy in the standard of care of select cancers, thereby setting the foundations and enthusiasm for applicability to other cancers.
Most immune-based therapeutic strategies for GBM have focused on the concept of vaccines. The overwhelming majority of GBM vaccine applications have been based on dendritic cells (DCs). DCs are particularly attractive in vaccine applications in light of their exquisite efficient ability to present foreign antigens as antigen presenting cells (APCs) to the immune system thereby generating an antigen-specific adaptive immune response. With this approach, expanded clones of autologous DCs pulsed with either GBM cell lysates or tumor-derived peptides are used for the vaccine (Figure 1). It is anticipated that the DCs will recognize GBM cells bearing applicable antigens leading destruction of residual GBM tumor cells through adaptive immune-mediated mechanisms. A major feature of this approach is its personalized cancer care focus, and the potential to target abroad range of tumor antigens. Potential limitations of this strategy include the requirement for surgical resection, as well as the labor-intensive and complex process of vaccine manufacturing.
The safety, immunogenic potential, and effectiveness of DC vaccines pulsed with GBM tumor cell-lysates or tumor-eluted peptides have been well established in preclinical6-11 as well as clinical studies12-22. The preponderance of evidence suggests that the vaccine strategy is well tolerated, effective, and can improve overall survival in a tumor-specific immune-response dependent fashion. One of the largest clinical series of DC vaccines was by De Vleeschouwer and colleagues18. They safely treated 56 recurrent GBM patients with DCs pulsed with autologous tumor lysate as post-surgical adjuvant therapy. There was a marked tendency towards improvement in both progression free survival (PFS) and overall survival (OS) within the vaccination group. Currently, our center is participating in a multicenter Phase 3 randomized double-blinded clinical trial examining the efficacy of a DC vaccine, DCVax, derived from autologous dendritic cells pulsed with GBM lysates in newly diagnosed GBM.
One of the critical lessons from initial clinical trial efforts with DC vaccines was correlating therapeutics benefits with immunogenicity. The first attempt to establish vaccine efficacy with immunogenicity on the DC platform was in the clinical trial by Wheeler and colleagues19. When they treated 32 patients with GBM using DCs pulsed with GBM lysate, they clearly identified T-cell responsiveness as a variable that strongly correlated with a prolonged survival and prolonged disease progression time in the vaccinated cohort. Subsequent clinical studies have similarly assessed and confirmed correlation between immunogenicity and therapeutic benefit for such patients. Markers of immune- responsiveness could facilitate optimal stratification of patients in the future.
Synthetic peptides derived from tumor-associated antigens have been employed as well in DC vaccines. The ease of manufacture in substantial amounts makes this approach attractive. For GBMs in particular, the mutated epidermal growth factor receptor variant III (EGFRvIII) is a highly immunogenic target with surface expression in 30-40% of GBM23. In preclinical orthotopic GBM models, a synthetic peptide derived from a segment of EGFRvIII demonstrated immunogenicity, significant antitumor activity, inhibition of formation of tumor in 70 percent of vaccinated animals, and ultimately resulted in long-term survivors24. In a subsequent clinical study of newly diagnosed GBMs, the same group was able to demonstrate EGFRvIII-specific immune responses secondary to vaccination using DCs pulsed with the synthetic peptide derived from a segment of EGFRvIII25. Median PFS of 6.8 months and median OS of 18.7 months relative to onset of vaccination were realized representing a significant improvement compared to match controls. Several additional clinical trials are underway examining EGFRvIII as a vaccine target.
In order to broaden the antigen coverage of DC vaccines, another approach has been to pulse DCs simultaneously with a panel of several tumor-associated antigen peptide. Using DCs directed against a panel of 6 glioma-associated antigen peptides, Phuphanich and colleagues demonstrated an overall median survival of 38.4 months in newly diagnosed GBM who expressed at least 3 of the 6 antigens in a Phase 1 clinical trial26. Within the series of 15 patients, 5 patients demonstrated post-vaccination T-cell responsiveness as evidenced by CD8+ and interferon-gamma production. Based on these encouraging findings, placebo-controlled, randomized Phase 3 studies using this 6-peptide panel are underway.
Moreover, the strategy of employing DCs to target tumors antigens can be extrapolated to targeting cancer stem cells that serve as the ultimate drivers of therapeutic resistance and tumor propagation. There is preclinical evidence that DC vaccines can target the tumor stem cell resistant clones if pulsed with stem cell specific antigens27, 28. In a recent clinical study involving 7 GBM patients treated with DC pulsed with mRNA from cancer stem cells, the investigators demonstrated the safety, feasibility as well as the potential for such an approach to positively influence PFS 29. Additional studies are warranted for further validation of this approach.
In summary, DC vaccine strategies have demonstrable clinical feasibility, safety and efficacy in a subset of GBM patients. Efforts at identifying humoral factors that correlate with vaccine efficacy as well as strategies that enhance T-cell responsiveness secondary to vaccination could have a significant impact. The theoretical risk of unintended autoimmune reactions to this vaccine strategy remains extremely low. Several clinical trials are underway looking at whole tumor cell lysates, tumor-eluted peptides, as well as synthetic tumor associated peptides and nucleic acids with exciting prospects. Our center is involved with some of these endeavors notably the whole cell lysate approach for newly-diagnosed GBM in a Phase 3 clinical trial format.
By Arnold Etame, M.D., Ph.D.
Arnold Etame, MD, PhD is a neurological surgeon and scientist specializing in Neuro-Oncology at the Moffitt Cancer Center and assistant professor of oncology at the University of South Florida College of Medicine. He performs a substantial number of surgeries in eloquent and non-eloquent regions of the brain with image-guided stereotactic techniques entailing functional MRI and DTI tractography neuro-navigation. Dr. Etame also directs a very active awake-brain tumor resection program for patients with tumors close to critical areas for speech and movement. His research focuses on enhanced delivery of targeted therapeutics across the blood-brain barrier for malignant and metastatic brain tumors using nanotechnology and focused ultrasound disruption of the blood-brain barrier. He is a principal investigator for clinical trial protocols in patients with malignant brain tumors.