General Principles of Immunotherapy for Glioblastoma

Background

Despite trimodal therapy consisting of maximal safe resection and adjuvant partial radiotherapy with concurrent and subsequent temozolomide, glioblastoma (GBM) has a dismal prognosis, with a median survival of 14.6 months and an overall survival of only 9.8% at 5 years.

Further contributing to therapeutic challenge is the intertumoral and intratumoral heterogeneity of GBM. The strongest predictor for response to temozolomide according to Stupp and colleagues was methylation of the MGMT (O6-alkylguanine DNA alkyltransferase) promotor. However, intratumoral heterogeneity in MGMT methylation status has been noted, perhaps explaining the positive response to temozolomide in some patients with unmethylated GBMs. Furthermore, a small population of glioma cancer cells, postulated to be stemlike cells that are able to self-renew and produce diverse daughter cells, have been shown to be chemoresistant and radioresistant, leading to inevitable tumor recurrence.

In this context, there has been considerable interest in an alternate approach to cancer therapy to further prolong survival and perhaps even offer a cure, namely by harnessing the unique specificity of the immune system. The central nervous system (CNS) has classically been considered immunologically less active, based on early experiments showing prolonged survival of skin grafts within the CNS compared with other sites. This notion has historically been furthered by reports of isolation from the immune system imposed by the blood-brain barrier (BBB), absence of CNS lymphatic drainage, and immune incompetence of native antigen-presenting cells (APCs). In contrast, active immune responses in the CNS are common clinical entities, such as in multiple sclerosis or Alzheimer disease.

Recent developments argue against the dogma of the CNS as an immunologically isolated site. The BBB is disrupted by tissue injury and inflammation, notably in the setting of malignant gliomas, allowing entry of immune cells into the CNS. Even in the absence of BBB disruption, peripheral immune cells can circumvent the BBB using trafficking signals. Moreover, CNS T cells and antigens drain into cervical lymphatics via the subarachnoid space, along the olfactory nerve, and across the cribriform plate. In addition, the resident macrophages of the CNS, microglia, can express major histocompatibility complex (MHC) class II antigens and induce differentiation of naive T cells.

The association between infection and remission of cancer (the infection-remission coincidence) has been noted in the literature for more than a hundred years. More recently, a retrospective, single-center, cohort study has reported a 2-fold survival benefit in patients with GBM who developed postoperative bacterial infections. In addition, an inverse relationship between atopic disease and gliomas has also been observed in numerous reports (as reviewed in Ref. ). The US Food and Drug Administration (FDA) approval of the antigen-specific agent sipuleucel-T for prostate cancer in 2010, and an immune checkpoint inhibitor ipilimumab for metastatic melanoma in 2011, among others, has led to the emergence of cancer immunotherapy into the clinical mainstream. These developments, in addition to the dismal prognosis of patients with GBM with standard therapy, have sparked interest in the potential of harnessed immunotherapy for brain tumors.

Cancer immunotherapy can be broadly defined as therapy based on the immune system’s ability to target and kill tumor cells. It can be classified by mechanism into immunomodulatory, passive, and active immunotherapy. Immunomodulatory therapy involves administration of interleukins, cytokines, and chemokines to enhance the antitumor activity of native effector cells. A phase III trial in recurrent GBMs is currently underway that involves inactivation of checkpoint mediators CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) and PD-1 (programmed cell death protein 1). Passive immunotherapy historically refers to administration of antibodies to target tumor antigens (eg, antibodies against the HER2 [human epidermal growth factor receptor 2]/neu receptor in breast cancer), and is currently undergoing resurgence in the form of transfer of ex vivo expanded effectors cells. In addition, active immunotherapy depends on activation of the patient’s immune system with so-called tumor vaccines, a term that encompasses a variety of products. Examples of phase III trials of active immunotherapy in GBM include one based on dendritic cells (DCs) ( NCT00045968 ), and another based on peptides ( NCT01480479 ), the latter of which has recently been discontinued.

Peptide targeting

Considerable effort has been invested in the peptide vaccine rindopepimut (also called cdx-110) targeting the tumor antigen epidermal growth factor receptor variant III (EGFRvIII). Rindopepimut is a peptide that spans the mutation site of EGFRvIII, conjugated to the immunogenic carrier protein keyhole limpet hemocyanin. Peptide vaccines have the advantage of being simple to manufacture compared with vaccines that require harvesting tumor or serum samples from patients. Three phase II trials of rindopepimut conducted on patients with newly diagnosed, EGFRvIII-positive GBM have shown promising results, with reports of overall survival of 24 to 26 months ( Fig. 19.1 ). Most tumors showing progression no longer expressed EGFRvIII. According to the immunoediting hypothesis, this finding suggests that rindopepimut was effective in eradicating its target cell population via immunologic pressure. A phase II trial in recurrent GBM showed a survival advantage of 11.6 versus 9.3 months with the addition of rindopepimut to bevacizumab. In spite of these early successes, a phase III trial of rindopepimut for newly diagnosed GBM has recently been discontinued with an interim analysis showing equivalent overall survival in the treatment group (20.4 months) versus control (21.1 months). These results emphasize that further work is needed to refine the peptide vaccine approach for EGFRvIII.

EGFRvIII is expressed on the cell surfaces in approximately 30% of all human GBMs, representing the most common epidermal growth factor receptor (EGFR) mutation. More recently, this has been confirmed by custom next-generation sequencing–based assay on RNA from tissue specimens. EGFRvIII is known to enhance tumor invasiveness, cell motility, and chemoradioresistance, and is independently associated with a poorer prognosis. Because this mutation promotes tumorigenesis, it likely represents a driver mutation unique to cancer cells, and indeed it has shown specificity for tumor cells. Moreover, EGFRvIII has also been shown to be expressed in a subpopulation of GBM cells that share properties of cancer stem cells.

Peptides have also been targeted through the use of tumor-derived heat shock proteins (HSP). HSPs function in antigen carriage and in the targeting and activation of APCs, including DCs, which then leads to priming of the immune system’s effector cells. This family of proteins is upregulated by cellular stressors; for example, heat, hypoxia, infection, and malignant transformation. Only HSPs derived from tumor cells, and when complexed to tumor antigens, are capable of inducing antitumor immunity. Hence, HSP vaccines require HSPs complexed to antigen peptides (HSP-protein complexes), procured from the patient’s tumor.

Most HSP vaccine trials have been based on prophages (also called HSP-96 protein complex), using a tumor-lysate approach. HSP-96 has notable substrates implicated in tumorigenesis, including EGFRvIII, platelet-derived growth factor receptor (PDGFR), FAK, AKT, p53, and phosphatidylinositol 3 kinase (PI3K). In a phase II trial for recurrent GBM, median overall survival was 90.2% at 6 months, and 29.3% at 12 months. Another trial in newly diagnosed GBMs showed a median overall survival of 23.8 months. A phase II trial of prophages in combination with bevacizumab for recurrent GBMs is currently underway ( NCT01814813 ).