**5. Immunotherapy**

Glioblastomas undertake a host of immunosuppressive mechanisms, resulting in challenges for the immunotherapeutic interventions [16]. In this subsection, we discuss the immunother‐ apy and the use and rationale of trials and their application.

Optimal antitumor therapy needs to have an antigen as an immunological target along with the activation of the immune system for facilitating trafficking and infiltration of the now activated immune system targeting the tumor.

Let us start by overviewing the multiple key immunosuppressive mechanisms existing within the highly plastic glioblastoma microenvironment. Regulatory T cells (Tregs) are produced within the thymus (nTregs) or are induced (iTregs). Of the two, nTregs are noted in higher concentration within the glioblastoma tumor clusters [16]. Immunosuppressive mechanisms have been directly correlated to, by identifying the cytokines within the tumor cysts fluid secreted by the Tregs: transforming growth factor beta [TGF-β] and interleukin 10 [IL-10]. Inhibitors of TGF-β receptor kinase are currently in preclinical testing. Up to a tenth of the mass of glioblastomas consist of the tumor-associated macrophages (M2 linage) and micro‐ glia. The aggressiveness of glioma-stem cells is enhanced by the secretion of TGF-β by the tumor-associated macrophages. The glioblastoma stem cells increase the number of circulat‐ ing Tregs and also activate the signal transducer and activator of transcription 3 (STAT 3), which is found to be ubiquitously expressed in glioblastoma cells [16].

This refers to no progression of nonmeasurable lesions and no new lesions. Specifically, this is defined as ≥50% decrease in sum of all products of diameters (SPD) of all target lesions

This differs from partial response with having ≥25% increase in the sum of target lesions, with significant increase in nonenhancing lesions, with clinical deterioration with no

This is the radiological diagnosis of exclusion of neither complete nor partial response, with

*As per the RANO guidelines, criteria for progressive disease is met when the majority of new enhancement is noted beyond the 80% isodose line of radiotherapy or on histopathological confirmation. This is an important point for us to bear in mind, as a third of glioblastoma patients may be reported on as undergoing pseudoprogression, thus this term needs to be utilized in accordance with the RANO guidelines. Also of importance, is the pseudoresponse seen post-antiangiogenic therapy (anti-vascular endothelial growth) which decrease the permeability of the blood-brain barrier thereby decreasing the gadolinium enhancement* [14]. *Radiological surveillance with T2/FLAIR is sensitive in identifying vasogenic edema and used in combination with DWI is a increases the likelihood of identifying tumor burden* [15]. *An improvement in T2/FLAIR is associated with improved survival and decreased mortality, DWI remains an independent predictor of progression free survival at 6 months* [12, 15].

RANO guidelines state that all radiological responses must persist for four weeks prior to be

Glioblastomas undertake a host of immunosuppressive mechanisms, resulting in challenges for the immunotherapeutic interventions [16]. In this subsection, we discuss the immunother‐

Optimal antitumor therapy needs to have an antigen as an immunological target along with the activation of the immune system for facilitating trafficking and infiltration of the now

Let us start by overviewing the multiple key immunosuppressive mechanisms existing within the highly plastic glioblastoma microenvironment. Regulatory T cells (Tregs) are produced within the thymus (nTregs) or are induced (iTregs). Of the two, nTregs are noted in higher concentration within the glioblastoma tumor clusters [16]. Immunosuppressive mechanisms have been directly correlated to, by identifying the cytokines within the tumor cysts fluid secreted by the Tregs: transforming growth factor beta [TGF-β] and interleukin 10 [IL-10]. Inhibitors of TGF-β receptor kinase are currently in preclinical testing. Up to a tenth of the

considered 'true' progression or response: this is the crux of the RANO guidelines.

apy and the use and rationale of trials and their application.

activated immune system targeting the tumor.

with stable clinical symptomatology and a stable steroid dose.

decrease in steroid dose and/or a new radiological lesion.

**•** Progressive disease

272 Neurooncology - Newer Developments

**•** Stable disease

lack of progression seen.

**5. Immunotherapy**

High-grade glioma progression has also been shown to be enhanced in the presence of gliomasecreted colony stimulating factor 1 (CSF-1) to cause polarization of tumors toward the gliomasupportive (M2) phenotype [17]. Of note, the vascular endothelial growth factor (VEGF) has multiple functions of tumorigenesis and simultaneously of inhibition of dendritic cell function. We discuss the role of anti-VEGR receptor agents in further detail below in subsection 6.

Immune checkpoint programmed death PD-1 binds the ligand for PD-1 (PD-L1) to suppress CD4+ and CD8+ cells. PD-L1 is upregulated in gliomas, specifically the mesenchymal subtype of glioblastomas and has been associated with inhibition and apoptosis of T cells. Anti-PD1 blockade has been undertaken in the murine glioblastoma models successfully with an increase in survival, in combination with radiotherapy [18].

Cytotoxic T-Lymphocyte-associated protein 4 (CTLA-4) is an inhibitory surface receptor found on constitutionally active Tregs, and hence, is the other immune checkpoint inhibitor of great clinical interest [19, 20]. The FDA has recently approved ipilimumab, a monoclonal antibody directed against CTLA-4, after Phase III trials for melanoma patients showed an objective increase in survival. In the murine model, anti-CTLA-4 and IL-12 administration demonstrat‐ ed a reduction in Tregs and increased immune effector response, which is now under investigation for glioblastoma therapies [21, 22].

An investigational immunotherapeutic agent that has been in the limelight for the past couple of years is RINTEGA® (Rindopepmut CDX-110). RINTEGA® is administered intradermally and consists of the EGFRv*III*-specific peptide sequence conjugated to keyhole limpet hemo‐ cyanin, thereby stimulating pronounced EGFRvIII-specific humoral and cellular responses resulting in the production of anti-EGFRv*III* antibodies infiltrating and attacking the tumor. EGFR*vIII* is a tumor-specific oncogene and a mutated form of the epidermal growth factor receptor (EGFR), which is noted in one-third of all GBM cases with aggressive tumor prolif‐ eration and correspondingly poor median survival compared with other glioblastoma cases [23–25]. EGFR*vIII* is not expressed in normal tissue, hence making it a unique immunothera‐ peutic target.

Hence, at this point, we will dedicate a few lines to the discontinuation of the ACT IV study in March 2016 based on the recommendation of the Data Safety and Monitoring Board and an update has appeared on the Celldex Therapeutics website. ACT IV was a Phase III study conducted in newly diagnosed EGFRvIII-positive glioblastoma patients with RINTEGA® and granulocyte-macrophage colony stimulating factor added to standard of care temozolamide with the control arm regimen undergoing standard of care temozolamide plus intradermal keyhole limpet hemocyanin. The control arm significantly outperformed expectations (hazard ratio = 0.99; median OS: RINTEGA 20.4 months vs. control 21.1 months) and hence the study showed an inability to meet the primary outcome survival endpoint.

The ReACT study is the randomized, Phase II trial of RINTEGA® in combination with bevacizumab (Avastin®) in patients with recurrent EGFRvIII-positive glioblastoma. In November 2015, Celldex Therapeutics reported long-term survival data in group 1 (bevaci‐ zumab-naive patients randomized to receive either RINTEGA or a control injection of KLH in a blinded fashion; all patients also received bevacizumab) at the Society for Neuro-Oncology Annual Meeting. At two years, the survival rate was 25% for patients in the RINTEGA arm versus 0% for patients in the control arm, with continuing advantage shown across multiple endpoints [26].
