Immunotoxin Monotherapy and Combinatorial Therapy With Immune Checkpoint Inhibitors for Malignant Brain Tumors
Glioblastoma is the most common and aggressive malignant brain tumor among all primary brain and central nervous system (CNS) tumors. The median survival time for glioblastoma patients given the current standard of care treatment (surgery, radiation, and chemotherapy) is less than 15 months. Medulloblastoma is another major malignant brain tumor that most frequently occurs in children. Although recent advances in surgery, radiotherapy, and chemotherapy have led to an increase in 5-year survival rates of medulloblastoma patients, treatment-related toxicity often has a major impact on long-term quality of survival.
As a result, there is an urgent need to develop more efficient and novel therapeutic approaches that specifically target tumor cells while preserving the surrounding normal CNS to improve the poor survival and quality of life of patients with malignant brain tumors. To address this need, we have developed two novel targeted immunotoxins (ITs), D2C7-(scdsFv)-PE38KDEL (D2C7-IT) and NZ-1-(scdsFv)-PE38KDEL (NZ-1-IT). D2C7-IT was developed by fusing the single-chain variable fragment (scFv) of the D2C7 monoclonal antibody (mAb) with domains II and III of Pseudomonas exotoxin A (PE38KDEL), and NZ-1-IT was developed by fusing the scFv of the NZ-1 mAb with PE38KDEL. D2C7-IT reacts with both the wild-type epidermal growth factor receptor (EGFRwt) and the EGFR variant III (EGFRvIII), two overexpressed proteins in glioblastomas. NZ-1-IT reacts with podoplanin (PDPN), a protein that has a high expression in glioblastomas and medulloblastomas.
In vitro cytotoxicity data shows that both ITs effectively inhibited protein synthesis in a variety of epitope-expressing glioblastoma and medulloblastoma xenograft cells and human tumor cell lines. Furthermore, the direct anti-tumor efficacy of D2C7-IT was examined in orthotopic glioma models in immunocompromised mice, while the direct anti-tumor efficacy of NZ-1-IT was observed in medulloblastoma xenograft-bearing immunocompromised mice. Both immunotoxins showed a robust anti-tumor efficacy in the preclinical brain tumor models. D2C7-IT was first investigated in the subsequent studies to accelerate its translation to the clinic. The preclinical toxicity of intracerebral D2C7-IT infusion was subsequently determined in normal Sprague-Dawley (SD) rats. The maximum tolerated dose (MTD) of D2C7-IT was determined to be between a total dose of 0.10 and 0.35 μg, and the no-observed-adverse-effect level (NOAEL) of D2C7-IT was a total dose of 0.05 μg in SD rats. Both the MTD and NOAEL were utilized as references for the D2C7-IT clinical trial design.
In addition to direct tumor cell killing, immunotoxin monotherapy has been shown to induce a secondary anti-tumor immune response through the engagement of T cells. Therefore, the D2C7-IT-induced secondary anti-tumor immune response was investigated using syngeneic mouse glioma models in immunocompetent mice. Moreover, previous studies have demonstrated that immune checkpoint inhibitors have a robust anti-tumor efficacy by augmenting the T cell response to the tumor cells. Thus, immune checkpoint inhibitors were combined with D2C7-IT in order to enhance the immunotoxin-induced anti-tumor immune response to eliminate residual tumor cells and prevent tumor recurrence in the long term. Meanwhile, studies with NZ-1-IT remain preliminary; thus, this IT will not be as robustly discussed as D2C7-IT throughout this text.
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