Gene therapy and targeted toxins for glioma

Abstract

The most common primary brain tumor in adults is glioblastoma. These tumors are highly invasive and aggressive with a mean survival time of nine to twelve months from diagnosis to death. Current treatment modalities are unable to significantly prolong survival in patients diagnosed with glioblastoma. As such, glioma is an attractive target for developing novel therapeutic approaches utilizing gene therapy. This review will examine the available preclinical models for glioma including xenographs, syngeneic and genetic models. Several promising therapeutic targets are currently being pursued in pre-clinical investigations. These targets will be reviewed by mechanism of action, i.e., conditional cytotoxic, targeted toxins, oncolytic viruses, tumor suppressors/oncogenes, and immune stimulatory approaches. Preclinical gene therapy paradigms aim to determine which strategies will provide rapid tumor regression and long-term protection from recurrence. While a wide range of potential targets are being investigated preclinically, only the most efficacious are further transitioned into clinical trial paradigms. Clinical trials reported to date are summarized including results from conditionally cytotoxic, targeted toxins, oncolytic viruses and oncogene targeting approaches. Clinical trial results have not been as robust as preclinical models predicted, this could be due to the limitations of the GBM models employed. Once this is addressed, and we develop effective gene therapies in models that better replicate the clinical scenario, gene therapy will provide a powerful approach to treat and manage brain tumors.

Fig. (1)

Diagram outlining the mechanism by which viral vector administration may result in tumor regression. Adenoviral vectors delivering substances like HSV-1TK and hsFlt3L injected intratumorally cause local cell death (TK) generating tumor antigen and trigger the maturation of local and infiltrating antigen presentating cells (Flt3L). APCs then may activate various adaptive and innate immune system cell types to trigger a fully activated anti-tumor immune response. This immune response also results in memory T cell generation which protects against future recurrence of disease.

Fig. (2)

CNS-1 cell tumors treated by adenoviral gene therapy. Brains from rats implanted with CNS-1 cell ten days before adenoviral delivery of gene therapy (saline control, RAdhsFlt3L or RAdTK) were harvested five days or twelve (RAdTK+hsFlt3L) days after gene therapy. The combination of immune stimulation and conditional cytoxicity trigger tumor regression [Ali et al., 2005]. Without combined therapy, rats succumb to tumors within 20 days of tumor implantation however with TK + Flt3L treatment, animals survive long term and no tumor remnant is evident 12 days after viral therapy.

Fig. (3)

Diagram outlining the mechanism through which RAdFlt3L and RAdTK stimulate a powerful anti-tumor response. In the absence of treatment, (a), few dendritic cells (DC's) present in the cerebrospinal fluid (CSF) or peripheral tissues can gain access to the tumor mass growing within the brain parenchyma. This prevents DC's from taking up tumor antigen and migrating to peripheral lymph nodes where it can be presented to T_Helper (T_H) cells. Tumors that are injected with RAdFlt3L alone, (b), allows dendritic cell infiltration into the tumor mass and subsequent maturation. Dendritic cells can proliferate within the tumor, mature, and take up endogenous tumor antigen from necrotic areas within the tumor. These DC's subsequently migrate to peripheral lymph nodes where they present tumor antigen to T_H cells on MHC II molecules. This results in an immune response against the tumor and can successfully clear small tumors from rodents. Tumors that are injected with both RAdFlt3L and RAdTK, (c), cause the infiltration of DC's within the tumor mass just like with RAdFlt3L alone. However, cytotoxic effects of RAdTK result in necrosis and apoptosis of large areas of the tumor. This creates an inflammatory environment ideal for the uptake of tumor antigen by DC's. These DC's migrate to peripheral lymph nodes and display antigen to T _H lymphocytes, resulting in a potent anti-tumor response sufficient to clear even large tumors from the rodents.

Fig. (4)

Targeted toxins for glioma therapy. The targeting of IL-13α2 receptor overexpressed in glioma cells has been improved by mutating the human IL-13 gene to generate a mutated IL-13 (muIL-13). MuIL-13 has shown a higher affinity for the glioma-associated IL-13α2 receptor and negligible binding to the physiological receptor composed of IL-13 receptor and IL-4 receptor (1). The mutant Pseudomonas exotoxin (PE) does not bind to its ubiquitous α2-macroglobulin receptor due to the deletion of domain I and was fused to muIL-13 to promote its internalization into IL-13α2R-expressing glioma cells. PE Domain II (PEII) catalyzes the translocation of the toxin into the cytosol (2) and undergoes proteolytic cleavage that activates the exotoxin (3). Domain III (PEIII) directs the processed fragment of the toxin to the endoplasmic reticulum and ADP ribosylates elongation factor 2, inhibiting protein synthesis (4) and leading to glioma cell death (5).