Clinical development of experimental therapies for malignant glioma

Abstract

Advances in medical and surgical treatments in the last two to three decades have resulted in quantum leaps in the overall survival of patients with many types of non-central nervous system (CNS) malignant disease, while survival of patients with malignant gliomas (WHO grades 3 and 4) has only moderately improved. Surgical resection, external fractionated radiotherapy and oral chemotherapy, during and after irradiation, remain the pillars of malignant glioma therapy and have shown significant benefits. However, numerous clinical trials with adjuvant agents, most of them administered systemically and causing serious complications and side effects, have not achieved a noteworthy extension of survival, or only with considerable deterioration in patients' quality of life. Significant attention was focussed in the last decades on the cell biology and molecular genetics of gliomas. Improved understanding of the fundamental features of tumour cells has resulted in the introduction and increasing clinical use of local therapies, which employ spatially defined delivery methods and tumour-selective agents specifically designed to be used in the environment of a glioma-invaded brain. This review summarises the key findings of some of the most recent and important clinical studies of locally administered novel treatments for malignant glioma. Several such therapies have shown considerable anti-tumour activity and a favourable profile of local and systemic side effects. These include biodegradable polymers for interstitial chemotherapy, targeted toxins administered by convection enhanced delivery, and intra- and peritumourally injected genetically modified viruses conferring glioma-selective toxicity. Areas of possible improvement of these therapies and essential future developments are also outlined.

Keywords: Astrocytoma; Convection-enhanced delivery; Glioma; Immunotoxin; Virus.

Figure 1:

Handling and implantation of Gliadel^® wafers during brain tumour surgery. A: A single wafer is removed from the sterile aluminium foil protective packaging. B: All 8 Gliadel^® wafers belonging to a complete pack are removed from the packaging and prepared for intracerebral implantation. C: Gliadel^® wafers lining the wall of a glioblastoma (GBM) resection cavity. Note that the relatively small tumour resection cavity can only accommodate 5 wafers. Slight overlapping of wafers is acceptable. D: Implanted wafers are covered with oxidised cellulose (Surgicel®, Ethicon, Hamburg, Germany) to prevent dislocation and intracavitary movement.

Figure 2:

Schematic and 3D-ribbon representations of the structure of the recombinant fusion toxin IL13-PE38 (IL13-PE38QQR). A derivative of the native Pseudomonas exotoxin A (PE), PE38QQR, in which domain Ia (amino acids 1 to 252) has been completely deleted and domain Ib (amino acids 365 to 380) has been partially deleted; lysine residues at positions 590 and 606 have been replaced by glutamine, and at position 613 by arginine, and it has been fused to the carboxy terminus of the human IL-13 to produce the fusion toxin IL13-PE38QQR (IL13-PE38).

Figure 3:

Schematic representation of the mode of action of the recombinant fusion toxin IL13-PE38 (IL13-PE38QQR). IL-13 receptor (IL-13R-ß2 subunit) is a tumour-specific protein in human malignant glioma cells. Immune cells, endothelial cells and normal glia and neurons express none or very low amounts of IL-13R. IL13-PE38 is highly selective and potent in killing human glioblastoma multiforme cells by binding to IL-13R, but does not kill normal cells, which are IL-13R-negative.

Figure 4a:

Schematic diagrams of virus vectors. Schematic representation of a wild type retrovirus (RV) and a RV vector with the corresponding packaging cell line (VPC). Wild type RV has a double-stranded RNA genome which is converted to DNA by viral reverse transcriptase and then integrated into the host genome at non-specific sites. The genetically modified RV vector genome contains the cis-acting elements required for replication (long terminal repeats - LTR, packaging sequence - Ψ), but lacks the viral genes (capsid core proteins - gag, reverse transcriptase, integrase and protease - pol, and the envelope antigens - env), which are replaced by up to 8 kb of foreign coding sequences (transgene cassette, e.g. cDNA encoding HSV-tk). A selectable marker (e.g. neomycin resistance gene - neo) allows for antibiotic selection of cells expressing the RV vector. In order that RV vectors replicate, it is necessary to provide the missing viral genes in trans, e.g. expressed within a genetically engineered RV VPC. VPC are most frequently of murine fibroblast origin. Separation of the packaging function from the genetic material to be transferred ensures that a replication incompetent RV is generated and makes RV vectors biologically safe.

Figure 4b:

Schematic representation of the genome of a wild type adenovirus (AV) and recombinant AV vectors. The wild type AV genome is flanked by the inverted terminal repeats (ITR) and divided into E1A, E1B, E2, E3, and E4 regions, whose genes are expressed in a defined temporal sequence. It contains 36 kb of double-stranded DNA, of which several regions can be deleted to accommodate up to 10 kb of foreign DNA (e.g. insertion of a therapeutic transgene cassette in the E1 region, deletion of E3 region). Replication-deficient AV vectors are generated by placing the AV genome in a plasmid and replacing E1 with a transgene (e.g. HSV-tk), then transfecting the plasmid into a packaging cell line (VPC) that provides E1 functions in trans.

Figure 5:

Schematic representation of the mode of action of recombinant retrovirus (RV) vector-mediated suicide gene therapy in patients with malignant glioma. Up to 20 ml of a suspension of viable RV-producing cells (VPC) carrying the transgene coding for herpes simplex virus (HSV) thymidine kinase (HSV-tk) are manually injected into the walls of the tumour resection cavity at the end of the surgical removal of a human glioblastoma multiforme (GBM). RV vectors, which have been genetically engineered to become replication-deficient, are produced in high titres by the implanted VPC. GBM cells are highly mitotic and, after infection by RV, are able to produce most RV proteins, including HSV-tk. This enzyme converts the low-toxicity prodrug ganciclovir (GCV) into highly toxic metabolites, which block DNA replication during mitosis and render the tumour cells apoptotic. After repeated i.v. application of GCV to patients, all cells expressing HSV-tk are killed, including VPC. Cells not expressing HSV-tk may also be killed by the so-called “bystander effect” - transfer of toxic GCV metabolites from HSV-tk-expressing to HSV-tk-negative cells by direct cellular contact. Up to 10 non-expressing cells may be killed by one HSV-tk-expressing cells in cell culture.