Gene therapy for brain cancer: combination therapies provide enhanced efficacy and safety

Abstract

Glioblastoma multiforme (GBM) is the most common primary brain cancer in adults. Despite significant advances in treatment and intensive research, the prognosis for patients with GBM remains poor. Therapeutic challenges for GBM include its invasive nature, the proximity of the tumor to vital brain structures often preventing total resection, and the resistance of recurrent GBM to conventional radiotherapy and chemotherapy. Gene therapy has been proposed as a useful adjuvant for GBM, to be used in conjunction with current treatment. Work from our laboratory has shown that combination of conditional cytotoxic with immunotherapeutic approaches for the treatment of GBM elicits regression of large intracranial tumor masses and anti-tumor immunological memory in syngeneic rodent models of GBM. In this review we examined the currently available animal models for GBM, including rodent transplantable models, endogenous rodent tumor models and spontaneous GBM in dogs. We discuss non-invasive surrogate end points to assess tumor progression and therapeutic efficacy, such as behavioral tests and circulating biomarkers. Growing preclinical and clinical data contradict the old dogma that cytotoxic anti-cancer therapy would lead to an immune-suppression that would impair the ability of the immune system to mount an anti-tumor response. The implications of the findings reviewed indicate that combination of cytotoxic therapy with immunotherapy will lead to synergistic antitumor efficacy with reduced neurotoxicity and supports the clinical implementation of combined cytotoxic-immunotherapeutic strategies for the treatment of patients with GBM.

Fig. (1). Infitration of rat GBM cells into the adjacent brain parenchyma

Representative confocal microphotograph shows rat GBM CNS-1 cells expressing Green Fluorescent Protein (GFP) infiltrating the adjacent brain parenchyma. Astrocytes (magenta) were stained using an anti-GFAP antibody. Nuclei were stained with DAPI (blue). T: tumor area. Width of illustrated field: 2mm.

Fig. (2). Combined cytotoxic-immunotherapy induces tumor regression and immunological memory

While single immunotherapy or cytotoxic therapy fails in inducing GBM regression, combination therapy leads to long term survival and immunological memory. We developed a gene therapeutic approach that combines one adenovirus that expresses Flt3L, which acts to increase the number of dendritic cells within the tumor microenvironment, with a second adenovirus, which expresses thymidine kinase, that together with the prodrug ganciclovir, kills brain tumor cells, releasing tumor antigens and inflammatory molecules from dying tumor cells. These tumor antigens are then taken up by the infiltrating dendritic cells, transported to the lymph nodes, where T cells are primed to induce a cytotoxic T cell response, which leads to tumor regression and immunological memory

Fig. (3). Proliferating cells, the target for conditionally cytotoxic approaches, are limited to the GBM mass

Confocal microphotographs show detection of proliferating cells stained with an anti-Ki67 antibody (green) in 9-day intracranial CNS-1 GBM in rats. Tumor cells were labeled with anti-vimentin antibodies (red). Nuclei were stained with DAPI (blue). T: tumor area. Width of illustrated field: 500 μm (Inset = 30 μm).

Fig. (4). Efficacy of combined cytotoxic-immunotherapy in several rodent models of brain cancer

A, Kaplan Meier survival curves show the survival of rats bearing intracranial CNS-1, F98 or 9L GBM tumors that were treated at 7-9 days with saline (n=6-10) or Ad-Flt3L+Ad-TK (n=8-10) followed by GCV administration. B, Kaplan Meier survival curves show the survival of C57/B6 mice that were implanted with GL26 or GL261 GBM cells or B16 melona cells in the brain. Mice were treated 17 days later with saline (n=5-6) or Ad-Flt3L+Ad-TK (n=5-6), followed by GCV administration.