665. Therapeutic Efficacy of Third Generation Oncolytic HSV-1 (G47Δ) for Glioma Cells with Stem Cell Property

Abstract

Glioblastoma is the most malignant brain tumor with highly dismal prognosis. Recent studies suggest that glioblastoma cells form a subpopulation with stem cell properties, termed glioma initiating cells (GICs). GICs grow less rapidly but are resistant to standard therapies, and are deemed to be a major reason for the disease to be incurable. In this study, the efficacy of G47Δ, a third generation oncolytic herpes simplex virus type 1, was evaluated using GICs established from surgically removed clinical specimens. The GICs grew in vitro as CD133-positive, nestin-positive neurospheres in serum-free medium supplemented with growth factors, and the stemness was confirmed by sphere forming assay. As few as 5×103 GICs implanted in the brain of athymic mice were capable of forming intracerebral tumors that retained histopathological appearances consistent with glioblastoma. On magnetic resonance imaging, the GIC-derived intracerebral tumors appeared as iso intensity in T1 weighted images, high intensity in T2 weighted images, and non contrast-enhanced mass lesions. In in vitro cytotoxicity assays, G47Δ killed 60 to 80% of GICs by day 4 when cells were infected at MOI of 0.01. Infecting GIC spheres with G47Δ caused significant inhibition in secondary sphere formation when compared with mock. In mice harboring GIC-derived intracerebral tumors, a single G47Δ inoculation (8 × 104 pfu, 10 days after tumor implantation) significantly prolonged the survival when compared with mock. Immunohistochemical study of G47Δ injected tumors demonstrated widespread distribution of G47Δ within the tumor but selective to tumor cells. These results indicate that G47Δ efficiently kills GICs as well as non-GIC glioma cells. Taken into account that a phase 2 clinical study with G47Δ in glioblastoma patients is ongoing in Japan, G47Δ may soon become a new therapeutic option for glioma patients with a potential of curing glioblastoma.