Neural Stem Cell-Mediated Enzyme/Prodrug Therapy for Glioma

Abstract

Glioblastoma (GBM), the most common primary brain tumor in adults, portends a grim prognosis with median survival rates typically less than a year.1 The aggressive, infiltrative nature of GBM precludes a surgical cure, and the blood-brain barrier has limited the efficacy of systemic chemotherapeutic interventions. Previous research has demonstrated the tumor-tropic behavior of neuronal stem cells (NSC), whereby these cells preferentially aggregate to glioma foci when implanted in the brain.2 Thus, stem cells have become an attractive tumor-specific delivery device surmounting the limitations of surgery or chemotherapyin the treatment of high grade glioma.3 Recent research published in this month’s issue of Science Translational Medicine by Aboodyet al, has exploited the tumor localizing characteristics of NSC and tested a novel enzyme/prodrug approach for the treatment of gliomas.4 Utilizing a cytosine deaminase (CD) expressing-human NSC line, HB1.F3.CD, the group hypothesized that the stem cells would co-localize to tumor cells and convert a systemically administered prodrug, 5-fluorocytosine (5-FC) to the active, nucleic acid analog 5-fluorouracil (5-FU), thereby leading to cell death. Their results demonstrate a potential breakthrough in the treatment of this most challenging pathology. To begin, the team created a stable human NSC line from HB1.F3 using a transgene for CD, then expanded this colony for all future experiments. They subsequently assessed the immunogenicity of the HB1.F3.CD NSC line and determined that they expressed HLA class I antigens but not class II antigens. Therefore, their cell line was unlikely to induce an immunogenic response, a caveat that has plagued other attempts in utilizing this approach. The investigators then evaluated the tropism of their NSC in a murine model under different simulated conditions that human patients experience. Specifically, they determined that when implanted into mouse brains, their NSC co-localized to glioma cells, despite the mouse brains having been previously irradiated or treated with corticosteroids. Additionally, their NSC were able to migrate to the tumor sites when injected adjacent to the tumor or in the opposite hemisphere, regardless of prior radiation or corticosteroid therapy. The safety profile and the tumorgenicity of their NSC line was then evaluated. Using a murine model injected intracranially with HB1.F3.CD, they wanted to determine if their NSC’s were independently tumorogenic and whether there was a maximum tolerated dose of NSC HB1.F3.CD. At both 4 and 12 weeks post NSC implantation, the mice were euthanized and their organs harvested. All organ systems failed to demonstrate any evidence of human NSC, and no tumors were discovered regardless of the amount of NSC injected. Having demonstrated tumor tropism and safety, the team analyzed the in vivo efficacy of their co-localizing NSC to preferentially kill the surrounding, dividing tumor cells. Mice were injected with glioma cells followed by the HB1.F3.CD NSC’s 7 days later. Peritoneal injections of 5-FC were performed on days 11 through 17, and the brains were evaluated. Typical response to treatment demonstrated extensive areas of tumor necrosis surrounded by areas of residual viable tumor (Figure). The therapeutic efficacy was then determined by measuring the viable brain tumor after 1 round of treatment with 5-FC in 24 mice. Their volumetric analysis demonstrated that mice receiving the NSC/5-FC combination therapy had a significantly smaller tumor burden than those mice that did not receive combination therapy.Figure: Diagramof CD-expressing NSCs localized to tumor cells, and CD conversion of 5-FC to 5-FU, which readily diffuses out of the NSCs to selectively kill surrounding, dividing tumor cells. (B and C) H&E-stained brain tumor sections from U251 glioma-bearing mice that received HB1.F3.CD NSCs only (B) or HB1.F3.CD NSCs in combinationwith 5-FC (C). White arrows indicate tumor region.NSCs-neural stem cells. CD-cytosine deaminase. Modified from Aboody KS, Najbauer J, Metz MZ, et al. Neural stem cell -mediated enzyme/prodrug therapy for glioma: preclinical studies. Sci Transl Med. 2013;5(184):184ra59. Reprinted with permission from AAAS.In conclusion, the researchers provide convincing evidence that an NSC-prodrug delivery system to combat glioblastoma is safe, effective and feasible in a murine model. These investigators have provided the necessary pilot data to translate these experimental results from the benchtop to the bedside in the first human clinical trial (NCT01172964) using a NSC-prodrug delivery system in patients with recurrent high-grade glioma. Future research in this area will hopefully offer new therapeutic options to treat this challenging disease.