Neural Stem Cell–Mediated Enzyme/Prodrug Therapy for Glioma: Preclinical Studies

Neural stem cells home to gliomas in mice where they convert a prodrug to 5-fluorouracil, leading to tumor regression. Cellular Assassins Derived from the supporting cells of the brain, gliomas are deadly tumors that can be only temporarily held at bay, but not cured. New ways to treat these cancers are needed. To get regulatory approval to test a new stem cell–based therapy in patients, Aboody et al. performed a series of preclinical experiments in mice with artificially implanted gliomas in their brains. By mimicking closely the treatments that they hoped to perform in humans, these authors were able to show to the satisfaction of the regulatory agency that the treatment was safe and effective enough in the mice to warrant a first-in-human trial in patients. The authors used a neural stem cell line carrying a v-myc gene and a gene for cytosine deaminase. These cells exhibit tropism to human glioma cells. When injected into mice with gliomas, they migrate to the site of the tumor, even when the mice are treated with steroids or radiation, as might be the case for human patients. The cytosine deaminase in the cells provides another anticancer weapon. This enzyme converts the prodrug 5-fluorocytosine (5-FC) to the toxic 5-flurouracil (5-FU), delivering a high concentration of the therapeutic agent directly in and around the tumor and causing it to shrink significantly. Injection of excess numbers of cells or increasing the dose of 5-FU did not result in any abnormalities in the animals; in fact, by 12 weeks after injection, no cells were to be seen in the brain or elsewhere, even when a highly sensitive polymerase chain reaction method was used to look for the v-myc DNA. This targeted cell-based approach to cancer therapy that concentrates the therapeutic agent in the vicinity of the tumor is expected to reduce toxicity to other tissues. Thus, a higher local dose is possible, potentially improving efficacy against the tumor. The phase 1 trial derived from these preclinical results is ongoing; its end will allow evaluation of how well these preclinical in vivo studies set the stage for humans. High-grade gliomas are extremely difficult to treat because they are invasive and therefore not curable by surgical resection; the toxicity of current chemo- and radiation therapies limits the doses that can be used. Neural stem cells (NSCs) have inherent tumor-tropic properties that enable their use as delivery vehicles to target enzyme/prodrug therapy selectively to tumors. We used a cytosine deaminase (CD)–expressing clonal human NSC line, HB1.F3. CD, to home to gliomas in mice and locally convert the prodrug 5-fluorocytosine to the active chemotherapeutic 5-fluorouracil. In vitro studies confirmed that the NSCs have normal karyotype, tumor tropism, and CD expression, and are genetically and functionally stable. In vivo biodistribution studies demonstrated NSC retention of tumor tropism, even in mice pretreated with radiation or dexamethasone to mimic clinically relevant adjuvant therapies. We evaluated safety and toxicity after intracerebral administration of the NSCs in non–tumor-bearing and orthotopic glioma–bearing immunocompetent and immunodeficient mice. We detected no difference in toxicity associated with conversion of 5-fluorocytosine to 5-fluorouracil, no NSCs outside the brain, and no histological evidence of pathology or tumorigenesis attributable to the NSCs. The average tumor volume in mice that received HB1.F3. CD NSCs and 5-fluorocytosine was about one-third that of the average volume in control mice. On the basis of these results, we conclude that combination therapy with HB1.F3. CD NSCs and 5-fluorocytosine is safe, nontoxic, and effective in mice. These data have led to approval of a first-in-human study of an allogeneic NSC-mediated enzyme/prodrug-targeted cancer therapy in patients with recurrent high-grade glioma.