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

TLDR

High‑grade gliomas are highly invasive and refractory to surgery, and the toxicity of current chemo‑ and radiation therapies limits dose escalation, but neural stem cells’ tumor‑tropic properties allow selective delivery of enzyme/prodrug therapy to tumors. The study employed a cytosine deaminase‑expressing human NSC line (HB1.F3.CD) that homes to gliomas in mice and locally converts the prodrug 5‑fluorocytosine to 5‑fluorouracil, with safety and toxicity evaluated after intracerebral administration in both non‑tumor‑bearing and glioma‑bearing immunocompetent and immunodeficient mice. In vitro and in vivo analyses showed that the NSCs are genetically stable, retain tumor tropism even after radiation or dexamethasone pretreatment, exhibit no off‑target toxicity or tumorigenesis, and reduce tumor volume to one‑third of controls, demonstrating safe, nontoxic, and effective therapy that has progressed to a first‑in‑human trial.

Abstract

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.