High-grade gliomas, the most common primary brain tumors in adults, have a poor prognosis and remain incurable with current therapies. These devastating tumors present significant treatment challenges: 1) surgery may cause permanent neurologic damage; 2) surgery misses cancer cells that have invaded beyond the edge of the tumor or to other sites in the brain; 3) many, if not most, chemotherapy drugs cannot enter the brain because of the blood-brain barrier; and 4) due to the highly toxic nature of chemotherapy agents the therapeutic window (the difference between the dose that kills the tumor and the dose that causes toxic side effects) is very small, resulting in undesirable side-effects. Therefore, if therapeutic agents could be localized and concentrated selectively to the tumor sites, treatment efficacy may be improved while toxic side effects are minimized.
The overarching goal of this project is to develop a human Neural Stem Cell (NSC)-based treatment strategy that produces potent localized anti-tumor effects while minimizing toxic side effects. NSCs hold the promise of improved treatment for brain cancers because they have an innate ability to distribute within and around a tumor mass and to seek out tumor cells that have invaded further into surrounding brain tissue. By homing to cancer cells, NSCs offer a way to selectively deliver concentrated chemotherapy to brain tumor sites. We are modifying NSCs to make the protein carboxylesterase (CE), which will convert a systemically administered prodrug, CPT-11 (irinotecan) to an active, potent anti-cancer drug, SN38 at the tumor sites.
Our second year of funding was highly productive and informative. We validated key elements of our system, successfully negotiating Go/No Go milestones, yielding substantial progress:
(1) We have selected the optimal genetically modified human CE to efficiently convert CPT-11 to SN-38. This CE is being developed for clinical grade use.
(2) We have determined the volume of tumor coverage by NSCs injected directly into the brain versus injecting them intravenously. We found that we achieve more tumor coverage with direct injection of the NSCs into the brain, and will focus on developing this approach for initial NSC.CE/CPT-11 clinical trials. However, following intravenous injections we found the NSCs localize prominently at the invasive tumor edges, which may prove therapeutically efficacious as well. Due to the significant clinical and commercial advantages that intravenous administration presents, this approach will also be developed toward patient trials. We have determined the starting NSC dose range for both approaches.
(3) We have shown that CPT-11 + CE is1,000 fold more toxic to glioma cells than CPT-11 alone. Importantly, microdialysis studies in our preclinical models have confirmed the conversion of CPT-11 to SN-38 by our CE-secreting NSCs in the brain.
(4) We have completed studies labeling our NSCs with iron (Feraheme) nanoparticles, which allows for non-invasive cell tracking by Magnetic Resonance Imaging (MRI). Safety studies for clinical use of this iron-labeling method were completed and submitted to the FDA, for consideration of use in brain tumor patients enrolled in our current NSC.CD/5-FC recurrent glioma clinical trial. This would be the first-in-human use of Feraheme-labeled stem cells for MRI tracking.
Our results to date robustly support the original hypothesis that an effective, glioma-specific therapy can be developed using NSCs that home to tumors and express CE to convert CPT-11 to the potent anti-cancer agent SN-38. Pre-clinical therapeutic efficacy studies to optimize CPT-11 regimens are now in progress.