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 disseminated 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. Therefore, if therapeutic agents could be concentrated and localized 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 other, secondary and smaller tumor nodules in the brain. By homing to cancer cells, NSCs offer a way to selectively deliver concentrated chemotherapy to brain tumor sites. After modifying NSCs by adding the gene to make the protein carboxylesterase (CE), NSCs deliver CE to convert the drug CPT-11 (irinotecan) to its more potent form, SN-38 at primary and secondary brain tumor sites.
The major milestone in our third year of funding was that we completed our pre-IND package and held our pre-IND meeting with the FDA. To this end, we validated the following:
(1) NSCs can potentiate the in vivo efficacy of irinotecan (CPT-11) using a low dose (7.5 mg/kg) daily x 5 schedule. Both real time Xenogen and integrated morphometric analysis of immunohistochemically stained sections of tumor were used to determine tumor volumes.
(2) In vivo pharmacokinetics demonstrated increased accumulation of SN-38 in tumor over that of tumor interstitium. The concentrations of tumor SN-38 were approximately 3-fold higher in tumor-bearing brain tissue than in corresponding normal tissue supporting the hypothesis that NSCs can direct toxic chemotherapy in a tumor localized manner.
(3) Following FDA approval of the incorporation of iron (Feraheme) into NSCs, three patients were treated with FeHe-labeled HB1.F3.CD, the first generation NSCs undergoing clinical trial. There were no adverse effects from the treatment demonstrating relative safety and lack of toxicity of this method.
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 SN-38. During the fourth and coming year of CIRM funding, we will conduct experiments to determine the optimal schedule for NSC/CPT-11 therapy and demonstrate the safety and lack of toxicity of the treatment schema in rodents to fulfill requirements for IND submission and clinical trial in humans.