Optimization of a Neural Stem-Cell-Mediated Carboxylesterase/Irinotecan Gene Therapy for Metastatic Neuroblastoma

Despite improved survival for children with newly diagnosed neuroblastoma (NB), recurrent disease is a significant problem, with treatment options limited by anti-tumor efficacy, patient drug tolerance, and cumulative toxicity. We previously demonstrated that neural stem cells (NSCs) expressing a modified rabbit carboxylesterase (rCE) can distribute to metastatic NB tumor foci in multiple organs in mice and convert the prodrug irinotecan (CPT-11) to the 1,000-fold more toxic topoisomerase-1 inhibitor SN-38, resulting in significant therapeutic efficacy. We sought to extend these studies by using a clinically relevant NSC line expressing a modified human CE (hCE1m6-NSCs) to establish proof of concept and identify an intravenous dose and treatment schedule that gave maximal efficacy. Human-derived NB cell lines were significantly more sensitive to treatment with hCE1m6-NSCs and irinotecan as compared with drug alone. This was supported by pharmacokinetic studies in subcutaneous NB mouse models demonstrating tumor-specific conversion of irinotecan to SN-38. Furthermore, NB-bearing mice that received repeat treatment with intravenous hCE1m6-NSCs and irinotecan showed significantly lower tumor burden (1.4-fold, p = 0.0093) and increased long-term survival compared with mice treated with drug alone. These studies support the continued development of NSC-mediated gene therapy for improved clinical outcome in NB patients.