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A Virus-directed Enzyme Prodrug Therapy Approach to Purging Neuroblastoma Cells from Hematopoietic Cells Using Adenovirus Encoding Rabbit Carboxylesterase and CPT-11¹

Department of Molecular Pharmacology, St. Jude Children’s Research Hospital, Memphis, Tennessee 38105

Tumor cells that contaminate hematopoietic cell preparations contribute to the relapse of neuroblastoma patients who receive autologous stem cell rescue as a component of therapy. Therefore, effective purging methods are needed. This study details in vitro experiments to develop a viral-directed enzyme prodrug purging method that specifically targets neuroblastoma cells. The approach uses an adenovirus to deliver the cDNA encoding a rabbit liver carboxylesterase that efficiently activates the prodrug irinotecan,7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin (CPT-11). The data show that an adenoviral multiplicity of infection of 50 transduces 100% of cultured neuroblastoma cells and primary tumor cells, irrespective of the level of tumor cell line contamination. Exposure of neuroblastoma cell lines or of mixtures of these cell lines with CD34⁺ cells at a ratio of 10:90 to replication-deficient AdRSVrCE for 24 h and subsequent exposure of cells to 1–5 µM CPT-11 for 4 h increased the toxicity of CPT-11 to three neuroblastoma cell lines (SJNB-1, NB-1691, and SK-N-SH) from 20–50-fold and eradicated their clonogenic potential. Also, after "purging," RNA for neuroblastoma cell markers (tyrosine hydroxylase, synaptophysin, and N-MYC) was undetectable by reverse transcription-PCR. In contrast, the purging protocol did not affect the number or type of colonies formed by CD34⁺ cells in an in vitro progenitor cell assay. No bystander effect on CD34⁺ cells was observed. The method described is being investigated for its potential clinical utility, particularly its efficacy for use with patients having relatively high tumor burdens, because no published methods have been shown to be efficacious when the tumor burden exceeds 1%.

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