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Efficacy and Toxicity of a Virus-directed Enzyme Prodrug Therapy Purging Method

Preclinical Assessment and Application to Bone Marrow Samples from Neuroblastoma Patients¹

Department of Molecular Pharmacology [L. M. W., S. M. G., R. A. B., C. L. M., L. C. H., P. J. H., P. M. P., M. K. D.], Animal Resource Center [C. M. S.], and Department of Tumor Cell Biology [R. A. A.], St. Jude Children’s Research Hospital, Memphis, Tennessee 38105

Autologous stem cell transplantation is used to rescue cancer patients from myelosuppression caused by high-dose chemotherapy. However, autologous grafts often contain tumor cells that can contribute directly to relapse. Current purging methods are useful when fewer than 1% tumor cells contaminate the bone marrow, and patients with tumor burdens of >1% are considered ineligible for chemotherapy that necessitates stem cell rescue. Using neuroblastoma (NB) as a model system, we developed a method that is effective even with tumor burdens of 10–25%. Mixtures of NB-1691 NB cells and CD34⁺ hematopoietic cells purged by this method showed no evidence of viable tumor cells as assessed by clonogenic assays or reverse transcription-PCR for the NB cell markers tyrosine hydroxylase and N-MYC. The efficacy and lack of toxicity of the method were verified using in vivo mouse models. Severe combined immunodeficient mice that received purged cell preparations containing 10% NB-1691 cells survived without evidence of disease for the observation period (>1 year), whereas mice that received unpurged cells developed disseminated disease requiring euthanasia 73–96 days after injection of cells. No evidence of toxicity to the mice was detected by numerous laboratory values for bone marrow, liver, and kidney function, and no difference was seen in the ability of purged cell mixtures versus unmanipulated CD34⁺ cells to reconstitute the marrow of non-obese diabetic severe combined immunodeficient mice. In a pilot study, marrow was obtained from eight patients who had 1% metastatic tumor burden. All eight samples were purged to the level of detection by reverse transcription-PCR (samples from seven patients) or clonogenic potential (sample from one patient), whichever assay was used. The described adenovirus/rabbit carboxylesterase/CPT-11 (irinotecan, 7-ethyl-10[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin) virus-directed enzyme prodrug method may be useful for patients whose tumor burdens exceed 1% at the time of stem cell harvest. Assessment of purging efficacy with additional samples from NB patients is ongoing.

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