The Role of Multi-Drug Resistance Associated Protein 4 and P-glycoprotein in Resistance of Neuroblastoma to Topotecan and Irinotecan

Abstract

Clinton Stewart for his guidance and support. I will always appreciate his patience, the numerous opportunities he has provided, and all of the time he has spent teaching me. iii Abstract High-risk neuroblastoma presents a significant therapeutic challenge because the 5-year survival rate remains less than 30% despite the use of surgery, multi-agent chemotherapy, radiation, and autologous bone marrow transplant. Novel therapeutic modalities are under development. The camptothecin analogs topotecan and irinotecan have been identified as successful cytotoxic agents. For topotecan, pharmacokinetically guided dosing to achieve a systemic exposure associated with preclinical anti-tumor activity in neuroblastoma xenograft models is feasible and has elicited favorable responses in children with high-risk neuroblastoma. However, some children with highrisk disease did not respond to the putatively effective topotecan systemic exposure.These children represent a subset of the disease intrinsically resistant to topotecan.Furthermore, mRNA expression of the adenosine triphosphate (ATP)-binding cassette (ABC) transporters P-glycoprotein (Pgp) and multidrug resistance associated protein 1 (MRP1), which efflux many drugs used in neuroblastoma therapy, has been implicated in poor outcome in neuroblastoma. Therefore, the purpose of our studies was to determine the role of ABC transport protein expression in neuroblastoma resistance to the camptothecin analogs topotecan and irinotecan. The results of the in vitro studies demonstrate that MRP4 and Pgp confer resistance to topotecan and SN-38. In the xenograft studies, MRP4 expression was associated with failure to respond to topotecan. However, this phenotype was not recapitulated in children treated with topotecan. These results may be confounded by small sample size and timing of sample acquisition. Further investigation of the role of ABC transporters in children with neuroblastoma who receive either topotecan or irinotecan may be warranted. In addition to the camptothecin analogs, patients will receive other drugs effluxed by the ABC transporters (e.g., doxorubicin, vincristine, etoposide, and cyclophosphamide). Therefore, analyzing ABC transporter expression by immunohistochemistry in diagnostic tumor specimens may help to select agents not subject to efflux by ABC transporters expressed in the tumor. However, eliminating drugs effluxed by ABC transporters from the treatment regimen creates a potential gap in therapy and may reduce drug intensity. Therefore, further rational design and development of drugs that evade ABC transporter-mediated efflux, and potentially other resistance mechanisms in neuroblastoma, is also warranted.