Cell context of BCRP (ABCG2) affects clofarabine cytotoxicity

AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA 3248 Breast cancer resistance protein (BCRP) is a member of ATP-binding cassette transporters, also known as ABCG2, and is involved in resistance to multiple structurally diverse compounds. We have recently demonstrated that BCRP reduces the intracellular accumulation of purine analogues (Cancer Res 67: 6965, 2007). Among these is cladribine. Based upon these findings we speculated that a structural analog, clofarabine, a promising agent in the treatment of patients with hematological malignancies, might also be a BCRP substrate. To address this issue we used cell lines engineered to overexpress BCRP. One cell line was the human osteosarcoma cell line, Saos-2, and the other cell line was an acute myeloid leukemia cell line, OCI-AML3. Immunoblot analysis indicated that both cell lines expressed similar levels of BCRP. Both cell lines showed that BCRP reduced intracellular accumulation of clofarabine and that a specific BCRP inhibitor, fumitremorgin C (FTC) increased intracellular retention. Further studies showed that in the BCRP overexpressing Saos-2 cell line addition of FTC sensitized these cells to clofarabine thus demonstrating that intracellular concentration of clofarabine was limiting to its cytotoxicity. Unexpectedly, OCI-AML3 cells overexpressing BCRP did not exhibit reduced sensitivity to clofarabine. These studies suggest that cell context plays a fundamental role in determining how BCRP impacts a cell’s cytotoxic response to a chemotherapeutic agent.