Cyclophosphamide, but not melphalan or carboplatin, synergistically enhanced topotecan activity against Ewing’s family tumor cell lines in hypoxia

Proc Amer Assoc Cancer Res, Volume 46, 2005 4712 Ewing’s family tumors (ESFT) are neuroectodermal tumors of bone and soft tissues that are often a major therapeutic challenge. ESFT therapy is heavily based upon alkylating agents. The role of hypoxia in resistance to various alkylating agents is largely unknown, especially when alkylating agents are combined with other drugs. We determined the effect of hypoxia on the cytotoxicity of cyclophosphamide (cyclo) (as 4-hydroperoxycyclophosphamide = 4-HC), melphalan (L-PAM) or carboplatin (CBDCA) when combined with topotecan (TPT) in 2 drug-sensitive and 4 multi-drug resistant (MDR) ESFT cell lines. Cytotoxicity was determined by DIMSCAN digital imaging fluorescence cytotoxicity assay with a 4 log dynamic range. CI (combination index) was calculated by the method of Chou. In normoxia L-PAM and CBDCA in high doses synergized TPT activity in 5 of 6 cell lines (CI 1) (see [Table][1]). These data suggest that hypoxia diminished the activity of L-PAM or CBDCA + TPT against ESFT. Combination Index for ESFT cell lines in normoxia (20%) and hypoxia (2%). At levels that may be clinically achievable with stem cell support, 4-HC synergistically (CI 4 logs of cell kill in hypoxic conditions. We determined treated/control survival ratios (T/C) and time to progression in days (TTP) for TC-71 (drug-sensitive) and CHLA-258 (post-ABMT relapse, multi-drug resistant) xenografts in athymic (nu/nu) mice treated daily x 5 with cyclo, TPT, and cyclo + TPT. At drug levels achievable in athymic mice, TPT did not show significant activity against ESFT as a single agent or combined with cyclo. For cyclo + TPT in TC-71 xenografts TTP=182 and T/C = 18, whereas for cyclo alone TTP =164 and T/C = 16. In the CHLA-258 xenografts cyclo + TPT was no more active (TTP=78, T/C =2.1) than cyclo alone (TTP = 91, T/C =2.1). Thus, unless significant dose escalation of cyclo + TPT is possible with stem cell support, this drug combination does not appear to offer an advantage over cyclo alone for ESFT. ![Figure][2] [1]: #F1 [2]: pending:yes