We have shown previously that 17-AAG enhances oxaliplatin cytotoxicity in a panel of colon cancer cell lines through inhibition of NF-κB. Since in the clinic oxaliplatin is used in combination with fluorouracil, which has been shown to modulate NF-κB activity in various cell line models, we sought to determine whether the addition of 17-AAG to oxaliplatin plus fluorouracil would be similarly synergistic as with oxaliplatin alone. First, we established concentrations of oxaliplatin and fluorouracil in combination sufficient to achieve 20% of cell death in clonogenic assays after incubation for 24 hours: 0.25 μM for oxaliplatin and 5 μM or 10 μM for fluorouracil (HT29 and HCT116 cells, respectively). Further clonogenic assays were carried out in the presence of increasing amounts of 17-AAG to establish the IC50 for the combination of three drugs. The concentrations of 17-AAG in these combinations were 3-fold lower than compared to the IC50 of 17-AAG as a single agent for both cell lines. Flow cytometry was also performed to evaluate the effects of the combination on apoptosis induction. In HT29 cells, 17-AAG alone did not induce apoptosis, but when added to oxaliplatin, increased the fraction of apoptotic cells from 2.3 to 9.1 percent compared to oxaliplatin alone, and when added to oxaliplatin/fluorouracil, increased the fraction of apoptotic cells from 7.9 to 13.2 percent. In HCT116 cells, 17-AAG as a single agent did not induce apoptosis, but when added to fluorouracil, increased the fraction of apoptotic cells from 8.43 to 21.4 percent. The addition of 17-AAG to the oxaliplatin/fluorouracil combination increased the fraction of apoptotic cells from 8.9 to 25.5 percent. In conclusion, 17-AAG enhances the cytotoxicity of oxaliplatin/fluorouracil combination in HT29 and HCT116 cells.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]