Abstract P3-03-07: Combination of eribulin and PI3K inhibitors in triple negative and HER2 expressing breast cancer cell lines results in synergistic growth inhibition and enhanced inhibition of the PI3K pathway

Abstract

Abstract Background. Patients with triple negative breast cancer (TNBC) have high levels of pAkt expression and activation of the PI3K-mTOR pathway. Eribulin is a microtubule-targeting agent with benefits in treating taxane and anthracycline refractory breast cancer. Our objective was to evaluate its efficacy in inhibiting PI3K pathway activity and cell growth both alone and in combination with PI3K/MTOR inhibitors BEZ 235 and BKM 120. Methods. TNBC cell lines MDA468, BT549, HS587T, MDA231, and HER2 expressing breast cancer cell line SKBR3 were used for this study. Both MDA468 and BT549 have PI3K- related mutations. The tetrazolium salt, 3-4,5 dimethylthiazol-2,5 diphenyl tetrazolium bromide (MTT) assays were used to assess growth inhibition after 72 hour treatment with eribulin, BEZ 235 and BKM 120 both alone and in combination. Combination indices (CI) generated by Chou-Talalay plots were used to quantify synergy. Western blots were used to evaluate the expression of phosphorylated Akt (pAkt), S6K1 (pS6K1) and S6 (pS6) from 30 min to 24 hours of treatment at different doses. Results. Eribulin has IC50 ranging from 60 pM to 300 pM, BEZ 235 has IC50 ranging from 50 nM to 80 nM, and BKM has IC50 ranging from 500 nM or higher. Standard dilutions of eribulin in combination with BEZ 235 resulted in synergistic growth inhibition (CI<1) in both MDA468 and BT549 cells at all doses tested, but required higher concentrations (500 to 2000 nM) for BKM 120. Western blot analysis for all cell lines treated with eribulin showed pAkt inhibition by eribulin alone with doses as low as 1 nM and as early as 4 hours. PI3K inhibitor alone confirmed inhibition of pAkt, pS6K1 and pS6 at early time points with feedback increase in pAkt at 24h. While both BEZ 235 and BKM 120 treatment increased pAkt in a dose dependent fashion at 24 hours, combination treatment with eribulin showed a dose dependent decrease in pAkt. Conclusion. Our study shows significant synergistic growth inhibition with the combination of eribulin and PI3K inhibitors. This may be related to inhibition by eribulin of the feedback increase in pAkt seen with PI3K inhibitors alone at later time points. These findings point to a potential role for combination therapy of both eribulin and PI3K inhibitors in treating refractory metastatic disease. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P3-03-07.