Abstract P3-03-08: A comparison of PI3K inhibition by eribulin, other microtubule targeting agents and a DNA-damaging chemotherapeutic in triple negative and HER2 expressing breast cancer cell lines

Background. Eribulin is a microtubule-targeting agent with significant benefits in treating refractory metastatic breast cancer. Tumors from patients with Triple negative breast cancer (TNBC) have high levels of Akt expression and consistently show activation of the PI3K-mTOR pathway. Our objective was to compare Eribulin9s ability to inhibit PI3K pathway activity and cell growth with two other microtubule targeting agents, Paclitaxel and Vinblastine, as well as a conventional DNA damaging chemotherapeutic Cisplatin. Methods. MDA468 and BT549 TNBC cell lines and SKBR3 HER2 overexpressing breast cancer cell lines were used for this study. Western blot analysis was used to evaluate the expression of phosphorylated Akt-Ser473 (pAkt) and S6K1 (pS6K1) at different time points from 2 to 24 hours of treatment with Eribulin, Paclitaxel, Vinblastine, or Cisplatin. MTT assays were used to assess growth inhibition after 72 hours of treatment. Results. Western blot analysis for MDA468 cells treated with Eribulin in varying concentrations confirm partial inhibition of pAkt expression as early as 4 hours at 100 pM concentration. Complete inhibition is reached at 50 nM. Partial inhibition of pS6K1 can be seen as early as 4 hours at 500 nM. Western analysis for MDA468 cells treated with Vinblastine in varying concentrations confirms inhibition of pAkt and pS6K1 beginning at 50 nM at 24 hours. Western analysis for MDA468 cells treated with Paclitaxel in varying concentrations showed increases in pAkt expression in a dose responsive fashion with significant increase in pAkt beginning at 5 nM concentration as well as increase in pS6K1 at 24 hours. Cisplatin markedly increases pAkt at 24 hours in a dose responsive fashion and decreases pS6K1 at 500 nM to 1000 nM concentration range in BT549. The IC509s for Eribulin ranged from 0.06 nM to 0.3 nM at 72 hours by MTT assay. The IC509s for Vinblastine ranged from 0.5 nM to 0.9 nM. Paclitaxel has reported IC509s in the 2 nM to 75 nM range in these cell lines, and Cisplatin has IC509s ranging from to ∼500 nM to ∼2000 nM at 72 hours in these cell lines. Conclusion . Our study shows that for microtubule targeting agents such as Eribulin and Vinblastine that block polymerization of tubulin into microtubules, both pAkt and pS6K1 expression is suppressed. Growth inhibition is also confirmed, and is seen at doses when pAkt and pS6K1 are not suppressed. Eribulin inhibits pAkt and pS6K1 at lower concentrations than Vinblastine. With Paclitaxel, a microtubule-targeting agent that enhances polymerization of tubulin to microtubules, and Cisplatin, a conventional DNA damaging chemotherapeutic we observe an increase in pAkt expression, with variable effects on pS6K1. Enhancement of Akt activity is a likely survival response by cancer cells to chemotherapy, yet in the case of microtubule polymerization blockade as seen with Eribulin and Vinblastine, Akt activity is suppressed, along with downstream mTOR activity. The potential role of microtubule polymerization blockade in inhibition of the PI3K pathway needs further study. It may be a factor in the novel finding of pAkt and pS6K1 inhibition by Eribulin. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P3-03-08.