Abstract 3502: Combination of BKM120 and erlotinib in squamous cell carcinoma of the head and neck: mechanism ofin vitroandin vivosynergy

Purpose: Molecularly targeted agents will play a major role in the next generation of personalized cancer therapies. The EGFR-targeted monoclonal antibody cetuximab is currently the only FDA-approved targeted treatment for head and neck cancers (HNC) with a response rate of less than 15%. Recent large scale genomic studies including TCGA suggest that >50% of HNC cases have activation of the PI3K/AKT/mTOR pathways, suggesting PI3K as an excellent target for HNC. The purpose of the current study is to investigate whether co-targeting EGFR and PI3K has synergistic antitumor effects and to understand the mechanism of synergy. Methods: In a panel of 10 malignant and 1 premalignant HN cell lines, we evaluated cell growth inhibition (by SRB assay); IC50, combination index, and dose reduction index (by CalcuSyn), and apoptosis (by Annexin-V staining). mRNA and protein expression were measured by qPCR and Western blotting, respectively. Small molecule chemical inhibitors and siRNA-mediated knockdown strategies were used to inactivate and shut down the expression of the relevant proteins, respectively. Results: Single targeting of EGFR with erlotinib or PI3K with BKM120 (pan-PI3K inhibitor) suppressed cellular growth without inducing significant apoptosis. Co-targeting EGFR and PI3K had in vitro synergy in all except one cell line (based on combination index and dose reduction index) and more effectively inhibited HNC xenograft growth in vivo. The combination of erlotinib and BKM120 induced variable apoptosis: some cell lines were very sensitive (Tu686, 686LN, 93-VU-147T), some moderately (Fadu, SqCCy1, 1483, UMSSC90) and others were resistant (UD-SCC2, MSK-LEUK1, JHU022) to apoptosis induction. Only pan-PI3K inhibitors (BKM120 and BEZ235) induced effective apoptosis in combination with erlotinib, since the PI3K-α inhibitor BYL279 or allosteric mTORC1 inhibitor RAD001 failed to induce apoptosis in combination with erlotinib. Erlotinib strongly inhibited p-EGFR and p-ERK in vitro and in vivo, but only partially inhibited p-AKT. On the other hand, BKM120 completely inhibited p-AKT without affecting p-ERK. We also found that the combination of BKM120 and erlotinib strongly inhibited both axes of the AKT-mTORC1 pathway in sensitive cell lines, but failed to inhibit p-4EBP1 in one resistant cell line (UD-SCC2). In addition, the combination of BKM120 and erlotinib strongly inhibited Bcl-2, Bcl-xL and MCL-1 at the translational level in the sensitive cell lines but not the resistant one. siRNA-mediated knockdown of eIF4E (to inhibit 4EBP1-eIF4E-dependent translation) sensitized UD-SCC2 cells to the combination of erlotinib and BKM-induced apoptosis. Conclusions: Our results strongly suggest that co-targeting of EGFR and PI3K is synergistic and induces apoptosis of HNC cell lines by inhibiting both axes of the AKT-mTOR pathway and translational regulation of anti-apoptotic Bcl-2 proteins. Citation Format: Abu Anisuzzaman, Abedul Haque, Zhuo Chen, Dong M. Shin, A.R.M. Ruhul Amin. Combination of BKM120 and erlotinib in squamous cell carcinoma of the head and neck: mechanism of in vitro and in vivo synergy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3502.