Abstract P6-03-01: A combination of dual inhibition in HER2-network by T-DM1 and GDC-0980 provides maximal antitumor efficacy in preclinical model of HER2+ breast cancer
Authors: De PK , Carlson JH , Sun Y , Lin X , Friedman L , Dey N , Leyland-Jones B 
Published: March 2017

Abstract
Background: PIK3CA mutation is associated with a lower pCR rate in primary HER2+ breast cancer (BC) treated with trastuzumab and lapatinib in addition to chemotherapy (from five clinical trials, PMID: 27177864). The BOLERO-1 study showed that the efficacy of a combination of mTOR inhibitor (everolimus) plus trastuzumab (T) and paclitaxel was not very efficacious with HER2+ advanced BC patients except for HER2+/ER- BC patients (PMID: 26092818). In the same line, BOLERO 3 trial data showed the same combination (T + everolimus + vinorelbine) is also not efficacious in T-resistant, HER2+ advanced breast cancer women (median PFS 7 months with everolimus and 5.78 months with placebo, HR: 0.78) (PMID: 24742739). T-DM1 does not have typical adverse events of chemotherapy. Therefore, there has been interest in combining it with other targeted agent. Here we tested the efficacy of a combination of T-DM1 plus GDC-0980 (a dual PI3K/mTOR inhibitor) in HER2+/T-resistant BC cell lines in vitro and in vivo. Methodology: Here we have studied the in vitro and in vivo effects of GDC-0980 along with T-DM1 in HER2+/T-sensitive (BT474), HER2+/T-resistant (BT474HerR), and HER2+/PIK3CA (HCC1954, MDA-MB453) mutated models. We assessed in vitro anti-proliferative, pro-apoptotic and activation status of the PI3K-AKT-mTOR signaling pathway following the combination of GDC-0980 plus T-DM1 in HER2+ BC cell lines. We next evaluated the impact of GDC-0980 plus T-DM1 on tumor growth and angiogenesis using xenograft models. Results: 1) GDC-0980 inhibited downstream activation of the PI3K-mTOR signaling pathway effectors, p-AKT (Ser473, The308), p-P70S6K, p-S6RP and p-4EBP1, and this inhibition was more pronounced when GDC-0980 was combined with T-DM1, 2) similarly the anti-proliferative activity of a combination of GDC-0980 plus T-DM1 was significantly higher by 3D-ON-TOP clonogenic assay following heregulin stimulation, 3) consistent with anti-proliferative effects of GDC-0980, the proportion of cells in the G1 phase of the cell cycle increased in HER2+ cell lines with a concomitant decrease in the S phase of their treatment with GDC-0980, 4) the initiation of apoptotic activity (annexin V) of GDC-0980 was significantly superior to that of an allosteric inhibitor of mTOR, RAD001. GDC-0980 also induced apoptotic markers like cleaved CASPASE3, cleaved PARP1, BIM in HER2+ BC cells and 5) a combination of GDC-0980 plus T-DM1 significantly blocked tumor growth to tumor regression in the HER2+/T-sensitive, HER2+/T-resistant and HER2+/PIK3CA mutated BC xenograft models. Along with its anti-tumor effect, this combination effectively decreased tumor angiogenesis (tumor micro-vessel density via CD31 staining). Conclusions: A combination of GDC-0980 plus T-DM1 significantly blocked in vitro and in vivo HER2+ breast tumor cells growth irrespective of PIK3CA mutation status. This strategy warrants further clinical investigation.
Citation Format: De PK, Carlson JH, Sun Y, Lin X, Friedman L, Dey N, Leyland-Jones B. A combination of dual inhibition in HER2-network by T-DM1 and GDC-0980 provides maximal antitumor efficacy in preclinical model of HER2+ breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-03-01.