Abstract A126: Acquired resistance to trastuzumab/pertuzumab, or to T-DM1 in vivo can be overcome by HER2 inhibition with TAS0728

Background: HER2-targeting antibodies (trastuzumab, pertuzumab) and an antibody-drug conjugate (trastuzumab emtansine: T-DM1) are available for the treatment of HER2 overexpressed breast cancer. TAS0728 is a small molecule, covalent-binding, selective inhibitor of HER2 kinase, and its assessment in humans is ongoing. Although HER2 targeting therapies with trastuzumab/pertuzumab, or T-DM1 initially show clinical activity, those treatments eventually become ineffective due to acquired resistance. Various resistant models to the therapies have been established in vitro to explore novel therapies for resistance. However, those models may not mimic patients’ tumors, because antibody dependent cellular cytotoxicity (ADCC) and/or pharmacokinetics of the drugs were not involved in the establishment of the models. In our previous report, we have established an acquired resistant model to T-DM1 in vivo. The model was sensitive to TAS0728. In the present study, we analyzed the molecular mechanism of T-DM1 resistance and mode of action of TAS0728 in the established model. Furthermore, we established another model with acquired resistance to trastuzumab and pertuzumab in mice, and investigated the sensitivity to TAS0728. Materials and Methods: To establish a resistant model to T-DM1, T-DM1 was administered at 10 mg/kg (q3w, i.v.) in NCI-N87 xenograft model until tumor regression and the subsequent re-growth of tumor were observed. Pharmacodynamics of T-DM1 at 10 mg/kg (q3w, i.v.) or TAS0728 at 60 mg/kg (q.d., p.o.) was evaluated in the tumors. To clarify the resistant mechanisms to T-DM1 and mode of action of TAS0728, molecular profiling of the resistant tumors was conducted by phospho-RTK array, RNA sequencing, and Western blotting. To establish a resistant model to trastuzumab/pertuzumab in vivo, the antibodies were administered concomitantly at 20 mg/kg each (q1w, i.p.) in the NCI-N87 xenograft model until tumor regression and the subsequent re-growth of tumor were observed. The mice were treated with the antibodies at 20 mg/kg each (q1w, i.p.) or TAS0728 at 60 mg/kg (q.d., p.o.) to compare antitumor effects. Results: In the T-DM1 resistant model, TAS0728 exerted antitumor effect associated with HER2 signal inhibition. RNA sequencing revealed that expression level of a candidate gene, which may be involved in the intracellular metabolism of T-DM1, was decreased in the T-DM1 resistant tumor. Alternative RTK activation was not observed. Next, we established a resistant model to trastuzumab/pertuzumab combination therapy in vivo. Although the antibodies initially induced tumor regression in the NCI-N87 xenograft model, ultimately, regrowth of tumors was observed. Once the tumors regrew during the treatment, the antibodies were no longer effective. On the other hand, switching to TAS0728 resulted in significant antitumor effect. Collectively, these results suggested that tumors with acquired resistance to trastuzumab/pertuzumab or T-DM1 are still addicted to the active HER2 signal, and were vulnerable to HER2 inhibition by TAS0728. Conclusion: The acquired resistance models to trastuzumab/pertuzumab or T-DM1 were established in vivo. Selective inhibition of HER2 kinase by TAS0728 is effective in the both resistant models. These results provide a rationale for therapy with TAS0728 in patients refractory to the established anti-HER2 therapy. Citation Format: Hiroki Irie, Rumi Kawabata, Yayoi Fujioka, Fumio Nakagawa, Hideki Nagase, Hiraku Itadani, Kimihiro Ito, Junji Uchida, Shuichi Ohkubo, Kenichi Matsuo, Takeshi Sagara, Teruhiro Utsugi. Acquired resistance to trastuzumab/pertuzumab, or to T-DM1 in vivo can be overcome by HER2 inhibition with TAS0728 [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A126. doi:10.1158/1535-7163.TARG-19-A126