Acquired resistance to trastuzumab/pertuzumab or to T-DM1 in vivo can be overcome by HER2 kinase inhibition with TAS0728.

HER2-targeting antibodies (trastuzumab, pertuzumab) and a HER2-directed antibody-drug conjugate (trastuzumab emtansine: T-DM1) are used for the treatment of HER2-overexpressing breast cancer. However, these treatments eventually become ineffective due to acquired resistance and there is an urgent need for alternative therapies. TAS0728 is a small-molecule, irreversible selective HER2 kinase inhibitor. In the present study, we established new in vivo models of cancer resistance by continuous exposure to a combination of trastuzumab and pertuzumab or to T-DM1 for evaluating the effect of TAS0728 on HER2 antibody-resistant populations. Treatment with trastuzumab and pertuzumab or with T-DM1 initially induced tumor regression in NCI-N87 xenografts. However, tumor regrowth during treatment indicated loss of drug effectiveness. In tumors with acquired resistance to trastuzumab and pertuzumab or to T-DM1, HER2-HER3 phosphorylation was retained. Switching to TAS0728 resulted in a significant anti-tumor effect associated with HER2-HER3 signal inhibition. No alternative receptor tyrosine kinase activation was observed in these resistant tumors. Furthermore, in a patient-derived xenograft model derived from breast cancer refractory to both trastuzumab/pertuzumab and T-DM1, TAS0728 exerted a potent anti-tumor effect. These results suggest that tumors with acquired resistance to trastuzumab and pertuzumab and to T-DM1 are still dependent on oncogenic HER2-HER3 signaling and are vulnerable to HER2 signal inhibition by TAS0728. These results provide a rationale for TAS0728 therapy for breast cancers that are refractory to established anti-HER2 therapies.