Abstract
<div>Abstract<p>Pharmacologically targeting the HER2 oncoprotein with therapeutics such as the mAb, trastuzumab, provides clinical benefit for patients with HER2-positive (HER2<sup>+</sup>) cancers. However, a significant number of patients eventually progress on these therapies. Efforts to overcome therapeutic resistance through combination therapy with small-molecule inhibitors of HER2 have been limited by toxicities associated with off-target activity and/or limited efficacy. In this preclinical study, we explore single-agent and combined activity of tucatinib, a novel HER2-selective small-molecule inhibitor. Tucatinib demonstrated potent, selective activity in a panel of 456 human cancer cell lines, with activity restricted to cell lines (breast and non-breast) with HER2-amplification, including models of acquired resistance to trastuzumab. Within the HER2<sup>+</sup> population, tucatinib response tracked strongly with HER2-driven signaling. Single-agent tucatinib induced tumor regressions in xenograft models of HER2<sup>+</sup> breast cancer and combination with trastuzumab induced a complete and sustained blockade of HER2/PI3K/AKT signaling. Efficacy of the tucatinib/trastuzumab combination matched that induced by current standard-of-care trastuzumab/pertuzumab/docetaxel combination, with the exception that the chemotherapy-sparing tucatinib/trastuzumab combination did not require a dosing holiday to achieve the same efficacy. In xenograft models of HER2<sup>+</sup> breast cancer that also express estrogen receptor (ER; HER2<sup>+</sup>/ER<sup>+</sup>), tucatinib showed combined efficacy with inhibitors of CDK4/6 and ER, indicating potential novel therapeutic strategies for difficult-to-treat subtypes of HER2<sup>+</sup> breast cancer. These data support expanded clinical investigations of tucatinib as a combination partner for other novel and approved targeted therapies for HER2-driven malignancies.</p></div>
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