Tucatinib also potently inhibited the phosphorylation of HER2 in a V777L mutant MCF-10A cell line, and in a colorectal PDX model containing a V777L mutation, tucatinib alone or in combination with trastuzumab induced tumor regressions...Taken together, these results support the idea that HER2 selective inhibition by tucatinib can block oncogenic signaling elicited by mutant HER2, and that it could potentially provide clinical benefit in patients with cancers with activating HER2 mutations.