Tucatinib was tested in 6 HER2 mutant PDX tumor models derived from colorectal, non-small cell lung cancer, gall bladder and gastric cancers with L755S, V77L or S310Y mutations...In two PDX models containing the S310Y extracellular domain mutation of HER2, tucatinib alone, or in combination with trastuzumab, resulted in tumor growth delays.