ERBB2 and breast cancer: Tucatinib-treated BT474 tumors had significantly decreased HER2 expression (p < 0.05); however, no significant HER2 change was observed in BCM3472 tumors.<h4>Conclusion</h4>Tucatinib significantly decreases intratumoral proliferation and hypoxia in both cell-line and patient-derived xenograft models of HER2 + breast cancer, which can be longitudinally quantified with PET imaging.