ERBB2 and breast cancer: 2017). It was developed for the treatment of HER2-positive solid tumours, including breast cancer (BC) and colorectal cancer (Cesca et al. 2020). On April 2020, a marketing authorization for tucatinib for the treatment of adult patients with HER2-positive locally advanced or metastatic BC was granted by US Food and Drug Administration (FDA) (Lee 2020; Shah et al. 2021).