Immunohistochemical features associated with sensitivity to lapatinib-plus-capecitabine and resistance to trastuzumab in HER2-positive breast cancer.

To identify immunohistochemical (IHC) features associated with sensitivity to lapatinib-plus-capecitabine (LX) and resistance to trastuzumab in human epidermal growth factor receptor (HER)-2-positive metastatic breast cancer. Expression levels of estrogen receptor, progesterone receptor, epidermal growth factor receptor, HER2, HER3/phosphorylated HER3 (pHER3), phosphatase and tensin homolog, thymidylate synthase (TYMS), and thymidine phosphorylase by IHC were compared between patients treated with LX following trastuzumab failure. In 35 patients, HER2 was the only biomarker associated with LX treatment outcomes. A high HER2 level was associated with significantly longer survival and a tendency towards longer time-to-progression and higher response rates. Acquisition of trastuzumab resistance was associated with higher pHER3 and TYMS expression. Elevated pHER3 was predictive of superior treatment outcomes. Up-regulation of pHER3 and TYMS was associated with trastuzumab resistance. High HER2 and increased pHER3 IHC levels correlated with favourable LX treatment outcomes in patients with HER2-positive metastatic breast cancer.