| Summary: | <p>HER2 overexpression has been implicated in breast cancer. Lapatinib is a dual EGFR/HER2 tyrosine kinase inhibitor used in advanced HER2-positive breast cancer patients who have previously progressed on trastuzumab treatment. Recent meta-analysis of several clinical trials showed that lapatinib was inferior to trastuzumab as a neoadjuvant treatment combined with chemotherapy; however, the mechanism underlying this difference remains unclear.</p> <p>This thesis aimed to assess the acute response and mechanism of resistance to lapatinib in HER2-overexpressing breast cancer cell lines SK-BR-3 and BT-474. Lapatinib inhibited the phosphorylation of EGFR, HER2 and HER3, but it recovered after eight hours of treatment. Lapatinib treatment induced NRG1 expression, correlating with this recovery. Lapatinib-resistant cell lines, SK250LR and BT250LR, were generated. These cells had an increased NRG1 mRNA expression, but the expression levels reduced within a week of lapatinib withdrawal.</p> <p>Exogenous NRG1 rescued breast cancer cells from lapatinib inhibition, because lapatinib only incompletely inhibited NRG1-induced HER3 and HER4 signalling pathways. Combining lapatinib with pertuzumab (L+P) improved the inhibition on NRG1-induced signalling. This difference in inhibitory abilities correlated with the effect of treatment, as L+P induced growth inhibition and apoptosis more than either drug alone. <em>In vivo</em> experiments also showed that L+P triggered greater tumour regression than monotherapy in a BT-474 xenograft model. L+P induced more apoptosis in BT250LR cells but not in SK250LR cells, but it required further validation in a lapatinib-resistant xenograft model. Analysis of RTKs activation suggested that SK250LR and BT250LR cell lines had different mechanisms of resistance. The SK250LR cell line underwent permanent changes in cell biology and became independent from EGFR/HER2 signalling. The BT250LR cell line developed a transient adaption by increasing HER signalling. Eight new RTK candidates were discovered, but their role in lapatinib resistance required further validation. This study suggested a novel strategy of drug combination between L+P in HER2-targeted therapy, which is promising for future investigation. It also uncovered new directions in RTK studies for their roles in lapatinib resistance.</p>
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