Identification of predictive biomarkers and the mechanisms of acquired resistance to epidermal growth factor receptor inhibitors

Abstract

Epidermal growth factor receptor (EGFR) is one of the most commonly altered genes in human cancer by way of over-expression, amplification, and mutation. Targeted inhibition of EGFR activity suppresses signal transduction pathways which control tumor cell growth, proliferation, and resistance to apoptosis. EGFR targeted therapies have been recently developed for the treatment of multiple cancer types. At the time when they were introduced in clinical practice, there was little knowledge of the molecular bases of tumor sensitivity and resistance to these novel targeted compounds. First, epidermal growth factor receptor tyrosine kinase inhibitor has been shown to exert a synergistic antitumor effect when combined with fluoropyrimidine. This synergy may be attributable to the downregulation of thymidylate synthase (TS), which is frequently overexpressed in fluoropyrimidine-resistant cancer cells. However, the molecular mechanism underlying the downregulation of TS has yet to be clearly elucidated. I have demonstrated that lapatinib, a dual inhibitor of EGFR and HER2 TK, effectively downregulates a variety of nucleotide synthesis-related genes, including TS, and exhibits activity superior to that of gefitinib not only in HER2-amplified cells, but also in wild-type cells. As a mechanism, I have determined, for the first time, that nuclear EGFR and HER2 activate TS gene transcription, and that EGFR and HER2-bound TS promoter activities are inhibited by lapatinib treatment. Second, although non-small cell lung cancer (NSCLC) cells with somatic mutations in EGFR initially show a dramatic response to tyrosine kinase inhibitor, these cells eventually develop resistance to TKI. This resistance may be caused by a secondary T790M mutation in the EGFR tyrosine kinase, which leads to the substitution of methionine for threonine in 790. I showed that lapatinib revealed moderately enhanced cytotoxicity against gefitinib-resistant T790M cells in vitro and in vivo. Based on its dedimerization effect on EGFR and HER2, lapatinib may partially attenuate the resistance to EGFR TKI in T790M lung cancer cells. Third, human epidermal growth factor receptor 2 (HER2)-directed treatment using trastuzumab has shown clinical benefit in HER2-positive gastric cancer. Clinical trials using lapatinib in HER2-positive gastric cancer are also currently underway. As with other molecularly targeted agents, the emergence of acquired resistance to HER2-directed treatment is an imminent therapeutic problem for HER2-positive gastric cancer. The measurement of basal gene expression levels allowed us to identify Testican-1-mediated EMT as a mechanism of acquired resistance to lapatinib in HER2-gastric cancer cells. Testican-1 silencing partially restored sensitivity to lapatinib in resistant cells, implying that other signaling pathways may also be responsible for drug resistance. However, owing to the limited number of cell lines used in our experiments, it is possible that other important biological factors exist that contribute to drug resistance in HER2-positive gastric cancer.