Anti-tumor Activity Of Trastuzumab In Combination With Cetuximab In Trastuzumab-resistant Gastric Cancer And Its Mechanism Of Action

Gastric cancer is one of the most common malignant carcinomas worldwide,and it has the second highest cause of cancer death. Researchs show that about15%-45%of gastric cancer patients with HER2overexpression. Trastuzumab，ahumanized monoclonal antibody directed against the dimerization interfaces indomain IV of HER2, has currently been approved for clinical treatment in patientswith erbB2-positive metastatic gastric and gastro-esophageal junction cancer.Clinical data showed that clinical response rate of trastuzumab reachs almost30%,but half of the trastuzumab-responsive patients develop resistance a year aftertreatment. Thus, futher studies are needed to better understand other signaltransduction pathways of acquired resistance after long-term exposure totrastuzumab in gastric cancers. Identification of mechanism for acquiredresistance will lead to design of clinical trials with newer approaches to delay orreverse resistance.The ErbB family belongs to the type I receptor tyrosine kinases and includesEGFR (HER1), ErbB2(Her2), ErbB3(HER3) and ErbB4(HER4). ErbB familymembers are activated by various ligands except HER2, which may not havephysiological ligands. Although HER2is unable to bind to any ligand, it couldform homodimers or heterodimers with both ligand-free and ligand-bound formsof EGFR、HER3or HER4and therefore becomes activated and triggers potentmechanisms of cell proliferation and survival. It has been reported that EGFRreceptors activate various common signaling pathways includingRas/Raf/MEK/ERK and PI3K/AKT. Recent studies have highlighted thatprolonged treatment with trastuzumab may induce tumor cells to reprogram themselves by overexpressing various receptor tyrosine kinases (RTKs) to developalternative compensatory pathways to sustain cell proliferation, ultimately leadingto trastuzumab resistance.In this study, we generated a trastuzumab-resistant subline of NCI-N87froma trastuzumab-sensitive gastric cancer cell line in vivo by exposingtrastuzumab-sensitive xenografts to increasing concentrations of trastuzumabfollowed by validation of the resistant phenotype in vivo and in vitro using MTTassays. Western blot and immunoprecipitation were used to examine the role ofEGFR on mediating trastuzumab resistance both in vitro and in vivo. Our resultsindicated that the amounts of the EGFR and the EGFR/HER2signaling pathwayswere remarkably up-regulated. We investigated the antitumor effect oftrastuzumab in combination with the cetuximab on the trastuzumab-resistantgastric cancer cell line, as well as its mechanism of action. Our data show thatcombining trastuzumab and cetuximab leads to a significant decrease inEGFR/HER2heterodimers and the AKT and MAPK signaling pathwayscompared with either antibody alone, and the combination results in greaterantitumor activity against the trastuzumab-resistant NCI-N87R cell line in vivo,suggesting that a combined EGFR/HER2inhibition may overcome trastuzumabresistance.Thus, the use of a combination cetuximab and trastuzumab inhibitingEGFR/HER2signaling pathways that confer trastuzumab resistance may have apotential clinical benefit.