Removal of MET kinase inhibition causes tumor rebound

MET oncogene amplification is emerging as a major mechanism of acquired resistance to EGFR-directed therapy in lung and colorectal cancers. Further, MET amplification predicts responsiveness to MET inhibitors currently in clinical trials. Among the anti-MET drugs available, ATP-competitive small molecule kinase inhibitors abrogate receptor autophosphorylation and downstream activation of ERK1/2 and AKT, resulting in cell cycle arrest. However, this anti-proliferative effect allows persistence of a pool of cancer cells that are quiescent but alive. Once the inhibition is removed, rebound activation of MET-driven cell proliferative pathways and tumor growth may occur, an adverse event observed frequently in clinical settings after drug discontinuation. Here we show that inhibitor withdrawal prompts receptor phosphorylation to levels higher than those displayed at steady-state and generates a rebound effect pushing quiescent cancer cells back into the cell cycle, both in vitro and in experimental tumor models in vivo. Mechanistically, we found that inhibitor treatment blocks MET endocytosis, causing a local increase in the number of receptors at the plasma membrane. Upon inhibitor washout, the receptor is readily re-phosphorylated. The initial phosphorylation is not only increased but also prolonged in duration due to downmodulation of a phosphatase-mediated MET negative feedback loop which accompanies receptor internalization. Notably, treatment with a MET therapeutic antibody that induces proteolytic cleavage of the receptor at the cell surface substantially prevents this rebound effect, providing a rationale to combine or alternate these mechanistically different types of MET-targeted therapy.

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