Despite documented HER2 receptor amplification, patients may have de novo or acquired resistance to trastuzumab. 39 Potential mechanisms of trastuzumab resistance include altered receptorantibody interaction (MUC4, p95HER2, ErbB2 mutations), altered downstream signaling (reduced p27, reduced PTEN, activation of PI3K, activation of Akt) and crosstalk with other signaling pathways (IGF-R1, ER, VEGFR).40 The p95HER2 receptor is created either by cleavage and shedding of the extracellular domain of the HER2 receptor or by specific mRNA splicing, resulting in a constitutively active truncated receptor which is associated with a more aggressive phenotype.
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