As a negative regulator of the PI3K pathway, loss of PTEN function through mutational inactivation or downregulation of expression results in activation of PI3KCAKT signaling and prevents trastuzumab-mediated growth arrest in HER2-amplified breast cancer cells. a constitutively active form of HER2, and crosstalk of HER2 with other growth factor receptors. The identification of these specific mechanisms of trastuzumab resistance has provided a rationale for the development of several novel HER2-targeted brokers as the mechanisms have largely suggested a continued tumor dependence on HER2 signaling. We explore the emerging data for the treatment of trastuzumab-refractory disease with novel brokers including lapatinib, neratinib, pertuzumab, trastuzumab-DM1, HSP90 and PI3K pathway inhibitors, and the future potential for these inhibitors which, if combined with reliable biomarkers of resistance, may ultimately usher in a new era of personalized medicine for this disease. gene encodes a transmembrane tyrosine kinase receptor that belongs to the EGF receptor (EGFR) family. This family of receptors includes four users (EGFR/HER1, HER2, HER3 and HER4) that function by stimulating growth factor signaling pathways such as the PI3KCAKTCmTOR pathway [4]. Receptors of this family contain an extracellular ligand-binding domain name, a lipophilic transmembrane domain name, and an intracellular tyrosine kinase domain name. Activation of receptor kinase function occurs predominantly via ligand-mediated hetero- or homo-dimerization. In the case of HER2, activation is also thought to occur in a ligand-independent manner, particularly when the receptor is found to be mutated or overexpressed [5]. Overexpression of HER2 enables constitutive activation of growth factor signaling pathways and thereby serves as an oncogenic driver Aesculin (Esculin) in breast malignancy. Through both genetic and pharmacologic methods it was decided that HER2 was both necessary and sufficient for tumor formation and maintenance in models of HER2-amplified breast cancer. Given that HER2 amplification mediates the transformed phenotype, direct pharmacologic targeting of HER2 was proposed. Trastuzumab (herceptin), a humanized, recombinant monoclonal antibody that binds to the extracellular domain name of HER2, has been shown to selectively exert anti-tumor effects in malignancy models and patients with HER2-amplified breast malignancy, and not in tumors with normal HER2 expression [6C8]. Although an unconfirmed analysis has suggested possible added benefits of Rabbit Polyclonal to Retinoic Acid Receptor beta trastuzumab for adjuvant patients with HER2 normal disease, the wealth of pre-clinical and clinical data point to the benefits of this drug exclusively in HER2-amplified disease [9]. Trastuzumab improves overall survival when given in combination with chemotherapy for metastatic disease and reduces the risk of disease recurrence and death when given in the adjuvant setting, making the drug the foundation for systemic therapy of HER2-overexpressing tumors [7,10C16]. Mechanisms of action Trastuzumab has been demonstrated to exert a variety of anti-tumor effects selectively in HER2-overexpressing tumor cells (Physique 1A). Trastuzumab binds to the Aesculin (Esculin) juxtamembrane domain name of HER2 and upon receptor binding, the antibody downregulates the expression of HER2 [17]. More recent work has exhibited that trastuzumab selectively blocks ligand-independent HER2CHER3 dimerization [18]. In addition, trastuzumab binding to HER2 blocks proteolytic cleavage of the extracellular domain name of HER2, resulting in diminished levels of the more active p95CHER2 form of HER2 [19]. As a result of these effects around the HER2 receptor, trastuzumab causes downregulation of PI3K pathway signaling and downstream mediators of cell cycle progression such as cyclin D1 [20]. Trastuzumab Aesculin (Esculin) not only inhibits HER2 signaling pathways but also triggers immune-mediated responses against HER2-overexpressing cells. Trastuzumab binding engages Fc receptors on immune effector cells leading to antibody-dependent cellular cytotoxicity [21,22]. Beyond these effects, trastuzumab has been shown to have antiangiogenic effects and to lower the proapoptotic threshold for chemotherapy [23]. Combinations of trastuzumab with several different chemo-therapeutic brokers have been tested in HER2-amplifed cell lines and xenograft models, and demonstrate additive or synergistic interactions for doxorubicin, epirubicin, paclitaxel carboplatin, docetaxel and gemcitabine [24C26]. As a result of these actions, the drug yields a clinical benefit for patients with all stages of HER2-positive breast cancer. Open in a separate window Physique 1 Proposed mechanisms of resistance to trastuzumab(A) HER2 transmission transduction. Activation of the receptor tyrosine kinase occurs by homodimerization Aesculin (Esculin) or heterodimerization with other HER family members. Activated HER2 initiates downstream signaling through the PI3KCAKTCmTOR pathway, promoting cell proliferation and survival..