Herceptin: Mechanism of action

Herceptin is a humanized monoclonal antibody that acts on the HER2/neu (erbB2) receptor. Trastuzumab's principal use is as an anti-cancer therapy in breast cancer in patients whose tumors over express (produce more than the usual amount of) this receptor. Trastuzumab is administered either once a week or once every three weeks intravenously for 30 to 90 minutes.

Amplification of HER2/neu (ErbB2) occurs in 25-30% of early-stage breast cancers.[1] It encodes the extracellular domain of her2. Although the signaling pathways induced by the HER2/neu receptor are incompletely characterized, it is thought that activation of the PI3K/Akt pathway is important. This pathway is normally associated with mitogenic signaling involving the MAPK pathway. However in cancer the growth promoting signals from HER2/neu are constitutively transmitted — promoting invasion, survival and angiogenesis of cells.[2] Furthermore overexpression can also confer therapeutic resistance to cancer therapies. The prime mechanism that causes increase in proliferation speed is due to induction of p27Kip1, an inhibitor of cdk2 and of cell proliferation, to remain in the cytoplasm instead of translocation in to the nucleus.[3] This is caused by phosphorylation by Akt.

Herceptin is a humanized monoclonal antibody which binds to the extracellular segment of the HER2/neu receptor. Cells treated with trastuzumab undergo arrest during the G1 phase of the cell cycle so there is reduced proliferation. It has been suggested that trastuzumab induces some of its effect by downregulation of HER2/neu leading to disruption of receptor dimerization and signaling through the downstream PI3K cascade. P27Kip1 is then not phosphorylated and is able to enter the nucleus and inhibit cdk2 activity, causing cell cycle arrest.[3] Also, trastuzumab suppresses angiogenesis by both induction of antiangiogenic factors and repression of proangiogenic factors. It is thought that a contribution to the unregulated growth observed in cancer could be due to proteolytic cleavage of HER2/neu that results in the release of the extracellular domain. Trastuzumab has been shown to inhibit HER2/neu ectodomain cleavage in breast cancer cells.[4] There may be other undiscovered mechanisms by which trastuzumab induces regression in cancer.

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