Her2-positive breast cancer patients may not respond to Herceptin® (trastuzumab) for the following reasons:
- Inaccurate testing, especially by immunohistochemistry (IHC)
- Inaccurate scoring, especially by IHC
- Other molecular markers expressed simultaneously, such as Topo IIa and p53
- Phosphorylation of HER2 itself
- Other inhibitors in the PI3K pathway, such as PTEN
