How does herceptin (trastuzumab) work?

Many ways: Described by dr. Clifford hudis in nejm in 2007. It can reduce her2 signaling, it can recruit cells to directly kill cancer cells. Her2 can activate a protein called p95. Herceptin (trastuzumab) can block this thereby reduce signaling leading to cell death. It is a multifaceted medication so this is why it is probably the most important drug in her2 positive breast cancer. Hope this helps.

Binds to tumor: Herceptin (trastuzumab) is a man-made antibody that binds to cancer cells if they express the her2 receptor. It acts like "seasoning" for the immune system, attracting immune cells which kill the tumor cells. That's a simplistic explanation, because no one really understands what happens when Herceptin (trastuzumab) binds to tumor cells. There are many theories, but we know immune cells are necessary for it to work.