Scientists at Hokkaido University have published research on the effects of chemotherapy on the cells that line the blood vessels inside tumors. They found that, in response to first-line chemotherapy, these tumor endothelial cells (TEC) can become drug-resistant, which allows the tumors themselves to survive chemotherapy and metastasize.
More specifically, the researchers found that the chemotherapy drug paclitaxel causes upregulation of ABCB1, a cell membrane protein that allows TEC to pump the drug out of the cell and thereby become drug-resistant. They also found that a combination of paclitaxel and an ABCB1 inhibitor were better at fighting tumor growth and metastasis than paclitaxel alone. Read more about it here.