Bortezomib (velcade) is a new drug that works by blocking a structure in the cells called proteasome. This proteasome acts like a system that eliminates abnormal or old proteins from the cells. Cancer cells may be sensitive to drugs like bortezomib (velcade), because tumor cells have more abnormal proteins than normal cells. Bortezomib (velcade) enters the tumor cells and affects the way they divide. When cancer cells cannot divide, they die.