These drugs target Heat Shock Protein 90. HSP90 is one of a class of chaperone proteins, whose normal job is to help other proteins acquire and maintain the shape required for those proteins to do their jobs. Chaperone proteins work by being in physical contact with other proteins. Hsp90 can also enable cancer cells to survive and even thrive despite genetic defects which would normally cause such cells to die. Thus, blocking the function of HSP90 and related chaperone proteins may cause cancer cells to die, especially if blocking chaperone function is combined with other strategies to block cancer cell survival.
Here is a link to information about HSP90 in Wikipedia.
Here is a link to a very detailed scientific article in Medscape about