 IPI-504 is an HSP90 inhibitor and is chemically related to 17-AAG and it has been studied in a clinical trial in combination with trastuzamab and a response rate of 26 727 was demonstrated in patients with pretreated, HER2-positive breast cancer. These data provide a strong scientific rationale for clinical testing of IPI-504 plus trastuzumab in patients with pretreated, locally advanced or metastatic HER2-positive breast cancer 