The phase Ib part of the trial will assess the MTD of AUY922 in combination with Trastuzumab in patients with Trastuzumab-refractory locally advanced or metastatic HER2+. The MTD is defined as the highest drug dosage not causing in the first cycle of treatment (28 days) medically unacceptable dose limiting toxicity (DLT).

The phase II part of the trial will assess any potential effect on efficacy of adding AUY922 to Trastuzumab in patients with Trastuzumab-refractory locally advanced or metastatic HER2+ breast cancer.

Both AUY922 and Trastuzumab will be administered as a weekly IV infusion. Treatment should be continued as long as the patient does not have disease progression and tolerates the treatment. The following reasons are examples of acceptable reasons for discontinuing the study; tumor progression (by RECIST, as assessed by the investigator), unacceptable toxicity, death, or discontinuation from the study for any other reason, such as patient refusal, withdrawn consent, lost to follow-up or investigator decision.

Inclusion Criteria:

• Female patients with confirmed HER-2 positive , non-operable locally advanced or metastatic breast cancer
• All patients must have received at least 1 but no more than 2 prior anti HER2 based regimens including at least 1 regimen containing Trastuzumab.
• All patients must have at least one measurable lesion as defined by RECIST criteria.
• All patients must have documented progressive disease following the last line of therapy before entering the study
• ECOG Performance status ≤ 1

Exclusion Criteria:

• Patients with known CNS metastasis which are: symptomatic or require treatment for symptom control and/or growing
• Prior treatment with any HSP90 or HDAC inhibitor
• Impaired cardiac function
• Acute or chronic liver or renal disease
• Patients who are currently receiving treatment with any medication which has a relative risk of prolonging the QTc interval or inducing Torsades de Pointes and cannot be switched or discontinued to an alternative drug prior to commencing AUY922
• Patients with a history of another primary malignancy that is currently clinically significant or currently requires active intervention
• Patients who do not have either an archival tumor sample available or are unwilling to have a fresh tumor sample collected at baseline.

Other protocol-defined inclusion/exclusion criteria may apply