Study Evaluating Bosutinib-Letrozole Combination Versus Letrozole Alone In Post Menopausal Women With Breast Cancer

Inclusion Criteria:

* Surgically sterile or post-menopausal women.
* Confirmed pathologic diagnosis of breast cancer.
* Locally advanced or metastatic, or loco-regional recurrent breast cancer not amenable to curative treatment with surgery or radiotherapy.
* Documented ER+ and/or PgR+ and erbB2- tumor based on most recently analyzed biopsy, as documented by a local laboratory.
* At least 1 radiologically measurable lesion as defined by Response Evaluation Criteria in Solid Tumors (RECIST).
* Eastern Cooperative Oncology Group (ECOG) performance status 0-2.

Exclusion Criteria:

* Prior letrozole (except in adjuvant setting), prior bosutinib, or any other prior Src inhibitor.
* Prior endocrine treatment for locally advanced or metastatic breast cancer (up to one prior adjuvant Aromatase Inhibitor (AI) agent/regimen is permitted).
* More than 1 prior chemotherapy regimen in locally advanced or metastatic breast cancer.
* Adjuvant endocrine therapy \<=12 months prior to day 1 of treatment.
* Disease refractory (ie, Progressive disease (PD) within 6 months from initiation of therapy) to previous adjuvant antiestrogen therapy.
* Bone or skin as the only site of disease.
* Extensive visceral disease or active Central Nervous System (CNS) disease.
* Any other cancer within 5 years of screening with the exception of ER+ contralateral breast carcinoma, adequately treated cervical carcinoma in situ, or adequately treated basal or squamous cell carcinoma of the skin.
* Major surgery or radiotherapy within 14 days of treatment day.
* Inadequate hepatic/renal/bone marrow function.
* History of clinically significant or uncontrolled cardiac disease.
* Serious concurrent illness.