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

This is a phase 2 study of bosutinib administered in combination with letrozole versus letrozole alone in post-menopausal women with breast cancer. This is a 2-part study. Subjects in part 1 will receive bosutinib and letrozole daily, and will be closely monitored for 28 days. The second part will proceed with subjects receiving a dose that is determined to be safe based on the safety evaluation of the first part. Eligible subjects will be randomly assigned to receive either bosutinib daily combined with daily letrozole, or daily letrozole alone for a specified period of time. Subjects will be followed up for survival after study drug discontinuation.