An Open Label Study to Evaluate the Safety, Tolerability and Effect on Tumor Response of R1507 in Combination With Letrozole in Postmenopausal Women With Advanced Breast Cancer.

Inclusion Criteria:

- postmenopausal female patients;

- stage IV or locally advanced breast cancer;

- measurable disease;

- letrozole failure (Part 2);

- ECOG performance status <=2.

Exclusion Criteria:

- previous chemotherapy for metastatic breast cancer;

- concurrent antibody or immunotherapy for advanced disease, within 4 weeks prior to
receipt of study drug;

- history of primary malignancy in 5 years prior to study, excluding adequately treated
basal or squamous cell cancer of skin, or cancer in situ of cervix;

- concurrent radiotherapy, or radiotherapy within 28 days prior to receipt of study
drug.