A Study to Compare the Steady-State Bioavailability of Injectable Letrozole SIE and Oral Letrozole in Post-Menopausal Women With Hormone Receptor Positive Early Breast Cancer

Inclusion Criteria: * Women with weight of ≥50 kg and a Body Mass Index (BMI) ≥19 kg/m2 and ≤39 kg/m2 * Postmenopausal women. * Women with confirmed diagnosis of HR+/HER2- or HR+/HER2+ early stage breast cancer who have completed at least 5 years of endocrine therapy, at least 2 years of which were with letrozole. Participants at a low risk of relapse, as assessed by the investigator, can also be eligible after completing 4 years of adjuvant endocrine therapy, at least 2 of which were with letrozole. * Women in good health. * Women who have undergone breast imaging per applicable guidelines (e.g., mammogram) within the last 12 months with no evidence of malignancy (documentation required), and if not done, must be willing to have 1 performed prior to baseline. Exclusion Criteria: * Presence of an uncontrolled, unstable, clinically significant medical condition. * Have used estrogen or progesterone systemic or topical therapy, oral contraceptives, androgens, LH-releasing hormone analogs, prolactin inhibitors, or antiandrogens within 3 months prior to screening. * Use of inducers or inhibitors of CYP3A4 and CYP2A6. * Diagnosed with osteoporosis. * Preexisting cardiovascular disease * Positive result for hepatitis B surface antigen, hepatitis B core antibody, hepatitis C antibody, or HIV antibodies.