Breast density changes in a randomized controlled trial evaluating bazedoxifene/conjugated estrogens

Objective: Breast density is associated with an increased risk of breast cancer. This study assessed changes in mammographic breast density after 24 months of treatment with bazedoxifene (BZA)/conjugated estrogens (CE) in postmenopausal women. Methods: This was an ancillary study in a subset of nonhysterectomized postmenopausal women enrolled in a randomized, double-blind, placebo-controlled, and active-controlled phase 3 study. Treatments evaluated were BZA 20 mg/CE 0.45 mg, BZA 20 mg/CE 0.625 mg, raloxifene 60 mg, and placebo. Women who were eligible for participation in the ancillary study must have completed 24 months of treatment and have mammograms at baseline and 24 months. The left craniocaudal views from each mammogram pair were digitized and analyzed by a radiologist who was blinded to treatment arm and mammogram date. The percent breast density was determined using validated software. Results: Mammogram pairs were obtained from 507 evaluable participants (mean age range, 55.2-56.3 y). The mean changes (95% CI) in mammographic breast density from baseline to 24 months were comparable among groups (-0.39% [-0.69 to -0.08], -0.05% [-0.38 to 0.27], -0.23% [-0.54 to 0.08], and -0.42% [-0.72 to -0.11] for BZA 20 mg/CE 0.45 mg, BZA 20 mg/CE 0.625 mg, raloxifene 60 mg, and placebo, respectively). These reductions from baseline were statistically significant for BZA 20mg/CE 0.45 mg and placebo. The effect of both BZA/CE doses on breast density was generally consistent among subgroups based on age, body mass index, and years since menopause. Conclusions: Treatment with BZA 20 mg/CE 0.45 mg or BZA 20 mg/CE 0.625 mg for 24 months did not affect mammographic breast density in this population of postmenopausal women.