Evolution in the risk of adverse events of adjuvant endocrine therapy in postmenopausal women with early-stage breast cancer

Background: Adjuvant endocrine therapy is a gold standard in early-stage, hormone receptor positive breast cancer. In postmenopausal women, aromatase inhibitors (AIs) are associated with improved outcome compared to tamoxifen monotherapy. Differences in the toxicity profiles of these drugs are described; however, little is known about whether the risk of adverse events changes over time. Methods: Sequential reports of large, randomized, adjuvant endocrine therapy trials comparing AIs to tamoxifen were reviewed. Data on pre-specified adverse events were extracted including cardiovascular events, bone fractures, cerebrovascular disease, endometrial cancer, secondary malignancies excluding breast cancer, venous thrombosis and death without recurrence. Odds ratios (ORs) were calculated for each adverse event at each time over the course of follow-up. The change in the ORs for adverse events over time was evaluated using weighted linear regression. Results: Analysis included 21 reports of 7 trials comprising 30,039 patients and reporting outcomes between 28 and 128 months of follow-up. Compared to tamoxifen, AIs use was associated with a significant reduction in the magnitude of increased odds of bone fracture over time (β = − 0.63, p = 0.013). There was a non-significant decrease in the magnitude of reduced odds of secondary malignancies over time (β = 0.448, p = 0.094). The differences in other toxicity profiles between AIs and tamoxifen did not change significantly over time. Conclusions: The increased risk of bone fractures associated with adjuvant AIs falls over time and after discontinuation of treatment. Differences in other toxicities between AIs and tamoxifen do not change significantly over time including a persistently elevated risk of cardiovascular events.