Cardiovascular Safety Profiles of Aromatase Inhibitors

Third-generation aromatase inhibitors (AIs) are now being used for the adjuvant treatment of postmenopausal women with breast cancer, and are challenging tamoxifen, the previous ‘gold standard’ of care, in this setting. This review evaluates the potential clinical impact of anastrozole, letrozole and exemestane on the cardiovascular (CV) system of postmenopausal women with breast cancer. Some data for CV safety are available for AIs from the advanced disease setting; however, most derive from patients being treated for early disease. CV data on anastrozole for the treatment of early breast cancer were taken from the ATAC trial, in which anastrozole was compared with tamoxifen in the primary adjuvant setting, and the ABCSG trial 8/ARNO 95 combined analysis, in which switching to 3 years of anastrozole after 2 years of tamoxifen was compared with the standard 5 years of tamoxifen adjuvant therapy. Letrozole has been studied in the primary adjuvant setting and the adjuvant sequencing setting in the BIG 1–98 study, as well as in extended adjuvant endocrine therapy after 5 years of tamoxifen in the MA-17 trial. For exemestane, results were reviewed from the IES trial, in which switching to exemestane following 2–3 years of adjuvant tamoxifen was compared with continued tamoxifen treatment.

All these trials clearly confirmed that all three AIs significantly reduce the risk of thromboembolic events compared with tamoxifen. Data on anastrozole versus tamoxifen from the ATAC trial (68 months’ follow-up) showed a similar incidence of myocardial infarctions (MIs), CV deaths and overall deaths for both therapies; however, anastrozole appeared to be associated with a lower incidence of cerebrovascular events compared with tamoxifen. In addition, the ABCSG trial 8/ARNO 95 study reported no difference in terms of MIs for patients switching to anastrozole compared with patients continuing tamoxifen treatment.

Data from BIG 1–98 (26 months’ follow-up) suggested that primary adjuvant treatment with letrozole may be associated with a significantly greater incidence of CV events and a numerical increase of cerebrovascular and cardiac deaths compared with tamoxifen. However, no increase in CV events with letrozole was reported from the MA-17 trial.

In the IES, updated data at 55 months’ median follow up showed no significant difference in the incidence of MIs and cardiac deaths between patients who switched to exemestane compared with those who continued tamoxifen.

In conclusion, a significantly reduced risk of thromboembolic disease was observed for all three AIs compared with tamoxifen. Anastrozole is, at this point, the only AI with a detailed benefit-risk profile from over 5 years’ follow-up in the adjuvant setting. Thus far, no apparent CV-safety concerns have emerged. Preliminary data on letrozole and exemestane suggest that longer follow-up is needed for these two AIs before being able to fully assess their respective long-term CV toxicity profile. The present differences in CV-safety profiles suggest that third-generation AIs should not be considered as equivalents in clinical practice.