Thromboembolism (TE) and adjuvant endocrine therapy (AET) in hormone receptor positive (HR+) early breast cancer (EBC): Did the evolution of treatment change the incidence of the adverse event? A metanalysis

Background: The AET of HR+ EBC has been changing in the recent years. Aromatase inhibitors (AI) as upfront, or in a planned switch strategy after Tamoxifen (T), have been added to the choice of T alone. An increased risk of TE is well known in the T treated patients while AIs have showed a reduced rate of TE. Recently, adding the cyclin dependent kinase 4/6 inhibitors (CDK4/6) abemaciclib to AIs, has showed a positive impact in the high risk HR+ EBC subgroups, but we are seeing an increase of the TE rate. We conducted this meta-analysis to evaluate the impact of the new AETs on the incidence of TE if compared to the standard monotherapy. Methods: We performed a meta-analysis of the randomized phase III trials comparing the experimental AETs and the endocrine standard therapy. A random-effect model to find differences in the rate of TE events between the experimental treatments and the standard therapy has been used. Results: Twelve phase III trials were included. Five trials evaluated the upfront strategy, 6 studies the switch and one the combination with a CDK4/6inhibitor (i.e. abemaciclib). Overall, the new AETs did not significantly modify or affect the rate of TE events (OR 0.708, 0.444-1.130, p = 0.148) with high heterogeneity among studies (I2 87, p < 0.0001). Excluding the abemaciclib trial, the incidence of TE is reduced (OR 0.609, 0.462-0.802, p < 0.0001) with a moderate heterogeneity among the studies (I2 59, p < 0.006). Considering the upfront strategies with AIs, the TE events are reduced (OR 0.507, 0.394-0.651, p < 0.0001) but they are not if we consider the trials in which T is used upfront before AIs (OR 0.762, 0.546-1.065, p = 0.112). Conclusions: Overall, the new treatments (AIs alone or plus CDK4/6 inhibitors) did not affect the rate in TE events. AIs as upfront strategy is the safest AETs of HR+ EBC, being associated to the lowest incidence of TE. The switch strategy increases the TE rate whereas the addition of abemaciclib to the standard AET showed to significantly increase the TE events. The results of the currently ongoing trials with the CDK4/6 inhibitors will help to obtain additional data to evaluate any differences among the different CDK4/6 inhibitors and to clarify the weight of the TE adverse events in the balance of benefit/risk of this new adjuvant strategy.