Body composition changes during breast cancer preventive treatment with anastrozole: Findings from the IBIS-II trial

Background: Uptake to anastrozole for breast cancer prevention is low, partly due to women’s concerns about side effects including gains in weight and specifically gains in body fat. Previous evidence does not link anastrozole with gains in weight, but there is a lack of data on any effects on body composition i.e. changes in fat and fat free mass. Here we assess association of anastrozole with body composition changes in a prospective sub-study from the second international breast intervention trial (IBIS-II). Methods: Participants had DXA scans at baseline and for five years of anastrozole/placebo and beyond (between March 2004 and September 2017. Primary outcomes were changes in body weight, body fat and fat free mass at 9–18 months. A linear model was used to estimate the size of a differential effect in these outcomes by randomised treatment allocation adjusted for baseline value and time since last scan, age, 10-year breast cancer risk, smoking and HRT status. Results: 203 postmenopausal women were recruited (n = 95 anastrozole, n = 108 placebo), mean age 58 years (SD = 5.4), BMI 28.0 kg/m2 (SD = 5.5). There was no evidence of a strong association between anastrozole or placebo and endpoints at 9–18 months; effect size (95 %CI) for anastrozole minus placebo for body weight (per/kg) −0.11 (−1.29–1.08); body fat 0.11 (−0.75–0.96) and fat free mass −0.30 (−0.79–0.19). Conclusions: There is unlikely to be a clinically significant change to body composition with anastrozole for breast cancer prevention.