Interaction between body mass index and bone turnover during aromatase inhibition: Evidence from the letrozole (L), exemestane (E), and anastrozole (A) pharmacodynamics (LEAP) trial

560 Background: High body mass index (BMI) protects against postmenopausal osteoporotic fracture, mediated in part by higher endogenous levels of oestradiol. Third-generation aromatase inhibitors (AIs) show superior efficacy and tolerability than tamoxifen in the adjuvant treatment of postmenopausal women with hormone receptor-positive early breast cancer. We examined the interactions between BMI and bone turnover during treatment with 3 aromatase inhibitors in the LEAP trial, an open, randomized, pharmacodynamic study in postmenopausal women. Methods: Healthy volunteers from the UK and Hungary with normal bone density and BMI between 18 and 34 kg/m2 were randomized to receive A (1 mg/day), L (2.5 mg/day), or E (25 mg/day) orally, once daily for 24 weeks. Effects on bone resorption (log transformed serum C-telopeptide crosslinks, CTX) and bone formation (bone alkaline phosphatase and propeptide of type I procollagen, PINP) were compared. Changes in biochemical markers of bone resorption and formation during treatment were all statistically inter- correlated (p<0.01) with no notable differences between the 3 agents studied, so that the groups were pooled for analysis. Spearman correlation coefficients and the p-values were computed. Results: A total of 90 participants were evaluable at baseline and at 24 weeks (29 A, 29 L, 32 E). As expected, baseline BMI correlated with pre-treatment circulating endogenous oestradiol (r=0.28, p=0.007) and both BMI and oestradiol negatively correlated with baseline serum CTX (r=-0.32, p=0.0025 and r=-0.35, p=0.0007 respectively) and PINP (r=-0.30, p=0.004 and r=-0.26, p=0.016 respectively). Baseline BMI and oestradiol were significantly correlated with the increase in serum CTX (r=0.26, p=0.015 and r=0.23, p=0.032 respectively) and BMI also correlated significantly with the increase in serum PINP (r=0.23, p=0.030). Conclusions: Steroidal and non-steroidal AIs increase bone turnover with the greatest increase in women with high BMI and higher oestradiol levels at baseline. As increased bone turnover is an independent risk factor for fracture, a high BMI may not confer a reduced fracture risk in the setting of AI use in early breast cancer. No significant financial relationships to disclose.