Aromatase inhibitors versus tamoxifen in early breast cancer: patient-level meta-analysis of the randomised trials

Background The optimal ways of using aromatase inhibitors or tamoxifen as endocrine treatment for early breast cancer remains uncertain. Methods We undertook meta-analyses of individual data on 31 920 postmenopausal women with oestrogen-receptor-positive early breast cancer in the randomised trials of 5 years of aromatase inhibitor versus 5 years of tamoxifen; of 5 years of aromatase inhibitor versus 2-3 years of tamoxifen then aromatase inhibitor to year 5; and of 2-3 years of tamoxifen then aromatase inhibitor to year 5 versus 5 years of tamoxifen. Primary outcomes were any recurrence of breast cancer, breast cancer mortality, death without recurrence, and all-cause mortality. Intention-to-treat log-rank analyses, stratified by age, nodal status, and trial, yielded aromatase inhibitor versus tamoxifen first-event rate ratios (RRs). Findings In the comparison of 5 years of aromatase inhibitor versus 5 years of tamoxifen, recurrence RRs favoured aromatase inhibitors significantly during years 0-1 (RR 0.64, 95% CI 0.52-0.78) and 2-4 (RR 0.80, 0.68-0.93), and non-significantly thereafter. 10-year breast cancer mortality was lower with aromatase inhibitors than tamoxifen (12.1% vs 14.2%; RR 0.85, 0.75-0.96; 2p=0.009). In the comparison of 5 years of aromatase inhibitor versus 2-3 years of tamoxifen then aromatase inhibitor to year 5, recurrence RRs favoured aromatase inhibitors significantly during years 0-1 (RR 0.74, 0.62-0.89) but not while both groups received aromatase inhibitors during years 2-4, or thereafter; overall in these trials, there were fewer recurrences with 5 years of aromatase inhibitors than with tamoxifen then aromatase inhibitors (RR 0.90, 0.81-0.99; 2p=0.045), though the breast cancer mortality reduction was not significant (RR 0.89, 0.78-1.03; 2p=0.11). In the comparison of 2-3 years of tamoxifen then aromatase inhibitor to year 5 versus 5 years of tamoxifen, recurrence RRs favoured aromatase inhibitors significantly during years 2-4 (RR 0.56, 0.46-0.67) but not subsequently, and 10-year breast cancer mortality was lower with switching to aromatase inhibitors than with remaining on tamoxifen (8.7% vs 10.1%; 2p=0.015). Aggregating all three types of comparison, recurrence RRs favoured aromatase inhibitors during periods when treatments differed (RR 0.70, 0.64-0.77), but not significantly thereafter (RR 0.93, 0.86-1.01; 2p=0.08). Breast cancer mortality was reduced both while treatments differed (RR 0.79, 0.67-0.92), and subsequently (RR 0.89, 0.81-0.99), and for all periods combined (RR 0.86, 0.80-0.94; 2p=0.0005). All-cause mortality was also reduced (RR 0.88, 0.82-0.94; 2p=0.0003). RRs differed little by age, body-mass index, stage, grade, progesterone receptor status, or HER2 status. There were fewer endometrial cancers with aromatase inhibitors than tamoxifen (10-year incidence 0.4% vs 1.2%; RR 0.33, 0.21-0.51) but more bone fractures (5-year risk 8.2% vs 5.5%; RR 1.42, 1.28-1.57); non-breast-cancer mortality was similar. Interpretation Aromatase inhibitors reduce recurrence rates by about 30% (proportionately) compared with tamoxifen while treatments diff er, but not thereafter. 5 years of an aromatase inhibitor reduces 10-year breast cancer mortality rates by about 15% compared with 5 years of tamoxifen, hence by about 40% (proportionately) compared with no endocrine treatment. Copyright (C) Early Breast Cancer Trialists' Collaborative Group (EBCTCG). Open Access article distributed under the terms of CC BY.