The Effect of Zoledronic Acid on Aromatase Inhibitor-Associated Bone Loss in Postmenopausal Women with Early Breast Cancer Receiving Adjuvant Letrozole: The Z-FAST Study 5-Year Final Follow-Up.

Abstract Background: Aromatase inhibitor (AI) therapy effectively increases disease-free survival in postmenopausal women (PMW) with ER+ and/or PR+ breast cancer (BCa). However, the use of AIs results in nearly complete ablation of estrogen production which can lead to accelerated bone loss and increased fracture risk. The Z-FAST study evaluated the efficacy and safety of zoledronic acid (ZOL) in preventing AI associated bone loss in PMW with early breast cancer (EBC) who received adjuvant letrozole (LET).Material and Methods: 602 PMW with stage I-IIIa ER+ and/or PR+ BCa starting LET (2.5 mg qd x 5 yrs) were randomized (1:1) to upfront ZOL (4 mg IV q 6 mos) vs delayed ZOL. The delayed arm (D) received ZOL when either the post-baseline T-score decreased to <-2 or a clinical fracture occurred. All patients (pts) were treated with calcium and vitamin D. The primary endpoint, the percent change in lumbar spine (LS) bone mineral density (BMD) at 12 mos, was previously reported (JCO; 25:829, 2007). The 5 year (5y) final study results are reported here.Discussion: Baseline characteristics were similar between groups. 180 pts in upfront ZOL arm (U) and 175 pts in D completed full 5y study. Of pts with BMD data available, U (n=140) showed a mean increase of 6.2% in LS BMD while D (n=132) showed a mean decrease of 2.4%, resulting in an absolute difference of 8.6% (p<0.001). U (n=141) showed a mean increase of 2.6% in total hip (TH) BMD while D (n=132) showed a mean decrease of 4.1%, resulting in an absolute difference of 6.7% (p<0.001). When BMD data in D was excluded after pts started ZOL (censored analysis), the absolute difference in LS and TH BMD between the two arms was 11.3% and 8.7%, respectively. Among pts with baseline LS T-score between -1 and -2, 27.9% (19) U pts [8.6% (7) D pts] returned to normal T-score (T-score >-1), and no U pts as compared to 4.9%(4) D pts became severely osteopenic (T <-2). 17.7% (53) D pts met criteria that required initiation of ZOL. Although the study was not designed to detect a significant difference in the fracture rate between treatment arms, fractures occurred in 10.7% (29) U pts and 12.4% (33) D pts. Administration of ZOL q 6 mos for up to 5y was safe and well tolerated. No serious renal adverse events suspected related to ZOL and no confirmed osteonecrosis of the jaw cases (ONJ) were reported. Disease recurrence including death due to disease progression was reported in 7.0%, 95% C.I. (3.7%-10.3%) from K-M (16) pts in U, and 8.8%, 95% C.I. (5.2%-12.5%) (21) pts in D.Conclusion: The 5y follow-up of the Z-FAST trial show that the overall difference in the percentage change in BMD between U and D, at both LS and TH, progressively increased from baseline through 5y. These data demonstrate that ZOL 4mg IV q 6 mos is effective in preventing bone loss associated with adjuvant AI therapy in PMW with EBC. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4083.