Zoledronic Acid Dosing in Patients With Metastatic Breast Cancer

To the Editor Hortobagyi et al conducted a noninferiority study for zoledronic acid dosing every 12 weeks (12W), comparing it with its counterpart of dosing every 4 weeks (4W) in women with breast cancer metastatic to bone. The primary end point was whether the patient had at least 1 debilitating skeletal-related event (SRE) by 12 months. The SREs occurred in 44 patients (22.0%) in the 4W zoledronic acid group and 47 patients (23.2%) in the 12W zoledronic acid group. The treatment difference (12W minus 4W) is 1.2%, with the upper bound of the 1-sided 97.5% confidence interval (CI) being 9.8%, barely within the prespecified noninferiority margin of 10%. That is, potentially, 12W zoledronic acid can be 9.8% worse than 4W zoledronic acid with respect to the SRE rate. Using such a large observed noninferiority margin to claim that 12W is as good as 4W is debatable.

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