Dr Mardones on Giredestrant Plus Palbociclib in Estrogen Receptor+ Breast Cancer

“[With] the combination of giredestrant and palbociclib, the [median PFS was] 33.1 months vs 28.2 months in the comparator arm, which [translated to] a numerical difference of [4.9] months. However, the HR in this group was 0.89, so [the trial] did not meet statistical significance [for this end point].”

Mabel Mardones, MD, a medical oncologist at Rocky Mountain Cancer Centers, discussed findings from the primary analysis of the phase 3 persevERA Breast Cancer trial (NCT04546009) evaluating first-line giredestrant plus palbociclib (Ibrance) in patients with estrogen receptor–positive, HER2-negative locally advanced or metastatic breast cancer.

Mardones began by describing the enrolled patient population, which included patients who had received more than 1 year of prior endocrine therapy or who had progressed within 1 year of that therapy, with the proportion of de novo metastatic disease capped at 20% of patients. In the giredestrant arm (n = 495), the median age was 63.0 years (range, 26-87), 59.8% of patients were White, and 28.1% of patients were Asian. Mardones emphasized that the cohort reflected the demographics of patients typically treated in community practice, as 68.9% had treatment-free intervals exceeding 1 year, and 60.8% presented with visceral metastases.

Turning to the efficacy data, Mardones explained that the primary end point was investigator-assessed progression-free survival (PFS), with the prespecified boundary for statistical significance defined as an HR of 0.85. In the giredestrant-plus-palbociclib arm, the median PFS was 33.1 months (95% CI, 30.2-38.3) compared with 28.2 months (95% CI, 25.0-33.1) among patients who received the comparator regimen of letrozole plus palbociclib (n = 276), representing a numerical difference of 4.9 months. However, she noted that the HR in this analysis was 0.89 (95% CI, 0.76-1.05; P = .1553), which did not cross the threshold for statistical significance. Mardones also highlighted a secondary observation of interest, noting that the median duration of response was numerically longer with the experimental combination, at 38.5 months (95% CI, 30.4-48.7) vs 30.4 months (95% CI, 25.3-36.1) in the control arm.

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