ER Antagonist With and Without Palbociclib Under Study in Breast Cancer

By: Amy MacDonald, MS
Posted: Tuesday, January 2, 2024

Sara A. Hurvitz, MD, of Fred Hutchinson Cancer Center, Seattle, and colleagues reported the final outcomes of the coopERA Breast Cancer study in The Lancet Oncology. The phase II study was designed to evaluate the oral estrogen receptor (ER) antagonist giredestrant in patients with untreated ER-positive, HER2-negative, early-stage breast cancer, as either a single agent or in combination with the CDK4/6 inhibitor palbociclib. The investigators found the use of giredestrant as a single agent and in combination with palbociclib yielded antitumor activity in this patient population and so warrant further investigation.

“This study adds to the current knowledge regarding the efficacy and safety of novel selective ER antagonists and degraders by confirming that the highly potent, nonsteroidal oral molecule giredestrant significantly reduces Ki67 compared with anastrozole after 2 weeks of treatment,” the investigators stated.

Between September 2020 and June 2021, the coopERA trial enrolled 221 postmenopausal patients, who were randomly assigned to receive giredestrant plus palbociclib (n = 112) or anastrozole plus palbociclib (n = 109). At data cutoff for the primary analysis, the mean relative reduction in the cell proliferation marker Ki67 index from baseline to week 2 was –75% with giredestrant and –67% with anastrozole (P = .043), meeting the primary endpoint.

At the final analysis (November 2021), the most common grade 3 or 4 adverse events observed in both arms were neutropenia and decreased neutrophil count. Serious adverse events were reported in five patients (4%) given giredestrant plus palbociclib and in two patients (2%) given anastrozole plus palbociclib. No treatment-related deaths were reported.

“In summary, coopERA Breast Cancer is the first randomized study, to our knowledge, demonstrating statistically superior suppression of tumor cell proliferation of an oral [selective ER degrader] over an aromatase inhibitor in ER-positive, HER2-negative, early breast cancer,” the study authors stated.

Disclosure: For full disclosures of study authors, visit thelancet.com.

The Lancet Oncology

---