Background: The treatment of choice as the first line for patients with metastatic breast cancer (MBC) who are hormone receptor-positive (HR+)/HER2-negative (HER2-) is the combination of endocrine therapy (ET) with cyclin-dependent kinase 4/6 inhibitors (CDK4/6i). Identifying prognostic or predictive factors of response could have important clinical implications. This study analysed the prognostic value of Ki-67 and progesterone receptor (PR) expression on progression-free survival (PFS) in this population.

Methods: A retrospective cohort study was conducted in patients with HR+/HER2- MBC, who had received first-line treatment with CDK4/6i combined with ET. For the PFS analysis, the log-rank test was used and for the multivariate analysis, a Cox regression analysis was performed.

Results: A total of 155 patients were analysed. Patients with PR values <20% showed a trend in univariate analysis towards shorter PFS, with a median of 20.7 months compared to those with a value ≥20%, with a median PFS of 33.0 months (p = 0.090). The Ki-67 value showed no statistically significant association with PFS. The prognostic role of PR was confirmed in the multivariate analysis with an HR of 0.59 (95% CI 0.36–0.98, p = 0.041) in patients with PR >20%.

Conclusion: Patients with PR values <20% tended to have shorter PFS, unlike the Ki-67 value, which did not demonstrate an impact on PFS. This suggests a prognostic value of PR expression levels in this scenario.