• Hye-In Jung, Sun-Hong Kwon, Jin Hyun Nam, Jeong-Yeon Cho, and Eui-Kyung Lee.
• School of Pharmacy, Sungkyunkwan University, Suwon, South Korea.
• Curr Med Res Opin. 2024 May 9: 191-9.

ObjectiveWe assessed the real-world effectiveness of cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors as first-line treatments in postmenopausal patients with HR+/HER2- advanced breast cancer, focusing on younger (<45 years) and older (>78 years) populations not considered in clinical trials.MethodsWe analyzed nationwide claims data from the Health Insurance Review and Assessment Service between November 2016 and February 2021. In this retrospective cohort study, patients using CDK4/6 inhibitors and aromatase inhibitors were selected and grouped by age as follows: 45-78 years (trial-enrolled), <45 years (younger), and >78 years (older). We estimated the median real-world progression-free survival (rwPFS) and overall survival (OS) using the Kaplan-Meier method. We conducted Cox regression analysis using a sub-distribution hazard model to evaluate risk factors (age, history of prior systemic treatment, presence of metastasis, comorbidity index, and type of provider) and estimated hazard ratios (HR).ResultsAmong the 2,830 patients who received CDK4/6 inhibitors as first-line therapy, we identified 358 (12.65%) younger and 148 (5.23%) older underrepresented patients. The younger patient group (50.84%) had the highest rate of prior systemic therapy, followed by the trial-enrolled (25.39%) and older patient groups (8.11%). The median rwPFS was shorter in the older group (19.30 months) than those in the younger and the trial-enrolled age groups (30.33 and 34.53 months, respectively; p = .002). The HR of older age for death was 1.59 (95% confidence interval (CI) = 1.24-2.03). For rwPFS, the HR of prior systemic therapy was 1.19 (95% CI = 1.04-1.37).ConclusionsThe younger age group, which was underrepresented in the trial, did not show a significant difference in risk compared with the enrolled age group. However, the older age group, which was also underrepresented in the trial, faces a risk of mortality but not progression. Patients who fall outside the specified age groups for the clinical trial can still expect the same level of effectiveness in terms of progression.

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