Cyclin-Dependent Kinase 4/6 Inhibitor in Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Breast Cancer: Global Challenges and Tibremciclib Perspective.

Abstract

Hormone receptor-positive (HR+), human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer represents a substantial proportion of breast cancer cases globally. Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors combined with endocrine therapy are established first-line treatments in high-income settings, improving progression-free survival (PFS) and quality of life. However, access remains limited in low- and middle-income countries (LMICs) because of cost, infrastructure, and policy barriers. We critically evaluated global challenges in CDK4/6 inhibitor implementation and assessed recent phase III data on tibremciclib plus fulvestrant in HR+/HER2-negative advanced breast cancer after prior endocrine therapy. Comparative analyses were conducted with landmark MONALEESA-3 and MONARCH-2 trials, focusing on study populations, outcomes, safety, and generalizability. The tibremciclib trial demonstrated a significant PFS benefit and high objective response rates in a single-country cohort without prior CDK4/6 inhibitor exposure. Overall survival (OS) data remain immature (19.7% maturity), and grade ≥3 toxicities-including neutropenia (15.2%), hypokalaemia (12.0%), and anemia (12.0%)-pose feasibility challenges in LMICs with limited monitoring capacity. Compared with ribociclib and abemaciclib trials, tibremciclib data are less generalizable because of narrow inclusion criteria and limited international representation. This highlights the need for future trials in more ethnically and geographically diverse populations, with extended OS and quality-of-life follow-up. Tibremciclib shows promise as an additional CDK4/6 inhibitor; however, its global integration requires broader, more diverse clinical trials, robust safety and survival data, and strategies to mitigate toxicity risks. Bridging access gaps will require coordinated efforts across policy, infrastructure, and global partnerships to ensure equitable benefit from emerging therapies.