| Sumario: | Simple Summary: Drugs known as cyclin-dependent kinase 4/6 inhibitors are commonly used together with hormone therapy as the first treatment for people with hormone receptor-positive, HER2-negative advanced breast cancer. However, these drugs have not been directly compared in large clinical trials. This research reviewed published studies that used real-world patient data to compare how well the three available drugs, palbociclib, ribociclib, and abemaciclib, work in routine clinical practice. Most studies showed that patients had similar outcomes regardless of which drug was used, including similar time before the disease worsened and similar overall survival. Differences between studies, such as how long patients were followed and how results were analyzed, made some findings difficult to compare. Overall, this review suggests that all three treatments may have broadly similar effectiveness, which can help researchers and clinicians better understand treatment choices in everyday care. Background/Objective: Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) combined with endocrine therapy are the standard first-line treatment for HR+/HER2− advanced/metastatic breast cancer (a/mBC), yet no head-to-head randomized trials have compared palbociclib, ribociclib, and abemaciclib. This systematic literature review (SLR) aimed to synthesize available real-world evidence (RWE) on the comparative effectiveness of these three CDK4/6i in the first-line setting. Methods: An SLR was conducted following PRISMA guidelines. Searches of MEDLINE, Embase, Cochrane, and gray literature (January 2015–September 2025) identified RWE studies reporting real-world progression-free survival (rwPFS) and/or overall survival (OS) for first-line CDK4/6i regimens. Eligible studies included adults with HR+/HER2− a/mBC receiving palbociclib, ribociclib, or abemaciclib, combined with endocrine-based therapy. Comparative outcomes were summarized qualitatively; study quality was assessed using the Newcastle–Ottawa Scale, ISPOR-AMCP-NPC questionnaire, and ESMO-GROW checklist. Results: From 13,345 records, 39 publications (32 unique studies) were identified, of which 21 were full-text studies meeting inclusion criteria. Most included studies used retrospective cohort designs, and approximately 50% evaluated all three CDK4/6is. Given the comparative nature of the included studies, quality issues were found to be related to study design and analytical approaches. Several analyses were unadjusted, and reporting of follow-up duration varied across studies, with some analyses providing incomplete follow-up information. Study sample sizes differed substantially across agents, with smaller populations generally reported for ribociclib and abemaciclib. Among the studies reporting rwPFS hazard ratios (HRs), 8/11 studies showed comparable rwPFS and 5/6 studies reported comparable OS outcomes between CDK4/6i regimens. Three full-text studies reported HRs favoring ribociclib or abemaciclib relative to palbociclib; these analyses primarily included populations receiving palbociclib, with smaller comparator cohorts for ribociclib and abemaciclib, as well as varied follow-up times. Conclusions: Most real-world studies included in the SLR suggested similar effectiveness of the three CDK4/6is in first-line HR+/HER2− a/mBC. Heterogeneity in patient characteristics, definitions of outcomes, follow-up time, sample size, and statistical approach may have important implications on study interpretability.
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