The AI–CDK4/6i synergy matrix: A de-novo meta-analysis decoupling aromatase-inhibitor identity from survival outcomes in 1L HR+/HER2− advanced breast cancer.

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Background: Clinical guidelines currently treat letrozole (LET) and anastrozole (ANA) as interchangeable non-steroidal aromatase inhibitor (NSAI) backbones for CDK4/6 inhibitors (CDK4/6i). However, the marked divergence in overall survival (OS) across pivotal trials—despite nearly identical progression-free survival (PFS)—suggests that specific agent combinations may possess unique biological synergies. This study is the first de-novo meta-analysis to decouple AI identity from CDK4/6i selection, aiming to identify the optimal therapeutic matrix for 1L metastatic disease. Methods: We performed a systematic investigation of pivotal 1L RCTs (PALOMA-2, MONALEESA-2, MONARCH-3) comparing CDK4/6i + AI vs. AI alone. Utilizing fixed-effect pooling of published Hazard Ratios (HR) and 95% Confidence Intervals (CI), we calculated "Survival Efficiency" for Letrozole-fixed backbones versus permissive-NSAI backbones (which include ANA). We specifically modeled the "PFS-to-OS Conversion Ratio" to detect signal-intensity across clinical phenotypes, including high-risk visceral and liver metastases. Results: Pooled analysis confirms a rigid class effect for PFS in letrozole-backbone trials (HR 0.574, 95% CI: 0.49–0.67), mirroring the mixed-NSAI effect in MONARCH-3 (HR 0.535). However, OS outcomes diverged sharply by inhibitor agent rather than AI partner. Ribociclib + LET demonstrated a definitive OS benefit (HR 0.76), while Palbociclib + LET remained neutral (HR 0.96). Abemaciclib + NSAI showed a strong numerical trend (HR 0.804). Our computed pooled OS HR for a letrozole-only backbone was 0.85 (95% CI: 0.736–0.977), a result driven by Ribociclib’s performance. Conclusions: This analysis identifies a critical "Synergy Gap" where 1L PFS gains do not guarantee OS extension. We conclude that AI identity is a passive clinical substrate; the specific CDK4/6i agent is the master regulator of survival. Ribociclib + Letrozole represents the current optimal OS-validated pairing. Clinicians should prioritize specific inhibitor OS data, particularly in high-risk visceral phenotypes, rather than assuming AI-partner interchangeability. These findings suggest a need to refine guidelines to reflect agent-specific survival hierarchies.

Pooled OS and PFS estimates by therapeutic matrix.