CDK4/6 inhibitors — particularly ribociclib and abemaciclib — have cemented their role as the standard of care in first-line HR-positive, HER2-negative metastatic breast cancer, with ribociclib showing the most consistent overall survival benefit across patient subgroups. Palbociclib, by contrast, has not demonstrated an OS benefit. For most patients, combining a CDK4/6 inhibitor with hormonal therapy (typically an aromatase inhibitor) remains the preferred approach, even in those with extensive visceral disease.

Despite hopes that swapping aromatase inhibitors for newer oral SERDs might improve outcomes, recent phase 3 data tell a different story. Both the persevERA trial (giredestrant + palbociclib) and the SERENA-4 trial (camizestrant + palbociclib) failed to show a PFS benefit over aromatase inhibitor-based combinations, dimming prospects for FDA approval of oral SERDs in this setting.

Key Takeaways:

• Ribociclib shows the most consistent OS benefit among CDK4/6 inhibitors; palbociclib does not
• Oral SERDs (giredestrant, camizestrant) have not demonstrated PFS superiority over aromatase inhibitors when combined with CDK4/6 inhibitors
• Inavolisib (PI3K inhibitor) showed benefit in PIK3CA-mutated patients post-adjuvant progression; gedatolisib received FDA approval in July 2026 for a related indication
• Next-gen approaches — selective CDK4 inhibitors, CDK4/6 PROTACs, and allosteric PI3K inhibitors — are in active development

Why it matters: Endocrine resistance remains the field's biggest unmet need. With targeted therapies offering only modest 5–10 month gains, identifying better sequencing strategies — especially when ESR1 and PIK3CA mutations co-exist — is critical to delaying chemotherapy and improving patient outcomes.