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In FOURLIGHT-1, a CDK4-only inhibitor designed to avoid the low blood counts of current CDK4/6 drugs improved progression-free survival in second line.
FOURLIGHT-1, trial NCT06105632 sponsored by Pfizer and reported in early 2026, showed in a randomised phase 2 that atirmociclib, a CDK4-only inhibitor designed to avoid the low blood counts of current CDK4/6 drugs, improves progression-free survival when added to fulvestrant after progression on a CDK4/6 inhibitor in HR-positive, HER2-negative advanced breast cancer. The trial, which enrolled 333 patients according to ClinicalTrials.gov, met its primary endpoint in topline results on 17 March 2026 with lower neutropenia than dual CDK4/6 inhibitors, and the phase 3 FOURLIGHT-3 trial in the first line is enrolling. OnCo links it to CDK4/6 as a target, atirmociclib and the idea of CDK4-selective inhibitors as the new first-line backbone. It is the first randomised evidence that a selective CDK4 inhibitor works after CDK4/6 progression, and whether that holds in the first-line phase 3 is the open question.
Numbers are from the trial as recorded here; see the source links in the table below. This is orientation, not medical advice: ask your team how closely the trial population matches you.
333 enrolled.
Randomised phase 2, post-CDK4/6 inhibitor; medians pending presentation (topline 17 March 2026)
Source| Endpoint | Arm | n | Value | HR (95% CI) | p | Source |
|---|---|---|---|---|---|---|
| Progression-free survival (investigator-assessed)primary | Atirmociclib + fulvestrant | - | Randomised phase 2, post-CDK4/6 inhibitor; medians pending presentation (topline 17 March 2026) | 0.6 (0.44 to 0.825) | 0.0007 | link |
| Fulvestrant, or everolimus + exemestane | - | - |
Shares Atirmociclib, CDK4/6 inhibitors, Pfizer (incl. Seagen), HR-positive / HER2-negative breast cancer.
Shares Atirmociclib, CDK4/6 inhibitors, Pfizer (incl. Seagen).
Shares CDK4/6, Hormonal therapy roadmap: removing the ovaries → tamoxifen → oral degraders switched by a blood test, CDK4/6 inhibitors, HR-positive / HER2-negative breast cancer.
Shares Atirmociclib, CDK4/6 inhibitors, Pfizer (incl. Seagen).
Shares Atirmociclib, CDK4/6 inhibitors, Pfizer (incl. Seagen).
Shares CDK4/6, Hormonal therapy roadmap: removing the ovaries → tamoxifen → oral degraders switched by a blood test, CDK4/6 inhibitors, HR-positive / HER2-negative breast cancer.
Shares CDK4/6, Hormonal therapy roadmap: removing the ovaries → tamoxifen → oral degraders switched by a blood test, CDK4/6 inhibitors, Pfizer (incl. Seagen).
Shares Atirmociclib, CDK4/6 inhibitors.