# CDK4-selective inhibitors as the new first-line backbone

Source: https://onco.cc/ideas/idea-cdk4-selective-first-line/  
OnCo record `idea-cdk4-selective-first-line` (Idea). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

If a CDK4-only drug matches CDK4/6 inhibitors on efficacy with less neutropenia, continuous dosing and better adherence could translate into longer control.

## Summary

The idea is to replace palbociclib, ribociclib and abemaciclib with a CDK4-selective drug such as atirmociclib, paired with letrozole, as the first-line backbone in HR-positive, HER2-negative breast cancer. Luminal breast cancer depends on cyclin D1 and CDK4, whereas CDK6 dependence is mainly haematopoietic, so sparing CDK6 should remove the neutropenia that forces dose holds and reductions with the current class. The hypothesis is non-inferior or superior progression-free survival with fewer severe blood-count events and higher dose intensity, so that continuous dosing and better adherence give longer control. FOURLIGHT-1 showed activity after CDK4/6 failure, and the enrolling phase 3 FOURLIGHT-3 trial is the test; the idea is at an early clinical stage.

## Fields

- Kind: Idea
- Last checked: 2026-09-07
- Hypothesis: CDK4-selective inhibition + letrozole is non-inferior or superior in PFS to CDK4/6 + letrozole in first-line HR+/HER2- disease, with fewer grade 3+ haematologic events and higher relative dose intensity.
- Rationale: Luminal breast cancer depends on cyclin D1-CDK4; CDK6 dependence is mainly haematopoietic. FOURLIGHT-1 showed activity after CDK4/6 failure.
- Proposed test: FOURLIGHT-3 phase 3 (enrolling); secondary analyses of dose intensity and quality of life.
- Maturity: early-clinical

## Sources

- ClinicalTrials.gov NCT06105632: FOURLIGHT-1: https://clinicaltrials.gov/study/NCT06105632

## Connected records

- cancers: [HR-positive / HER2-negative breast cancer](https://onco.cc/cancers/breast-hr-positive/)
- targets: [CDK4/6](https://onco.cc/targets/cdk4-6/)
- drugs: [Abemaciclib](https://onco.cc/drugs/abemaciclib/), [Atirmociclib](https://onco.cc/drugs/atirmociclib/), [Palbociclib](https://onco.cc/drugs/palbociclib/), [Ribociclib](https://onco.cc/drugs/ribociclib/)
- trials: [FOURLIGHT-1](https://onco.cc/trials/fourlight-1/)
- key papers: [Palbociclib and Letrozole in Advanced Breast Cancer](https://onco.cc/key-papers/paper-paloma-2-n-engl-j-med-2016/), [Ribociclib as First-Line Therapy for HR-Positive, Advanced Breast Cancer](https://onco.cc/key-papers/paper-monaleesa-2-n-engl-j-med-2016/), [The cyclin-dependent kinase 4/6 inhibitor palbociclib in combination with letrozole versus letrozole alone as first-line treatment of oestrogen receptor-positive, HER2-negative, advanced breast cancer (PALOMA-1/TRIO-18): a randomised phase 2 study](https://onco.cc/key-papers/paper-palbociclib-breast-hr-positive-lancet-oncol-2015/)

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