# Atirmociclib

Source: https://onco.cc/drugs/atirmociclib/  
OnCo record `atirmociclib` (Treatment). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

A next-generation pill that blocks only CDK4, not CDK6, to keep the benefit of today's drugs without the low blood counts.

## Summary

Pfizer's CDK4-selective inhibitor. Phase 2 FOURLIGHT-1 (second line, with fulvestrant) positive in early 2026 with lower neutropenia than dual inhibitors; phase 3 FOURLIGHT-3 compares atirmociclib + letrozole with a CDK4/6 inhibitor + letrozole in first line. Rationale: CDK6 inhibition drives neutropenia while CDK4 carries most of the antitumour effect in luminal breast cancer.

## Fields

- Kind: Treatment
- Status: phase-3
- Last checked: 2026-09-07
- Code: PF-07220060
- Modality: Small-molecule CDK4-selective inhibitor
- Mechanism: ATP-competitive inhibitor selective for CDK4 over CDK6 (~70-fold).
- Dosing: Oral; Twice daily (phase 2/3 dosing under evaluation)

## 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/), [HR-positive metastatic breast cancer after CDK4/6 inhibitors](https://onco.cc/cancers/hr-positive-metastatic-post-cdk46/)
- technologies: [CDK4/6 inhibitors](https://onco.cc/technologies/cdk46-inhibitor/)
- targets: [CDK4/6](https://onco.cc/targets/cdk4-6/)
- companies: [Pfizer (incl. Seagen)](https://onco.cc/companies/pfizer/)
- trials: [A Study to Learn About the Study Medicine (Called PF-07220060 in Combination With PF-07104091) In Participants With Breast Cancer and Solid Tumors](https://onco.cc/trials/nct05262400/), [A Study to Learn About Vepdegestrant When Given With PF-07220060 to People With Advanced or Metastatic Breast Cancer.](https://onco.cc/trials/nct06206837/), [FOURLIGHT-1](https://onco.cc/trials/fourlight-1/), [Study of PF-07220060 With Letrozole in Adults With HR-positive HER2-negative Breast Cancer Who Have Not Received Anticancer Treatment for Advanced/Metastatic Disease](https://onco.cc/trials/nct06760637/), [Study of PF-07248144 in Advanced or Metastatic Solid Tumors](https://onco.cc/trials/nct04606446/), [Study to Evaluate the Safety and Tolerability of Camizestrant in Combination With Atirmociclib in Women With Advanced Breast Cancer](https://onco.cc/trials/nct07427394/), [Study to Test the Safety and Tolerability of PF-07220060 in Participants With Advance Solid Tumors](https://onco.cc/trials/nct04557449/)
- ideas: [CDK4-selective inhibitors as the new first-line backbone](https://onco.cc/ideas/idea-cdk4-selective-first-line/)
- pathways: [The cell-cycle engine (cyclins & CDKs)](https://onco.cc/pathways/cell-cycle-engine-cdks/)
- roadmaps: [Hormonal therapy roadmap: removing the ovaries → tamoxifen → oral degraders switched by a blood test](https://onco.cc/roadmaps/hormonal-therapy-roadmap/)

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