{"entity":{"id":"atirmociclib","kind":"drug","name":"Atirmociclib","aka":[],"tldr":"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.","status":"phase-3","asOf":"2026-09-07","links":[{"label":"ClinicalTrials.gov NCT06105632 (FOURLIGHT-1)","url":"https://clinicaltrials.gov/study/NCT06105632"}],"tags":[],"related":[],"cancers":["breast-hr-positive"],"sections":[],"technologies":["cdk46-inhibitor"],"targets":["cdk4-6"],"drugs":[],"companies":["pfizer"],"institutions":[],"pathways":[],"terms":[],"trials":["fourlight-1","nct04557449","nct04606446","nct06206837","nct07427394","nct05262400","nct06760637"],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"code":"PF-07220060","modality":"Small-molecule CDK4-selective inhibitor","mechanism":"ATP-competitive inhibitor selective for CDK4 over CDK6 (~70-fold).","approvals":[],"mechanismSteps":["Cyclin D1-CDK4 phosphorylates RB in luminal breast cancer cells","Atirmociclib blocks CDK4 but largely spares CDK6","RB stays bound to E2F; cells arrest in G1","Haematopoietic progenitors, which depend on CDK6, keep dividing","Less neutropenia allows continuous dosing"],"dosing":{"route":"Oral","schedule":"Twice daily (phase 2/3 dosing under evaluation)"},"toxicity":[],"access":[],"regulatoryEvents":[{"date":"2026-02","type":"designation","region":"Global","note":"Positive topline phase 2 FOURLIGHT-1","source":"https://www.pfizer.com/news/press-release/press-release-detail/pfizer-announces-positive-topline-phase-2-results-next"}]},"route":"/drugs/atirmociclib/","neighbours":{"cancer":[{"id":"breast-hr-positive","kind":"cancer","name":"HR-positive / HER2-negative breast cancer","route":"/cancers/breast-hr-positive/"},{"id":"hr-positive-metastatic-post-cdk46","kind":"cancer","name":"HR-positive metastatic breast cancer after CDK4/6 inhibitors","route":"/cancers/hr-positive-metastatic-post-cdk46/"}],"technology":[{"id":"cdk46-inhibitor","kind":"technology","name":"CDK4/6 inhibitors","route":"/technologies/cdk46-inhibitor/"}],"target":[{"id":"cdk4-6","kind":"target","name":"CDK4/6","route":"/targets/cdk4-6/"}],"company":[{"id":"pfizer","kind":"company","name":"Pfizer (incl. Seagen)","route":"/companies/pfizer/"}],"trial":[{"id":"nct05262400","kind":"trial","name":"A Study to Learn About the Study Medicine (Called PF-07220060 in Combination With PF-07104091) In Participants With Breast Cancer and Solid Tumors","route":"/trials/nct05262400/"},{"id":"nct06206837","kind":"trial","name":"A Study to Learn About Vepdegestrant When Given With PF-07220060 to People With Advanced or Metastatic Breast Cancer.","route":"/trials/nct06206837/"},{"id":"fourlight-1","kind":"trial","name":"FOURLIGHT-1","route":"/trials/fourlight-1/"},{"id":"nct06760637","kind":"trial","name":"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","route":"/trials/nct06760637/"},{"id":"nct04606446","kind":"trial","name":"Study of PF-07248144 in Advanced or Metastatic Solid Tumors","route":"/trials/nct04606446/"},{"id":"nct07427394","kind":"trial","name":"Study to Evaluate the Safety and Tolerability of Camizestrant in Combination With Atirmociclib in Women With Advanced Breast Cancer","route":"/trials/nct07427394/"},{"id":"nct04557449","kind":"trial","name":"Study to Test the Safety and Tolerability of PF-07220060 in Participants With Advance Solid Tumors","route":"/trials/nct04557449/"}],"idea":[{"id":"idea-cdk4-selective-first-line","kind":"idea","name":"CDK4-selective inhibitors as the new first-line backbone","route":"/ideas/idea-cdk4-selective-first-line/"}],"pathway":[{"id":"cell-cycle-engine-cdks","kind":"pathway","name":"The cell-cycle engine (cyclins & CDKs)","route":"/pathways/cell-cycle-engine-cdks/"}],"roadmap":[{"id":"hormonal-therapy-roadmap","kind":"roadmap","name":"Hormonal therapy roadmap: removing the ovaries → tamoxifen → oral degraders switched by a blood test","route":"/roadmaps/hormonal-therapy-roadmap/"}]}}