Medicine:Atirmociclib

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Atirmociclib (development code PF-07220060) is an investigational orally bioavailable and CDK4-specific inhibitor being developed by Pfizer for the treatment of various solid tumors, particularly hormone receptor-positive, HER2-negative breast cancer.[1][2] The safety and efficacy of atirmociclib have not been established, as it remains in clinical development as of September 2025.[3][4][5]

Mechanism of action

Atirmociclib is designed as a CDK4-specific inhibitor, distinguishing it from dual CDK4/6 inhibitors currently approved for cancer treatment.[6] The drug targets cyclin-dependent kinase 4, which plays a role in cell cycle regulation.[1][7][8]

Atirmociclib functions as a selective inhibitor of the CDK4/cyclin D complex, which plays a crucial role in cell cycle regulation.[4] The drug works by targeting the CDK4 kinase, rendering the retinoblastoma (Rb)/E2F transcription system inactive, which ultimately leads to cell cycle arrest in the G1 phase.[4] This mechanism is particularly effective in tumors that have lost Rb cell cycle-suppressive function, a common feature in various solid tumors.[5]

The selective nature of atirmociclib represents a significant advancement over existing dual CDK4/6 inhibitors.[6] By specifically targeting CDK4 while limiting CDK6 inhibition, atirmociclib is designed to maintain antitumor efficacy while potentially reducing dose-limiting hematologic toxicities, particularly neutropenia, which is believed to be primarily driven by CDK6 inhibition.[9]

Clinical development

Atirmociclib is currently being evaluated in clinical trials for the treatment of advanced solid tumors.[1] Clinical studies are ongoing with estimated completion dates extending to 2027–2028, reflecting the early stage of development for this investigational compound.[1]

Preclinical research published in Cancer Cell in March 2025 reported atirmociclib as a next-generation CDK4-selective inhibitor with enhanced anti-tumor activity and reduced predicted toxicity compared to FDA-approved dual CDK4/6 inhibitors, though these findings require validation in clinical studies.[6]

Preclinical studies

Preclinical research has demonstrated that atirmociclib exhibits enhanced anti-tumor activity compared to FDA-approved dual CDK4/6 inhibitors while showing reduced predicted toxicity.[6] Studies have shown that CDK4-selective inhibition can provide improved preclinical anti-tumor efficacy and safety profiles compared to dual CDK4/6 inhibition strategies.[10]

The preclinical development program has explored combination approaches with various therapeutic modalities, including endocrine therapy, CDK2 inhibition, HER2 antibodies, and immune checkpoint inhibitors.[6] These combination strategies are designed to counter resistance mechanisms to CDK4 inhibition and expand the potential therapeutic applications of cell cycle targeting therapy.[6]

Clinical trials

Atirmociclib has entered clinical development as part of Pfizer's extensive oncology pipeline.[11] The clinical program is evaluating atirmociclib both as a single agent and in combination with other therapeutic approaches, particularly focusing on patients with hormone receptor-positive, HER2-negative breast cancer.[9][12][13][14][15][16][17]

Early clinical studies have included heavily pretreated patient populations, including those who have previously received CDK4/6 inhibitor therapy.[9] This approach allows for the evaluation of atirmociclib's potential to overcome resistance to existing CDK4/6 inhibitors and provide therapeutic benefit in patients with limited treatment options.[9]

Safety profile and toxicity

One of the key differentiating features of atirmociclib is its potential for improved safety profile compared to existing dual CDK4/6 inhibitors.[6] The selective targeting of CDK4 while limiting CDK6 inhibition is specifically designed to reduce neutropenia, the most common dose-limiting toxicity associated with current CDK4/6 inhibitors.[18]

The rationale for this approach is based on preclinical evidence suggesting that neutropenia is primarily driven by CDK6 inhibition rather than CDK4 inhibition.[18] By selectively targeting CDK4, atirmociclib aims to maintain therapeutic efficacy while potentially allowing for higher or more sustained dosing without the dose-limiting hematologic toxicities that can compromise treatment outcomes with existing agents.[18]

Regulatory status

As of September 2025, atirmociclib remains an investigational drug that has not received approval from the FDA or other regulatory agencies.[5] The compound is part of Pfizer's oncology development pipeline.[5]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 Pfizer (2025-02-02). A Phase 1/2A Study Evaluating the Safety, Tolerability, Pharmacokinetics, Pharmacodynamics, and Anti-Tumor Activity of Pf-07220060 as a Single Agent and as Part of Combination Therapy in Participants With Advanced Solid Tumors (Report). clinicaltrials.gov. https://clinicaltrials.gov/study/NCT04557449. 
  2. ↑ "The evolving role of cyclin-dependent kinase inhibitors in cancer management". Clinical Advances in Hematology & Oncology 15 (3): 174–177. March 2017. PMID 28398270. 
  3. ↑ "CDK4 inhibitor PF-07220060" (in en). 2011-02-02. https://www.cancer.gov/publications/dictionaries/cancer-drug/def/atirmociclib. 
  4. ↑ 4.0 4.1 4.2 "Pfizer Pipeline". https://www.pfizer.com/science/drug-product-pipeline. 
  5. ↑ 5.0 5.1 5.2 5.3 "Atirmociclib PF-07220060". https://www.pfizeroncologydevelopment.com/molecule/cdk4-inhibitor. 
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 "Single-cell multi-stage spatial evolutional map of esophageal carcinogenesis". Cancer Cell 43 (3): 380–397.e7. March 2025. doi:10.1016/j.ccell.2025.02.009. PMID 40068596. 
  7. ↑ "Cyclin D-Cdk4,6 Drives Cell-Cycle Progression via the Retinoblastoma Protein's C-Terminal Helix". Molecular Cell 74 (4): 758–770.e4. May 2019. doi:10.1016/j.molcel.2019.03.020. PMID 30982746. 
  8. ↑ "Cell-Cycle Gene Alterations in 4,864 Tumors Analyzed by Next-Generation Sequencing: Implications for Targeted Therapeutics". Molecular Cancer Therapeutics 15 (7): 1682–1690. July 2016. doi:10.1158/1535-7163.MCT-16-0071. PMID 27196769. 
  9. ↑ 9.0 9.1 9.2 9.3 "ESMO 2024 – combos could be the way forward for CDK2". September 15, 2024. https://www.oncologypipeline.com/apexonco/esmo-2024-combos-could-be-way-forward-cdk2. 
  10. ↑ "CDK4 selective inhibition improves preclinical anti-tumor efficacy and safety". Cancer Cell 43 (3): 464–481.e14. March 2025. doi:10.1016/j.ccell.2025.02.006. PMID 40068598. 
  11. ↑ "Pfizer Highlights Diverse Oncology Portfolio and Combination Approaches at ESMO 2024". 2024. https://www.pfizer.com/news/press-release/press-release-detail/pfizer-highlights-diverse-oncology-portfolio-and. 
  12. ↑ Pfizer (2025-08-12). A Phase 1/2a Dose Escalation and Expansion Study to Evaluate Safety, Tolerability, Pharmacokinetic, Pharmacodynamic, and Anti-Tumor Activity of Pf-07248144 in Participants With Advanced or Metastatic Solid Tumors (Report). clinicaltrials.gov. https://clinicaltrials.gov/study/NCT04606446. 
  13. ↑ Pfizer (2025-07-02). An Interventional Safety and Efficacy Phase 1/2, Open-Label Study to Investigate Tolerability, Pk, and Antitumor Activity of Vepdegestrant (Arv-47/Pf-07850327), an Oral Proteolysis Targeting Chimera, in Combination With Pf-07220060 in Participants Aged 18 Years and Older With Er+/her2- Advanced or Metastatic Breast Cancer (Report). clinicaltrials.gov. https://clinicaltrials.gov/study/NCT06206837. 
  14. ↑ Pfizer (2024-11-14). A Phase 1/2, Open-Label, Multicenter, Dose Escalation and Dose Expansion Study to Evaluate the Safety, Tolerability, Pharmacokinetics, Pharmacodynamics, and Antitumor Activity of PF-07220060 in Combination With Pf-07104091 Plus Endocrine Therapy in Participants With Advanced Solid Tumors (Report). clinicaltrials.gov. https://clinicaltrials.gov/study/NCT05262400. 
  15. ↑ Pfizer (2025-06-17). (FOURLIGHT-3) (Report). clinicaltrials.gov. https://clinicaltrials.gov/study/NCT06760637. 
  16. ↑ Pfizer (2025-03-13). An Interventional, Open-Label, Randomized, Multicenter Phase 3 Study of PF-07220060 Plus Letrozole Compared to cdk4/6 Inhibitor Plus Letrozole in Participants Over 18 Years of Age With Hormone Receptor (Hr)-Positive, her2-Negative Advanced/Metastatic Breast Cancer Who Have Not Received Any Prior Systemic Anticancer Treatment for Advanced/Metastatic Disease (FOURLIGHT-1) (Report). clinicaltrials.gov. https://clinicaltrials.gov/study/NCT06105632. 
  17. ↑ Pfizer (2024-11-15). An Interventional, Open-Label, Randomized, Multicenter, Phase 2 Study of Pf-07220060 Plus Letrozole Compared to Letrozole Alone in Postmenopausal Women 18 Years or Older With Hormone Receptor-Positive, her2-Negative Breast Cancer in the Neoadjuvant Setting (Report). clinicaltrials.gov. https://clinicaltrials.gov/study/NCT06465368. 
  18. ↑ 18.0 18.1 18.2 "Pfizer dials down its atirmociclib ambitions". May 1, 2025. https://www.oncologypipeline.com/apexonco/pfizer-dials-down-its-atirmociclib-ambitions. 



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