Genomic Insights Tackle CDK4/6 Resistance in Breast Cancer

MSK researchers discover genetic factors that predict resistance to common breast cancer drugs

Published on Mar. 6, 2026

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Researchers at Memorial Sloan Kettering Cancer Center (MSK) have made an important discovery about how genetic mutations in breast cancer patients can interact and drive resistance to certain drugs called CDK4/6 inhibitors. The study found that patients with inherited BRCA2 mutations are more likely to also lose the RB1 gene, making them less responsive to CDK4/6 inhibitors. The researchers propose treating these patients with PARP inhibitors instead as their initial therapy to delay or prevent resistance.

Why it matters

This research represents a major advance in understanding and predicting how breast cancer will respond to specific therapies based on a tumor's genetic profile. By identifying genetic factors that drive resistance, the researchers can now design more personalized treatment approaches to improve outcomes for breast cancer patients.

The details

The study analyzed data from over 5,800 MSK breast cancer patients and found that patients born with BRCA2 mutations are more likely to also have additional mutations in the RB1 gene. Tumors carrying only a single copy of the RB1 gene before starting CDK4/6 inhibitor treatment are much more likely to develop complete RB1 loss, a key mechanism of resistance. The researchers also found that underlying DNA repair defects, especially homologous recombination deficiency (HRD), further drive this resistance. In preclinical models, drugs called PARP inhibitors resulted in better outcomes than CDK4/6 inhibitors in tumors with HRD.

  • The research was published in Nature in 2026.
  • A global, randomized phase 3 clinical trial called EvoPAR-Breast01 is now enrolling patients to test the new approach.

The players

Pedram Razavi

A physician-scientist at Memorial Sloan Kettering Cancer Center (MSK) who led the study with Sarat Chandarlapaty.

Sarat Chandarlapaty

A physician-scientist at MSK who led the study with Pedram Razavi.

Anton Safonov

A physician-scientist in the MSK Breast Translational Program and the first author of the study.

Minna Lee

A co-first author who used patient-derived xenograft models from BRCA2-mutant breast cancers in Dr. Chandarlapaty's laboratory.

AstraZeneca

A pharmaceutical company collaborating with MSK on the EvoPAR-Breast01 clinical trial.

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What they’re saying

“This represents a major advance in understanding and predicting cancer behavior in response to treatment.”

— Pedram Razavi, Physician-scientist, Memorial Sloan Kettering Cancer Center (Mirage News)

“To our knowledge, this is the first example showing that a complete genomic analysis of breast cancer, including both inherited and tumor-specific alterations, can predict the precise biological mechanism of resistance before therapy even begins.”

— Pedram Razavi, Physician-scientist, Memorial Sloan Kettering Cancer Center (Mirage News)

“Cancers don't have endless ways to escape treatment. They are one- or two-trick ponies, and those tricks are often determined by their inherited or tumor-specific genetic features. If we can predict what they're capable of, we can intercept it before the resistance happens. That's what we're trying to do in this trial — forecast the mechanism of resistance and hopefully improve the outcomes for our patients.”

— Pedram Razavi, Physician-scientist, Memorial Sloan Kettering Cancer Center (Mirage News)

“This study gives us the opportunity to address drug resistance proactively, rather than reactively. This will allow us to stay one step ahead of breast cancer by gaining the ability to peek at its 'battle plans'.”

— Anton Safonov, Physician-scientist, Memorial Sloan Kettering Cancer Center (Mirage News)

“This highlights the strength of our program and how we are able to very quickly translate our findings to a potentially practice-changing clinical trial. There aren't many examples where translational data were compelling enough to move directly into a phase 3 study without developing earlier clinical evidence.”

— Pedram Razavi, Physician-scientist, Memorial Sloan Kettering Cancer Center (Mirage News)

What’s next

The global, randomized phase 3 EvoPAR-Breast01 clinical trial is now enrolling patients to test the new approach of using PARP inhibitors instead of CDK4/6 inhibitors as the initial therapy for breast cancer patients with HRD-positive tumors.

The takeaway

This research represents a significant advancement in understanding the genetic factors that can drive resistance to common breast cancer drugs. By identifying these predictive biomarkers, the researchers can now design more personalized treatment strategies to improve outcomes for breast cancer patients.