Researchers at Memorial Sloan Kettering Cancer Center and The University of Texas MD Anderson Cancer Center have published separate studies addressing resistance to CDK4/6 inhibitors, a standard treatment for certain types of breast cancer. One study identified genetic mechanisms driving resistance and proposed a new predictive approach and alternative therapies, while the other explored a dual-targeting drug strategy to overcome resistance across various breast cancer models. Both lines of research aim to improve treatment outcomes for patients facing drug resistance.
The Challenge of CDK4/6 Inhibitor Resistance
Cyclin-dependent kinase (CDK) 4/6 inhibitors, when combined with endocrine therapy, are a primary first-line treatment for hormone receptor (HR)-positive, HER2-negative (HR+/HER2-) metastatic breast cancer. Despite their efficacy, many patients develop resistance over time, leading to treatment limitations. The potential benefit of these inhibitors in triple-negative breast cancer (TNBC), an aggressive subtype with limited targeted options, also remains an area of ongoing investigation.
Despite their efficacy, many patients develop resistance over time, leading to treatment limitations.
Uncovering Genetic Drivers of Resistance at Memorial Sloan Kettering
A study published in Nature by researchers at Memorial Sloan Kettering (MSK) Cancer Center identified specific genetic mutations that contribute to resistance against CDK4/6 inhibitors.
The MSK findings suggest a potential pathway for predicting and preventing drug resistance based on a tumor's genetic profile.
The findings suggest a potential pathway for predicting and preventing drug resistance based on a tumor's genetic profile. Key observations from analyzing data from over 5,800 MSK breast cancer patients included:
- Approximately 10% of breast cancer patients developed resistance due to the loss of the protective RB1 gene in their cancer cells.
- Two primary indicators for this resistance were identified: DNA repair deficiencies, specifically homologous recombination deficiency (HRD, a defect in the cell's ability to repair certain types of DNA damage), and the initial genetic makeup of the tumor.
- Patients with inherited BRCA2 gene mutations were observed to be more prone to additional RB1 gene mutations and exhibited poor responses to standard CDK4/6 inhibitor therapy.
- Tumors with only a single copy of the RB1 gene before treatment showed a high likelihood of developing complete RB1 loss during CDK4/6 inhibitor therapy.
- Underlying DNA repair defects, particularly HRD, were found to further contribute to this resistance mechanism.
These findings suggest a method for identifying high-risk tumors and tailoring treatment decisions. Based on this research, the global, randomized phase 3 clinical trial EvoPAR-Breast01 is enrolling patients with newly diagnosed ER-positive, HRD-positive metastatic breast cancer. This trial is investigating an alternative approach that replaces CDK4/6 inhibitors with therapies targeting HRD, such as PARP inhibitors, which preclinical models and clinical data suggest may be more effective in HRD-positive tumors. The trial compares the PARP inhibitor saruparib combined with the hormonal therapy camizestrant against standard CDK4/6 inhibitors and hormonal therapy.
Developing a Dual-Targeting Strategy at MD Anderson
Separately, a preclinical study published in Nature Communications by researchers at The University of Texas MD Anderson Cancer Center identified a dual-targeting strategy to overcome breast cancer drug resistance.
This dual-targeting strategy involves simultaneously targeting two cell-cycle regulators: CDK2 and CDK4/6, demonstrating strong and durable anti-tumor effects in preclinical models.
This approach involves simultaneously targeting two cell-cycle regulators: CDK2 and CDK4/6. The research team, led by Dr. Linjie Luo and Dr. Khandan Keyomarsi, found that cancer cells can adapt to CDK4/6 inhibitors by increasing their reliance on CDK2, thereby circumventing treatment. The study investigated combining the selective CDK2 inhibitor BLU-222 with existing CDK4/6 inhibitors.
Preclinical models of breast cancer, including those with established treatment resistance and aggressive triple-negative breast cancer, demonstrated strong and durable anti-tumor effects with this combination.
Mechanism of Action
The mechanism of this combination treatment involved BLU-222, alone and in combination with CDK4/6 inhibitors, triggering the cancer cells' natural mechanisms to halt cell division. This was achieved by increasing the levels of p21 and p27 proteins, which regulate cell growth and are often suppressed in drug-resistant tumors. The restoration of p21 and p27 activity blocked both CDK2 and CDK4, preventing cancer cells from continued division. Genetic removal of p21 or p27 using CRISPR technology eliminated the synergistic effect of the drug combination, indicating their critical role. RNA sequencing further indicated that the combination therapy activated cellular senescence, a permanent cessation of cancer cell growth, and interferon signaling.
This study provides preclinical evidence for a potential therapeutic approach, coinciding with the advancement of multiple next-generation CDK2 inhibitors toward clinical trials. The data suggest that targeting CDK2, in conjunction with CDK4/6 inhibitors, may re-establish control over the cell cycle in resistant tumors, potentially addressing unmet clinical needs for patients with CDK4/6 inhibitor-resistant HR-positive breast cancer and those with triple-negative disease.