A novel mechanism underlying acquired resistance to CDK 4/6 inhibitor treatment for breast cancer has been identified by a multi-institutional research team. The team discovered new mutations in the RB1 gene, a key part of the pathway targeted by CDK 4/6 inhibitors, in women with hormone-receptor (HR)-positive breast cancer whose tumors had progressed on CDK 4/6 inhibitor treatment.
The study published in the Annals of Oncology was led by investigators at Massachusetts General Hospital (MGH), Institute Gustave Roussy in Paris, and Texas Oncology-Baylor Sammons Cancer Center/U.S. Oncology in Dallas.
"CDK 4/6 inhibitors have changed the treatment landscape for HR-positive breast cancer , and it is estimated that more than 70,000 patients in the U.S. have been treated with palbociclib, the first of three such drugs to receive FDA approval," says Aditya Bardia, MD, MPH, of the MGH Cancer Center, co-senior author of the study. "However, after months of responding to treatment, patients' tumors eventually progress. The mechanisms governing the development of resistance have been unknown, and this first report of the emergence of RB1 mutations by our collaborative team will hopefully lead to additional research and development of therapeutic strategies to target and perhaps even prevent clinical resistance."
All three had developed metastases after several types of previous treatment, and their tumors had been genotyped before CDK 4/6 inhibitor treatment - either palbociclib (Ibrance) or ribocliclib (Kisqali) - was initiated. When their tumors progressed after treatment with a CDK 4/6 inhibitor, repeat genotyping identified several new RB1 mutations that had led to loss of RB1 function and consequently resistance to CDK 4/6 inhibitors. The authors note that, while their findings do not prove the mutations caused resistance, the detection of these mutations by investigators at three different institutions supports their potential role in clinical resistance and validates observations from other centers' studies in cell lines and animal models that reported the loss of RB1 function in CDK 4/6-inhibitor-resistant cells or tumors.
Addressing whether resistance to CDK 4/6 inhibitor can be prevented or delayed, Bardia says, "Preclinical studies have shown that adding a PI3K inhibitor to combined CDK 4/6 inhibitor and hormone treatment could prevent or delay CDK 4/6 inhibitor resistance; however, that needs validation in clinical studies. While combination therapy can enhance efficacy, it can also enhance toxicity. We are eagerly awaiting results from ongoing clinical trials evaluating triple combination treatment with endocrine therapy, a CDK 4/6 inhibitor and a PI3K inhibitor - or downstream mTOR inhibitor - in metastatic breast cancer." He is the director of Precision Medicine at the Center for Breast Cancer of the MGH Cancer Center and an assistant professor of Medicine at Harvard Medical School.