Abemaciclib had already been approved for use in the treatment of HR+, HER2– advanced or metastatic breast cancer.
Now it is also approved for use in HR+, HER2– early breast cancer for patients who have high-risk, node-positive disease and whose tumors have a Ki-67 score of 20% or higher, as determined by a U.S. Food and Drug Administration–approved test.
The FDA also approved the Ki-67 IHC MIB-1 pharmDx (Dako Omnis) assay for use as a companion diagnostic test.
This is the first CDK4/6 inhibitor to be approved for use in this patient population.
Approximately 70% of all breast cancers are of the HR+, HER2– subtype.
The approval is based on some of the results from the monarchE study, which was presented last year at the annual meeting of the European Society of Medical Oncology and was simultaneously published in the Journal of Clinical Oncology.
The results showed that the addition of abemaciclib to endocrine therapy (tamoxifen or aromatase inhibitors) significantly improved invasive disease-free survival (IDFS), which was defined on the basis of the length of time before breast cancer comes back, any new cancer develops, or death.
The 2-year IDFS rates were 92.2% with the combination vs. 88.7% for endocrine therapy alone for the overall patient population.
“This is the first time in more than 20 years that we have seen an advance in the adjuvant treatment of this form of breast cancer,” lead investigator Stephen Johnston, MD, PhD, from the Royal Marsden Hospital NHS Foundation Trust, London, said at the meeting, as reported at the time by this news organization.
Reacting to the findings, Giuseppe Curigliano, MD, PhD, head of the division of early drug development at the European Institute of Oncology, Milan, said, “This is a very important trial and the findings will change practice.”
He predicted that once the drug is approved for use in high-risk HR+, HER2– early breast cancer, “the new standard of care for these patients will be to add 2 years of abemaciclib to endocrine therapy.”
In a press release about the new approval from the manufacturer (Lilly), another investigator on the monarchE study, Sara M. Tolaney, MD, MPH, Harvard Medical School and the Dana-Farber Cancer Institute, Boston, agreed that the results are practice changing. She said that the combination of abemaciclib and endocrine therapy is a potential new standard of care for this patient population. “We are encouraged by the marked reduction in the risk of recurrence even beyond the 2-year treatment period in these patients, and I’m grateful to be able to offer this as a treatment option to my patients,” she said.
On Twitter, she commented that restricting the indication to patients who show Ki67 ≥20% is “interesting,” inasmuch as benefits were seen in patients with both low and high Ki67.
Hal Burstein, MD, from Dana-Farber, also found this detail “interesting, as Ki67 testing remains a very controversial topic and difficult to standardize.”
Replying, Pedro Exman, MD, from the Hospital Alemão Oswaldo Cruz, in São Paulo, said: “Does it make sense to approve only in a subset of patients based in a positive subgroup analysis of a positive ITT study that was not even described in the JCO publication?”
Other experts said they were eagerly awaiting further results, particularly on overall survival, from the monarchE trial. New data are due to be presented on Oct. 14 at an ESMO virtual plenary session.
Commenting late last year about these results, George W. Sledge Jr, MD, professor of medicine at Stanford University Medical Center, Palo Alto, Calif., said that the median follow-up time “is still quite short for a study of ER+ adjuvant therapy, where the majority of recurrences and deaths occur after 5 years in many studies.”
Consequently, “we still have a long way to go to understand the ultimate effects of CDK4/6 inhibition on early-stage ER+ breast cancer, particularly on late recurrences,” he told this news organization at the time.
Agreed, said C. Kent Osborne, MD, codirector of the San Antonio Breast Cancer Symposium and founding director of the Duncan Cancer Center at Baylor College of Medicine, Houston, Tex. The results are “very encouraging, especially in the subgroup of tumors with high proliferation” (identified by the K1-67 score).
However, Dr. Osborne also urged caution in the interpretation of the results, “given the still rather short follow-up, given that that ER+ disease is known for its persistent recurrence rate, even past 10 years.”
He also noted that “this class of inhibitors is likely cytostatic, rather than cytocidal, meaning that it blocks cell proliferation rather than killing the cells.” Questions therefore remain over whether the survival curves for combination therapy will come together with those for endocrine therapy alone once patients stop taking the drug.