CDK 4/6 Blocker Prolongs Survival in HER2+ Breast Cancer

SAN ANTONIO — Adding the CDK 4/6 blocker palbociclib to standard endocrine and antihuman epidermal growth factor receptor 2 (HER2) therapies in metastatic hormone receptor (HR)–positive, HER2-positive breast cancer extended patients’ median progression-free survival more than a year, according to the results of the phase 3 PATINA study.

This regimen “may represent a new standard of care” for these patients, said principal investigator and presenter Otto Metzger, MD, a medical breast oncologist at the Dana-Farber Cancer Institute in Boston, who presented the findings at the San Antonio Breast Cancer Symposium (SABCS) 2024.

The open-label PATINA trial, which was conducted in Europe, Australia, New Zealand, and the United States, included a total of 518 patients. Patients received first-line treatment of six to eight cycles of induction chemotherapy plus anti-HER2 therapy. Researchers then randomized patients to either palbociclib plus anti-HER2 and endocrine therapy (n = 261) or to anti-HER2 and endocrine therapy alone (n = 257).

Patients did not progress on induction therapy, which likely would have signaled early resistance to anti-HER2 treatment. For anti-HER2 therapy, 97.3% received a combination of trastuzumab and pertuzumab. For endocrine therapy, 90.9% received an aromatase inhibitor.

Metzger and colleagues found that median progression-free survival was 1.3 years longer in patients receiving palbociclib — 3.7 years in the palbociclib arm vs 2.4 years in the control group (hazard ratio [HR], 0.74; P = .0074).

Although overall survival outcomes are immature, 5-year survival rates were slightly better in the palbociclib arm — 74.3% with palbociclib vs 69.8% without it — but the difference was not statistically significant.

Grade 3 neutropenia was the most frequent adverse event in the palbociclib arm (63.2% vs 2%). Grades 2 and 3 fatigue, stomatitis, and diarrhea were also more common with palbociclib. Grade 4 adverse events occurred in 12.3% of those receiving palbociclib and 8.9% of those who did not. There were no treatment-related deaths.

“We’re very impressed with the results,” said Metzger.

On the basis of previous studies, it’s believed that CDK 4/6 inhibition counteracts the development of resistance to anti-HER2 and endocrine therapies, which likely explains the benefit found in the trial.

But even without CDK 4/6 inhibition, the progression-free survival of 2.4 years in the control arm “far exceed[ed] our expectations,” Metzger reported. This may have occurred because the control arm received endocrine therapy, something previous trials of anti-HER2 therapy have avoided due to tolerability and other concerns.

These findings, however, support “the common use of endocrine therapy,” Metzger said.

‘Incredible’ Results

The progression-free survival as well as overall survival results in the trial are “incredible,” said study discussant Sara Hurvitz, MD, a medical breast oncologist at the Fred Hutch Cancer Center in Seattle. This is “historic and very important data.”

Hurvitz even suggested the results might mean that patients who fit the PATINA criteria can avoid the toxicity of upfront trastuzumab deruxtecan and use the PATINA regimen instead, potentially preserving their quality of life for longer.

Another study discussant, Virginia Kaklamani, MD, a medical breast oncologist at MD Anderson Cancer Center, San Antonio, had a similar thought.

In PATINA, “we’re talking about patients being on a treatment that’s well tolerated, where patients continue to work and continue with their lives despite being on treatment for metastatic breast cancer for 4 years, which is remarkable,” Kaklamani said.

Many of us have dabbled with giving CDK 4/6 inhibitors in triple-positive breast cancer, but “now we have more definitive data,” she said. The approach can help “maintain the quality of life of our patients for a longer period of time” and delay the use of chemotherapy in the second line, she added.

Metzger said Pfizer, the maker of palbociclib, plans to file for a HER2-positive indication with the US Food and Drug Administration based on the trial results.

For now, the CDK 4/6 blocker is only indicated in combination with endocrine therapy for HR-positive, HER2-negative metastatic disease.

In response to a question about using the PATINA regimen in patients who don’t get chemotherapy induction, Metzger noted that, “while the study didn’t test this directly, I would argue that this data is quite compelling” for using palbociclib plus anti-HER2 and endocrine therapy, even without chemotherapy induction.

The work was funded by palbociclib maker Pfizer. Metzger had no disclosures. Hurvitz has numerous industry ties, including being a researcher and advisor to Pfizer. Kaklamani also has numerous industry ties, including reporting personal/consulting fees from Pfizer Canada.

M. Alexander Otto is a physician assistant with a master’s degree in medical science and a journalism degree from Newhouse. He is an award-winning medical journalist who worked for several major news outlets before joining Medscape Medical News. Alex is also an MIT Knight Science Journalism fellow. Email: [email protected]