Conference Coverage

ER+/HER2– breast cancer: Is first or second line CDK4/6 inhibitor therapy better?


AT ASCO 2023

– Patients with advanced estrogen receptor–positive, HER-negative breast cancer receiving first line treatment with CDK4/6 inhibitors (CDK4/6i) experienced no progression-free survival (PFS) or overall survival (OS) advantages over those who received the same therapy as second line treatment.

That was the conclusion of the phase 3 SONIA study, which was presented at the annual meeting of the American Society of Clinical Oncology.

The benefit from first line therapy is not maintained and almost completely disappears when patients in the control arm cross over to receive CDK4/6 inhibition in second line,” said Gabe Sonke, MD, PhD, during his presentation at the meeting.

CDK4/6 inhibitors have shown benefit in both the first-and second-line setting, according to Dr. Sonke, who is a medical oncologist at the Netherlands Cancer Institute, Amsterdam. He added that most guidelines suggest use of CDK4/6 inhibitors in the first line, but there hasn’t been a direct comparison between use in the first and second line.

“Many patients do very well on endocrine therapy alone [in the first line]. Combination treatment leads to a higher risk of the emergence of resistant patterns such as ESR1 mutations, and CDK4/6 inhibitors also come with added costs and toxicities. Given the absence of comparative data between first line and second line, we designed the SONIA trial,” said Dr. Sonke.

Study methods and results

The researchers recruited 1,050 pre- and postmenopausal women who were randomized to a nonsteroidal AI in the first line followed by second-line CDK4/6i plus the estrogen receptor antagonist fulvestrant, or a nonsteroidal AI plus a CDK4/6i in the first line and fulvestrant in the second line. The most commonly used CDK4/6i was palbociclib at 91%, followed by ribociclib at 8%, and abemaciclib at 1%.

After a median follow-up of 37.3 months, the median duration of CDK4/6i exposure was 24.6 months in the first-line CDK4/6i group and 8.1 months in the second-line CDK4/6i group.

The median PFS during first-line therapy was 24.7 months in the first-line CDK4/6i group and 16.1 months in the second-line CDK4/6i group (hazard ratio, 0.59; P < .0001), which was consistent with the results seen in CDK4/6i pivotal trials in the first-line setting, according to Dr. Sonke. However, PFS after two lines of therapy was not significantly different between the groups (31.0 months vs. 26.8 months, respectively; HR, 0.87; P =.10).

The safety profile was similar to what had been seen in previous trials with respect to adverse events like bone marrow and liver function abnormalities and fatigue, but there were 42% more grade 3 or higher adverse events in the first-line CDK4/6i group than in the second-line CDK4/6i group. Dr. Sonke estimated that the increase in costs related to adverse events amounted to about $200,000 per patient receiving CDK4/6i as first line.

There were no significant differences between the two groups in quality of life measurement.

Subgroup analyses of patient categories including prior adjuvant or neoadjuvant chemotherapy or endocrine therapy, de novo metastatic disease, visceral disease, bone-only disease, and treatment with palbociclib or ribociclib showed no difference in outcome for first- versus second-line CDK4/6i treatment.


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