“Triple-negative breast cancer is an aggressive subtype of breast cancer often affecting younger women and, unfortunately, the current treatment options for metastatic disease remain limited,” said Peter Schmid, MD, PhD, Director of the St. Bartholomew’s Breast Centre at St. Bartholomew’s Hospital and Barts Cancer Institute in London.
This study involves the largest cohort of patients with metastatic breast cancer treated with immunotherapy to be presented to date, and it is the first study to report data on survival for this subgroup, according to Dr. Schmid.
“The most significant finding is the difference in the overall survival between patients who responded to atezolizumab and patients who did not respond. While all responders were alive after 1 year, the 1-year survival rate for nonresponders was only 38%,” Dr. Schmid said.
He added, “Another noteworthy finding is that metastatic triple-negative breast cancer patients treated with atezolizumab had a prolonged median duration of response of 21 months, which is substantially longer than what has been seen with any other treatment to date for this patient population.”
Dr. Schmid and colleagues recruited patients with metastatic triple-negative breast cancer to one of the expansion cohorts of the phase I trial. Of the 112 patients evaluable for response, 19 received atezolizumab as first-line treatment, and 93 had received at least two lines of prior therapy. At the time of enrollment, patients’ tumors were evaluated for the presence of the PD-L1 (programmed death-ligand 1) protein on tumor-infiltrating immune cells.
Per RECIST v1.1, 11 patients responded to treatment, for an overall response rate, which included complete and partial responses, of 10%.
Both 1- and 2-year overall survival (OS) rates for responders were 100%, and for nonresponders, OS rates were 33% and 11%, respectively. Of the 11 RECIST v1.1 responders, five received atezolizumab as first-line therapy, and nine had disease with high PD- L1 expression (IC2/3).
One- and 2-year OS for patients who received first-line atezolizumab were 63% and 47%, respectively; for those who were previously treated, OS rates were 37% and 18%, respectively. One-year OS for patients with high PD-L1 expression (IC2/3) was 45%, vs 37% for those with low to no PD-L1 expression (IC0/1).
Only 11% of patients experienced treatment-related grade 3 or 4 side effects, and side effects led to treatment discontinuation in 3% of patients, Dr. Schmid noted.
“Atezolizumab has yielded durable responses in a small population of both previously untreated and pretreated triple-negative breast cancer patients and is associated with an excellent safety profile. The results provide further evidence that immunotherapy may play a significant role in the treatment of breast cancer,” Dr. Schmid said. “It will be down to other ongoing and future studies to further improve on these treatment outcomes by optimizing treatment regimens and combinations for this hard-to-treat group of patients.”
A limitation is that the study did not have a randomized control group with standard therapy; the survival data could therefore only been seen in the context of historical controls, Dr. Schmid said.