Treatment with the anti-PD-L1 cancer immunotherapy atezolizumab produced a durable clinical benefit in patients with metastatic triple-negative breast cancer who responded to treatment, according to results from a phase I study.

Overall survival (OS) rates were 41% at 1 year and 22% at both year 2 and year 3. Patients with PD-L1 on 5% or more of tumor-infiltrating immune cells (IC2/3) achieved even better clinical outcomes: Their OS rates at 1, 2, and 3 years were 45%, 28%, and 28%.

The findings from this early phase I trial, which were presented at the annual meeting of the American Association for Cancer Research, also demonstrated that response rates were higher in the first-line setting, and an exploratory biomarker analysis suggested that higher CD8 T cell and tumor-infiltrating lymphocyte counts also contributed to a better response.

“We have no targeted therapy at the moment for triple-negative breast cancer,” said study lead author Dr. Peter Schmid , director of the St. Bartholomew’s Breast Centre at St. Bartholomew’s Hospital and Barts Cancer Institute in London, during a media briefing. “The treatment we have is chemotherapy, and most patients develop resistance relatively quickly.”

Dr. Schmid noted that the median survival for these patients is still relatively short – about 9-12 months – so, the data from this trial need to be seen in that context.

“On the other hand, triple-negative breast cancer is probably the best subtype of breast cancer in terms of selecting patients for immune therapy,” said Dr. Schmid. “This is based on a high degree of genetic instability, a high rate of mutations, higher levels of PD-L1 expression, and tumor infiltrating lymphocytes inside the tumor.”

Atezolizumab is a humanized monoclonal antibody that disrupts the PD pathway, inhibits the binding of PD-L1 to PD-1 and B7.1, and, in doing so, restores tumor-specific T-cell immunity.

In this study, Dr Schmid and colleagues recruited patients with metastatic triple-negative breast cancer to one of the expansion cohorts of a phase I trial. A total of 112 patients were evaluable for response. Of this group, 19 received atezolizumab as first-line treatment, and 93 had received at least two lines of prior therapy.

Atezolizumab was administered every 3 weeks at 15 mg/kg or 20 mg/kg, and the level of PD-L1 expression on tumor-infiltrating immune cells was evaluated. The primary endpoint of the study was safety, with overall response rate, duration of response, and progression-free survival as key secondary endpoints.

The 1- and 2-year overall survival rates for responders were 100%, but that dropped to 33% and 11%, respectively, for nonresponders. Of the 11 responders, 5 received atezolizumab as first-line therapy, while 9 had high PD-L1 expression (IC2/3).

For patients who received atezolizumab in the first-line setting, 1-year overall survival was 63%, and 2-year overall survival was 47%. The rates were lower for second-line and beyond; 37% and 18%, respectively.

For IC2/3 patients, 1-year overall survival was 45%, compared with 37% for those with low to no PD-L1 expression (IC0/1).

Only 11% of patients experienced treatment-related grade 3 or greater adverse events, and side effects led to treatment discontinuation in 3% of patients.

A key message was that the duration of response had a median of 21 months, and that is significant in this disease setting, explained Dr. Schmid. Another important point was that “overall survival was significantly longer that what we see with chemotherapy.”

Genentech funded the study. Dr Schmid’s spouse is a consultant to Roche/Genentech.