Immunotherapy advances
Immune checkpoint inhibitors represent another promising treatment avenue for metastatic TNBC. Pembrolizumab and atezolizumab, recently approved by the FDA, show moderate progression-free and overall survival benefits in patients with metastatic TNBC expressing PD-L1. Estimates of PD-L1 immune cells present in TNBC tumors vary widely, from about 20% to 65%.
Yet, data on which patients will benefit are not so clear-cut. “These drugs give us more choices and represent the fast-evolving therapeutic landscape in TNBC, but they also leave a lot of unanswered questions about PD-L1 as a biomarker,” Dr. Yuan said.
Take two recent phase 3 trials evaluating atezolizumab: IMpassion130 and IMpassion131. In IMpassion130, patients with PDL1–positive tumors exhibited significantly longer median overall survival on atezolizumab plus nab-paclitaxel (25.0 months) compared with nab-paclitaxel alone (15.5 months). As with the trend observed in the TROP2 data for sacituzumab govitecan, all patients survived longer on atezolizumab plus nab-paclitaxel regardless of PD-L1 status: 21.3 months vs. 17.6 months with nab-paclitaxel alone.
However, in IMpassion131, neither progression-free survival nor overall survival significantly improved in the PD-L1–positive group receiving atezolizumab plus paclitaxel compared with paclitaxel alone: Progression-free survival was 5.7 months vs. 6 months, respectively, and overall survival was 28.3 months vs. 22.1 months.
“It is unclear why this study failed to demonstrate a significant improvement in progression-free survival with the addition of atezolizumab to paclitaxel,” Dr. Nanda said. “Perhaps the negative finding has to do with how the trial was conducted, or perhaps the PD-L1 assay used is an unreliable biomarker of immunotherapy benefit.”
Continued efforts to understand TNBC
Given the diversity of metastatic TNBC and the absence of clear molecular targets, researchers are exploring a host of therapeutic strategies in addition to antibody-drug conjugates and immunotherapies.
On the oncogene front, researchers are investigating common mutations in TNBC. About 11% of TNBC tumors, for instance, carry germline mutations in BRCA1 and BRCA2. These tumors may be more likely to respond to platinum agents and PARP inhibitors, such as FDA-approved olaparib. In a phase 3 trial, patients with metastatic HER2-negative breast cancer and a germline BRCA mutation who received olaparib exhibited a 2.8-month longer median progression-free survival and a 42% reduced risk for disease progression or death compared with those on standard chemotherapy.
When considering signaling pathways, the PI3K/AKT/mTOR pathway has been the target of numerous clinical trials. Dysregulation of signaling through the PI3K and AKT signaling pathway occurs in 25%-30% of patients with advanced TNBC, and AKT inhibitors have been shown to extend survival in these patients. Data show, for instance, that adding capivasertib to first-line paclitaxel therapy in patients with metastatic TNBC led to longer overall survival – 19.1 months vs. 12.6 with placebo plus paclitaxel – with better survival results in patients with PIK3CA/AKT1/PTEN altered tumors.
But there’s more to learn about treating metastatic TNBC. “Relapses tend to occur early in TNBC, and some tumors are inherently resistant to chemotherapy from the get-go,” said Charles Shapiro, MD, medical oncologist, Icahn School of Medicine at Mount Sinai, New York. “Understanding the causes of drug response and resistance in patients with metastatic TNBC represents the holy grail.”
Dr. Nanda agreed, noting that advancing treatments for TNBC will hinge on identifying the key factors driving metastasis. “For TNBC, we are still trying to elucidate the best molecular targets, while at the same time trying to identify robust biomarkers to predict benefit from therapies we already have available,” she said.
A version of this article first appeared on Medscape.com.