Immune Checkpoint Inhibitors in the Treatment of Breast Cancer Brain Metastases

The management of breast cancer brain metastases (BCBM) has historically involved local therapies. However, as novel systemic treatments have become more effective in controlling visceral disease, BCBM have also been better controlled. Immune checkpoint inhibitors (ICIs) have demonstrated efficacy in brain metastases in patients with lung cancer and melanoma and represent a promising option for patients with triple-negative BCBM, a group with limited systemic therapy options. In this review we summarize current data about the role of ICIs in the treatment BCBM. We identified 15 clinical trials that evaluated ICIs ± chemotherapy in patients with breast cancer. The studies were mostly focused on triple-negative breast cancer (TNBC). Of these trials, 4 excluded patients with BCBM, while 11 allowed patients with stable, treated or asymptomatic BCBM. In total, 2692 patients were enrolled in the identified clinical trials, but only 91 trial patients (3.3%) had BCBM. Furthermore, only 2 of these clinical trials reported BCBM-specific outcomes and none of the clinical trials reported BCBM-specific adverse events. Up to 45% of patients with TNBC will develop BCBM; however, only 3.3% of the patients included in the clinical trials that led to the U.S. Food and Drug Administration approvals for ICIs in advanced breast cancer had brain metastases. This review reinforces that efficacy data are greatly needed for patients with BCBM—this is an area of unmet need in oncology. More inclusive clinical trials and real-world data that evaluate the safety and efficacy of ICIs in patients with BCBM are greatly needed.