Triple negative breast cancer (TNBC) is a deadly form of breast cancer due to the development of resistance to chemotherapy affecting over 30% of patients. New therapeutics and companion biomarkers are urgently needed. Recognizing the elevated expression of glucose transporter 1 (GLUT1, encoded by SLC2A1) and associated metabolic dependencies in TNBC, we investigated the vulnerability of TNBC cell lines and patient-derived samples to GLUT1 inhibition. We report that genetic or pharmacological inhibition of GLUT1 with BAY-876 impairs the growth of a subset of TNBC cells displaying high glycolytic and lower oxidative phosphorylation (OXPHOS) rates. Pathway enrichment analysis of gene expression data suggests that the functionality of the E2F pathway may reflect to some extent OXPHOS activity. Furthermore, the protein levels of retinoblastoma tumor suppressor (RB1) strongly correlate with the degree of sensitivity to GLUT1 inhibition in TNBC, where RB1-negative cells are insensitive to GLUT1 inhibition. Collectively, our results highlight a strong and targetable RB1-GLUT1 metabolic axis in TNBC and warrant clinical evaluation of GLUT1 inhibition in TNBC patients stratified according to RB1 protein expression levels.

Historically, breast cancer tumors have been considered immunologically quiescent, with the majority of tumors demonstrating low lymphocyte infiltration, low mutational burden, and modest objective response rates to anti-PD-1/PD-L1 monotherapy. Tumor and immunologic profiling has shed light on potential mechanisms of immune evasion in breast cancer, as well as unique aspects of the tumor microenvironment (TME). These include elements associated with antigen processing and presentation as well as immunosuppressive elements, which may be targeted therapeutically. Examples of such therapeutic strategies include efforts to (1) expand effector T-cells, natural killer (NK) cells and immunostimulatory dendritic cells (DCs), (2) improve antigen presentation, and (3) decrease inhibitory cytokines, tumor-associated M2 macrophages, regulatory T- and B-cells and myeloid derived suppressor cells (MDSCs). The goal of these approaches is to alter the TME, thereby making breast tumors more responsive to immunotherapy. In this review, we summarize key developments in our understanding of antitumor immunity in breast cancer, as well as emerging therapeutic modalities that may leverage that understanding to overcome immunologic resistance.

Addition of pembrolizumab to neoadjuvant chemotherapy followed by adjuvant pembrolizumab improved outcomes in patients with high-risk, early-stage, triple-negative breast cancer. However, whether the addition of neoadjuvant pembrolizumab to chemotherapy would improve outcomes in high-risk, early-stage, estrogen receptor-positive/human epidermal growth factor receptor 2-negative (ER⁺/HER2⁻) breast cancer remains unclear. We conducted a double-blind, placebo-controlled phase 3 study (KEYNOTE-756) in which patients with previously untreated ER⁺/HER2⁻ grade 3 high-risk invasive breast cancer (T1c-2 (≥2 cm), cN1–2 or T3–4, cN0–2) were randomly assigned (1:1) to neoadjuvant pembrolizumab 200 mg or placebo Q3W given with paclitaxel QW for 12 weeks, followed by four cycles of doxorubicin or epirubicin plus cyclophosphamide Q2W or Q3W. After surgery (with/without adjuvant radiation therapy), patients received adjuvant pembrolizumab or placebo for nine cycles plus adjuvant endocrine therapy. Dual primary endpoints were pathological complete response and event-free survival in the intention-to-treat population. In total, 635 patients were assigned to the pembrolizumab−chemotherapy arm and 643 to the placebo−chemotherapy arm. At the study’s prespecified first interim analysis, the pathological complete response rate was 24.3% (95% confidence interval (CI), 21.0–27.8%) in the pembrolizumab−chemotherapy arm and 15.6% (95% CI, 12.8–18.6%) in the placebo−chemotherapy arm (estimated treatment difference, 8.5 percentage points; 95% CI, 4.2–12.8; P = 0.00005). Event-free survival was not mature in this analysis. During the neoadjuvant phase, treatment-related adverse events of grade ≥3 were reported in 52.5% and 46.4% of patients in the pembrolizumab−chemotherapy and placebo−chemotherapy arms, respectively. In summary, the addition of pembrolizumab to neoadjuvant chemotherapy significantly improved the pathological complete response rate in patients with high-risk, early-stage ER⁺/HER2⁻ breast cancer. Safety was consistent with the known profiles of each study treatment. Follow-up continues for event-free survival. ClinicalTrials.gov identifier: NCT03725059.

CX-5461 is a G-quadruplex stabilizer that exhibits synthetic lethality in homologous recombination-deficient models. In this multicentre phase I trial in patients with solid tumors, 40 patients are treated across 10 dose levels (50–650 mg/m²) to determine the recommended phase II dose (primary outcome), and evaluate safety, tolerability, pharmacokinetics (secondary outcomes). Defective homologous recombination is explored as a predictive biomarker of response. CX-5461 is generally well tolerated, with a recommended phase II dose of 475 mg/m² days 1, 8 and 15 every 4 weeks, and dose limiting phototoxicity. Responses are observed in 14% of patients, primarily in patients with defective homologous recombination. Reversion mutations in PALB2 and BRCA2 are detected on progression following initial response in germline carriers, confirming the underlying synthetic lethal mechanism. In vitro characterization of UV sensitization shows this toxicity is related to the CX-5461 chemotype, independent of G-quadruplex synthetic lethality. These results establish clinical proof-of-concept for this G-quadruplex stabilizer. Clinicaltrials.gov NCT02719977.

Patient-derived tumor organoids (PDOs) are a highly promising preclinical model that recapitulates the histology, gene expression, and drug response of the donor patient tumor. Currently, PDO culture relies on basement-membrane extract (BME), which suffers from batch-to-batch variability, the presence of xenogeneic compounds and residual growth factors, and poor control of mechanical properties. Additionally, for the development of new organoid lines from patient-derived xenografts, contamination of murine host cells poses a problem. We propose a nanofibrillar hydrogel (EKGel) for the initiation and growth of breast cancer PDOs. PDOs grown in EKGel have histopathologic features, gene expression, and drug response that are similar to those of their parental tumors and PDOs in BME. In addition, EKGel offers reduced batch-to-batch variability, a range of mechanical properties, and suppressed contamination from murine cells. These results show that EKGel is an improved alternative to BME matrices for the initiation, growth, and maintenance of breast cancer PDOs.