Triple-negative breast cancer (TNBC) is an aggressive entity lacking hormone receptors, and HER2 expression, limiting treatment to chemotherapy and radiotherapy. However, frequent resistance leads to relapses and metastasis, reducing patient survival, highlighting the urgent need of novel treatment strategies. Cells regulate death through a balance of pro- and anti-apoptotic proteins, and disruptions in this balance contribute to malignancy. In this line, targeting anti-apoptotic BCL-2 family members is a promising strategy, as their overexpression is linked to recurrence and resistance. This study shows that targeting BCL-2 proteins can overcome TNBC resistance, reducing tumor growth and metastasis in vivo.

Gene expression association with prognosis and response to chemotherapy was analyzed using KM and ROC plotter databases, respectively. BCL2 levels were measured in TNBC patient biopsies via immunohistochemistry. A drug screening with BH3 mimetics was performed on a panel of 8 TNBC cell lines. Resistant TNBC cell models (MDA-MB-231R, BT-549R) were generated using a cisplatin pulse-based strategy. BCL2, MCL1, and BCL-XL expression levels were evaluated by Western blot. Effects on proliferation, colony formation, migration, invasion, and tumor progression were assessed in resistant cells, while apoptosis was analyzed by flow cytometry. Drug’s efficacy was validated in vivo models, including patient-derived xenografts (PDX) and MMTV-PyMT. A PROTAC based strategy was used to validate the experimental approach.

Gene expression levels of anti-apoptotic BCL2 family members are associated with a lack of chemotherapy response in TNBC. Moreover, high BCL-2 and BCL-XL levels correlated with poor prognosis in patients with neoadjuvant therapy. A drug screening using BH3 mimetics drugs that directly induce apoptosis by inhibiting anti-apoptotic BCL-2 family members identified Obatoclax, a pan-inhibitor, as the most efficient compound to reduce proliferation in all tested TNBC cell lines, regardless of chemotherapy sensitivity. Functional in vitro analysis in chemo-persistent cells demonstrated that Obatoclax effectively reduced proliferation, migration, invasiveness, and clonogenic potential, while also inducing apoptosis in persistent cells. In vivo, the drug inhibited tumor progression and metastatic potential in mouse models: an orthotopic xenograft with resistant cells, a syngeneic TNBC model, and a taxane-refractory PDX model. Our results were reproduced using a PROTAC with a dual effect on the targets, showing no cross-resistance

Our findings suggest that the simultaneous targeting of BCL2 anti-apoptotic members could represent a promising therapeutic approach for chemotherapy-resistant TNBC.