Obscurin-PH as a chemo-sensitizer in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer due to its proclivity to migrate, invade, and metastasize. Given the lack of targetable cell surface markers, the standard of care for TNBC is systemic chemotherapy and surgery. The anthracycline doxorubicin (dox) is an efficacious mainstay chemotherapy for TNBC despite its toxicity, as it triggers dilated cardiomyopathy in up to 20% of patients. Thus, tolerable targeted therapeutics for TNBC are an urgent priority. Obscurin is a giant cytoskeletal protein richly expressed in normal breast epithelium; its expression is often lost in breast cancer, leading to lower survival and reduced responsiveness to anthracyclines. Intriguingly, administration of sublethal dox in TNBC cells upregulates the oncogenic PI3K/AKT axis, potentially conferring a survival advantage. Obscurin's pleckstrin homology domain (obscurin-PH) tightly binds and sequesters the p85 regulatory subunit of PI3K, suppressing PI3K/AKT activation and preventing migration, invasion, and metastasis. We hypothesized that obscurin-PH may act as a chemo-sensitizer by synergizing with dox, improving its efficacy and allowing for lower, less toxic doses to be used for TNBC treatment. The potency of a cocktail combining obscurin-PH and dox was evaluated across 3 TNBC cell lines with distinct PI3K activation patterns (Table 1). Excitingly, our data shows a strong synergy between obscurin-PH and dox across all cell lines, allowing for the use of sub-cardiotoxic dox doses, with the minimal effective synergistic dose varying based on the PI3K activity in each cell line. Mechanistically, the dox-induced survival advantage is blunted via inhibition of the pro-survival/metastatic PI3K/AKT/NF-κB axis. This data underscores the potential of obscurin-PH as a novel non-chemical PI3K inhibitor exhibiting anti-growth, anti-metastatic, and chemo-sensitizing properties. Moreover, it highlights the importance of precision medicine to avoid overtreatment and toxicity. Table 1. Obscurin-PH synergizes with dox in TNBC cells with varying modes of PI3K activation. Cell line PI3K pathway mutation(s) Clinical significance IC50 DOX (µM) IC50 OBSCURIN-PH (MOI) Relative Inhibition (RI) DOX Relative Inhibition (RI) OBSCURIN-PH Combination Sensitivity Score (CSS) CSS > RI = synergy; CSS < RI = antagonism MDA-MB-231 N/A N/A 3.82 941.48 15.43 -2.47 30.58 Synergy, CSS > RI BT-549 PTEN mutant [pVal275fs*1] Pathogenic, oncogenic 4.33 1765.09 23.2 6.3 30.46 Synergy, CSS > RI SUM159 PIK3CA mutant [p.His1047Leu] Pathogenic, oncogenic 5.0 8549.62 42.43 13.81 49.74 Synergy, CSS > RI