Precise Ratiometric Drug Delivery for the Treatment of Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) remains a significant clinical challenge due to its high aggressiveness, poor prognosis, and lack of targeted therapies. Combining paclitaxel (PTX) with rapamycin (RAP), a PI3K/AKT/mTOR pathway inhibitor, has shown promise in preclinical and clinical studies, but the approach is limited by pharmacokinetic disparities and toxicity concerns. Here, we introduce Rapaxane, a formulation composed of polymeric micelles coloaded with oligo(lactic acid)8 conjugated prodrugs of PTX (oLA8-PTX) and RAP (oLA8-RAP) at an optimized synergistic ratio (5:1). We evaluated the efficacy of Rapaxane in vitro and in preclinical TNBC models, comparing it to the benchmark formulation Abraxane, the combination of parent drugs, and monotherapies using the prodrugs. In vitro, Rapaxane demonstrated notable cytotoxicity against the 4T1 and MDA-MB-231 TNBC cell lines. Ratiometric encapsulation, stability, synchronized drug release, and conversion were confirmed using dynamic light scattering (DLS) and reverse-phase high-performance liquid chromatography (RP-HPLC). Hemolysis assays indicated negligible toxicity, confirming the safety of Rapaxane for intravenous administration. In vivo, Rapaxane significantly reduced tumor growth and metastasis while improving survival rates in subcutaneous and orthotopic TNBC mouse models. Histological analysis using H&E staining, complemented by Ki-67 immunohistochemical staining, demonstrated effective inhibition of lung metastasis in Rapaxane-treated groups compared to control groups. Rapaxane's ability to maintain precise ratiometric dosing, sustain drug release, and enhance therapeutic efficacy while mitigating adverse effects underscores its potential as a next-generation therapy for TNBC. This study highlights the feasibility of nanotechnology-based ratiometric drug delivery systems in overcoming the limitations of conventional combination therapies, paving the way for more effective treatment options for aggressive cancers like TNBC. Keywords: paclitaxel; polymeric micelle; rapamycin; ratiometric drug delivery; triple-negative breast cancer.