Hypoxia-Responsive Polymersomes for Stemness Reduction in Patient-Derived Solid Tumor Spheroids

Aggressive solid tumors are associated with rapid growth, early hypoxia, the lack of targeted therapies, and a poor prognosis. The hypoxic niches within the rapidly growing solid tumors give rise to a stem cell-like phenotype with higher metastasis and drug resistance. To overcome the drug resistance of these regions, we used hypoxia-responsive polymersomes with an encapsulated anticancer drug (doxorubicin, Dox) and a stemness modulator (all-trans-retinoic acid, ATRA). Reductase enzymes overexpressed in the hypoxia reduce the azobenzene linker of the polymers, disrupt the bilayer structure of the polymersomes, and release the encapsulated drugs. We used triple-negative breast cancer (TNBC) as a representative of aggressive and hypoxic solid tumors. We observed that ATRA synergistically enhanced the efficacy of Dox in killing the cancer cells. A synergistic combination of the two drug-encapsulated polymersomes reduced volumes of patient-derived TNBC spheroids by 90%. In contrast, Dox alone decreased the spheroid volumes by 70% and encapsulated Dox by 19%. Mechanistic studies revealed that ATRA inhibited the efflux pumps, leading to a higher concentration of doxorubicin within the TNBC cells. In addition, the combination of encapsulated Dox and ATRA significantly decreased stemness expression of the TNBC cells in hypoxia compared to Dox alone.