Introduction. High-frequency proton therapy is a promising breast cancer (BC) treatment. Earlier, we revealed that the extent of nanoparticle (NPs) uptake in BC cells indicates their metastatic potential (MP). The research investigated BC cell nanoparticles absorption, comparing parental cells with their proton irradiation survivors in different environmental stiffnesses.

Materials and Methods. Low-MP MCF7 and high-MP MDA-MB-231 cell lines and their fractions survived after 6Gr-dose proton beams MCF7RP and MDA-MB-231RP, respectively, were seeded onto either rigid plastic flasks or soft fibrin gels for 24 hours, followed by incubation with fluorescent 200 nm nanoparticles and nuclear stain. After washing, cells were imaged in fluorescence channels, and Pearson’s colocalization coefficient was calculated.

Results. On rigid plastic, MDA-MB-231 cells internalized 1.35-fold more nanoparticles than MCF7 cells (p<0.0017). NP uptake further increased 1.6-fold in MDA-MB-231RP cells (p<0.0001), whereas MCF7RP cells showed no change. Overall, MDA-MB-231RP uptake was 1.86-fold higher than MCF7RP (p<0.0001), suggesting increase in MP. On soft fibrin, NP uptake by MDA-MB-231 cells decreased 1.33-fold, while uptake by MCF7 cells increased 1.18-fold compared with rigid plastic, resulting in comparable uptake levels between the two cell-lines. In contrast, MCF7RP cells showed reduced NP uptake, while MDA-MB-231RP cells were 1.47-fold more efficient (p<0.0172).

Conclusions. Proton irradiation BC cell survivors exhibited a pattern of nanoparticle uptake that correlates with their MP and the stiffness of the microenvironment. Hence, combining proton therapy with methods to soften the tumor microenvironment could help reduce the risk of metastasis after treatment.