Optimizing targeting molecule density on drug carriers to improve cellular uptake

Purpose: The purpose of this work is to determine whether the number of targeting molecules on a carrier correlates with improved cell uptake. The xCT transporter is a cell-surface facilitative transporter that imports cystine into the cell and exports glutamate. Using liposomes as the carrier, we conjugated varying amounts of cystine, the targeting agent, to their surfaces to harness the xCT transporter. We evaluated the cellular uptake of liposomal formulations with different levels of the targeting molecule conjugated to the liposomal surface (A: medium, B: low, C: high) in several cell lines. Methods: Liposomes with different levels of the targeting molecule were formulated, and sulforhodamine B (SRB), a fluorescent dye, was encapsulated in the liposomes. The formulations tested were: three targeted formulations with varying levels of the targeting molecule (A: medium, B: low, C: high), one non-targeted formulation (NT), and an untreated control. These formulations were incubated for 6 hours with five human cancer cell lines (BxPC-3, MIA PaCa-2, A549, OVCAR-8, and PANC-1). After incubation, the cell lines were washed three times, and liposome uptake was quantified by measuring the mean fluorescence intensity of SRB using flow cytometry (Attune NxT, Acoustic Focusing Cytometer, Thermo Fisher Scientific, MA). Results: Targeted liposomes consistently demonstrated significantly higher uptake than the non-targeted formulation across all five cell lines tested. However, the correlation between targeting-molecule density and uptake was cell-line-dependent. The formulation with the highest targeting molecule density (Formulation C) showed the highest uptake in three of the five cell lines (BxPC-3, MIA PaCa-2, and A549). In contrast, varying the targeting molecule density did not result in a statistically significant difference in uptake among the targeted formulations in the OVCAR-8 and PANC-1 cell lines. Conclusion: The data suggest that cells' ability to internalize liposomes depends on whether the liposomes carry a targeting molecule. However, the extent of internalization is cell line dependent.