Riboflavin transporters (RFTs) and the
riboflavin carrier protein (RCP) are highly upregulated in many
tumor cells, tumor stem cells, and
tumor neovasculature, which makes them attractive targets for nanomedicines. Addressing cells in different
tumor compartments requires
drug carriers, which are not only able to accumulate via the EPR effect but also to extravasate, target specific cell populations, and get internalized by cells. Reasoning that
antibodies are among the most efficient targeting systems developed by nature, we consider their size (∼10-15 nm) to be ideal for balancing passive and active
tumor targeting. Therefore, small, short-circulating (10 kDa, ∼7 nm, t1/2 ∼ 1 h) and larger, longer-circulating (40 kDa, ∼13 nm, t1/2 ∼ 13 h)
riboflavin-targeted branched PEG
polymers were synthesized, and their biodistribution and target site accumulation were evaluated in mice bearing angiogenic
squamous cell carcinoma (A431) and desmoplastic
prostate cancer (PC3) xenografts. The
tumor accumulation of the 10 kDa PEG was characterized by rapid intercompartmental exchange and significantly improved upon active targeting with
riboflavin (RF). The 40 kDa PEG accumulated in
tumors four times more efficiently than the small
polymer, but its accumulation did not profit from active RF-targeting. However, RF-targeting enhanced the cellular internalization in both
tumor models and for both
polymer sizes. Interestingly, the nanocarriers' cell-uptake in
tumors was not directly correlated with the extent of accumulation. For example, in both
tumor models the small RF-PEG accumulated much less strongly than the large passively targeted PEG but showed significantly higher intracellular amounts 24 h after iv administration. Additionally, the size of the
polymer determined its preferential uptake by different
tumor cell compartments: the 10 kDa RF-PEGs most efficiently targeted
cancer cells, whereas the highest uptake of the 40 kDa RF-PEGs was observed in tumor-associated macrophages. These findings imply that
drug carriers with sizes in the range of therapeutic
antibodies show balanced properties with respect to passive accumulation, tissue penetration, and active targeting. Besides highlighting the potential of RF-mediated (
cancer) cell targeting, we show that strong
tumor accumulation does not automatically mean high cellular uptake and that the nanocarriers' size plays a critical role in cell- and compartment-specific drug targeting.