Shedding light on surface exposition of poly(ethylene glycol) and folate targeting units on nanoparticles of poly(ε-caprolactone) diblock copolymers: beyond a paradigm

Polymeric nanoparticles (NPs) of poly(ε-caprolactone) (PCL) covered with a hydrophilic poly(ethylene glycol) (PEG) shell are usually prepared from diblock PEG-PCL copolymers through different techniques. Furthermore PEG, NPs can be decorated with targeting ligands to accumulate in specific cell lines. However, the density and conformation of PEG on the surface and its impact on the exposition of small targeting ligands has been poorly considered so far although this has a huge impact on biological behaviour. Here, we focus on PEG-PCL NPs and their folate-targeted version to encourage accumulation in cancer cells overexpressing folate receptor α. NPs were prepared with mixtures of PEG-PCL with different PEG length (short 1.0kDa, long 2.0kDa,) and a folate-functionalized PEG-PCL (PEG 1.5kDa) by the widely employed solvent displacement method. In depth characterization of NPs surface by 1H NMR, fluorescence and photon correlation spectroscopy evidenced a PEGylation extent below 7% with PEG in a mushroom conformation and the presence of folate more exposed to water pool in the case of copolymer with short PEG. NPs with short PEG adsorbed HSA forming a soft corona without aggregating. Although limited, PEGylation overall reduced NPs uptake in human macrophages. Uptake of NPs exposing folate prepared with short PEG was higher in KB cells (FR+) than in A549 (FR-), occurred via FR-receptor and involved lipid rafts-dependent endocytosis. In conclusion, the present results demonstrate that PEG length critically affects protein interaction and folate exposition with a logical impact on receptor-mediated cell uptake. Our study highlights that the too simplistic view suggesting that PEG-PCL gives PEG-coated NPs needs to be re-examined in the light of actual surface properties, which should always be considered case-by-case.