Stealth properties of poly(ethylene oxide)-based triblock copolymer micelles: A prerequisite for a pH-triggered targeting system K. Van Butsele a,1 , M. Morille b,c,1 , C. Passirani b,c,⇑ , P. Legras d , J.P. Benoit b,c , S.K. Varshney e , R. Jérôme a , C. Jérôme a a Center for Education and Research on Macromolecules, University of Liege, B6 Sart-Tilman, B-4000 Liege, Belgium b LUNAM Université, Ingénierie de la Vectorisation Particulaire, F-49933 Angers, France c INSERM, U646, F-49933 Angers, France d Service Commun Animalerie Hospito-Universitaire, University of Angers, 49100 Angers, France e Polymer Source, 124 Avro Street, Montreal, Canada H9P 2X8 article info Article history: Received 14 March 2011 Received in revised form 18 May 2011 Accepted 8 June 2011 Available online 17 June 2011 Keywords: Multifunctional nanocarriers Environment responsiveness Stealth Poly(ethylene oxide) Copolymer abstract Evaluation of the biocompatibility of pH-triggered targeting micelles was performed with the goal of studying the effect of a poly(ethylene oxide) (PEO) coating on micelle stealth properties. Upon proton- ation under acidic conditions, pH-sensitive poly(2-vinylpyridine) (P2VP) blocks were stretched, exhibit- ing positive charges at the periphery of the micelles as well as being a model targeting unit. The polymer micelles were based on two different macromolecular architectures, an ABC miktoarm star terpolymer and an ABC linear triblock copolymer, which combined three different polymer blocks, i.e. hydrophobic poly(e-caprolactone), PEO and P2VP. Neutral polymer micelles were formed at physiological pH. These systems were tested for their ability to avoid macrophage uptake, their complement activation and their pharmacological behavior after systemic injection in mice, as a function of their conformation (neutral or protonated). After protonation, complement activation and macrophage uptake were up to twofold higher than for neutral systems. By contrast, when P2VP blocks and the targeting unit were buried by the PEO shell at physiological pH, micelle stealth properties were improved, allowing their future sys- temic injection with an expected long circulation in blood. Smart systems responsive to pH were thus developed which therefore hold great promise for targeted drug delivery to an acidic tumoral environment. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction Nanoparticles, liposomes and polymer micelles have been extensively studied as targeting drug carrier systems [1–3]. An important requirement to the systemic intravenous use of these targeting particles is the ability to circulate in the blood stream for a prolonged period of time [4–8]. The rapid uptake of intrave- nously injected particulate drug carriers by cells of the mononu- clear phagocyte system (MPS) is the main limitation of drug targeting to sites other than the liver and spleen in the human body [9–11]. Colloidal carriers are generally recognized by the macro- phages due to physicochemical characteristics linked to their size, surface charge and surface hydrophobicity. Poly(ethylene oxide) (PEO) polymers have been the most commonly used materials to modify particulate surfaces in order to prevent recognition by MPS cells, with their efficiency increased as functions of chain size, density and conformation [8,12–17]. A prolonged circulation time is required to allow accumulation of the particles in solid tumors via the enhanced permeability and retention (EPR) effect, thus resulting in ‘‘passive targeting’’ [18–21]. Nevertheless, after localiz- ing in the tumor site, the particles have to be internalized because most cytotoxic drugs act intracellularly. Unfortunately, PEO mole- cules are known to have the antagonist effect of impeding cell entry [22]. Binding pilot molecules to nanocarriers is a straightforward way to improve the drug targeting [23,24]. However, coupling the ligands at the distal end of the PEO chains can lead to accelerated removal of the targeting particles from circulation [25,26]. Thus, attempts have been made to enhance the therapeutic effi- cacy of sterically stabilized particles by means of transient shield- ing of the targeting unit [27]. For this purpose, a polymer corona is generally used to coat the targeting units during circulation and therefore prevent the rapid clearance of the carrier. The removal of this corona, once the particle has arrived at the targeted site by the EPR effect, then restores the interaction potential of the targeting unit with the cell membrane receptors to activate the cellular uptake. Generally, the removal of the external polymer 1742-7061/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2011.06.014 ⇑ Corresponding author at: INSERM, U646, F-49933 Angers, France. E-mail address: Catherine.Passirani@univ-angers.fr (C. Passirani). 1 Both authors contributed equally. Acta Biomaterialia 7 (2011) 3700–3707 Contents lists available at ScienceDirect Acta Biomaterialia journal homepage: www.elsevier.com/locate/actabiomat