Phosphorylcholine-Based pH-Responsive Diblock Copolymer Micelles as Drug Delivery Vehicles: Light Scattering, Electron Microscopy, and Fluorescence Experiments Cristiano Giacomelli, Lucile Le Men, and Redouane Borsali* Laboratoire de Chimie des Polyme ` res Organiques (LCPO)-ENSCPB, Universite ´ Bordeaux 1, 16 Av. Pey Berland, 33607 Pessac Cedex, France Jose ´ phine Lai-Kee-Him and Alain Brisson Laboratoire d’Imagerie Mole ´ culaire et Nano-Bio-Technologie (IECB), Universite ´ Bordeaux 1, 2 Rue Robert Escarpit, 33607 Pessac Cedex, France Steven P. Armes* Department of Chemistry, Dainton Building, Brook Hill, Sheffield, South Yorkshire, S3 7HF United Kingdom Andrew L. Lewis Biocompatibles UK Ltd, Chapman House, Farnham Business Park, Weydon Lane, Farnham, Surrey, GU9 8QL United Kingdom Received November 22, 2005; Revised Manuscript Received January 13, 2006 The micellization behavior of a diblock copolymer comprising a highly hydrophilic and biocompatible poly(2- methacryloyloxyethyl phosphorylcholine) (PMPC) corona-forming block and a pH-sensitive poly(2-(diisoprop- ylamino)ethyl methacrylate) (PDPA) core-forming block (PMPC-b-PDPA) has been studied by static and dynamic light scattering (SDLS), transmission electron microscopy (TEM), and potentiometry. Self-assembly of PMPC- b-PDPA copolymers with two different DPA volume fractions (φ DPA ) leads to narrowly distributed and structurally distinct spherical micelles, as evidenced by their molecular weight (M w,mic ), aggregation number (N agg ), hydrodynamic radius (R H ), corona width (W), and core radius (R c ). The excellent potential of these pH-responsive micelles as nanosized drug delivery vehicles was illustrated by the encapsulation of dipyridamole (DIP), a model hydrophobic drug that dissolves in acid solutions and becomes insoluble above pH 5.8, which is comparable to the pK a of the PDPA block. The influence of micelle structure (namely M w,mic , N agg , R H , W, and R c ) on drug loading content, drug loading efficiency, partition coefficient, and release kinetics was investigated and confirmed by fluorescence spectroscopy studies. The maximum dipyridamole loadings within PMPC 30 -b-PDPA 30 (R H ) 14.0 nm; W ) 4.8 nm; R c ) 9.2 nm) and PMPC 30 -b-PDPA 60 (R H ) 27.1 nm; W ) 11.0 nm; R c ) 16.1 nm) micelles were 7 and 12% w/w p , respectively. This preferential solubilization of DIP into micelles formed by copolymer chains having longer core-forming blocks (i.e., possessing larger core volumes) reflects the larger partition coefficient (K V ) of DIP between the aqueous phase and PMPC 30 -b-PDPA 60 micelles (K V ) 5.7 × 10 4 ) compared to PMPC 30 -b-PDPA 30 micelles (K V ) 1.1 × 10 4 ). This enhanced ability of PMPC 30 -b-PDPA 60 aggregates to entrap/stabilize small hydrophobic molecules also produces slower release kinetics. Rapid release can be triggered by lowering the pH to induce micellar dissociation. 1. Introduction Over the past few decades, cellular membrane mimicking in relatively simple models has inspired many advances in the biomedical and nanotechnology fields, especially in terms of self-assembly processes involving phospholipid-like molecules. 1 These naturally occurring compounds usually comprise double hydrophobic tails and a polar headgroup, which in many cases contains the phosphorylcholine (PC) motif. On this basis, PC- based macromolecules of clinically proven biocompatibility have been successfully synthesized either by grafting PC moieties onto a reactive polymer backbone or by polymerizing PC- containing vinyl monomers such as 2-(methacryloyloxy)ethyl phosphorylcholine (MPC). 2,3 For example, Winnik et al. 4,5 reported the synthesis of hydrophobically modified PC-based polybetaines via reductive amination of phosphorylcholine glyceraldehyde by primary amine groups attached to the polymer. On the other hand, atom transfer radical polymerization (ATRP) 6 has been used by Armes and co-workers 7-10 to copolymerize MPC with various stimulus-responsive (pH, temperature, ionic strength) vinyl monomers to give a range of well-defined amphiphilic diblock and triblock copolymers. One of the most interesting and fascinating properties of amphiphilic AB diblock copolymers is their ability to self- assemble into micelles (and other ordered structures such as lamellae, vesicles, etc.) if dissolved in a so-called “selective” solvent, i.e., a solvent that is thermodynamically good for one block and poor for the other. 11-17 Such micelles are character- ized by their core-shell structure. In an aqueous environment, the hydrophobic blocks of the copolymer are segregated from * To whom correspondence should be addressed. E-mail: borsali@ enscpb.fr (R.B.); s.p.armes@sheffield.ac.uk (S.P.A.). 817 Biomacromolecules 2006, 7, 817-828 10.1021/bm0508921 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/10/2006