Phosphorylcholine-based pH-responsive diblock copolymer micelles as drug delivery vehicles: Light scattering, electron microscopy, and fluorescence experiments

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-(diisopropylamino)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 (Phi DPA) leads to narrowly distributed and structurally distinct spherical micelles, as evidenced by their molecular weight (M-w,M-mic), aggregation number (N,(agg),,), hydrodynamic radius (RH), corona width (M, 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), RH, W, and R,) 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 PMPC30-b-PDPA(30) (RH = 14.0 nm; W = 4.8 nm; R-c = 9.2 nm) and PMPC30-b-PDPA(60) (RH = 27.1 nm; W = 11.0 nm; R, = 16.1 nm) tr:ticelles 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) inicelles (Kv = 5.7 x 101) compared to PMPC30-b-PDPA(30) micelles (Kv = 1.1 x 10(4)). This enhanced ability Of PMPC30-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.