Solubilization and Release of a Model Drug Nimesulide from PEO-PPO-PEO Block Copolymer Core-Shell Micelles: Effect of Size of PEO Blocks

The commercially available polypropylene oxide (PPO)-polyethylene oxide (PEO) symmetrical triblock copolymers (Pluronics(A (R))) have been recognized as pharmaceutical excipients and used in a variety of applications. This paper reports studies on micellar and solubilization behavior of three PEO-PPO-PEO block copolymers, viz. P103, P104 and P105 (same PPO mol. wt = 3250 g center dot mol(-1) but different % PEO = 30, 40 and 50 %, respectively) in aqueous solutions. Critical micellization concentrations (CMCs), critical micellization temperatures (CMTs), and micelle size/polydispersity for copolymers with and without the drug, nimesulide (NIM), are reported. The solubilization of NIM is significantly enhanced with increasing hydrophobicity (P103 > P104 > P105), concentration, temperature and in the presence of added salt. The copolymer hydrophobicity, temperature and the drug loading strongly affect micelle behavior. The micelle-water partition coefficient (P) and thermodynamic parameters of solubilization, viz. Gibbs energy (), enthalpy () and entropy (), were calculated. The solubilization site of the drug in different micellar solutions and its release from Pluronics(A (R)) micelles in phosphate buffer saline (PBS) solution at 37 A degrees C were examined. The kinetics of NIM exhibits burst release characteristics, which are believed to be controlled by degradation of the copolymers. These studies were carried out to investigate the feasibility of using Pluronics(A (R)) as a release vehicle of nimesulide in vitro. From the results, it was concluded that Pluronic(A (R)) based formulations might be practical for drug delivery.