Sulfonamide based pH-sensitive polymeric micelles: physicochemical characteristics and pH-dependent aggregation

Polymeric micelles composed of block copolymers of poly(l-lactide) (PLLA)/poly(ethylene glycol) (PEG) of which end was modified with a novel pH sensitive element of a sulfonamide, sulfadimethoxine (SD; pKa 6.1), or a polymer (PSD; Mw: 3000) of its derivative were investigated. PSD was synthesized by radical polymerization of a vinylated SD (SD monomer; SDM) using 2-aminoethanethiol as a chain transfer agent. SD or PSD was coupled to carboxylic PEG (Mw=2000)/PLLA (Mw=3500) block copolymer. The polymeric micelles were prepared by a diafiltration method and their physicochemical properties in aqueous media were characterized by dynamic light scattering (DLS) and fluorescence spectroscopy. The mean diameter of all samples was smaller than 60 nm with a unimodal size distribution. The critical micelle concentration (CMC) and mean aggregation number of PLLA block in one hydrophobic domain of PLLA/PEG–PSD micelle in borate buffer (pH 9.0) were slightly higher than those of PLLA/PEG and PLLA/PEG–SD micelles due to the enhanced hydrophilic nature of PSD when ionized. The pH-sensitive properties were examined by the measurement of pH-dependent solution turbidity and the size of micellar aggregates. The PLLA/PEG–PSD micelle aggregated below pH 7.0 due to deionized SDM on the surface, while the other micelles were stable. The polymeric micelle prepared in this study sharply responded to the change in pH around the physiological pH.