Tumor pH-responsive flower-like micelles of poly(L-lactic acid)-b-poly(ethylene glycol)-b-poly(L-histidine)

Abstract

Polymeric micelles were constructed from poly(l-lactic acid) (PLA; M(n) 3K)-b-poly(ethylene glycol) (PEG; M(n) 2K)-b-poly(l-histidine) (polyHis; M(n) 5K) as a tumor pH-specific anticancer drug carrier. Micelles (particle diameter: approximately 80 nm; critical micelle concentration (CMC): 2 microg/ml) formed by dialysis of the polymer solution in dimethylsulfoxide (DMSO) against pH 8.0 aqueous solution, are assumed to have a flower-like assembly of PLA and polyHis blocks in the core and PEG block as the shell. The pH-sensitivity of the micelles originates from the deformation of the micellar core due to the ionization of polyHis at a slightly acidic pH. However, the co-presence of pH-insensitive lipophilic PLA block in the core prevented disintegration of the micelles and caused swelling/aggregation. A fluorescence probe study showed that the polarity of pyrene retained in the micelles increased as pH was decreased from 7.4 to 6.6, indicating a change to a more hydrophilic environment in the micelles. Considering that the size increased up to 580 nm at pH 6.6 from 80 nm at pH 7.4 and that the transmittance of micellar solution increased with decreasing pH, the micelles were not dissociated but rather swollen/aggregated. Interestingly, the subsequent decline of pyrene polarity below pH 6.6 suggested re-self-assembly of the block copolymers, most likely forming a PLA block core while polyHis block relocation to the surface. Consequently, these pH-dependent physical changes of the PLA-b-PEG-b-polyHis micelles provide a mechanism for triggered drug release from the micelles triggered by the small change in pH (pH 7.2-6.5).

Fig. 1

Schematic diagram depicting the central concept of flower-like pH-sensitive micelles based on PLA-b-PEG-b-polyHis triblock copolymer. During circulation in blood micelles preserve a flower-like shape but at the tumor site micelles undergo interior swelling and triggers antitumor drug release.

Fig. 2

The pH-profile of PLA-b-PEG-b-polyHis tribock copolymer (●) and NaCl (■) by acid-base titration. The average value from triplicate titrations was plotted.

Fig. 3

The determination of the CMC from the fluorescence intensity ratio I₃₃₇/I₃₃₄ from excitation spectra vs. log concentration of the PLA-b-PEG-b-polyHis micelles (pH 7.4).

Fig. 4

The particle size distribution of the PLA-b-PEG-b-polyHis micelles (pH 7.4)

Fig. 5

The pH-sensitivity of the PLA-b-PEG-b-polyHis micelles from (a) a plot of the intensity ratio I₃₃₇/I₃₃₄ (n=3), (b) the relative transmittance change (n=3), and (c) the particle size change (n=3), according to the pH of micellar solution. The micellar solution (ionic strength: 0.15) was kept to 0.1 g/l.

Fig. 6

(a) The pH-dependent cumulative DOX release from the PLA-b-PEG-b-polyHis micelle: pH 7.4 (●), pH 7.0 (■), pH 6.8 (▲), pH 6.4 (▼), pH 6.0 (◆) (n=3), and (b) the pH-dependent cytotoxicity of DOX loaded flower-like micelles (■) or free DOX (■) against MCF-7 cells (n=3).

Fig. 7

Confocal images of MCF-7 cells treated with the DOX-loaded PLA-b-PEG-b-polyHis micelles at different pH condition (pH 7.4 and 7.0).

Scheme 1

Overall scheme for the synthesis of PLA-b-PEG-b-polyHis