Control of rhGH Release Profile from PEG-PAF Thermogel.

Poly(ethylene glycol)-poly(l-alanine-co-l-phenyl alanine) diblock copolymers (PEG-PAF) of 2000-990 Da (P2K) and 5000-2530 Da (P5K) with the different molecular weights of PEGs, but having a similar molecular weight ratio of hydrophobic block to hydrophilic block were synthesized to compare their solution behavior and corresponding protein drug release profiles from their in situ formed thermogels. The PEG-PAF aqueous solutions underwent heat-induced sol-to-gel transition in a concentration range of 18.0-24.0 wt % and 8.0-12.0 wt % for P2K and P5K, respectively. P5K formed bigger micelles than P2K, of a broad distribution, whereas the PAF blocks of P5K developed richer in α-helix than those of P2K in the core of the micelles. As the temperature increased, the micelles underwent dehydration of the PEG, which led to the aggregation of micelles, while the secondary structure of PAF was slightly affected during the sol-to-gel transition. The P5K exhibited higher tendency to aggregate and formed a tighter gel than P2K. Upon injection into the subcutaneous layer of rats, both polymer aqueous solutions formed a biocompatible gel with typical mild inflammatory tissue responses. Recombinant human growth hormone (rhGH) maintained its stability without forming any aggregates in both sol (4 °C) and gel (37 °C) states of the PEG-PAFs. Even though P2K and P5K have a similar molecular weight ratio of hydrophobic block to hydrophilic block, the P5K system exhibited a reduced initial burst release, improved bioavailability, and prolonged therapeutic duration of the rhGH, compared to the P2K system. The current research suggests that a drug release profile is a complex function of self-assembling carriers and incorporated drugs, and thus, a promising protein delivery system could be designed by adjusting the molecular parameters of a thermogel.

PEG-Poly(L-alanine) thermogel as a 3D scaffold of bone-marrow-derived mesenchymal stem cells.

Bone-marrow-derived mesenchymal stem cells (BMSCs) were cultured in three-dimensional (3D) scaffolds formed by temperature-sensitive sol-to-gel transition of BMSC-suspended poly(ethylene glycol)-poly(L-alanine) (PEG-PA) aqueous solutions. A commercialized thermogelling 3D scaffold of Matrigel™ was used for the comparative study. The cells maintained their spherical shapes in the PEG-PA thermogel, whereas fibrous cell morphologies were observed in the Matrigel™. Type II collagen and myogenic differentiation factor 1 were dominantly expressed in the PEG-PA thermogel. On the other hand, a significant extent of type III ß-tubulin was expressed in the Matrigel™ in addition to type II collagen and myogenic differentiation factor 1. After confirming the dominant chondrogenic differentiation of the BMSCs in the PEG-PA thermogel in in vitro study, in vivo study was performed for injectable tissue engineering application of the BMSCs/PEG-PA system. The cell-growing implant was formed in situ by subcutaneous injection of the BMSC-suspended PEG-PA aqueous solution to mice. In vivo study also proved the excellent expressions of chondrogenic biomarkers including collagen type II and sulfated glycosaminoglycan in the mouse model. This paper suggests that the PEG-PA thermogel is a very promising as a 3D culture matrix as well as an injectable tissue-engineering system for preferential chondrogenic differentiation of the BMSCs.

Ion and temperature sensitive polypeptide block copolymer.

A poly(ethylene glycol)/poly(L-alanine) multiblock copolymer incorporating ethylene diamine tetraacetic acid ([PA-PEG-PA-EDTA(m)) was synthesized as an ion/temperature dual stimuli-sensitive polymer, where the effect of different metal ions (Cu(2+), Zn(2+), and Ca(2+)) on the thermogelation of the polymer aqueous solution was investigated. The dissociation constants between the metal ions and the multiblock copolymer were calculated to be 1.2 × 10(-7), 6.6 × 10(-6), and 1.2 × 10(-4) M for Cu(2+), Zn(2+), and Ca(2+), respectively, implying that the binding affinity of the multiblock copolymer for Cu(2+) is much greater than that for Zn(2+) or Ca(2+). Atomic force microscopy and dynamic light scattering of the multiblock copolymer containing metal ions suggested micelle formation at low temperature, which aggregated as the temperature increased. Circular dichroism spectra suggested that changes in the α-helical secondary structure of the multiblock copolymer were more pronounced by adding Cu(2+) than other metal ions. The thermogelation of the multiblock copolymer aqueous solution containing Cu(2+) was observed at a lower temperature, and the modulus of the gel was significantly higher than that of the system containing Ca(2+) or Zn(2+), in spite of the same concentration of the metal ions and their same ionic valence of +2. The above results suggested that strong ionic complexes between Cu(2+) and the multiblock copolymer not only affected the secondary structure of the polymer but also facilitated the thermogelation of the polymer aqueous solution through effective salt-bridge formation even in a millimolar range of the metal ion concentration. Therefore, binding affinity of metal ions for polymers should be considered first in designing an effective ion/temperature dual stimuli-sensitive polymer.