Self-assembling nanocomposites for protein delivery: Supramolecular interactions between PEG-cholane and rh-G-CSF

PEG_{5 kDa}-cholane, PEG_{10 kDa}-cholane and PEG _{20 kDa}-cholane self-assembling polymers have been synthesised by the end-functionalisation of 5, 10 and 20 kDa linear amino-terminating monomethoxy-poly(ethylene glycol) (PEG-NH₂) with 5β-cholanic acid. Spectroscopic studies and isothermal titration calorimetry showed that the CMC of the PEG-cholane derivatives increased from 23.5 ± 1.8 to 60.2 ± 2.4 μM as the PEG molecular weight increased. Similarly, light scattering analysis showed that the micelle size increased from 15.8 ± 4.9 to 23.2 ± 11.1 nm with the PEG molecular weight. Gel permeation studies showed that the polymer bioconjugates associate with recombinant human granulocyte colony stimulating factor (rh-G-CSF) to form supramolecular nanocomposites according to multi-modal association profiles. The protein loadings obtained with PEG_{5 kDa}-cholane, PEG_{10 kDa}-cholane and PEG_{20 kDa}-cholane were 7.4 ± 1.1, 2.7 ± 0.3 and 2.1 ± 0.4% (protein/polymer, w/w %), respectively. Scatchard and Klotz analyses showed that the protein/polymer affinity constant increased and that the number of PEG-cholane molecules associated to rh-G-CSF decreased as the PEG molecular weight increased. Isothermal titration calorimetry confirmed the protein/polymer multi-modal association. Circular dichroism analyses showed that the polymer association alters the secondary structure of the protein. Nevertheless, in vitro studies performed with NFS-60 cells showed that the polymer interaction does not impair the biological activity of the cytokine. In vivo studies performed by intravenous and subcutaneous administrations of rh-G-CSF to rats showed that the association with PEG_{5 kDa}-cholane prolongs the body exposure of the protein. After subcutaneous administration, the protein t_max values obtained with rh-G-CSF and 1:14 and 1:21 rh-G-CSF/PEG_{5 kDa}-cholane (w/w ratio) nanocomplexes were 2, 8 and 24 h, respectively. The 1:21 (w/w) rh-G-CSF/PEG_{5 kDa}-cholane formulation resulted in 149% relative bioavailability, and the pharmacokinetic behaviour was similar to that obtained with an equivalent protein dose of rh-G-CSF chemically conjugated with one linear 20-kDa PEG. A single administration of a 1.5 mg/kg dose of a 1:21 (w/w) rh-G-CSF/PEG_{5 kDa}-cholane formulation induced a high production of white blood cells for 96 h.