Relationship between molecular mass and duration of activity of polyethylene glycol conjugated granulocyte colony-stimulating factor mutein

Proteins conjugated with polyethylene glycol (PEG) have increased in vivo activity compared to native proteins. We examined the activity of a variety of PEG conjugates prepared with a recombinant mutein of granulocyte colony-stimulating factor (nartograstim [NTG], KW-2228). The total PEG mass was varied by the number and size of the PEG molecules conjugated. In vitro activity, determined using a proliferation assay with G-NFS-60 cells, demonstrated an inverse relationship between PEG mass and concentration required for half-maximal proliferation. In vivo activity was examined by injecting compounds subcutaneously into normal mice and determining neutrophil counts at various times. Initial experiments in C57BL/6J mice indicated that neutrophil levels were significantly elevated 5 days after a single injection of 25 μg/mouse of each PEG-NTG preparation. More detailed experiments were performed with several of the preparations in C3H/HeJ mice lacking endotoxin receptors. The results demonstrated that the time after injection at which neutrophil numbers reached a maximum increased with increasing size of PEG. Similar results were obtained with purified preparations containing 1, 2, or 3 units of 20-kDa PEG per molecule of NTG, showing that increasing the extent of PEGylation also increases in vivo activity. Dose–response studies with the 20-kDa PEG-NTG demonstrated a plateau at doses >2.7 μg/mouse at day 3. The plateau dose increased to 8.4 μg/mouse at day 5, and no plateau was evident at the highest dose tested (50 μg/mL) at days 7 and 10. These results demonstrate that elevated neutrophil levels can be maintained for extended periods following single administration of high-molecular-weight PEG-NTG.