Relationship between molecular mass and duration of activity of polyethylene glycol conjugated granulocyte...

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 micrograms/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 micrograms/mouse at day 3. The plateau dose increased to 8.4 micrograms/mouse at day 5, and no plateau was evident at the highest dose tested (50 micrograms/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.