Pharmacokinetics of recombinant human granulocyte colony-stimulating factor in the rat. Single and multiple dosing studies.

The pharmacokinetics of recombinant human granulocyte colony-stimulating factor (rhG-CSF) were investigated in male Sprague-Dawley rats at a dose of 5 micrograms/kg. The serum concentrations of rhG-CSF were monitored using a specific sandwich enzyme immunoassay. In single-dose studies, the influence of routes of administration were evaluated. For iv administration, the serum concentration-time data showed the rapid disappearance of rhG-CSF from the systemic blood and a mean residence time (MRT) of 1.341 hr. For sc, im, and ip administration, lower peak serum levels were observed, but after 2 to 3 hr, rhG-CSF levels were higher than those for iv administration. The MRTs after sc, im, and ip injections were 3.918, 2.894, and 3.538 hr, respectively. The serum concentration profiles after extravascular injections showed that an im injection gave slightly faster absorption kinetics of rhG-CSF from the injection site into systemic blood than did sc and ip injections. In multiple-dose studies, rhG-CSF was injected into animals iv and sc at 5 micrograms/kg/day for 7 days. On the day 7 the serum concentration-time profiles after rhG-CSF administration were compared between single and multiple dosing. The AUC after iv multiple dosing decreased by 17.4%, although half-lives and the volume of distribution were not significantly different between single and multiple dosing groups. The AUC after sc multiple dosing decreased by 25.6%; however, the bioavailability and observed maximum serum concentration of rhG-CSF were not significantly different. These results showed that the clearance of rhG-CSF increased after multiple dosing, although the mechanism of increased clearance was not apparent.