Kinetic analysis of the in vivo tissue distribution of the recombinant human granulocyte colony-stimulating factor (rhG-CSF) derivative, nartograstim (NTG; nartograstim is an international nonproprietary name of KW-2228, Kyowa Hakko Kogyo, Tokyo, Japan) was studied in rats to determine the mechanism of the growth factor's nonlinear pharmacokinetic properties. The early-phase tissue uptake clearance (CL(uptake)) by each tissue was determined within 5 min after the i.v. administration of 125I-NTG. Coadministration of various amounts (0.6-150 microgram/kg) of unlabeled NTG did not produce any significant change in CL(uptake) by kidneys and liver. The hepatic and renal extraction ratios of NTG were low, 0.10 to 0.15, which suggests that there is no saturable uptake system in either of these organs. CL(uptake) by the kidneys was comparable to the glomerular filtration rate of unbound NTG. By contrast, a dose-dependent reduction in CL(uptake) by bone marrow and spleen was clearly observed with increasing doses of unlabeled NTG, i.e., at the maximal dose, these values were 14% and 20%, respectively, of those after a tracer dose. A saturable process was, therefore, involved in the tissue uptake of 125I-NTG by bone marrow and spleen. The intrinsic clearance (Vmax/Km) of this saturable uptake by bone marrow, 1.4 ml min-1 kg-1, was greater than that by the spleen, 0.09 ml min-1 kg-1. The sum of the intrinsic clearances of the saturable process was comparable with the value for the clearance of the saturable elimination process of NTG obtained previously by nonlinear pharmacokinetic analysis.(ABSTRACT TRUNCATED AT 250 WORDS)