RECEPTOR-MEDIATED CLEARANCE OF G-CSF DERIVATIVE NARTOGRASTIM IN BONE-MARROW OF RATS

To clarify the role of the granulocyte colony-stimulating factor (G-CSF) receptor in the nonlinear elimination of a recombinant human G-CSF derivative, nartograstim (NTG), the accompanying changes in the in vivo NTG total body clearance at steady state (CL(ss)) or the early-phase tissue uptake clearance (CL(uptake)) in rats were compared with the change in the number of G-CSF receptors in bone marrow. The infusion rate-dependent decrease in CL(ss) in control rats confirmed the existence of a saturable elimination mechanism for NTG. The Michaelis-Menten constant (K-m) and maximal velocity for this saturable process were estimated to be 107 pM and 15.5 pmol . h(-1). kg(-1), respectively. The K-m for this saturable process was comparable with the dissociation constant (K-d) for the specific binding of NTG to bone marrow cells. After administration of excess NTG, the CL(uptake) Of tracer amounts of I-125-NTG by bone marrow and spleen, which corresponds to the receptor density in the tissues, was reduced at 2 h but gradually recovered. This change in CL(uptake) corresponds well to the change in the in vitro NTG-binding capacity in each isolated cell. This reduction in CL(uptake) might be due to the downregulation of G-CSF receptors on the cell surface. On the other hand, the saturable CL(ss) in cyclophosphamide-treated rats was 17% of that in control rats, whereas the saturable CL(ss) in rats given NTG repeatedly was twofold greater than in controls, which is associated with the upregulation of G-CSF receptors. These changes in saturable CL(ss) indeed corresponded well with those of the in vitro NTG-binding capacity in bone marrow. These findings indicate that the saturable uptake and elimination of NTG in rats are due to G-CSF receptor-mediated endocytosis.