THE DOWN AND UP-REGULATION OF THE RECEPTOR-MEDIATED ENDOCYTOSIS FOR THE G-CSF DERIVATIVE
1994 Volume 9 Issue supplement Pages 58-61
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1994 Volume 9 Issue supplement Pages 58-61
The non-linear pharmacokinetics of the recombinant human granulocyte colony-stimulating factor derivative nartograstim (NTG) has been reported. To clear the role of G-CSF receptor in the non-linear elimination of NTG, in vivo total clearance of NTG at steady-state (CLss) in the rats was compared with in vitro NTG binding capacity in the bone marrow. The dosedependent decrease of CLss in the rats indicated that a part of NTG elimination process was saturated. The Km and Vmax values for this saturable process were 107 pM and 16 pmol/hr/kg, respectively. The saturable CLss in the cyclophosphamide-treated rat with a low NTG binding capacity in the bone marrow, was 17 % of that in control. Also, the saturable CLss in the NTG-repeated administered rats with a high NTG binding capacity was 2-fold greater than that in control. On the other hands, the initial uptake clearance (CLuptake) by the bone marrow and spleen were down-regulated within 8 hr after a subcutaneous administration of NTG at a saturable dose. These finding indicated that the saturable elimination of NTG from the circulation might be due to the receptormediated endocytosis (RME) of G-CSF receptor.
In vitro RME of NTG in the rat bone marrow cells was kinetically analyzed. The internalization rate constant of occupied receptor (kint) and degradation rate constant (kdeg), obatained by three experiments (pulsechase, initial binding and steady-state binding experiments), were ranged 0.033-0.108 min-1 and 0.041-0.046 min-1. Also, from the steady-state plot, the ratio of kint to internalization constant of free receptor could be calculated as 6.6. This indicated that the internalization of the receptor was accelerated by the NTG binding, resulting in the down-regulation of the receptor. The in vitro clearance at steady-state, 104 ml/hr/kg, was comparable with the in vivo saturable CLss. This suggests that the saturable elimination of NTG from the circulating plasma comes mainly from RME in the bone marrow.
