[Metabolism, Distribution and Excretion of Recombinant Human Thrombopoietin in Mice]

The metabolism, distribution and excretion profiles of recombinant human thrombopoietin (rhTPO) in mice were studied by means of (125)I-labeled rhTPO ((125)I-rhTPO) combined with size exclusive high performance liquid chromatography (SHPLC) or trichloroacetic acid (TCA) precipitation analysis. (125)I-rhTPO was prepared by iodogen method. Purification was performed on Sephacryl S-200 HR gel. Radioactive-purity of (125)I-rhTPO identified by SHPLC was (96.9 +/- 1.5)% (n = 3). The proliferation effect of TPO dependent cell line (TD-3) and the increase of peripheral platelet counts in mouse by (125)I-rhTPO demonstrated that (125)I-labeled protein maintained the biological activities of TPO both in vitro and in vivo. SHPLC analysis of serum and urine samples taken after sc 1 micro g/mouse (345 kBq/mouse) of (125)I-rhTPO revealed that there were two lower molecular weight (125)I-degradation metabolites ((125)I-MI and (125)I-MII) other than parent molecule. (125)I-MI was mainly found in urine, and (125)I-MII was detected both in serum and in urine. The maximal concentration of (125)I-rhTPO was reached at 2 hours after injection. The terminal half-life was 10.8 hours, which was much longer than those of other peptides. TCA precipitable radioactivity in tissue showed that the radioactivity in bone marrow was rather high. The highest level was found in urinary system. Levels in adrenals, lymph nodes, and fat were near to that in serum. Lowest was found in brain. The main excretion route was urinary system and (98 +/- 5.6)% of (125)I-rhTPO was excreted within 72 hours after dosing.