Comparison of receptor-mediated endocytosis kinetics between wild-type t-PA and recombinant pamiteplase in isolated rat hepatocytes and liver cell plasma membranes

1. Differences in receptor-mediated endocytosis kinetics between pamiteplas e, an engineered t-PA, and an unmodified rt-PA were examined using liver ce ll plasma membranes and isolated rat hepatocytes. 2. Whereas the binding site of pamiteplase on hepatocytes was the same as t hat of rt-PA, the K-d of pamiteplase was 5.1-7.7 times larger than that of rt-PA, indicating a lower affinity of pamiteplase for the t-PA receptor. 3. k(e) for pamiteplase measured using parenchymal cells or non-parenchymal cells was slightly smaller than that for rt-PA, whereas k(on) for pamitepl ase were much lower than that of rt-PA, suggesting that the interaction bet ween pamiteplase and the receptor is slower than that of rt-PA because of i ts structural modification. 4. Therefore, the difference in drug disposition between pamiteplase and rt -PA is mainly due to the difference in the hepatic clearance caused by a ch ange in the interaction rate between the ligand and its cell-surface recept or.