Pharmacokinetic analysis of scavenger receptor-mediated uptake of anionized proteins in the isolated perfused rat liver

The hepatic uptake characteristics of ¹¹¹In-labeled succinylated lysozyme (Suc-LZM), superoxide dismutase (Suc-SOD), bovine serum albumin (Suc-BSA), catalase (Suc-CAT), and maleylated SOD (Mal-SOD), BSA (Mal-BSA) were studied in rat liver perfusion experiments. During a single-pass constant infusion mode, [¹¹¹In]Suc-BSA, [¹¹¹In]Suc-CAT, and ¹¹¹In]Mal-BSA were significantly extracted while extraction of [¹¹¹In]Suc-LZM, [¹¹¹In]Suc-SOD, and [¹¹¹In]Mal-SOD were small, suggesting the importance of molecular weight or total number of anionic charges per one protein molecule for the hepatic uptake of anionized proteins. The extraction ratio at steady state (E(ss)) for [¹¹¹In]Suc-BSA was significantly decreased by co-administration of Mal-BSA or dextran sulfate, which is known to be taken up via scavenger receptor, and NH₄Cl, dinitrophenoI, or cytochalasin B, suggesting that hepatic uptake of [¹¹¹In]Suc-BSA proceeds via receptor-mediated endocytosis. The internalization rate constant (k(int)) for [¹¹¹In]Suc-BSA. was calculated to be 0.27 min^-1 in liver perfusion experiments using the acid-wash method. The outflow patterns of [¹¹¹In]Suc-BSA at various inflow concentrations were simultaneously fitted to a physiological one-organ pharmacokinetic model, in which the hepatic uptake was represented by division into the processes of binding to the cell surface and internalization, by the use of the MULTI (RUNGE) program. The obtained pharmacokinetic parameters (maximum binding amount X(∞), binding constant K(int) and internalization rate constant k(int)) for [¹¹¹In[Suc-BSA clearly characterized the difference in their hepatic uptake mechanisms compared with lactosylated and cationized BSA. The present study has demonstrated that large succinylated and maleylated proteins should be useful as a carrier for the intracellular delivery of drugs specifically into the liver endothelial cells.