They are also equal to the slopes of the terminal phases for the Ab 12B9mH25 complex, which achieves serum concentration levels 8,890-fold higher than serum free Ab 12B9m concentrations at the same time. rate constantkaof 0.0278 h1, with 86% bioavailability. The model suggested a rapid hepcidin clearance of approximately 800 mL h1kg1. Only the highest-tested Ab 12B9m dose of 300 mg kg1week1was able to maintain free hepcidin level below the baseline during the dosing intervals. Free Ab 12B9m and free hepcidin concentrations were simulated, and their PK profiles were nonlinear as affected by their binding to each other. Additionally, the total amount of FcRn receptor involved in Ab 12B9m recycling at a given time was calculated empirically, and the temporal changes in the free FcRn levels upon Ab 12B9m administration were inferred. KEY WORDS:FcRn, hepcidin, modeling, monkey, pharmacokinetics == INTRODUCTION == Iron homeostasis in vertebrates is dominated by the lack of an excretory route for excess iron. Serum iron level is regulated by the rate of iron entry through the duodenal mucosa, which affects net iron absorption, and by the rate of iron release from macrophages recycling iron from aged or damaged erythrocytes (1,2). Export of iron from duodenal enterocytes and macrophages into plasma is regulated by the plasma membrane transporter ferroportin (3,4). Hepcidin, a hormone peptide of 25 amino acids synthesized by the liver, binds to ferroportin, causes ferroportin internalization and degradation, and thereby blocks the iron export (5,6). The presence of hepcidin in urine (7,8) suggests hepcidin elimination by the kidney. Hepcidin production is increased in response to high circulating iron levels (9). Increased hepcidin levels decrease iron release from intestinal enterocytes and iron-storage cells (e.g., macrophages) by reducing ferroportin expression on these target cells. This leads to decreased circulating iron levels, which in turn remove the stimulus for further hepcidin production. When the hepcidin level falls, the ferroportin level recovers, resulting in increased iron availability in circulation. Hepcidin is therefore the key regulator responsible for systemic iron homeostasis (10) and has been suggested to be a strategic target Mouse monoclonal to PTK6 for iron Betamethasone regulation in the treatment of various iron disorders such as hyporesponsiveness to erythropoietin (1113). Ab 12B9m is a fully human immunoglobulin G subtype 2 (IgG2) monoclonal antibody that binds to monkey and human hepcidin with similar affinities (Kd ~ 1 pM). In cynomolgus monkey studies, a significant total hepcidin accumulation was observed, suggesting a fast turnover rate for free hepcidin and/or limited renal elimination of the Ab 12B9mhepcidin complex as compared with free hepcidin. Although the role of hepcidin in iron regulation has been elucidated in recent years, quantitative information regarding hepcidin production, elimination, and turnover rate has been lacking. In this paper, total concentrations of Ab 12B9m and hepcidin obtained after single and multiple intravenous and subcutaneous doses of Ab 12B9m were used to jointly characterize their pharmacokinetics through the development of a semi-mechanistic model based on target mediated drug disposition (TMDD) and saturable FcRn-mediated IgG recycling. TMDD occurs when the time course of the concentration is influenced by the interaction between the drug and its pharmacological target (14). General pharmacokinetic models have been developed to account for drugreceptor binding, internalization, and degradation, as well as the receptor turnover (1523). Similar principles have also been applied to drugs targeting soluble ligands (2427). Betamethasone In addition, FcRn is an endosomal salvage receptor that binds to and protects IgGs from degradation during endosomal recirculation (28,29). The influence of the FcRn on the IgG disposition has been studied using physiologically based pharmacokinetic models (3032). In addition Betamethasone to FcRn-mediated disposition, the reticuloendothelial system might play a role in phagocytosis and elimination of IgGs and their immuno-complexes (33). Smaller proteins, such as hepcidin, are often filtered by the kidney glomeruli and undergo tubular reabsorption and/or elimination (34). Due to the large molecular size of monoclonal antibodies, clearance of intact Ab 12B9m and hepcidinAb 12B9m complex through the kidney is negligible. Consequently, Ab 12B9m and Ab 12B9mhepcidin complex were thought to undergo two parallel elimination processes: (1) nonspecific distribution and elimination via the reticuloendothelial system and (2) FcRn-mediated endosomal recycling and degradation. However, the relative contribution of each clearance pathway was unknown. Therefore, one objective of this study was to characterize Betamethasone the pharmacokinetics of hepcidin and Ab 12B9m in cynomolgus monkeys,.