A Mechanism-Based Population Pharmacokinetics Model of Erythropoietin in Premature Infants and Healthy Adults Following Multiple Intravenous Doses

A Full Target-Mediated Drug Disposition (TMDD) Model to Explain the Changes in Recombinant Human Erythropoietin (rhEpo) Pharmacokinetics in Patients with Different Bone Marrow Integrity Following Hematopoietic Transplantation

Our aim was to build a mechanistic full target-mediated drug disposition (TMDD) model for rhEpo to better understand rhEpo disposition, Epo receptor (EpoR) synthesis, and degradation in hematopoietic transplant patients with four distinct bone marrow conditions. All PK data were analyzed simultaneously using the nonlinear mixed effect modeling approach with NONMEM. The final model was a two-compartmental full TMDD model, which adequately characterizes rhEpo PK in patients and provides insight into the dynamics of free EpoR, rhEpo-EpoR, and total EpoR. The model predicted association rate constant (kon), dissociation rate constant (koff), and internalization rate constant (kint) were 0.0276 pM-1h-1, 0.647 h-1, and 0.255h-1, respectively, which were supported by experimental data. Also, the EpoR degradation rate constant (kdeg) was estimated to be 0.461 h-1. EpoR production rate was estimated to be 37.5 pM/h for adults at pre-ablation baseline and 5.91 pM/h, and 4.19 pM/h in the early post-transplant post-engraftment, and late post-transplant full engraftment. Our model provides extensive information on the dynamics of free EpoR, total EpoR and rhEpo-EpoR, and proven to be more robust and can provide more physiologically relevant binding parameters than previous models.

The Utility of Pharmacometric Models in Clinical Pharmacology Research in Infants

Purpose of commentary

Acquiring knowledge on drug disposition and action in infant is challenging because of the problem of sparse and unbalanced data obtained for each individual infant due to the limited blood volume as well as the issue of extensive inter-subject and intra-subject variability in drug exposure and response due to the fast growth and dynamic maturation changes in infants. This commentary highlights the importance of using population-based pharmacometric models to improve knowledge on drug disposition and action in infants.

Recent findings

Pharmacometric modeling remains to be critical in clinical pharmacology research in infants. Many pediatric covariate models developed for scaling of drug clearance use a combination of allometric weight scaling to account for size change and a sigmoid function of antenatal development and postnatal maturation to characterize the age-related maturation. To expedite the development of safe and effective dosing regimens in infants, a number of strategies have been proposed recently, including the use of pediatric covariate model obtained from one drug for extrapolation to other drugs undergoing similar elimination pathways, as well as the combination of opportunistic clinical studies and population-based pharmacometrics models.

Summary

Population-based pharmacometric modeling plays a pivotal role in clinical pharmacology research in infants. Most of the covariate models reported so far focus on antibiotics undergoing renal elimination. Novel modeling strategies have been proposed recently to facilitate clinical pharmacology research and expedite the dose optimization process in infants.

The Effect of Size, Maturation, Global Asphyxia, Cerebral Ischemia, and Therapeutic Hypothermia on the Pharmacokinetics of High-Dose Recombinant Erythropoietin in Fetal Sheep

High-dose human recombinant erythropoietin (rEPO) is a promising potential neuroprotective treatment in preterm and full-term neonates with hypoxic-ischemic encephalopathy (HIE). There are limited data on the pharmacokinetics of high-dose rEPO in neonates. We examined the effects of body weight, gestation age, global asphyxia, cerebral ischemia, hypothermia and exogenous rEPO on the pharmacokinetics of high-dose rEPO in fetal sheep. Near-term fetal sheep on gestation day 129 (0.87 gestation) (full term 147 days) received sham-ischemia (n = 5) or cerebral ischemia for 30 min followed by treatment with vehicle (n = 4), rEPO (n = 8) or combined treatment with rEPO and hypothermia (n = 8). Preterm fetal sheep on gestation day 104 (0.7 gestation) received sham-asphyxia (n = 1) or complete umbilical cord occlusion for 25 min followed by i.v. infusion of vehicle (n = 8) or rEPO (n = 27) treatment. rEPO was given as a loading bolus, followed by a prolonged continuous infusion for 66 to 71.5 h in preterm and near-term fetuses. A further group of preterm fetal sheep received repeated bolus injections of rEPO (n = 8). The plasma concentrations of rEPO were best described by a pharmacokinetic model that included first-order and mixed-order elimination with linear maturation of elimination with gestation age. There were no detectable effects of therapeutic hypothermia, cerebral ischemia, global asphyxia or exogenous treatment on rEPO pharmacokinetics. The increase in rEPO elimination with gestation age suggests that to maintain target exposure levels during prolonged treatment, the dose of rEPO may have to be adjusted to match the increase in size and growth. These results are important for designing and understanding future studies of neuroprotection with high-dose rEPO.

Concept of Pharmacologic Target-Mediated Drug Disposition in Large-Molecule and Small-Molecule Compounds

Target-mediated drug disposition (TMDD) is a term to describe a nonlinear pharmacokinetic (PK) phenomenon that is caused by high-affinity binding of a compound to its pharmacologic targets. As the interaction between a drug and its pharmacologic target belongs to the process of pharmacodynamics (PD), TMDD can be viewed as a consequence of "PD affecting PK." Although there are numerous TMDD-related articles in the literature, most of them focus on characterizing TMDD using various mathematical models, which may not be suitable for those readers who have little interest in mathematical modeling and only want to have an understanding of the basic concept. The goal of this review is to serve as a "primer" on TMDD. This review explains (1) how TMDD happens; (2) why large-molecule and small-molecule compounds exhibiting TMDD demonstrate substantially different nonlinear PK behaviors; (3) what nonlinear PK profiles look like in large-molecule and small-molecule compounds exhibiting TMDD, using pegfilgrastim, erythropoietin, ABT-384, and linagliptin as case examples; and (4) how to identify whether the nonlinear PK of a compound is because of TMDD.