A comparison of nonlinear pharmacokinetics of erythropoietin in sheep and humans

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.

Target-mediated disposition population pharmacokinetics model of erythropoietin in premature neonates following multiple intravenous and subcutaneous dosing regimens

Routine erythropoietin (Epo) therapy for neonatal anemia is presently controversial due to its modest response. We speculate that an important contributor to this modest response is that previous clinical study designs were not driven by rigorous mechanistic and kinetic insights into the complex pharmacokinetics (PK) and pharmacodynamics (PD) of Epo in this population. To address this therapeutic opportunity, we conducted a prospective clinical study to investigate the PK of Epo in very-low-birth-weight (VLBW) premature neonates using a unique Epo dosing algorithm that accounts for complex neonatal erythropoietic physiology. Twenty-seven subjects received up to 10 intravenous or subcutaneous exogenous doses of Epo (600 or 1200 U/kg) during the first 4 weeks of life. Subjects were administered two doses of Epo 1200 U/kg on days 2 and 16, and eight doses of Epo 600 U/kg on days 4, 5, 6, 7, 9, 14, 15, and 28 following birth. We have developed for the first time a mechanistic, target-mediated disposition model that provides novel insights into the mechanisms driving Epo PK in VLBW neonates. Epo association rate, k_on, was estimated to be 0.00610 pM^-1h^-1, and the dissociation rate k_off was 0.112 h^-1. Internalization of the Epo-target complex (k_int) and the total receptor concentration (R_max) were estimated to be 0.118 h^-1 and 133 pM, respectively. Following s.c. administration, the absorption rate (k_a) of Epo was 0.0738h^-1 and bioavailability was 78.0%. Our mechanism-based population pharmacokinetic analysis provided quantitative insight into Epo kinetics in VLBW neonates; the information gained will assist in deriving dosing strategies for neonatal anemia and for neuroprotection efficacy studies.

High-dose erythropoietin population pharmacokinetics in neonates with hypoxic-ischemic encephalopathy receiving hypothermia

BACKGROUND

High-dose erythropoietin (Epo) is a promising neuroprotective treatment in neonates with hypoxic-ischemic encephalopathy (HIE) receiving hypothermia. We evaluated the pharmacokinetics and dose-exposure relationships of high-dose Epo in this population to inform future dosing strategies.

METHODS

We performed a population pharmacokinetic analysis of 47 neonates with HIE treated with hypothermia who received up to six doses of Epo in two previous clinical trials. We compared the ability of different dosing regimens to achieve the target neuroprotective Epo exposure levels determined from animal models of hypoxic-ischemia (i.e., area under the curve during the first 48 h of treatment (AUC_{48 h}) 140,000 mU*h/ml).

RESULTS

Birth weight scaled via allometry was a significant predictor of Epo clearance and volume of distribution (P < 0.001). After accounting for birth weight, variation in Epo pharmacokinetics between neonates was low (CV% 20%). All 23 neonates who received 1,000 U/kg every 24 h for the first 2 d of therapy achieved the target AUC_{48 h} 140,000 mU*h/ml. No neonate who received a lower dosing regimen achieved this target.

CONCLUSION

In neonates with HIE receiving hypothermia, Epo 1,000 U/kg every 24 h for the first 2 d of therapy resulted in consistent achievement of target exposures associated with neuroprotection in animal models.

Clinical pharmacokinetics and pharmacodynamics of erythropoiesis-stimulating agents

The cloning of the EPO gene in the early 1980s allowed for the development of recombinant erythropoietins and analogues [erythropoiesis-stimulating agents (ESAs)], offering an alternative to transfusion as a method of raising haemoglobin (Hb) levels, which have been used for more than 20 years to treat anaemia in millions of anaemic patients. There are now a number of ESAs available worldwide for the treatment of anaemia, approved for different routes of administration (intravenous and subcutaneous) and dosing intervals (three times weekly, weekly, biweekly and monthly). In this review, we discuss the pharmacokinetic characteristics, including absorption, distribution and elimination processes, across the different ESAs. Incomplete and slow lymphatic absorption, with limited extravascular distribution, and minor contributions of the target-mediated drug disposition to the overall elimination are the common characteristics across the marketed ESA. Additionally, we assess the similarities and differences of ESAs related to pharmacodynamics in the context of the different biomarkers used to monitor the magnitude and duration of the effect, and introduce the concept of the minimum effective concentration of the ESA. The relationship between the minimum effective concentration and the half-life suggests that the time during which drug concentrations are above the minimum effective concentration is the main determinant of ESA efficacy in increasing Hb levels. The tolerance phenomenon and its physiological mechanism and implications for ESA dosing are discussed. Finally, the areas of future clinical pharmacology research are envisioned.

Multidose optimization simulation of erythropoietin treatment in preterm infants

INTRODUCTION

Preterm infants commonly develop anemia requiring red blood cell transfusions (RBCTx). Although an alternative therapy is recombinant human erythropoietin (Epo), it is not widely employed. To provide a rigorous scientific basis supporting the latter approach, a model-based simulation analysis of endogenous erythropoiesis was developed.

RESULTS

The pharmacodynamic/pharmacokinetic (PK/PD) model identified an optimal Epo dosing algorithm in preterm infants that demonstrated maximal efficacy when Epo was dosed frequently during the early weeks of life (when phlebotomy loss is greatest). Model-based simulations employing optimized Epo dosing predicted that 13 of the 27 (46%) infants would avoid RBCTx ("good responders"). Importantly, simulation results identified five subject-specific covariate factors predictive of good Epo response.

DISCUSSION

This simulation study provides a basis for possibly eliminating RBCTx in infants who can be selected for optimized Epo therapy.

METHODS

Epo PD hemoglobin production parameters were determined in 27 preterm infants studied intensively during the first 28 d of life. Model-derived Epo PD parameters were combined with PK parameters derived from the literature to simulate an optimized intravenous Epo bolus dosing schedule. The goal of this simulated optimized schedule was to eliminate RBCTx, as prescribed per current guidelines, in as many preterm infants as possible.

Differential pharmacokinetic analysis of in vivo erythropoietin receptor interaction with erythropoietin and continuous erythropoietin receptor activator in sheep

The two erythropoiesis stimulating agents (ESAs), short acting recombinant human erythropoietin (EPO) and long acting continuous erythropoietin receptor activator (CERA), have been hypothesized to share an in vivo elimination pathway that involves binding to erythropoietin receptor (EPOR) and subsequent internalization. A physiologically based recirculation model and a pharmacokinetic tracer interaction methodology (TIM) were used to compare the in vivo interaction kinetics with EPOR between the two ESAs in adult sheep. Animals treated with EPO experienced a greater EPOR up-regulation than those treated with CERA, as evidenced by an eightfold-higher initial EPOR normalized production rate constant, k(syn) /R(0) , versus a twofold-larger EPOR degradation rate constant, k(deg) . In agreement with in vitro studies, EPO had a lower in vivo equilibrium dissociation constant from EPOR than CERA (K(D) = 6 versus 88.4 pmol/l, respectively, p < 0.01). The internalization and/or degradation of the EPO-EPOR complex was faster than that of the CERA-EPOR complex (k(int) = 24 versus 2.41 h(-1) , respectively, p < 0.01). The adopted model enables a mechanism-based explanation for CERA's slower elimination and greater erythropoietic activity in vivo. As predicted by the model, the slower elimination of CERA is due to: (1) less EPOR up-regulation induced by CERA administration; (2) slower binding of CERA to EPOR; and (3) reduced internalization and/or degradation rate of surface-bound CERA. Slower CERA/EPOR complex elimination explains the greater in vivo erythropoiesis reported for CERA, despite its lower affinity to EPOR. A sensitivity analysis showed that the model parameters were reliably estimated using the TIM methodology.

Receptor-based dosing optimization of erythropoietin in juvenile sheep after phlebotomy

The primary objective of this work was to determine the optimal time for administration of an erythropoietin (Epo) dose to maximize the erythropoietic effect using a simulation study based on a young sheep pharmacodynamic model. The dosing optimization was accomplished by extending a Hb production pharmacodynamic model, which evaluates the complex dynamic changes in the Epo receptor (EpoR) pool from the changes in Epo clearance. Fourteen healthy 2-month-old sheep were phlebotomized to Hb levels of 3 to 4 g/dl. Epo clearance was evaluated longitudinally in each animal by administering tracer doses of (125)I-recombinant human Epo multiple times during the experiment. Kinetic parameters were estimated by simultaneously fitting to Hb data and Epo clearance data. The phlebotomy caused a rapid temporary increase in the endogenous Epo plasma level. The Hb began to increase after the increased in the Epo level with a lag time of 1.13 ± 0.79 days. The average correlation coefficients for the fit of the model to the Hb and clearance data were 0.953 ± 0.018 and 0.876 ± 0.077, respectively. A simulation study was done in each sheep with fixed individual estimated model parameters to determine the optimal time to administer a 100 U/kg intravenous bolus Epo dose. The optimal dose administration time was 11.4 ± 6.2 days after phlebotomy. This study suggests that the Hb produced from Epo administration can be optimized by considering the dynamic changes in the EpoR pool.

Mechanisms of disease: the hypoxic tubular hypothesis of diabetic nephropathy

Diabetic nephropathy is traditionally considered to be a primarily glomerular disease, although this contention has recently been challenged. Early tubular injury has been reported in patients with diabetes mellitus whose glomerular function is intact. Chronic hypoxia of the tubulointerstitium has been recognized as a mechanism of progression that is common to many renal diseases. The hypoxic milieu in early-stage diabetic nephropathy is aggravated by manifestations of chronic hyperglycemia-abnormalities of red blood cells, oxidative stress, sympathetic denervation of the kidney due to autonomic neuropathy, and diabetes-mellitus-induced tubular apoptosis; as such, tubulointerstitial hypoxia in diabetes mellitus might be an important early event. Chronic hypoxia could have a dominant pathogenic role in diabetic nephropathy, not only in promoting progression but also during initiation of the condition. Early loss of tubular and peritubular cells reduces production of 1,25-dihydroxyvitamin D3 and erythropoietin, which, together with dysfunction of their receptors caused by the diabetic state, diminishes the local trophic effects of the hormones. This diminution could further compromise the functional and structural integrity of the parenchyma and contribute to the gradual decline of renal function.

Interspecies comparisons of pharmacokinetics and pharmacodynamics of recombinant human erythropoietin

Erythropoietin (EPO) has a highly conserved structure among mammals, and thus recombinant human EPO (rHuEPO) has biological activity in various species. This study explores the interspecies relationships of the pharmacokinetics (PK) and pharmacodynamics (PD) of rHuEPO. The PK parameters such as clearance (CL) and volume of distribution (V(ss)) after i.v. doses of rHuEPO were obtained in several species via noncompartmental analysis and were assessed using the traditional allometric approach. Also, PK/PD modeling of rHuEPO concentrations and responses [reticulocytes, red blood cells (RBCs), and hemoglobin] was performed following a range of i.v. and s.c. doses in rats, monkeys, and humans. Nonlinear disposition (V(max), K(m)) and s.c. absorption rate and bioavailability parameters of rHuEPO were examined. A cascade, indirect, lifespan PD model was applied to recover efficacy (S(max)) and potency (SC(50)) of rHuEPO on erythropoiesis and erythroid cell lifespan parameters. Despite nonlinear rHuEPO disposition, CL and V(ss) were highly correlated with body weight (R(2) > 0.92) with allometric scaling exponents of 0.708 for CL and 0.853 for V(ss). The s.c. bioavailability increased with dose in monkeys and humans but appeared to be dose-independent in rats. A correlation between S(max) or SC(50) and body weight was not obvious. However, RBC lifespans obeyed allometric principles. Size dependence was found for PK and lifespan parameters, whereas pharmacologic parameters were independent of body weight.

Pharmacokinetics of high-dose recombinant erythropoietin in plasma and brain of neonatal rats

Recombinant human erythropoietin (rEpo) is neuroprotective in neonatal models of brain injury. Pharmacokinetic data regarding the penetration of circulating rEpo into brain tissue is needed to optimize neuroprotective strategies. We sought to determine the pharmacokinetics of rEpo given intraperitoneally or subcutaneously in plasma and brain. We hypothesized that 1) exogenous rEpo would penetrate the blood-brain barrier (BBB), 2) brain and plasma Epo would correlate, and 3) brain injury would enhance rEpo penetration. Two hundred and eighty-four 7-d-old control, sham, or brain-injured rats were treated with i.p. or s.c. rEpo (0, 250, 2500, or 5000 U/kg) and killed at scheduled intervals. Plasma and brain tissue were collected. Epo concentrations were measured by ELISA. Intraperitoneal injection yielded a faster and greater peak concentration of plasma rEpo (Tmax 3 h, Cmax 10,016 +/- 685 mU/mL) than s.c. injection (Tmax 9 h, Cmax 6224 +/- 753 mU/mL). Endogenous brain Epo was below detection even after hypoxia exposure. Systemic rEpo crossed the BBB in a dose-dependent manner, peaked in brain at 10 h, and was increased after brain injury. We conclude that high-dose rEpo is detectable in brain for >20 h after a single systemic injection. These pharmacokinetic data are valuable for planning of rEpo neuroprotection experiments.

Increased erythropoietin elimination in fetal sheep following chronic phlebotomy

PURPOSE

To determine by pharmacokinetic (PK) means the role of erythropoietin-receptor (EPO-R) upregulation in fetuses on the elimination of erythropoietin (EPO).

MATERIALS AND METHODS

Six fetal sheep were catheterized at a gestational age of 125-127 days and phlebotomized daily for 6 days. Paired tracer PK studies using recombinant human EPO (rHuEPO) were conducted in the sheep fetuses at baseline and post-phlebotomy, 7 days later. A PK model with Michaelis-Menten elimination was simultaneously fit to the PK data at baseline and post-phlebotomy for each fetus.

RESULTS

Daily phlebotomies reduced the hemoglobin levels from baseline values of 10.8 (5%) (mean (C.V.)) g/dl to a nadir of 4.5 (17%) g/dl post-phlebotomy. The endogenous EPO concentration rapidly increased after the first phlebotomy and remained elevated, although variable, thereafter. The Michaelis-Menten maximal rHuEPO elimination rate parameter, V(max), was significantly greater post-phlebotomy than at baseline (p < 0.05), increasing 1.31 fold. The fetal baseline "linear" clearance at very low concentrations of rHuEPO was determined to be 117 ml/kg/h, similar to that determined in newborn sheep but 2-3 fold higher than that determined in adult sheep.

CONCLUSIONS

The observed increase in V(max) is consistent with an up-regulation of EPO-R due to a positive feedback resulting from the phlebotomy-induced anemia.

Population pharmacokinetics meta-analysis of recombinant human erythropoietin in healthy subjects

OBJECTIVE

The aim of this analysis was to develop a population pharmacokinetic model to describe the pharmacokinetics of recombinant human erythropoietin (rHuEPO) in healthy subjects, after intravenous and subcutaneous administration over a wide dose range, and to examine the influence of demographic characteristics and other covariates on the pharmacokinetic parameters of rHuEPO.

METHODS

Erythropoietin serum concentration data were available from 16 studies comprising 49 healthy subjects who received rHuEPO intravenous doses from 10 to 300 IU/kg, 427 healthy subjects who received rHuEPO subcutaneous doses from 1 to 2400 IU/kg, and 57 healthy subjects who received placebo and where endogenous erythropoietin concentrations were measured. Different pharmacokinetic models were fitted to the dataset using nonlinear mixed-effects modeling software (NONMEM, Version V, Level 1). Several patient covariates were tested in order to quantify the effect on rHuEPO pharmacokinetic parameters. Model evaluation was examined using a posterior predictive check.

RESULTS

Erythropoietin showed a diurnal baseline variation of +/-20%, described with a dual cosine model. Disposition was described with a two-compartment model with a small volume of distribution (6L) and parallel linear and nonlinear clearance. Total clearance varied between 0.3 and 0.9 L/h over the concentration range studied. A dual absorption model was used to characterise the rHuEPO absorption from the subcutaneous formulation and consisted of a faster pathway described as a sequential zero- and first-order absorption process and a parallel slower pathway characterised as a zero-order process. The bioavailability of subcutaneous rHuEPO increased from 30% at low doses to 71% at the highest dose of 160 kIU and was described using a hyperbolic model. The most important covariate effects were a decrease in the first-order absorption rate constant (k(a)) with increasing age, an increase in subcutaneous bioavailability with increasing baseline haemoglobin, and a decrease in bioavailability with increasing bodyweight. A posterior predictive check showed no systematic deviation of the simulated data from the observed values.

CONCLUSION

The population pharmacokinetic model developed is suitable to describe the pharmacokinetic behaviour of rHuEPO after intravenous and subcutaneous administration in healthy subjects, over a wide dose range.

Pharmacokinetic and pharmacodynamic modeling of recombinant human erythropoietin after intravenous and subcutaneous administration in rats

The pharmacokinetics (PK) and pharmacodynamics (PD) of recombinant human erythropoietin (rHuEPO) were studied in rats after single i.v. and s.c. administration at three dose levels (450, 1350, and 4050 IU/kg). The plasma concentrations of rHuEPO and its erythropoietic effects including reticulocyte (RET), red blood cell (RBC), and hemoglobin (Hb) levels were determined. A two-compartment model with dual input rate and nonlinear disposition was used to characterize the PK of rHuEPO. The catenary indirect response model with several compartments reflecting the bone marrow and circulating erythropoietic cells was applied. The s.c. doses exhibited slow absorption (T(max) = 12 h) and incomplete bioavailability (F = 0.59). In placebo groups, RBC and Hb values gradually increased over time with growth of the rats, and the changes in the baselines monitored from 8 to 32 weeks of age were described by a nonlinear growth function. All doses resulted in dose-dependent increases in RET counts followed by an immediate decline below the baseline at around 6 days and returned to the predose level in 21-24 days after dosing. A subsequent steady increase of RBC and Hb levels followed and reached peaks at 6 days. A tolerance phenomenon observed at all dose levels was modeled by a negative feedback inhibition with the relative change in Hb level. The PK/PD model well described the erythropoietic effects of rHuEPO as well as tolerance, thereby yielding important PD parameters (S(max) = 1.87 and SC(50) = 65.37 mIU/ml) and mean lifespans of major erythropoietic cell populations in rats.

The absorption of darbepoetin alfa occurs predominantly via the lymphatics following subcutaneous administration to sheep

PURPOSE

To determine the contribution of the lymphatics to the systemic availability of darbepoetin alfa (DA) using an established sheep model.

MATERIALS AND METHODS

DA was administered either by intravenous (IV) injection (0.2, 0.5 or 2 microg/kg) or by subcutaneous (SC) administration (2 microg/kg) into the interdigital space of the hind leg. A SC control group was used to determine the absolute bioavailability (F (sys)). Cannulation of the peripheral lymphatics in a parallel SC group allowed the continuous collection of lymph draining the injection site and determination of the cumulative amount of DA absorbed via the lymphatics. Serum and lymph concentrations of DA were determined by ELISA. The fraction of the dose absorbed into the lymphatics (F (lymph)) relative to the fraction absorbed directly into the blood (F (blood)) was determined using a compartmental approach.

RESULTS

Dose-linear pharmacokinetics was observed within the dose range investigated. The bioavailability was virtually complete following SC injection into the interdigital space (88.4 +/- 15.7%). A high proportion of the administered dose was recovered in peripheral lymph (90.2 +/- 4.4%) resulting in a substantial reduction in the systemic availability in lymph cannulated animals (3.7%).

CONCLUSION

The high recovery of DA in the peripheral lymph demonstrated near complete absorption of this recombinant protein via the lymphatics in a lymph cannulated sheep model.

Lymphatic Absorption Is the Primary Contributor to the Systemic Availability of Epoetin Alfa following Subcutaneous Administration to Sheep

The contribution of the lymphatics to the absorption and systemic availability of recombinant human epoetin alfa (rHuEPO) following s.c. injection was examined using a cannulated sheep model. Parallel studies were conducted in sheep where a single bolus dose was administered either by i.v. (10, 100, or 1000 IU/kg) or s.c. (400 IU/kg) injection. The first s.c. group served as a control for the calculation of absolute bioavailability. In the second group, the efferent popliteal lymphatic duct was cannulated and peripheral lymph draining the injection site was continuously collected. In the third group, the thoracic duct was cannulated to allow collection of central lymph just prior to entry into the systemic circulation. Blood was periodically sampled from all animals, and concentrations in serum and lymph were determined by enzyme-linked immunosorbent assay. The cumulative amount of rHuEPO recovered in peripheral and central lymph was 83.9 +/- 6.6% and 75.3 +/- 3.9% of the administered dose, respectively, indicating almost complete absorption from the s.c. injection site and minimal clearance during transit through the lymphatic system. After i.v. administration, the systemic clearance of rHuEPO decreased with increasing dose, reflecting capacity-limited elimination kinetics. A pharmacokinetic model was developed to simultaneously fit experimental data for all treatment groups and estimate bioavailability. The direct measurement of >75% of the dose in peripheral and central lymph independently verifies the calculated bioavailability of 87% and demonstrates the major role of the lymphatic route in the overall s.c. bioavailability of rHuEPO after s.c. administration with this animal model.

Recombinant human erythropoietin for the treatment of renal anaemia in children: no justification for bodyweight-adjusted dosage

BACKGROUND

Drug doses for children are usually calculated by reducing adult doses in proportion to bodyweight. The clinically effective dose of recombinant human erythropoietin (epoetin) in children, however, seems to be higher than predicted by this calculation.

OBJECTIVE

To determine the quantitative relationship between epoetin dose, bodyweight and response in children with end-stage renal disease.

PATIENTS AND METHODS

The time-course of haemoglobin in 52 children during long-term treatment with epoetin beta was analysed by population pharmacodynamic modelling. Patients were 5-20 years old and weighed 16-53kg at the beginning of treatment. Epoetin beta was given intravenously three times per week after haemodialysis. Doses ranged from 110 to 7500IU (3-205 IU/kg). Haemoglobin versus time was described by assuming that the haemoglobin level rises after each dose due to the formation of new red blood cells, which then survive according to a logistic function. The initial rise after each dose was modelled in terms of absolute dose (not dose/kg). A parametric analysis was done with NONMEM, followed by a nonparametric analysis with NPAG.

RESULTS

Dose-response was best described by a sigmoid maximum-effect (E(max)) model with median E(max) = 0.29 g/dL, median 50% effective dose (ED(50)) = 2400IU and shape parameter gamma = 2. The estimated median survival time of the epoetin-induced red blood cells, tau, was 76 days. Neither of the dose-response parameters E(max) and ED(50) showed dependence on bodyweight. The median haemoglobin response to a standard dose, 0.042 g/dL for 1000IU, was similar to that reported for adults with intravenous administration.

CONCLUSIONS

Doses for children in this age range should be specified as absolute amounts rather than amounts per unit bodyweight. Initial doses can be calculated individually, based on haemoglobin level before treatment, the desired haemoglobin at steady state and the median population parameters E(max), ED(50) and tau.

Pharmacokinetic/Pharmacodynamic Analysis of Paradoxal Regulation of Erythropoietin Production in Acute Anemia

The regulatory mechanism responsible for a paradoxal, rapid drop in the erythropoietin (EPO) plasma level seen 2 to 4 days after acute, phlebotomy-induced anemia was investigated in seven adult sheep. To introduce acute anemia, each sheep underwent two phlebotomies where the hemoglobin (Hb) was reduced to 3 or 4 g/dl over 4 to 5 h. The phlebotomies were spaced 4 to 6 weeks apart in three animals, and 8 days apart in four other animals. EPO plasma levels, reticulocyte count, Hb, and p50 for oxygen-Hb dissociation were determined from frequent blood samplings throughout the study period. EPO's disposition pharmacokinetic (PK) and plasma clearance were determined from i.v. bolus injections of tracer amounts of a recombinant human EPO tracer. The controlled drop in Hb resulted in a rapid increase in plasma EPO to 836 +/- 52 mU/ml (mean +/- coefficient of variation percentage) that was followed by a paradoxical rapid drop 2 to 4 days after the phlebotomy while the animals were still very anemic (Hb = 4.3 +/- 15 g/dl). The rapid drop in plasma EPO level could not be explained by the up-regulated clearance (clearance increased by a factor of less than 2.5) or by physiological adaptation (no change in p50, p > 0.05, second phlebotomy to Hb = 3g/dl inadequately stimulated the EPO production). The PK/pharmacodynamic (PD) analysis supports the hypothesis of a limited sustained high EPO production rate in acute anemia, which indicates an apparent deficiency in the regulation of EPO production in acute anemia. The hypothesis was supported by a PK/PD feedback inhibition model that showed good agreement with the data (r = 0.973 +/- 1.57).

Erythropoietin production rate in phlebotomy-induced acute anemia

OBJECTIVE

To estimate the rate of erythropoietin (EPO) production under physiological, conditions and to examine the regulatory mechanism of EPO production in response to acute phlebotomy-induced anemia.

METHODS

Six sheep each underwent two phlebotomies in which the hemoglobin (Hb) was reduced to 3-4 g/dl over 4-5 h. The EPO plasma level, reticulocytes, Hb and EPO clearance were followed by frequent blood sampling. The EPO production rate was determined by a semi-parametric method based on a disposition decomposition analysis that accounts for the nonlinear disposition kinetics of EPO and corrects for time-dependent changes in the clearance.

RESULTS

The controlled drop in hemoglobin resulted in an abrupt increase in the plasma EPO concentration (peak level 812+/-40 mU/ml, mean+/-CV%) that was followed by a rapid drop 2-4 days after the phlebotomy at a time when the sheep were still anemic (Hb=4.3+/-16 g/dl). The EPO production rate at baseline was 43+/-52 U/day/kg and the amounts of EPO produced over an 8 day period resulting from the first and second phlebotomy were 2927+/-40 U/kg and 3012+/-31 U/kg, respectively.

CONCLUSIONS

The rapid reduction in the EPO plasma level observed 2-4 days following the phlebotomy cannot be explained solely by the increase in EPO clearance but also by a reduction in EPO production.

Analysis Methods and Recent Advances in Nonlinear Pharmacokinetics from In Vitro through In Loci to In Vivo

An attempt has been made to review the nonlinearities in the disposition in vitro, in situ, in loci and in vivo mainly from a theoretical point of view. Parallel Michaelis-Menten and linear (first-order) eliminations are often observed in the cellular uptake, metabolism and efflux of drugs. The well-stirred and parallel-tube models are mainly adopted under steady-state conditions in perfusion experiments, whereas distribution, tank-in-series and dispersion models are often used under nonsteady-state conditions with a pulse input. The analysis of the nonlinear local disposition in loci is reviewed from two points of view, namely an indirect method involving physiologically based pharmacokinetics (PBPK) and a direct (two or three samplings) method using live animals. The nonlinear global pharmacokinetics in vivo is reviewed with regard to absorption, elimination (metabolism and excretion) and distribution.

A differential pharmacokinetic analysis of the erythropoietin receptor population in newborn and adult sheep

Strong evidence indicates that erythropoietin (Epo) is eliminated via Epo receptors (EpoR). Epo receptors may be classified as erythropoietic receptors that are largely located on erythroid progenitor cells in the bone marrow (BM) and nonerythropoietic receptors present in most tissues. Epo's elimination kinetics was studied using a very sensitive tracer interaction method (TIM) before and after chemical ablation of BM as an indirect way of evaluating the EpoR through an assortment of pharmacokinetic parameters (VM, KM, K, and CL) used in differentiating the EpoR population in newborn and adult sheep. TIM identified a parallel nonlinear Michaelis-Menten (VM and KM), and linear (K) elimination pathway and found the latter pathway to be significantly (p < 0.01) more dominant in lamb: K/(VM/KM + K) = 0.309 (25.3) versus 0.0895 (18.4) mean (CV%) lambs versus adult sheep. The significantly (p < 0.01) larger total clearance found for lambs indicates a larger nonhematopoietic tissue clearance of Epo (CL = 118 (10.9) ml/h/kg versus 67.8 (19.3) lamb versus adult sheep). The VM/KM ratio for the nonlinear pathway was not found to be significantly different (p > 0.05) between newborn and adults with values of 1.10 (15.8) and 1.30 (3.81) h-1, respectively. We proposed the hypothesis that the linear pathway is via nonhematopoietic EpoR. Assuming that Epo's elimination largely depends not only on erythropoietic EpoR but also on nonhematopoietic EpoR, this work shows a significant difference in the relative proportions of the two EpoR populations in lamb and adult sheep. The larger dominance of the nonhematopoietic EpoR in lamb supports the hypothesis that these receptors are more needed in early life, e.g., providing neuroprotection from perinatal hypoxemic-ischemic episodes.

The enigma of the metabolic fate of circulating erythropoietin (Epo) in view of the pharmacokinetics of the recombinant drugs rhEpo and NESP

Recombinant human erythropoietin (rhEpo) is a mainstay in the treatment of anaemia, primarily in renal failure. Because the half-life of circulating rhEpo is relatively short (4-8 h), the drug is usually administered 2-3 times weekly. Recently, a novel erythropoiesis-stimulating protein (NESP) with a longer half-life (24-26 h) has been approved. NESP possesses two additional N-glycans compared to endogenous Epo or rhEpo. The pharmacokinetics of rhEpo and NESP in humans have been investigated in detail. The composition of the N-glycans is clearly important in determining the biological activity and the velocity of the degradation of Epo and its analogues. However, due to the lack of knowledge of the main site and mechanism of the removal of Epo from circulation, the difference in survival of rhEpo and NESP has remained phenomenological. Investigators have implicated the liver, kidneys, and bone marrow as possible sites of the catabolism of Epo. However, while hepatocytes take up desialylated Epo, the liver does not appear to play a major role in the degradation of intact Epo. Likewise, renal Epo clearance is apparently of secondary importance. Studies showing non-linear pharmacokinetics of Epo suggest that Epo is eliminated by saturable mechanisms. The hormone, as well as the recombinant drugs, can be incorporated by erythrocytic progenitors and other tissues expressing the Epo receptor. The affinity of the Epo receptor for rhEpo is 4.3-fold higher than for NESP. Taken together, it seems most likely that native Epo, rhEpo and NESP are degraded following Epo receptor-mediated uptake, mainly in bone marrow.

Receptor-based model accounts for phlebotomy-induced changes in erythropoietin pharmacokinetics

Previous clinical studies have demonstrated two distinctive pharmacokinetic behaviors of erythropoietin (EPO): changes in pharmacokinetics (PK) after a period of rhEPO treatment and nonlinear pharmacokinetics. The objective of this work was to study the temporal changes in EPO's PK following phlebotomy in order to propose possible mechanisms for this behavior. Five healthy adult sheep were phlebotomized on two separate occasions 4-6 weeks apart to hemoglobin levels of PK 3-4 g/dL. PK parameters were estimated from the concentration-time profiles obtained following repeated intravenous bolus PK studies using tracer doses of biologically active 125I-rhEPO. Based on the changes in clearances, a PK model was derived to provide a mechanistic receptor-based description of the observed phenomena. Phlebotomy resulted in a rapid increase in the EPO plasma concentration, which peaked at 760 +/- 430 mU/mL (mean +/- SD) at 1.8 +/- 0.65 days, and which coincided with a transient reduction in EPO clearance from prephlebotomy values, i.e., from 45.6 +/- 11.2 mL/hr/kg to 24.3 +/- 9.7 mL/hr/kg. As plasma EPO levels returned toward baseline levels in the next few days, a subsequent increase in EPO clearance was noted. EPO clearance peaked at 90.2 +/- 26.2 mL/hr/kg at 8.5 +/- 3.3 days and returned to baseline by 4-5 weeks postphlebotomy. The proposed model derived from these data includes positive feedback control of the EPO receptor (EPOR) pool. The model predicts that: 1) the initial reduction in EPO plasma clearance is due to a transient saturation of EPORs resulting from the phlebotomy-induced high EPO concentration; and 2) the EPOR pool is expandable not only to compensate for EPOR loss but also to adjust to a greater need for EPORs/progenitor cells to restore hemoglobin (Hb) concentration to normal levels.