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

CLEC-2 stimulates IGF-1 secretion from podoplanin-positive stromal cells and positively regulates erythropoiesis in mice

BACKGROUND

Erythropoiesis is a complex multistep process by which erythrocytes are produced. C-type lectin-like receptor 2 (CLEC-2) is a podoplanin (PDPN) receptor almost exclusively expressed on the surface of platelets and megakaryocytes. Deletion of megakaryocyte/platelet CLEC-2 was reported to cause anemia along with thrombocytopenia in mice. PDPN-expressing stromal cells in the bone marrow (BM) were also reported to facilitate megakaryocyte expansion and maturation depending on the CLEC-2/PDPN interaction.

OBJECTIVES

We investigated how specific deletion of CLEC-2 in megakaryocytes/platelets leads to anemia.

METHODS

We used flow cytometry to analyze maturation of erythroblasts, apoptotic cell death, and cell cycle distribution. CLEC-2 stimulated PDPN-expressing stromal cell-conditioned medium was analyzed by cytokine array and ELISA, and co-cultured with immature erythroblasts. Cytokine levels in serum and BM extracellular fluid were quantified by ELISA.

RESULTS

We observed increased apoptosis of BM erythroblasts in megakaryocyte/platelet-specific CLEC-2 conditional knockout (Clec1bΔPLT ) mice. Moreover, PDPN-expressing stromal cells in the BM secreted insulin-like growth factor 1 (IGF-1) depending on the CLEC-2/PDPN interaction. Pretreatment with IGF-1 receptor inhibitor increased apoptosis rate and decreased the proliferation of erythroblasts in vitro. Furthermore, in Clec1bΔPLT mice, IGF-1 concentrations in serum and BM extracellular fluid were decreased, and IGF-1 replacement in Clec1bΔPLT mice attenuated anemia.

CONCLUSIONS

Our findings suggest that IGF-1 secretion from PDPN-expressing stromal cells by CLEC-2 stimulation positively regulates erythroblasts. This novel mechanism of erythropoiesis regulation indicates that a microenvironment consisting of megakaryocytes and PDPN-expressing stromal cells supports erythropoiesis.

Semi-mechanistic Pharmacokinetic/Pharmacodynamic model of three pegylated rHuEPO and ior®EPOCIM in New Zealand rabbits

Marketed formulations of erythropoietin (EPO) ior®EPOCIM, MIRCERA® and two newly developed pegylated-EPO analogues (PEG-EPO 32 and 40 kDa) formulations were intravenously administered to New Zealand rabbits. A semi-mechanistic Pharmacokinetic/Pharmacodynamic (PK/PD) model describing in a simultaneous and integrated form the time course of reticulocytes, red blood cells and hemoglobin was built to account for the time course of hematopoiesis stimulation after erythropoietin administration. Data analysis was performed based on the population approach with the software NONMEM version 7.3. Erythropoietin disposition of each of the administered formulations was best described with a two compartment model and linear elimination. Different formulations show different clearance and apparent volume of distribution of the central compartment but share estimates of inter-compartmental clearance and apparent peripheral volume of distribution. A semi-mechanistic model including cell proliferation, maturation, and homeostatic regulation provided a good description of the data regardless the type of erythropoietin formulation administered. The system-, and drug-related parameters showed consistency and differed across formulations, respectively. A single IV administration of PEG-EPO 32 and 40 kDa formulations in New Zealand rabbits achieves a median change of 27% and 22% on RET levels, and of 47% and 63% on RBC and HGB levels, respectively compared to MIRCERA®. The administration of new branched PEG-chains formulations improves PK and PD properties of EPO, in terms of increasing elimination half-lives and pharmacological activity on RET, RBC and HGB compared to commercially available formulations (ior®EPOCIM and MIRCERA®).

Population pharmacodynamic analysis of erythropoiesis in preterm infants for determining the anemia treatment potential of erythropoietin

A population pharmacokinetics/pharmacodynamic (PK/PD) model was developed to describe changes in erythropoiesis as a function of plasma erythropoietin (EPO) concentration over the first 30 days of life in preterm infants who developed severe anemia requiring red blood cell (RBC) transfusion. Several covariates were tested as possible factors influencing the responsiveness to EPO. Discarded blood samples in 27 ventilated preterm infants born at 24-29 wk of gestation were used to construct plasma EPO, hemoglobin (Hb), and RBC concentration-time profiles. The amount of Hb removed for laboratory testing and that transfused throughout the study period were recorded. A population PK/PD model accounting for the dynamic Hb changes experienced by these infants was simultaneously fitted to plasma EPO, Hb, and RBC concentrations. A covariate analysis suggested that the erythropoietic efficacy of EPO is increased for preterm infants at later gestational ages. The PD analysis showed a sevenfold difference in maximum Hb production rate dependent on gestational age and indicated that preterm infants, when stimulated by EPO, have the capacity to produce additional Hb that may result in a decrease in RBC transfusions. The present model has utility in clinical trial simulations investigating the treatment potential of erythropoietic stimulating agents in the treatment of anemia of prematurity.

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.

Pharmacodynamic analysis of stress erythropoiesis: change in erythropoietin receptor pool size following double phlebotomies in sheep

A feedback receptor regulation model was incorporated into a pharmacodynamic model to describe the stimulation of hemoglobin (Hb) production by endogenous erythropoietin (EPO). The model considers the dynamic changes that take place in the EPO receptor (EPOR) pool under phlebotomy-induced anemia. Using a (125)I-rhEPO tracer the EPO clearance changes are evaluated longitudinally prior to and following phlebotomy-induced anemia indirectly to evaluate changes in the EPOR pool size, which has been shown to be linearly related to the clearance. The proposed model simultaneously captures the general behavior of temporal changes in Hb relative to EPO plasma clearance in five lambs (r = 0.95), while accounting for the confounding variables of phlebotomy and changes in the blood volume in the growing animals. The results indicate that under anemia the EPOR pool size is up-regulated by a factor of nearly two over baseline and that the lowest and highest EPOR pool sizes differ by a factor of approximately four. The kinetic model developed and the data-driven mechanism proposed serves as a starting point for developing an optimal EPO dosing algorithm for the treatment of neonatal anemia.

Selection between Michaelis-Menten and target-mediated drug disposition pharmacokinetic models

Target-mediated drug disposition (TMDD) models have been applied to describe the pharmacokinetics of drugs whose distribution and/or clearance are affected by its target due to high binding affinity and limited capacity. The Michaelis-Menten (M-M) model has also been frequently used to describe the pharmacokinetics of such drugs. The purpose of this study is to investigate conditions for equivalence between M-M and TMDD pharmacokinetic models and provide guidelines for selection between these two approaches. Theoretical derivations were used to determine conditions under which M-M and TMDD pharmacokinetic models are equivalent. Computer simulations and model fitting were conducted to demonstrate these conditions. Typical M-M and TMDD profiles were simulated based on literature data for an anti-CD4 monoclonal antibody (TRX1) and phenytoin administered intravenously. Both models were fitted to data and goodness of fit criteria were evaluated for model selection. A case study of recombinant human erythropoietin was conducted to qualify results. A rapid binding TMDD model is equivalent to the M-M model if total target density R ( tot ) is constant, and R ( tot ) K ( D ) /(K ( D ) + C) ( 2 ) << 1 where K ( D ) represents the dissociation constant and C is the free drug concentration. Under these conditions, M-M parameters are defined as: V ( max ) = k ( int ) R ( tot ) V ( c ) and K ( m ) = K ( D ) where k ( int ) represents an internalization rate constant, and V ( c ) is the volume of the central compartment. R ( tot ) is constant if and only if k ( int ) = k ( deg,) where k ( deg ) is a degradation rate constant. If the TMDD model predictions are not sensitive to k ( int ) or k ( deg ) parameters, the condition of R ( tot ) K ( D ) /(K ( D ) + C) ( 2 ) << 1 alone can preserve the equivalence between rapid binding TMDD and M-M models. The model selection process for drugs that exhibit TMDD should involve a full mechanistic model as well as reduced models. The best model should adequately describe the data and have a minimal set of parameters estimated with acceptable precision.

The effect of severe hepatic impairment on the pharmacokinetics and haematological response of C.E.R.A

AIM

To examine the effect of severe hepatic impairment (HI) on the pharmacokinetics (PK) and pharmacodynamics (PD) of the continuous erythropoietin receptor activator, C.E.R.A.

METHODS

A non-randomised, multicentre, single-dose, open-label study in patients with HI (n=12) and healthy subjects (n=12). After 2 weeks of screening, participants received a single intravenous dose of C.E.R.A. (200 mug), and were then followed for approximately 8 weeks. The area under the concentration-time curve (AUC) from drug administration to last measurable concentration (AUC(last)), and maximum C.E.R.A. concentration (C(max)) were calculated to assess PK. The baseline-corrected area under the effect curve over 22 days (AUE(corr)) for reticulocyte count was the primary PD parameter.

RESULTS

The PK profile was similar in patients and healthy subjects (AUC(last): 6678 vs 6985 ng*h/mL; C(max): 63 vs 75 ng/mL) C.E.R.A. produced a sustained erythropoietic response in bothgroups, with increases in reticulocyte counts peaking 7-9 days post-dose and returning to baseline by Day 22. Although mean AUE(corr) was 64% lower in patients, this may have been an artefact of higher baseline reticulocyte counts. Lower reticulocyte responses in patients did not translate into lower responses for haemoglobin, haematocrit or erythrocytes, suggesting that HI had no clinically relevant effect on the PD of C.E.R.A. C.E.R.A. was well tolerated. Four AEs (none considered drug related) were reported in three patients (mild myocardial ischaemia; mild pyrexia and liver transplant; severe bacterial peritonitis [serious AE]); no AEs were reported in healthy subjects.

CONCLUSIONS

Severe HI has no clinically relevant effect on PK parameters or haematological response after single-dose C.E.R.A.

Erythropoietin measurements in severely traumatized patients

BACKGROUND

Despite numerous studies in critically ill patients, physiological adaptation to acute anaemia and the pattern of erythropoietin (EPO) secretion has not been well described in severely injured patients. The aim of this study was to describe EPO secretion and its relationship with haemoglobin (Hb) levels in severely injured patients.

METHODS

We performed an observational, prospective clinical study in our intensive care unit (ICU). For all patients with severe trauma (Injury Severity Score>15), EPO measurement was obtained on admission, during the first 3 days and then when Hb level was measured. Maximal EPO level (EPOmax) and minimal Hb level (Hbmin) during the ICU stay was determined for all patients.

RESULTS

One hundred and seventy-one consecutives patients were included (440 EPO measurements). Seventy-nine patients (46.2%) showed an increased value (> or =25 UI/l) EPOmax value. Most EPOmax values were observed early after the trauma [within 4 days for 63 patients (82.8%)]. Plotting EPOmax to Hbmin values show that a threshold Hbmin value of 105 g/l best discriminated patients with and without an elevated EPO secretion. Less than 10% of the patients with Hbmin<105 g/l did not increase their EPO secretion.

CONCLUSION

In severely traumatized patients a marked response to acute anaemia is observed in most patients. In our study, Hb threshold for a significant EPO secretion following post-traumatic acute anaemia was 105 g/l. The peak level was achieved early in the course of the anaemia.

Mechanism of increased mortality risk with erythropoietin treatment to higher hemoglobin targets

Recent randomized, controlled trials indicate that there is a strong trend for increased risk for death or adverse composite outcomes with erythropoiesis-stimulating agent treatment in kidney disease to hemoglobin targets higher than those currently recommended. The failure of these trials to find a benefit of higher hemoglobin is in stark contrast to findings from large, observational, population-based studies that continue to demonstrate the association of low hemoglobin with adverse outcomes. The mechanisms for the adverse effect of higher hemoglobin targets that are seen in the randomized, controlled trials are poorly understood. This review explores hypotheses involving (1) the effect of achieved hemoglobin itself, (2) the role of erythropoiesis-stimulating agent treatment, (3) the use of iron supplementation, (4) increased blood pressure, and (5) erythropoiesis-stimulating agent hyporesponsiveness. Because the causal pathway has yet to be determined, further research is strongly encouraged. Clinical practice, however, should avoid erythropoiesis-stimulating agent treatment to higher hemoglobin targets, particularly in those with significant cardiovascular morbidity and those who require disproportionately high dosages of erythropoietin-stimulating agents to achieve recommended hemoglobin levels.

A 'bottom-up' approach for endo-PK/PD analysis

A 'bottom-up' PK/PD analysis approach employing system analysis principles of convolution/deconvolution and special nonparametric estimation procedures is presented to resolve the complex 'endo-PK/PD' of the endogenous form of recombinant drugs using erythropoietin (EPO) as an example. A novel cellular deconvolution algorithm is presented that facilitates the identification of the functional relationship between the variables involved in EPO's complex PK/PD. Five sheep each underwent two phlebotomies spaced 4-6 weeks apart when their hemoglobin levels were reduced from 12 g/dl to 3-4 g/dl. EPO levels and reticulocyte counts were frequently sampled. The data were analysed using end-constrained cubic splines. The rate of reticulocyte production was determined using the novel deconvolution methodology. The erythroid progenitor cells activation rate by EPO was estimated from the reticulocyte production rate using a lag-time parameter which determines the delay in the reticulocyte appearance in the blood relative to the activation of erythroid progenitors. Hysteresis minimization combined with cellular deconvolution was employed to determine the population PK/PD transduction function relating the progenitor activation rate to EPO concentrations in a nonparametric manner without assuming a specific structure. The proposed approach provides a rational informative starting point for developing parametric PK/PD models to resolve the complex endo-PK/PD of recombinant drugs.

Pharmacodynamic model for chemotherapy-induced anemia in rats

Anticancer agents often cause bone marrow toxicity resulting in progressive anemia which may influence the therapeutic effects of erythropoietic-stimulating agents. The objective of this study was to develop a pharmacodynamic (PD) model to describe chemotherapy-induced anemia in rats. Anemia was induced in male Wistar rats with a single intravenous (i.v.) injection of 60 mg/kg carboplatin. Hematological responses including reticulocytes, red blood cells (RBC), hemoglobin, and endogenous rat erythropoietin (EPO) were measured for up to 4 weeks. A catenary, lifespan-based, indirect response model served as a basic PD model to represent erythroid cellular populations in the bone marrow and blood involved in erythropoiesis. The model assumed that actively proliferating progenitor cells in the bone marrow are sensitive to anti-cancer agents and subject to an irreversible removal process. The removal rate of the target cells is proportional to drug activity concentrations and the cell numbers. An additional RBC loss from the circulation resulting from thrombocytopenia was described by a first-order process. The turnover process of rat EPO and EPO-mediated feedback inhibition mechanism regulated by hemoglobin changes were incorporated. Reticulocyte counts decreased rapidly and reached a nadir by day 3 after administration of carboplatin and returned to the baseline by day 13. This was followed by a gradual increase and the rebound peak occurred at about day 15. The hemoglobin nadir was approximately 9 g/dl observed at about 11-13 days compared to its normal value of 13 g/dl and hemoglobin returned to the baseline by day 30. The increase in endogenous rat EPO mirrored inversely hemoglobin changes and the maximum increase was observed soon after the hemoglobin nadir. The carboplatin-treated rats exhibited progressive anemia. The proposed model adequately described the time course of hematological changes after carboplatin in rats and can be a useful tool to explore potential strategies for the management of anemia caused by chemotherapy.

Pharmacodynamic model of interleukin-21 effects on red blood cells in cynomolgus monkeys

Interleukin-21 (IL-21) is a novel cytokine that is currently under clinical investigations as a potential anti-cancer agent. Like many other anti-cancer agents, including other interleukins, IL-21 is seen to produce a broad range of biological effects that may be related to both efficacy and safety of treatment. The present analysis investigates the observed pharmacodynamics effects on red blood cells following various treatment schedules of human IL-21 administrated to cynomolgus monkeys. These effects are described by a novel non-linear mixed-effects model that enabled separation of drug effects and sampling effects, the latter believed to be due partly to blood loss and partly to stress induced haemolysis in connection with blood sampling. Two different studies with a total of 9 different treatment groups of cynomolgus monkeys were used for model development. In conclusion, the model describes the IL-21 induced drop in red blood cells to be (1) caused by removal rather than suppression of production, consistent with increased reticulocyte concentration, and (2) considerably delayed compared to dosing, i.e. not related to the drop in red blood cells observed immediately post dose. It is believed that the structural model presented here can be used for other types of drug induced loss of red blood cells, whereas the mechanism for sampling related blood loss is relevant for investigations of anaemia in all pharmacological studies with smaller animals.

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.

Pharmacodynamic Analysis of Changes in Reticulocyte Subtype Distribution in Phlebotomy-Induced Stress Erythropoiesis

Changes in the reticulocyte subtype distribution (high, medium and low reticulocytes count (HR, MR, LR)) measured by flow cytometry following phlebotomy-induced stress erythropoiesis (abruptly dropping hemoglobin to 3-4 g/dl over 4-5 hr) and the pharmacodynamic (PD) relationship to the stimulated erythropoietin (EPO) was investigated in sheep. A PD model was developed that describes the relationship between EPO and the reticulocyte maturity distribution fractions (r=0.95+/-0.02, mean +/- SD). The lag-time between EPO activation of erythroid progenitor cells and the subsequent increase in the least mature HR fraction in the peripheral circulation was 0.72 +/- 0.08 days. The mean transition times (in days) for all three reticulocyte fractions changed at baseline from, T(HR) : 0.09 +/- 0.06, T(MR) : 0.06 +/- 0.04, and T(LR) : 0.46 +/- 0.24 to T(HR) : 0.13 +/- 0.08, T(MR) : 0.29 +/- 0.15, and T(LR) : 2.3 +/- 0.24 under stress erythropoiesis. The total mean residence time for a reticulocyte in the peripheral circulation, T(total) (T(HR) + T(MR) + T(LR)), increased from 0.60 +/- 0.33 days under basal to 2.8 +/- 0.09 days during stress erythropoiesis. The statistically significant increase observed for T(LR) and T(total) supports the hypothesis that stress erythropoiesis perturbs the mean reticulocyte transition times. A correlation analysis between various new, proposed metrics involving the HR, MR and LR fractions and the total reticulocyte count, with the latter indicative of stress erythropoiesis at higher total counts, revealed a highly significant correlation indicating these new metrics may be a valuable adjunct to the reticulocyte maturation index (RMI) and the immature reticulocyte fractions index (IRF) previously used in assessing erythropoietic activity in response to anemia.

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.