Pharmacokinetic differentiation of drug candidates using system analysis and physiological-based modelling. Comparison of C.E.R.A. and erythropoietin

Methoxy polyethylene glycol-epoetin beta for the treatment of anemia associated with chronic renal failure

Since more than two decades erythropoiesis-stimulating agents are the main pillar for treatment of anemia associated with chronic kidney disease. Methoxy polyethylene glycol-epoetin beta (MPG-EPO), also called continuous erythropoietin receptor activator, is the longest acting erythropoiesis-stimulating agent currently available. MPG-EPO is characterized by an elimination half-life of approximately 137 h and offers extended dosing intervals up to 4 weeks. Numerous phase I/II studies and a comprehensive clinical phase III program demonstrated the feasibility of MPG-EPO therapy for anemia correction and maintenance of stable hemoglobin levels in adult chronic kidney disease patients. Due to patent disputes MPG-EPO was only available outside the US market so far. In view of a prevailing US market introduction, this review focuses on efficacy and safety data from pivotal trials, summarizes recent clinical research and finally tries to substantiate potential benefits associated with the use of this anti-anemic drug.

Once-monthly continuous erythropoietin receptor activator (CERA) for haemoglobin maintenance in haemodialysis patients with chronic renal anaemia

Background

This study was conducted to evaluate the efficacy and safety of once-monthly continuous erythropoietin receptor activator (CERA) for maintenance of stable haemoglobin (Hb) levels in adult chronic renal anaemia patients on dialysis according to local clinical judgment in Turkey.

Methods

This was a prospective, open-label, single-arm, multi-centre study conducted in 20 centres in Turkey. After a 4-week screening period, eligible patients receiving conventional erythropoiesis-stimulating agents were converted to monthly intravenous CERA and entered a 16-week CERA dose-titration period (DTP) followed by an 8-week efficacy evaluation period (EEP) and a 4-week safety follow-up. The primary endpoint was the proportion of patients whose Hb concentration remained stable within ±1.0 g/dL of their reference Hb and within the range of 10.0–12.0 g/dL during the EEP.

Results

A total of 173 patients were screened, 132 entered the DTP and 84 completed the study. Thirty-nine patients [46.4% (95% confidence interval: 35.5–57.7%)] maintained stable target Hb concentrations. The mean change in time-adjusted average Hb concentration was 0.29 ± 1.08 g/dL between baseline and the EEP. The mean CERA monthly dose was 112.4 ± 76.78 µg during the EEP, and the CERA dose was adjusted in 39 patients (36.4%). Eleven patients (8.4%) reported 13 treatment-related adverse events, the most frequent adverse events being infections and infestations, gastrointestinal and vascular disorders.

Conclusions

Once-monthly CERA maintains stable Hb concentrations in chronic renal anaemia patients on dialysis in Turkey. The study results confirm the known efficacy and safety profile of CERA.

Applications of minimal physiologically-based pharmacokinetic models

Conventional mammillary models are frequently used for pharmacokinetic (PK) analysis when only blood or plasma data are available. Such models depend on the quality of the drug disposition data and have vague biological features. An alternative minimal-physiologically-based PK (minimal-PBPK) modeling approach is proposed which inherits and lumps major physiologic attributes from whole-body PBPK models. The body and model are represented as actual blood and tissue (usually total body weight) volumes, fractions (f ( d )) of cardiac output with Fick's Law of Perfusion, tissue/blood partitioning (K ( p )), and systemic or intrinsic clearance. Analyzing only blood or plasma concentrations versus time, the minimal-PBPK models parsimoniously generate physiologically-relevant PK parameters which are more easily interpreted than those from mammillary models. The minimal-PBPK models were applied to four types of therapeutic agents and conditions. The models well captured the human PK profiles of 22 selected beta-lactam antibiotics allowing comparison of fitted and calculated K ( p ) values. Adding a classical hepatic compartment with hepatic blood flow allowed joint fitting of oral and intravenous (IV) data for four hepatic elimination drugs (dihydrocodeine, verapamil, repaglinide, midazolam) providing separate estimates of hepatic intrinsic clearance, non-hepatic clearance, and pre-hepatic bioavailability. The basic model was integrated with allometric scaling principles to simultaneously describe moxifloxacin PK in five species with common K ( p ) and f ( d ) values. A basic model assigning clearance to the tissue compartment well characterized plasma concentrations of six monoclonal antibodies in human subjects, providing good concordance of predictions with expected tissue kinetics. The proposed minimal-PBPK modeling approach offers an alternative and more rational basis for assessing PK than compartmental models.

Pharmacokinetic analysis of continuous erythropoietin receptor activator disposition in adult sheep using a target-mediated, physiologic recirculation model and a tracer interaction methodology

The pharmacokinetics (PK) of continuous erythropoietin receptor activator (CERA), a PEGylated erythropoietin (EPO) derivative, was studied in sheep after bone marrow (BM) busulfan ablation by using a receptor-based recirculation model and tracer interaction method (TIM) experiments. The nontracer CERA component of the TIM was analyzed using a noncompartmental approach. In contrast to EPO elimination that is linear after the BM ablation, CERA elimination remains nonlinear. After busulfan treatment, initial EPO receptors (EPOR) normalized production rate constant, EPOR degradation rate constant, and CERA-EPOR complex internalization rate constant decreased (p < 0.01), whereas no change in CERA/EPOR equilibrium dissociation constant was detected (p > 0.05). After BM ablation, noncompartmental analysis showed that CERA-PK parameters underwent 1) a decrease in plasma clearance (p < 0.01); 2) a concomitant increase in elimination half-life and mean residence time; and 3) no significant change in volume of distribution, distribution half-life, or distributional clearance (p > 0.05). These results suggest that CERA elimination is mediated through saturable hematopoietic and nonhematopoietic EPOR pathways, with possible contribution of another EPOR-independent pathway(s). Compared with the nonhematopoietic EPOR population, the hematopoietic receptors have similar affinity to CERA but are significantly more involved in CERA's in vivo elimination. The saturable nature of the nonerythropoietic, non-BM pathway(s) for CERA in contrast to EPO predicts two fundamental differences: 1) an increasing fraction of CERA is used for erythropoiesis for increasing concentrations; and 2) the clearance of CERA becomes more limited for increasing concentrations. Taken together, these differences favor a more efficacious and prolonged action for CERA.

Pharmacodynamic effects of haematopoietic cytokines: the view of a clinical oncologist

The production of haematopoietic cells is under the tight control of a group of haematopoietic cytokines. Each cytokine has multiple actions mediated by receptors whose cytoplasmic domains contain specialized regions initiating survival, proliferation, differentiation commitment, maturation and functional activation. Granulocyte colony-stimulating factor (G-CSF), erythropoietin (EPO), and thrombopoiesis-stimulating agents are in routine clinical use to stimulate cell production and in total have been used in the management of many millions of patients. G-CSF regulates neutrophil production to maintain blood neutrophil counts in the normal range. G-CSF is used to prevent febrile neutropenia or to increase dose-density in chemotherapy regimens. Despite consistently showing a shorter duration of neutropenia, multiple prospective randomized trials have documented only modest clinical benefit. A clinical advantage of dose-dense chemotherapy has been shown only in specific chemotherapy regimens. Professional recommendations tailor the use of CSFs to patients with a high risk of adverse outcome of febrile neutropenia. EPO was used to prevent anaemia requiring red blood cell transfusion. However, recent studies strongly suggest a negative overall effect on mortality, without a plausible biological explanation. It is now proposed that its use should be restricted to patients in clinical trials. Thrombopoiesis-stimulating agents have only been recently introduced into the market for splenectomized and non-splenectomized patients with immune thrombocytopenic purpura, a rare disease. Before widely used in other conditions such as chemotherapy-induced thrombocytopenia, the lessons learned from the example of G-CSF and EPO should be taken seriously.

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.