A Mechanism Based Pharmacokinetic/Pharmacodynamic Model of the Effect of a Prolyl Hydroxylase Inhibitor on Erythropoietic Response In Mice

The oxygen sensing pathway is one of the major controlling mechanisms of erythropoiesis in humans and other mammals. The transcription factor, Hypoxia-Inducible-Factor (HIF) regulates the local gene expression of the hormone erythropoietin (EPO). Degradation of HIF1α is regulated via oxygen dependent pathway by Prolyl Hydroxylases (PHDs) enzymes and the inhibition of PHD enzyme activity stimulates the erythropoietic response. The purpose of this study was to develop a mechanistic Pharmacokinetic/Pharmacodynamic (PK/PD) model to describe the effect of a Prolyl Hydroxylase Inhibitor (PHI) on the erythropoietic process in mice.

PHI was given orally at three single doses of 10, 30 and 100mg/kg to male BALB/C mice (N=3-6, per time point). The blood plasma and kidney tissue samples were collected upto 96hrs. The PD markers were: kidney HIF1α concentration, kidney EPO mRNA expression, EPO plasma concentration and reticulocyte count. A two compartment model with first order absorption was fitted to describe the PK in plasma. Two different dose dependent apparent volume of distribution was assumed for the central (plasma) compartment at lower and higher doses. For the PD model, under quassi steady state assumption the enzymatic hydroxylation leading to degradation of HIF1α by PHDs is assumed to be an irreversible reaction. The drug present in plasma stimulates the removal of PHDs which results in accumulation of HIF1α and further EPO stimulation. This stimulation of kidney EPO mRNA, EPO protein level and reticulocytes were described by a cascade of indirect response models. ADAPT5 was used for all model fittings.

At higher PHI dose levels, ~30-40 fold increase in HIF1α was observed which caused ~1000 fold increase in EPO mRNA and EPO plasma concentrations within 6–8 hrs whereas the reticulocytes level increased by 3–4 fold within 3–5 days of single dose administration. Model parameters such as PHI elimination rate (kel, 0.43hr^-1), the dose dependent apparent volume of distribution (V/F, 2–8L), the drug potency (IC50, 0.68μ M) for the degradation of PHDs; rate of loss (kout, 0.26hr^-1) of HIF1α; the depletion rate of EPO mRNA (kd1, 0.48hr^-1), plasma EPO (kd2, 0.25hr^-1); and half maximal effective concentration (EC50, 7.01 ng/mL) of EPO for reticulocyte stimulation were estimated with high precision levels. Overall, the PK/PD model adaquetly captured the experimental data for all three doses.

The erythropoietic process follows a cascade of biological events which is well described by the presented PK/PD model. Our model uses the systems biology approach to vertically integrate the intracellular HIF1α and EPO mRNA responses with in vivo markers of erythropoiesis such as EPO plasma levels and reticulocyte counts.

No relevant conflicts of interest to declare.