Michaelis-Menten metabolite formation kinetics: equations relating area under the curve and metabolite recovery to the administered dose

A computational approach which concomitantly determines the capacity-limited rate constants of parent drug elimination and metabolite formation is presented. The approach applies both the presently derived total excretory recovery versus dose relationships of the metabolite and the AUC versus dose relationships of the parent drug to identify the parameters. Three parent drug elimination conditions were assessed: pooled first-order, pooled Michaelis-Menten, and parallel first-order and pooled Michaelis-Menten kinetics. Model and parameter identification criteria are discussed. Literature data for theophylline and two of its metabolites in rats were examined to reveal pooled Michaelis-Menten elimination kinetics of theophylline and capacity-limited formation of the metabolites. The proposed technique is useful for quantitating commonly obtained nonlinear drug disposition data such as AUC and amount of metabolites excreted.     

Application of Moment Analysis to Nonlinear Drug Disposition Described by the Michaelis–Menten Equation

An equation for the mean residence time (MRT) of drug in the body is derived for the system where drug is injected intravenously into a one-compartment model and eliminated by a single, capacity-limited process. This MRT is a complex function of dose, volume, V _m, and K _m but degenerates into the classical volume/clearance expression under limiting low-dose conditions (K _m C ₀). The equation was validated by comparison of the MRT obtained by direct calculation versus numerical area estimation for simulated data. The equation may be useful analytically in the estimation of the fundamental Michaelis–Menten parameters, V _m and K _m, from experimental data.     

Effects of Acute and Chronic Inflammation on the Pharmacokinetics of Prednisolone in Rats

The effects of acute and chronic stages of carrageenan-induced air-pouch inflammation on the pharmacokinetics of prednisolone were studied in male Wistar rats. Chronic inflammation produced a significant increase in the area under the curve (AUC) of prednisolone compared to control animals (6594 ± 2144 vs 3530 ± 2164 µg · hr/ L). The effect of acute inflammation was not significant (AUC = 4996 ± 3813). Both acute and chronic inflammation also reduced thein vitro plasma protein binding of prednisolone, the reduction being much greater after chronic inflammation. The AUC of free prednisolone after chronic inflammation was 3141 µg · hr/L, compared to 1121 µg · hr/L in the control group and 1823 µg · hr/L after acute inflammation. The mean values of half-life and apparent volume of distribution at steady-state in each group were similar. These results indicate that prednisolone must be used with caution in the treatment of inflammatory diseases because of higher free concentrations of the steroid.     

Prenatal and Postnatal Serum PCB Concentrations and Cochlear Function in Children at 45 Months of Age

Background: Some experimental and human data suggest that exposure to polychlorinated biphenyls (PCBs) may induce ototoxicity, though results of previous epidemiologic studies are mixed and generally focus on either prenatal or postnatal PCB concentrations exclusively.Objectives: Our aim was to evaluate the association between pre- and postnatal PCB concentrations in relation to cochlear status, assessed by distortion product otoacoustic emissions (DPOAEs), and to further clarify the critical periods in development where cochlear status may be most susceptible to PCBs.Methods: A total of 351 children from a birth cohort in eastern Slovakia underwent otoacoustic testing at 45 months of age. Maternal pregnancy, cord, and child 6-, 16-, and 45-month blood samples were collected and analyzed for PCB concentrations. At 45 months of age, DPOAEs were assessed at 11 frequencies in both ears. Multivariate, generalized linear models were used to estimate the associations between PCB concentrations at different ages and DPOAEs, adjusting for potential confounders.Results: Maternal and cord PCB-153 concentrations were not associated with DPOAEs at 45 months. Higher postnatal PCB concentrations at 6-, 16-, and 45-months of age were associated with lower (poorer) DPOAE amplitudes. When all postnatal PCB exposures were considered as an area-under-the-curve metric, an increase in PCB-153 concentration from the 25th to the 75th percentile was associated with a 1.6-dB SPL (sound pressure level) decrease in DPOAE amplitude (95% CI: –2.6, –0.5; p = 0.003).Conclusions: In this study, postnatal rather than maternal or cord PCB concentrations were associated with poorer performance on otoacoustic tests at age 45 months.Citation: Jusko TA, Sisto R, Iosif AM, Moleti A, Wimmerová S, Lancz K, Tihányi J, Šovčíková E, Drobná B, Palkovičová L, Jurečková D, Thevenet-Morrison K, Verner MA, Sonneborn D, Hertz-Picciotto I, Trnovec T. 2014. Prenatal and postnatal serum PCB concentrations and cochlear function in children at 45 months of age. Environ Health Perspect 122:1246–1252; http://dx.doi.org/10.1289/ehp.1307473     

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.     

Dose dependent pharmacokinetics of prednisone and prednisolone in man

Six healthy male volunteers were given 5, 20, and 50 mg of oral prednisone and 5, 20, and 40 mg doses of intravenous prednisolone. Plasma and urine concentrations of prednisone and prednisolone were determined by HPLC, and the binding of prednisolone to plasma proteins was measured by radioisotopic and equilibrium dialysis techniques. The pharmacokinetics of both oral prednisone and intravenous prednisolone were dose-dependent. The mean oral dose plasma clearances of prednisone ranged from 572 ml/min/ 1.73 m ² for the 5mg dose to 2271 ml/min/1.73 m ² for the 50 mg dose. Changes in prednisone half-life were insignificant, but increases in the half-life of its metabolite were dose-dependent. The systemic plasma clearance of i.v. prednisolone was dose-dependent and increased from 111 to 194 ml/min/1.73 m ² over the 5 to 40 mg i.v. dosage range. The steady-state volume of distribution also increased, but little change in mean transit time and half-life was found. The binding of prednisolone to plasma proteins was markedly concentration-dependent, and a two compartment, nonlinear equation was used to characterize the effective binding of prednisolone to transcortin and albumin. The apparent pharmacokinetic parameters of protein-free and transcortin-free prednisolone were relatively constant with dose. The interconversion of prednisone and prednisolone varied with time and dose, although prednisolone concentrations dominated by 4-to 10-fold over prednisone. In urine, 2–5% of either administered drug was excreted as prednisone and 11–24% as prednisolone. The apparent renal clearances of both steroids were also nonlinear and unrelated to protein binding. These studies indicate that the pharmacokinetics of prednisone and prednisolone are dose-dependent and that protein binding does not fully explain their apparent nonlinear distribution and disposition.This work was supported in part by Grant 24211 from the National Institutes of General Medical Sciences, National Institutes of Health.