Drug concentration, binding, and effect in vivo

Process development of periplasmatically produced single chain fragment variable against epidermal growth factor receptor in Escherichia coli

Prokaryotic production systems have been widely used to manufacture recombinant therapeutic proteins. Economically, the prokaryotic production – especially of small therapeutic molecules – is advantageous compared to eukaryotic production strategies. However, due to the potential endotoxin and host cell protein contamination, the requirements for the purification process are disproportionately higher and therefore more expensive and elaborate to circumvent. For this reason, the goal of this work was to develop and establish a rapid, simple, inexpensive and ‘up-scalable’ production and purification process, using the therapeutic relevant protein anti-EGFR scFv hu225 as model molecule. Configuring high cell density cultivation of Escherichia coli – using the rha-BAD expression system as production platform – a specific product concentration up to 20 mgscFv/gCDW was obtained. By combining freeze-and-thaw, osmotic shock and pH induced host cell protein precipitation, almost 70% of the product was extracted from the biomass. In a novel approach a mixed mode chromatography was implemented as a capturing and desalting step, which allowed the direct application of further ion exchange chromatography steps for purification up to pharmaceutical grade. Thereby, 50% of the produced scFv could be purified within 10 h while maintaining the biological activity.

Pharmacodynamics-mediated drug disposition (PDMDD) and precursor pool lifespan model for single dose of romiplostim in healthy subjects

The objective of this study was to characterize the pharmacokinetics and pharmacodynamics (PK-PD) of romiplostim after single-dose administration in healthy subjects. The mean serum romiplostim concentrations (PK data) and mean platelet counts (PD data) collected from 32 subjects receiving a single intravenous (0.3, 1 and 10 μg/kg) or subcutaneous (0.1, 0.3, 1, and 2 μg/kg) dose were fitted simultaneously to a mechanistic PK-PD model based on pharmacodynamics-mediated drug disposition (PDMDD) and a precursor pool lifespan concept. The two-compartment PK model incorporated receptor-mediated endocytosis and linear mechanisms as parallel elimination pathways. The maximal concentration of receptors (assumed to be proportional to the platelet count), the equilibrium dissociation constant, and the first-order internalization rate constant for endocytosis of the drug-receptor complex were 0.022 fg/platelet, 0.131 ng/mL, and 0.173 h⁻¹, respectively. Romiplostim concentration stimulates the production of platelet precursors via the Hill function, where the SC₅₀ was 0.052 ng/mL and S (max) was 11.2. The estimated precursor cell and platelet lifespans were 5.9 and 10.5 days, respectively. Model-based simulations revealed that the romiplostim exposure and the platelet response are both dependent on the dose administered and the baseline platelet counts. Also, weekly dosing produced a sustained PD response while dosing intervals ≥2 weeks resulted in fluctuating platelet counts. Thus, the mechanistic PK-PD model was suitable for describing the romiplostim PK-PD interplay (PDMDD), the dose-dependent platelet stimulation, and the lifespans of thrombopoietic cell populations.

Pharmacodynamic modelling of the drug-induced downregulation of a beta 2-adrenoceptor mediated response and lack of restoration of receptor function after a single high dose of prednisone

Changes in beta 2-adrenoceptor function by chronic dosing of beta 2-mimetics and the possible influence of a single dose of prednisone have been studied as changes over time in the concentration-effect relationship of the beta 2-adrenoceptor agonist terbutaline. Hypokalaemia was used as the specific beta 2-adrenoceptor mediated effect. 8 healthy volunteers were given subcutaneous terbutaline 0.01 mg.kg-1 BW on 3 occasions over a 10-day experimental protocol: 1 Control experiment on Day 1; 2 After 7 days of oral terbutaline 5 mg t.i.d. (Day 8); and 3 After 8 days on oral terbutaline and 12 h after prednisone 100 mg orally (Day 10). The time course of the terbutaline concentrations and hypokalaemia was related using a pharmacokinetic-pharmacodynamic model. A sigmoid and a threshold Emax model were used to relate drug concentrations to effects. The oral terbutaline treatment caused a 35% increase in the distribution volume of SC terbutaline. After one week on oral terbutaline the concentration-effect relationship was shifted to the right and was steeper, with a higher EC50 of terbutaline and higher values for the apparent threshold concentration. These observations are compatible with a decrease in receptor numbers after 7 days of terbutaline in a system characterised by the presence of spare receptors. The data after prednisone pretreatment showed an apparent decline in the baseline plasma potassium concentrations that could be included in the Emax model. There was no change in the concentration-effect relationship 12 hours after prednisone.

Simultaneous modeling of the pharmacokinetic and pharmacodynamic properties of benzodiazepines. II. Triazolam

This study compares the time course of triazolam effects on psychomotor and cognitive skills with triazolam plasma concentrations in a combined pharmacokinetic-pharmacodynamic (sigmoid-Emax) model. Ten male subjects received a single oral dose (1 mg) of triazolam or placebo. The CNS impairment effects were measured by using computerized tracking, body sway, and digit symbol substitution tests, and triazolam plasma concentration was measured by gas chromatography. The drug-induced effect changes lagged behind the plasma drug level changes. The magnitude of the time lag was quantified by the half-time of equilibration between concentrations in the hypothetical effect compartment and the plasma triazolam levels (t 1/2 keo). Essentially the same t 1/2 keo (approximately 6 min) was found for subcritical tracking, body sway, and digit symbol substitution tests. When using the predicted drug concentrations at the effect site, the hysteresis of the plasma concentration-effect disappears, suggesting that the hysteresis is not caused by drug induced tolerance. Moreover, the model allows for estimation of the effect site concentration that causes one-half of the maximal predicted effect (EC50, approximately 5 ng/ml) which is a measure of an individual's sensitivity to triazolam. On the basis of the EC50 values of the effect measures, body sway was slightly less sensitive to triazolam than subcritical tracking and digit symbol substitution tests.

Pharmacokinetics and pharmacodynamics of thiazinamium in asthmatic patients

The pharmacokinetics and pharmacodynamics of thiazinamium (Multergan) were studied after intravenous and intramuscular administration to 7 males with chronic reversible airways obstruction. Disposition after i.v. administration was described by a clearance of 0.54 l.min-1, central compartment volume of 14.8 l, distribution rate constant 0.092 min-1, and an elimination rate constant of 0.0044 min-1. The corresponding estimates after i.m. administration were 0.324 l.min-1, 34.1 l, 0.035 min-1, and 0.0018 min-1. The bronchodilator response (expressed as % predicted FEV1) after i.v. administration was characterized by maximum increase in FEV1 of 33.9%, with an EC50 of 12.8 ng.ml-1 and an equilibration half-time of 11 min. Corresponding parameter estimates after i.m. administration were 32.2%, 18.8 ng.ml-1, and 9 min. Anticholinergic activity, measured by the change in heart rate after i.v. administration, showed maximum increase of 76 beats.min-1, with an EC50 of 176 ng.ml-1 and an equilibration half-time of 1.3 min. After i.m. administration the corresponding values were 120 beats.min-1, 250 ng.ml-1, and 3 min. The optimal plasma concentration of thiazinamium was about 100 ng.ml-1, which should give a near maximal bronchodilator response (over 80% of predicted normal) and a heart rate of about 100 beats.min-1.

Kinetic-effect models and their applications

This article focuses on mathematical models that analyze the time course of drug effects in humans. Any such model, whether parametric or nonparametric, is termed a kinetic-effect model (KEM). These models serve to describe (interpolation) and to predict (extrapolation) the effect-time profile. KEMs are applicable to many problems in pharmaceutics, pharmacology, and clinical pharmacology.