Does nonlinear blood-brain barrier transport matter for (lower) morphine dosing strategies?

Morphine blood-brain barrier (BBB) transport is governed by passive diffusion, active efflux and saturable active influx. This may result in nonlinear plasma concentration-dependent brain extracellular fluid (brainECF) pharmacokinetics of morphine. In this study, we aim to evaluate the impact of nonlinear BBB transport on brainECF pharmacokinetics of morphine and its metabolites for different dosing strategies using a physiologically based pharmacokinetic simulation study. We extended the human physiologically based pharmacokinetic LeiCNS-PK3.0, model with equations for nonlinear BBB transport of morphine. Simulations for brainECF pharmacokinetics were performed for various dosing strategies: intravenous (IV), oral immediate (IR) and extended release (ER) with dose range of 0.25-150 mg and dosing frequencies of 1-6 times daily. The impact of nonlinear BBB transport on morphine CNS pharmacokinetics was evaluated by quantifying (i) the relative brainECF to plasma exposure (AUCu,brainECF/AUCu,plasma) and (ii) the impact on the peak-to-trough ratio (PTR) of concentration-time profiles in brainECF and plasma. We found that the relative morphine exposure and PTRs are dose dependent for the evaluated dose range. The highest relative morphine exposure value of 1.4 was found for once daily 0.25 mg ER and lowest of 0.1 for 6-daily 150 mg IV dosing. At lower doses the PTRs were smaller and increased with increasing dose and stabilized at higher doses independent of dosing frequency. Relative peak concentrations of morphine in relation to its metabolites changed with increasing dose. We conclude that nonlinearity of morphine BBB transport affects the relative brainECF exposure and the fluctuation of morphine and its metabolites mainly at lower dosing regimens.

Improving the Prediction of Local Drug Distribution Profiles in the Brain with a New 2D Mathematical Model.

The development of drugs that target the brain is very challenging. A quantitative understanding is needed of the complex processes that govern the concentration-time profile of a drug (pharmacokinetics) within the brain. So far, there are no studies on predicting the drug concentration within the brain that focus not only on the transport of drugs to the brain through the blood-brain barrier (BBB), but also on drug transport and binding within the brain. Here, we develop a new model for a 2D square brain tissue unit, consisting of brain extracellular fluid (ECF) that is surrounded by the brain capillaries. We describe the change in free drug concentration within the brain ECF, by a partial differential equation (PDE). To include drug binding, we couple this PDE to two ordinary differential equations that describe the concentration-time profile of drug bound to specific as well as non-specific binding sites that we assume to be evenly distributed over the brain ECF. The model boundary conditions reflect how free drug enters and leaves the brain ECF by passing the BBB, located at the level of the brain capillaries. We study the influence of parameter values for BBB permeability, brain ECF bulk flow, drug diffusion through the brain ECF and drug binding kinetics, on the concentration-time profiles of free and bound drug.

P-glycoprotein protein expression versus functionality at the blood-brain barrier using immunohistochemistry, microdialysis and mathematical modeling.

A proper understanding of P-gp mediated transport (functionality) at the blood-brain barrier (BBB) and beyond is needed to interpret, understand and predict pharmacokinetic (PK)- pharmacodynamic (PD) relationships of CNS drugs that are substrates of P-gp, especially since P-gp functionality may be different in different conditions. Often, P-gp expression is taken as a biomarker of transporter functionality. The aim of our study was to investigate whether brain capillary protein expression of P-gp is associated with changes in P-gp mediated drug efflux at the BBB. Status Epilepticus (SE) was induced by kainate in male rats. During 3-5 weeks post SE, hippocampal P-gp expression was determined using immunohistochemistry, while BBB P-gp functionality was assessed by microdialysis of quinidine, in absence and presence of the P-gp blocker tariquidar. The data were analyzed using Non-linear Mixed Effect Modeling implemented in NONMEM. Following SE, changes in brain capillary P-gp expression were observed. However, no relation between BBB P-gp protein expression and BBB P-gp mediated drug efflux was found. This warrants a critical view on the interpretation of reported changes in BBB P-gp expression as a biomarker of BBB P-gp functionality.

Bundling arrows: improving translational CNS drug development by integrated PK/PD-metabolomics.

INTRODUCTION: Diseases of the Central Nervous System (CNS) affect millions of people worldwide, with the number of people affected quickly growing. Unfortunately, the successful development of CNS-acting drugs is less than 10%, and this is attributed to the complexity of the CNS, unexpected side effects, difficulties in penetrating the blood-brain barrier and lack of biomarkers. Areas covered: Herein, the authors first review how pharmacokinetic/pharmacodynamic (PK/PD) models are designed to predict the dose-dependent time course of effect, and how they are used to translate drug effects from animal to man. Then, the authors discuss how pharmacometabolomics gives insight into system-wide pharmacological effects and why it is a promising method to study interspecies differences. Finally, the authors advocate the application of PK/PD-metabolomics modeling to advance translational CNS drug development by discussing its opportunities and challenges. Expert opinion: It is envisioned that PK/PD-metabolomics will increase our understanding of CNS drug effects and improve translational CNS drug development, thereby increasing success rates. The successful future development of this concept will require multi-level and longitudinal biomarker evaluation over a large dose range, multi-tissue biomarker evaluation, and the generation of a proof of principle by application to multiple CNS drugs in multiple species.

Analgosedation in paediatric severe traumatic brain injury (TBI): practice, pitfalls and possibilities.

Analgosedation is a fundamental part of traumatic brain injury (TBI) treatment guidelines, encompassing both first and second tier supportive strategies. Worldwide analgosedation practices continue to be heterogeneous due to the low level of evidence in treatment guidelines (level III) and the choice of analgosedative drugs is made by the treating clinician. Current practice is thus empirical and may result in unfavourable (often hemodynamic) side effects. This article presents an overview of current analgosedation practices in the paediatric intensive care unit (PICU) and addresses pitfalls both in the short and long term. We discuss innovative (pre-)clinical research that can provide the framework for initiatives to improve our pharmacological understanding of analgesic and sedative drugs used in paediatric severe TBI and ultimately facilitate steps towards evidence-based and precision pharmacotherapy in this vulnerable patient group.

Multivariate pharmacokinetic/pharmacodynamic (PKPD) analysis with metabolomics shows multiple effects of remoxipride in rats.

The study of central nervous system (CNS) pharmacology is limited by a lack of drug effect biomarkers. Pharmacometabolomics is a promising new tool to identify multiple molecular responses upon drug treatment. However, the pharmacodynamics is typically not evaluated in metabolomics studies, although being important properties of biomarkers. In this study we integrated pharmacometabolomics with pharmacokinetic/pharmacodynamic (PKPD) modeling to identify and quantify the multiple endogenous metabolite dose-response relations for the dopamine D2 antagonist remoxipride. Remoxipride (vehicle, 0.7 or 3.5mg/kg) was administered to rats. Endogenous metabolites were analyzed in plasma using a biogenic amine platform and PKPD models were derived for each single metabolite. These models were clustered on basis of proximity between their PKPD parameter estimates, and PKPD models were subsequently fitted for the individual clusters. Finally, the metabolites were evaluated for being significantly affected by remoxipride. In total 44 metabolites were detected in plasma, many of them showing a dose dependent decrease from baseline. We identified 6 different clusters with different time and dose dependent responses and 18 metabolites were revealed as potential biomarker. The glycine, serine and threonine pathway was associated with remoxipride pharmacology, as well as the brain uptake of the dopamine and serotonin precursors. This is the first time that pharmacometabolomics and PKPD modeling were integrated. The resulting PKPD cluster model described diverse pharmacometabolomics responses and provided a further understanding of remoxipride pharmacodynamics. Future research should focus on the simultaneous pharmacometabolomics analysis in brain and plasma to increase the interpretability of these responses.

The influence of drug distribution and drug-target binding on target occupancy: The rate-limiting step approximation.

The influence of drug-target binding kinetics on target occupancy can be influenced by drug distribution and diffusion around the target, often referred to as "rebinding" or "diffusion-limited binding". This gives rise to a decreased decline of the drug-target complex concentration as a result of a locally higher drug concentration that arises around the target, which leads to prolonged target exposure to the drug. This phenomenon has been approximated by the steady-state approximation, assuming a steady-state concentration around the target. Recently, a rate-limiting step approximation of drug distribution and drug-target binding has been published. However, a comparison between both approaches has not been made so far. In this study, the rate-limiting step approximation has been rewritten into the same mathematical format as the steady-state approximation in order to compare the performance of both approaches for the investigation of the influence of drug-target binding kinetics on target occupancy. While both approximations clearly indicated the importance of kon and high target concentrations, it was shown that the rate-limiting step approximation is more accurate than the steady-state approximation, especially when dissociation is fast compared to association and distribution out of the binding compartment. It is therefore concluded that the new rate-limiting step approximation is to be preferred for assessing the influence of binding kinetics on local target site concentrations and target occupancy.

Translational aspects of blood-brain barrier transport and central nervous system effects of drugs: from discovery to patients.

The development of CNS drugs is associated with high failure rates. It is postulated that too much focus has been put on BBB permeability and too little on understanding BBB transport, which is the main limiting factor in drug delivery to the brain. An integrated approach to collecting, understanding, and handling pharmacokinetic-pharmacodynamic information from early discovery stages to the clinic is therefore recommended in order to improve translation to human drug treatment.

Application of ED-optimality to screening experiments for analgesic compounds in an experimental model of neuropathic pain.

In spite of the evidence regarding high variability in the response to evoked pain, little attention has been paid to its impact on the screening of drugs for inflammatory and neuropathic pain. In this study, we explore the feasibility of introducing optimality concepts to experimental protocols, enabling estimation of parameter and model uncertainty. Pharmacokinetic (PK) and pharmacodynamic data from different experiments in rats were pooled and modelled using nonlinear mixed effects modelling. Pain data on gabapentin and placebo-treated animals were generated in the complete Freund's adjuvant model of neuropathic pain. A logistic regression model was applied to optimise sampling times and dose levels to be used in an experimental protocol. Drug potency (EC(50)) and interindividual variability (IIV) were considered the parameters of interest. Different experimental designs were tested and validated by SSE (stochastic simulation and estimation) taking into account relevant exposure ranges. The pharmacokinetics of gabapentin was described by a two-compartment PK model with first order absorption (CL = 0.159 l h(-1), V(2) = 0.118 l, V(3) = 0.253 l, Ka = 0.26 h(-1), Q = 1.22 l h(-1)). Drug potency (EC(50)) for the anti-allodynic effects was estimated to be 1400 ng ml(-1). Protocol optimisation improved bias and precision of the EC50 by 6 and 11.9. %, respectively, whilst IIV estimates showed improvement of 31.89 and 14.91 %, respectively. Our results show that variability in behavioural models of evoked pain response leads to uncertainty in drug potency estimates, with potential impact on the ranking of compounds during screening. As illustrated for gabapentin, ED-optimality concepts enable analysis of discrete data taking into account experimental constraints.

Effect of altered AGP plasma binding on heart rate changes by S(-)-propranolol in rats using mechanism-based estimations of in vivo receptor affinity (K(B,vivo)).

In contrast to the impact of plasma protein binding on pharmacokinetics, no quantitative in vivo information is available on its impact on pharmacodynamics. The pharmacokinetic-pharmacodynamic relationship of the model drug S(-)-propranolol was evaluated using mechanism-based estimations of in vivo receptor affinity (K(B,vivo)), under conditions of altered plasma protein binding resulting from different levels of alpha-1-acid glycoprotein (AGP). Male Wistar Kyoto rats with isoprenaline-induced tachycardia received an intravenous infusion of S(-)-propranolol, on postsurgery day 2 (n = 7) and day 7 (n = 8) with elevated and normal plasma protein binding, respectively. Serial blood samples were taken in parallel to heart rate measurements. AGP concentrations at 2 and 7 days postsurgery were 708 +/- 274 and 176 +/- 111 microg/mL (mean +/- SE), respectively. Using nonlinear mixed effects modeling, AGP concentration was a covariate for intercompartmental clearance for the third compartment of the pharmacokinetic model of S(-)-propranolol. Individual values of AGP concentrations ranged between 110 and 1150 microg/mL, and were associated with K(B,vivo) values of S(-)-propranolol from 7.0 to 30 nM. Using the K(B,vivo) for S(-)-propranolol with correction for average values for normal and elevated plasma protein binding, nearly identical values were found. This confirms, strictly quantitative, earlier indications that plasma protein binding restricts the pharmacodynamics of S(-)-propranolol.

Influence of plasma protein binding on pharmacodynamics: Estimation of in vivo receptor affinities of beta blockers using a new mechanism-based PK-PD modelling approach.

The objective of this investigation was to examine in a systematic manner the influence of plasma protein binding on in vivo pharmacodynamics. Comparative pharmacokinetic-pharmacodynamic studies with four beta blockers were performed in conscious rats, using heart rate under isoprenaline-induced tachycardia as a pharmacodynamic endpoint. A recently proposed mechanism-based agonist-antagonist interaction model was used to obtain in vivo estimates of receptor affinities (K(B,vivo)). These values were compared with in vitro affinities (K(B,vitro)) on the basis of both total and free drug concentrations. For the total drug concentrations, the K(B,vivo) estimates were 26, 13, 6.5 and 0.89 nM for S(-)-atenolol, S(-)-propranolol, S(-)-metoprolol and timolol. The K(B,vivo) estimates on the basis of the free concentrations were 25, 2.0, 5.2 and 0.56 nM, respectively. The K(B,vivo)-K(B,vitro) correlation for total drug concentrations clearly deviated from the line of identity, especially for the most highly bound drug S(-)-propranolol (ratio K(B,vivo)/K(B,vitro) approximately 6.8). For the free drug, the correlation approximated the line of identity. Using this model, for beta-blockers the free plasma concentration appears to be the best predictor of in vivo pharmacodynamics.

Influence of biophase distribution and P-glycoprotein interaction on pharmacokinetic-pharmacodynamic modelling of the effects of morphine on the EEG.

BACKGROUND AND PURPOSE: The aim was to investigate the influence of biophase distribution including P-glycoprotein (Pgp) function on the pharmacokinetic-pharmacodynamic correlations of morphine's actions in rat brain. EXPERIMENTAL APPROACH: Male rats received a 10-min infusion of morphine as 4 mg kg(-1), combined with a continuous infusion of the Pgp inhibitor GF120918 or vehicle, 10 or 40 mg kg(-1). EEG signals were recorded continuously and blood samples were collected. KEY RESULTS: Profound hysteresis was observed between morphine blood concentrations and effects on the EEG. Only the termination of the EEG effect was influenced by GF120918. Biophase distribution was best described with an extended catenary biophase distribution model, with a sequential transfer and effect compartment. The rate constant for transport through the transfer compartment (k(1e)) was 0.038 min(-1), being unaffected by GF120918. In contrast, the rate constant for the loss from the effect compartment (k(eo)) decreased 60% after GF120918. The EEG effect was directly related to concentrations in the effect compartment using the sigmoidal E(max) model. The values of the pharmacodynamic parameters E(0), E(max), EC(50) and Hill factor were 45.0 microV, 44.5 microV, 451 ng ml(-1) and 2.3, respectively. CONCLUSIONS AND IMPLICATIONS: The effects of GF120918 on the distribution kinetics of morphine in the effect compartment were consistent with the distribution in brain extracellular fluid (ECF) as estimated by intracerebral microdialysis. However, the time-course of morphine concentrations at the site of action in the brain, as deduced from the biophase model, is distinctly different from the brain ECF concentrations.

Population pharmacokinetic modelling of non-linear brain distribution of morphine: influence of active saturable influx and P-glycoprotein mediated efflux.

BACKGROUND AND PURPOSE: Biophase equilibration must be considered to gain insight into the mechanisms underlying the pharmacokinetic-pharmacodynamic (PK-PD) correlations of opioids. The objective was to characterise in a quantitative manner the non-linear distribution kinetics of morphine in brain. EXPERIMENTAL APPROACH: Male rats received a 10-min infusion of 4 mg kg(-1) of morphine, combined with a continuous infusion of the P-glycoprotein (Pgp) inhibitor GF120918 or vehicle, or 40 mg kg(-1) morphine alone. Unbound extracellular fluid (ECF) concentrations obtained by intracerebral microdialysis and total blood concentrations were analysed using a population modelling approach. KEY RESULTS: Blood pharmacokinetics of morphine was best described with a three-compartment model and was not influenced by GF120918. Non-linear distribution kinetics in brain ECF was observed with increasing dose. A one compartment distribution model was developed, with separate expressions for passive diffusion, active saturable influx and active efflux by Pgp. The passive diffusion rate constant was 0.0014 min(-1). The active efflux rate constant decreased from 0.0195 min(-1) to 0.0113 min(-1) in the presence of GF120918. The active influx was insensitive to GF120918 and had a maximum transport (N(max)/V(ecf)) of 0.66 ng min(-1) ml(-1) and was saturated at low concentrations of morphine (C(50)=9.9 ng ml(-1)). CONCLUSIONS AND IMPLICATIONS: Brain distribution of morphine is determined by three factors: limited passive diffusion; active efflux, reduced by 42% by Pgp inhibition; low capacity active uptake. This implies blood concentration-dependency and sensitivity to drug-drug interactions. These factors should be taken into account in further investigations on PK-PD correlations of morphine.

Pharmacokinetic-pharmacodynamic modelling of S(-)-atenolol in rats: reduction of isoprenaline-induced tachycardia as a continuous pharmacodynamic endpoint.

BACKGROUND AND PURPOSE: For development of mechanism-based pharmacokinetic-pharmacodynamic (PK-PD) models, continuous recording of drug effects is essential. We therefore explored the use of isoprenaline in the continuous measurement of the cardiovascular effects of antagonists of beta-adrenoceptors (beta-blockers). The aim was to validate heart rate as a pharmacodynamic endpoint under continuous isoprenaline-induced tachycardia by means of PK-PD modelling of S(-)-atenolol. EXPERIMENTAL APPROACH: Groups of WKY rats received a 15 min i.v. infusion of 5 mg kg(-1) S(-)-atenolol, with or without i.v. infusion of 5 microg kg(-1) h(-1) isoprenaline. Heart rate was continuously monitored and blood samples were taken. KEY RESULTS: A three-compartment model best described the pharmacokinetics of S(-)-atenolol. The PK-PD relationship was described by a sigmoid Emax model and an effect compartment was used to resolve the observed hysteresis. In the group without isoprenaline, the variability in heart rate (30 b.p.m.) approximated the maximal effect (Emax=43+/-18 b.p.m.), leaving the parameter estimate of potency (EC50=28+/-27 ng ml(-1)) unreliable. Both precise and reliable parameter estimates were obtained during isoprenaline-induced tachycardia: 517+/-13 b.p.m. (E0), 168+/-15 b.p.m. (Emax), 49+/-14 ng ml(-1) (EC50), 0.042+/-0.012 min(-1) (k(eo)) and 0.95+/-0.34 (n). CONCLUSIONS AND IMPLICATIONS: Reduction of heart rate during isoprenaline-induced tachycardia is a reliable pharmacodynamic endpoint for beta-blockers in vivo in rats. Consequently this experimental approach will be used to investigate the relationship between drug characteristics and in vivo effects of different beta-blockers.

The role of the efflux transporter P-glycoprotein in brain penetration of prednisolone.

In the present study, we have investigated the role of the multidrug resistance (mdr) P-glycoprotein (Pgp) at the blood-brain barrier in hampering the access of the synthetic glucocorticoid, prednisolone. In vivo, a tracer dose of [(3)H]prednisolone poorly penetrated the brain of adrenalectomised wild-type mice, but the uptake was more than threefold enhanced in the absence of Pgp expression in mdr1a (-/-) mice. In vitro, in stably transfected LLC-PK1 monolayers the human MDR1 P-glycoprotein was able to transport prednisolone present at a micromolar concentration. A specific Pgp blocker, LY 335979, could block this polar transport of [(3)H]prednisolone. Human Pgp does not transport all steroids, as cortexolone was not transported at all and aldosterone was only weakly transported. The ability of Pgp to export the synthetic glucocorticoid, prednisolone, suggests that uptake of prednisolone in the human brain is impaired, leading to a discrepancy between central and peripheral actions. Furthermore, the ensuing imbalance in activation of the two types of brain corticosteroid receptors may have consequences for cognitive performance and mood.