Population Pharmacokinetic Analysis for Simultaneous Determination of B max and K D In Vivo by Positron Emission Tomography

Pharmacokinetic-pharmacodynamic models that incorporate drug-target binding kinetics

Pharmacokinetic/pharmacodynamic (PK/PD) models predict the effect time course resulting from a drug dose. In this review, we summarize the development of mechanistic PK/PD models that explicitly integrate the kinetics of drug-target interactions into predictions of drug activity. Such mechanistic models are expected to have several advantages over approaches in which concentration and effect are linked using variations of the Hill equation, and where preclinical data are often used as a starting point for modeling drug activity. Instead, explicit use of the full kinetic scheme for drug binding enables time-dependent changes in target occupancy to be calculated using the kinetics of drug-target interactions and drug PK, providing a more precise picture of target engagement and drug action in the non-equilibrium environment of the human body. The mechanistic PK/PD models also generate target vulnerability functions that link target occupancy and effect, and inform on the sensitivity of a target to engagement by a drug. Key factors such as the rate of target turnover can also be integrated into the modeling which, together with target vulnerability, provide additional information on the PK profile required to achieve the desired pharmacological effect and on the utility of kinetic selectivity in developing drugs for specific targets.

Modelling the delay between pharmacokinetics and EEG effects of morphine in rats: binding kinetic versus effect compartment models

Drug–target binding kinetics (as determined by association and dissociation rate constants, k_on and k_off) can be an important determinant of the kinetics of drug action. However, the effect compartment model is used most frequently instead of a target binding model to describe hysteresis. Here we investigate when the drug–target binding model should be used in lieu of the effect compartment model. The utility of the effect compartment (EC), the target binding kinetics (TB) and the combined effect compartment–target binding kinetics (EC–TB) model were tested on either plasma (EC_PL, TB_PL and EC–TB_PL) or brain extracellular fluid (ECF) (EC_ECF, TB_ECF and EC–TB_ECF) morphine concentrations and EEG amplitude in rats. It was also analyzed when a significant shift in the time to maximal target occupancy (Tmax_TO) with increasing dose, the discriminating feature between the TB and EC model, occurs in the TB model. All TB models assumed a linear relationship between target occupancy and drug effect on the EEG amplitude. All three model types performed similarly in describing the morphine pharmacodynamics data, although the EC model provided the best statistical result. The analysis of the shift in Tmax_TO (∆Tmax_TO) as a result of increasing dose revealed that ∆Tmax_TO is decreasing towards zero if the k_off is much smaller than the elimination rate constant or if the target concentration is larger than the initial morphine concentration. The results for the morphine PKPD modelling and the analysis of ∆Tmax_TO indicate that the EC and TB models do not necessarily lead to different drug effect versus time curves for different doses if a delay between drug concentrations and drug effect (hysteresis) is described. Drawing mechanistic conclusions from successfully fitting one of these two models should therefore be avoided. Since the TB model can be informed by in vitro measurements of k_on and k_off, a target binding model should be considered more often for mechanistic modelling purposes.

Development of a population pharmacokinetic model to predict brain distribution and dopamine D2 receptor occupancy of raclopride in non-anesthetized rat

BACKGROUND

Raclopride is a selective antagonist of the dopamine D2 receptor. It is one of the most frequently used in vivo D2 tracers (at low doses) for assessing drug-induced receptor occupancy (RO) in animals and humans. It is also commonly used as a pharmacological blocker (at high doses) to occupy the available D2 receptors and antagonize the action of dopamine or drugs on D2 in preclinical studies. The aims of this study were to comprehensively evaluate its pharmacokinetic (PK) profiles in different brain compartments and to establish a PK-RO model that could predict the brain distribution and RO of raclopride in the freely moving rat using a LC-MS based approach.

METHODS

Rats (n=24) received a 10-min IV infusion of non-radiolabeled raclopride (1.61μmol/kg, i.e. 0.56mg/kg). Plasma and the brain tissues of striatum (with high density of D2 receptors) and cerebellum (with negligible amount of D2 receptors) were collected. Additional microdialysis experiments were performed in some rats (n=7) to measure the free drug concentration in the extracellular fluid of the striatum and cerebellum. Raclopride concentrations in all samples were analyzed by LC-MS. A population PK-RO model was constructed in NONMEM to describe the concentration-time profiles in the unbound plasma, brain extracellular fluid and brain tissue compartments and to estimate the RO based on raclopride-D2 receptor binding kinetics.

RESULTS

In plasma raclopride showed a rapid distribution phase followed by a slower elimination phase. The striatum tissue concentrations were consistently higher than that of cerebellum tissue throughout the whole experimental period (10-h) due to higher non-specific tissue binding and D2 receptor binding in the striatum. Model-based simulations accurately predicted the literature data on rat plasma PK, brain tissue PK and D2 RO at different time points after intravenous or subcutaneous administration of raclopride at tracer dose (RO <10%), sub-pharmacological dose (RO 10%-30%) and pharmacological dose (RO >30%).

CONCLUSION

For the first time a predictive model that could describe the quantitative in vivo relationship between dose, PK and D2 RO of raclopride in non-anesthetized rat was established. The PK-RO model could facilitate the selection of optimal dose and dosing time when raclopride is used as tracer or as pharmacological blocker in various rat studies. The LC-MS based approach, which doses and quantifies a non-radiolabeled tracer, could be useful in evaluating the systemic disposition and brain kinetics of tracers.

Small-Molecule Compounds Exhibiting Target-Mediated Drug Disposition (TMDD): A Minireview

Nonlinearities are commonplace in pharmacokinetics, and 1 special source is the saturable binding of the drug to a high-affinity, low-capacity target, a phenomenon known as target-mediated drug disposition (TMDD). Compared with large-molecule compounds undergoing TMDD, which has been well recognized due to its high prevalence, TMDD in small-molecule compounds is more counterintuitive and has not been well appreciated. With more and more potent small-molecule drugs acting on highly specific targets being developed as well as increasingly sensitive analytical techniques becoming available, many small-molecule compounds have recently been reported to have nonlinear pharmacokinetics imparted by TMDD. To expand our current knowledge of TMDD in small-molecule compounds and increase the awareness of this clinically important phenomenon, this minireview provides an overview of the small-molecule compounds that demonstrate nonlinear pharmacokinetics imparted by TMDD. The present review also summarizes the general features of TMDD in small-molecule compounds and highlights the differences between TMDD in small-molecule compounds and large-molecule compounds.

A positron emission tomography study in healthy volunteers to estimate mGluR5 receptor occupancy of AZD2066 - estimating occupancy in the absence of a reference region

AZD2066 is a new chemical entity pharmacologically characterized as a selective, negative allosteric modulator of the metabotropic glutamate receptor subtype 5 (mGluR5). Antagonism of mGluR5 has been implicated in relation to various diseases such as anxiety, depression, and pain disorders. To support translation from preclinical results and previous experiences with this target in man, a positron emission tomography study was performed to estimate the relationship between AZD2066 plasma concentrations and receptor occupancy in the human brain, using the mGluR5 radioligand [(11)C]-ABP688. The study involved PET scans on 4 occasions in 6 healthy volunteers. The radioligand was given as a tracer dose alone and following oral treatment with different doses of AZD2066. The analysis was based on the total volume of distribution derived from each PET-assessment. A non-linear mixed effects model was developed where ten delineated brain regions of interest from all PET scans were included in one simultaneous fit. For comparison the analysis was also performed according to a method described previously by Lassen et al. (1995). The results of the analysis showed that the total volume of distribution decreased with increasing drug concentrations in all regions with an estimated Kipl of 1170 nM. Variability between individuals and occasions in non-displaceable volume of distribution could explain most of the variability in the total volume of distribution. The Lassen approach provided a similar estimate for Kipl, but the variability was exaggerated and difficult to interpret.

Pharmacokinetic modeling of P-glycoprotein function at the rat and human blood-brain barriers studied with (R)-[11C]verapamil positron emission tomography

Background

This study investigated the influence of P-glycoprotein (P-gp) inhibitor tariquidar on the pharmacokinetics of P-gp substrate radiotracer (R)-[¹¹C]verapamil in plasma and brain of rats and humans by means of positron emission tomography (PET).

Methods

Data obtained from a preclinical and clinical study, in which paired (R)-[¹¹C]verapamil PET scans were performed before, during, and after tariquidar administration, were analyzed using nonlinear mixed effects (NLME) modeling. Administration of tariquidar was included as a covariate on the influx and efflux parameters (Q_in and Q_out) in order to investigate if tariquidar increased influx or decreased outflux of radiotracer across the blood–brain barrier (BBB). Additionally, the influence of pilocarpine-induced status epilepticus (SE) was tested on all model parameters, and the brain-to-plasma partition coefficient (V_T-NLME) was calculated.

Results

Our model indicated that tariquidar enhances brain uptake of (R)-[¹¹C]verapamil by decreasing Q_out. The reduction in Q_out in rats during and immediately after tariquidar administration (sevenfold) was more pronounced than in the second PET scan acquired 2 h after tariquidar administration (fivefold). The effect of tariquidar on Q_out in humans was apparent during and immediately after tariquidar administration (twofold reduction in Q_out) but was negligible in the second PET scan. SE was found to influence the pharmacological volume of distribution of the central brain compartment V_br1. Tariquidar treatment lead to an increase in V_T-NLME, and pilocarpine-induced SE lead to increased (R)-[¹¹C]verapamil distribution to the peripheral brain compartment.

Conclusions

Using NLME modeling, we were able to provide mechanistic insight into the effects of tariquidar and SE on (R)-[¹¹C]verapamil transport across the BBB in control and 48 h post SE rats as well as in humans.

Scaling pharmacodynamics from in vitro and preclinical animal studies to humans

An important feature of mechanism-based pharmacokinetic/pharmacodynamic (PK/PD) models is the identification of drug- and system-specific factors that determine the intensity and time-course of pharmacological effects. This provides an opportunity to integrate information obtained from in vitro bioassays and preclinical pharmacological studies in animals to anticipate the clinical and adverse responses to drugs in humans. The fact that contemporary PK/PD modeling continues to evolve and seeks to emulate systems level properties should provide enhanced capabilities to scale-up pharmacodynamic data. Critical steps in drug discovery and development, such as lead compound and first in human dose selection, may become more efficient with the implementation and further refinement of translational PK/PD modeling. In this review, we highlight fundamental principles in pharmacodynamics and the basic expectations for in vitro bioassays and traditional allometric scaling in PK/PD modeling. Discussion of PK/PD modeling efforts for recombinant human erythropoietin is also included as a case study showing the potential for advanced systems analysis to facilitate extrapolations and improve understanding of inter-species differences in drug responses.

Incorporating receptor theory in mechanism-based pharmacokinetic-pharmacodynamic (PK-PD) modeling

Pharmacokinetic-Pharmacodynamic (PK-PD) modeling helps to better understand drug efficacy and safety and has, therefore, become a powerful tool in the learning-confirming cycles of drug-development. In translational drug research, mechanism-based PK-PD modeling has been recognized as a tool for bringing forward early insights in drug efficacy and safety into the clinical development. These models differ from descriptive PK-PD models in that they quantitatively characterize specific processes in the causal chain between drug administration and effect. This includes target site distribution, binding and activation, pharmacodynamic interactions, transduction and homeostatic feedback mechanisms. Compared to descriptive models mechanism-based PK-PD models that utilize receptor theory concepts for characterization of target binding and target activation processes have improved properties for extrapolation and prediction. In this respect, receptor theory constitutes the basis for 1) prediction of in vivo drug concentration-effect relationships and 2) characterization of target association-dissociation kinetics as determinants of hysteresis in the time course of the drug effect. This approach intrinsically distinguishes drug- and system specific parameters explicitly, allowing accurate extrapolation from in vitro to in vivo and across species. This review provides an overview of recent developments in incorporating receptor theory in PK-PD modeling with a specific focus on the identifiability of these models.

The influence of labour on the pharmacokinetics of intravenously administered amoxicillin in pregnant women

AIMS

Many physiological changes take place during pregnancy and labour. These might change the pharmacokinetics of amoxicillin, necessitating adjustment of the dose for prevention of neonatal infections. We investigated the influence of labour on the pharmacokinetics of amoxicillin.

METHODS

Pregnant women before and during labour were recruited and treated with amoxicillin intravenously. A postpartum dose was offered. Blood samples were obtained and amoxicillin concentrations were determined using high-pressure liquid chromatography. The pharmacokinetics were characterized by nonlinear mixed-effects modelling using NONMEM.

RESULTS

The pharmacokinetics of amoxicillin in 34 patients was best described by a three-compartment model. Moderate interindividual variability was identified in CL, central and peripheral volumes of distribution. The volume of distribution (V) increased with an increasing amount of oedema. Labour influenced the parameter estimate of peripheral volume of distribution (V(2)). V(2) was decreased during labour, and even more in the immediate postpartum period. For all patients the population estimates (mean +/- SE) for CL and V were 21.1 +/- 4.1 l h(-1) (CL), 8.7 +/- 6.6 l (V(1)), 11.8 +/- 7.7 l (V(2)) and 20.5 +/- 15.4 l (V(3)) respectively.

CONCLUSIONS

The peripheral distribution volume of amoxicillin in pregnant women during labour and immediately postpartum is decreased. However, these changes are not clinically relevant and do not warrant deviations from the recommended dosing regimen for amoxicillin during labour in healthy pregnant patients.

Kinetic modelling in small animal imaging with PET

Small animal imaging with positron emission tomography has undergone a major evolution. This has been driven by technical improvements and the development of dedicated PET camera's for small animals. The focus has shifted from detection of tracer uptake and visualization of the tracer distribution towards the quantification of the physiological parameters necessary to use this technique for kinetic modelling of tracers. At the moment there are still several issues which need further research and evaluation before we can fully employ the possibilities of PET as an in-vivo measurement of underlying molecular biology. These issues relate to improved quantification of measurements, improved image reconstruction and processing, and the use of blood plasma data in combination with kinetic models. Besides the more technical issues, there are two more issues which need further clarification: the effect of the anaesthesia, and the effect of radiation on the experiment itself. In this review, we will give an overview of how the technique can be used but we will also discuss the issues mentioned above. The focus will be on the three major parts of the imaging procedure: acquisition, reconstruction of images, and kinetic modelling of the data.

Pharmacokinetics of amoxicillin in maternal, umbilical cord, and neonatal sera

The pharmacokinetics of amoxicillin were studied in umbilical cord and neonatal sera relative to maternal concentrations in prevention of neonatal group B streptococcus infection. The subjects were 44 pregnant women receiving amoxicillin as 1 or 2 g as an intravenous infusion. To measure the concentrations, blood samples were obtained from the mother, the arterial and venous umbilical cord, and the neonate. The pharmacokinetics were characterized by a five-compartment model by using nonlinear mixed-effects (population) modeling. The population estimates for the clearance, central volume of distribution, and the two peripheral maternal volumes of distribution were 19.7 +/- 0.99 liters/h, 6.40 +/- 0.61 liters, and 5.88 +/- 0.83 liters (mean +/- standard error), respectively. The volume of distribution of the venous umbilical cord and the neonatal volume of distribution were 3.40 liters and 11.9 liters, respectively. The pharmacokinetic parameter estimates were used to simulate the concentration-time profiles in maternal, venous umbilical cord, and neonatal sera. The peak concentration in the venous umbilical cord serum was 18% of the maternal peak concentration. It was reached 3.3 min after the maternal peak concentration. The concentration-time profile in neonatal serum was determined by the profile in venous umbilical cord serum, which in turn depended on the profile in maternal serum. Furthermore, the simulated concentrations in maternal, venous umbilical cord, and neonatal sera exceeded the MIC for group B streptococcus for more than 90% of the 4-h dosing interval. In a first approximation, the 2-g infusion to the mother appears to be adequate for the prevention of group B streptococcal disease. However, to investigate the efficacy of the prophylaxis, further studies of the interindividual variability in pharmacokinetics are indicated.

Positron Emission Tomography in Basic Epilepsy Research: A View of the Epileptic Brain

The neurobiological processes that result in epilepsy, known as epileptogenesis, are incompletely understood. Moreover, there is currently no therapy that effectively halts or impedes the development or progression of the condition. Positron Emission Tomography (PET) provides valuable information about the function of the brain in vivo, and is playing a central role in both clinical practice and research. This technique reliably reveals functional abnormalities in many epilepsy syndromes, particularly temporal lobe epilepsy. Unfortunately, epileptogenesis is extremely difficult to study in human patients who usually present with established epilepsy, rather than at the early stages of the process. Animal models offer the advantage of permitting the assessment of the pre-, developing, and chronic epileptic states. However, traditional techniques (e.g., histology) are only able to examine the brain at one time point during epileptogenesis in any one individual. Recent advances in dedicated small animal PET (saPET) allow researchers for the first time to study in vivo biomolecular changes in the brain during epileptogenesis by means of serial acquisitions in the same animal. Repeated application of in vivo imaging modalities in the same animal also decreases the effect of biological inter-individual variability and the number of animals to be used. The availability of novel PET tracers permits the investigation of a broad range of biochemical and physiological processes in the brain. Besides research on epileptogenesis, saPET can also be applied to investigate in vivo the biological effect of novel treatment strategies. saPET is widely used in many fields of pathophysiological investigation and is likely to significantly enhance epilepsy research.