Phamacokinetic-pharmacodynamic modelling of behavioural responses

Translation of drug effects from experimental models of neuropathic pain and analgesia to humans

Neuropathic pain research remains a challenging undertaking owing to: (i) the lack of understanding about the underlying disease processes; and (ii) poor predictive validity of the current models of evoked pain used for the screening of novel compounds. Common consensus is that experimental models replicate symptoms (i.e. have face validity but no construct validity). Another issue that requires attention is the sensitivity of endpoints to discriminate drug effects that are relevant to the disease in humans. In this paper we provide an overview of the pre-clinical models that can be used in conjunction with a model-based approach to facilitate the prediction of drug effects in humans. Our review strongly suggests that evidence of the concentration-effect relationship is necessary for translational purposes.

Pharmacokinetic-pharmacodynamic modeling of the effect of fluvoxamine on p-chloroamphetamine-induced behavior

The pharmacokinetic-pharmacodynamic (PK-PD) correlation of the effect of fluvoxamine on para-chloroamphetamine (PCA)-induced behavior was determined in the rat. Rats (n=66) with permanent arterial and venous cannulas received a 30-min intravenous infusion of 1.0, 3.7 or 7.3 mg kg(-1) fluvoxamine. At various time points after the start of fluvoxamine administration, a single dose of PCA (2.5 mg kg(-1)) was injected in the tail vein and resulting behavioral effects, excitation (EXC), flat body posture (FBP) and forepaw trampling (FT), were immediately scored (scores: 0, 1, 2 or 3) over a period of 5 min. In each individual animal the time course of the fluvoxamine plasma concentration was determined up to the time of PCA administration. Observed behavioral effects were related to fluvoxamine plasma concentrations. Fluvoxamine pharmacokinetics was described by a population three-compartment pharmacokinetic model. The effects of fluvoxamine on PCA-induced behavior (probability of EXC, FBP and FT) were directly related to fluvoxamine plasma concentration on the basis of the proportional odds model. For EXC, EC(50) values for the cumulative probabilities P(Y<1), P(Y<2), P(Y<3) were 237+/-39, 174+/-28 and 100+/-20 ng ml(-1), respectively. Slightly higher EC(50) values were obtained for the corresponding effects on FBP and FT. This investigation demonstrates the feasibility of PK-PD modeling of categorical drug effects in animal behavioral pharmacology. This constitutes a basis for the future development of a mechanism-based PK-PD model for fluvoxamine in this paradigm.

Population pharmacokinetic model of fluvoxamine in rats: utility for application in animal behavioral studies

The limitations of blood sampling in pharmacokinetic (PK)/pharmacodynamic (PD) studies in behavioral animal models could in part be overcome by a mixed effects modeling approach. This analysis characterizes and evaluates the population PK of fluvoxamine in rat plasma using nonlinear mixed effects modeling. The model is assessed for its utility in animal behavioral PK/PD studies. In six studies with a different experimental setup, study site and/or sampling design, rats received an intravenous infusion of 1, 3.7 or 7.3mg/kg fluvoxamine. A population three-compartment PK model adequately described the fluvoxamine plasma concentrations. Body weight was included as a covariate and mean population PK parameters for CL, V(1), V(2), Q(2), V(3) and Q(3) were 25.1 ml/min, 256 ml, 721 ml, 30.3 ml/min, 136 ml and 1.0 ml/min, respectively. Inter-individual variability was identified on CL (39.5%), V(1) (43.5%), V(2) (50.1%) and Q(2) (25.7%). A predictive check and bootstrap analysis confirmed the predictive ability, model stability and precision of the parameter estimates. Body weight was identified as a significant covariate of the inter-compartmental clearance Q(2). The pharmacokinetics was independent of factors such as dose, surgery (for instrumentation) and study site. The utility of the model in animal behavioral studies was demonstrated in a PK/PD analysis of the effects on REM sleep in which a sparse PK sampling design was used. By using the pertinent information from the population PK model, individual PK profiles and the PK/PD correlation could be adequately described.

The pharmacokinetics and pharmacodynamics of tandospirone in rats exposed to conditioned fear stress

The 5-HT1, agonist tandospirone is generally thought to have a weak anxiolytic effect with a slow onset of action. Our recent clinical study suggested that a comparatively high dose of tandospirone has excellent anxiolytic efficacy and is without significant adverse effects. The present study was designed to clarify the relationship between the anxiolytic effect of tandospirone and its plasma and brain concentrations. The anxiolytic effect was estimated by determining the conditioned fear stress-induced freezing behavior in rats after tandospirone administration. Obvious correlations between anxiolytic effect and brain concentration of tandospirone were observed 0.5 and 4 h after tandospirone administration, while the anxiolytic effect was dependent on the plasma concentration of at 0.5 h but not 4 h after tandospirone administration. The plasma concentration was significantly correlated with the brain concentration. These findings suggest that the potency of the anxiolytic effect is dependent on both the plasma and brain concentration.

Population pharmacokinetic–pharmacodynamic modelling of the anti-hyperalgesic effect of 5′deoxy-N6-cylopentyladenosine in the mononeuropathic rat

The objective of this investigation was to characterise the pharmacokinetic-pharmacodynamic correlation of 5'-deoxy-N6-cyclopentyl-adenosine (5'dCPA) in the chronic constriction injury model of neuropathic pain. Following intravenous administration of 5'dCPA (0.30 or 0.75 mg kg(-1)), the time course of the drug concentration in plasma was determined in conjunction with the effect on (1) the mechanical paw pressure and (2) the Von Frey Hair monofilament withdrawal threshold. Population pharmacokinetic-pharmacodynamic analysis was applied to derive individual concentration-effect relationships. For mechanical paw pressure a composite model consisting of an Emax model for the anti-hyperalgesic effect in combination with a linear model for the anti-nociceptive effect accurately described the data. The EC50 for the anti-hyperalgesic effect was 178+/-51 ng ml(-1) and the slope of the anti-nociceptive effect 0.055+/-0.008 g ml ng(-1). For the Von Frey Hair monofilament withdrawal threshold responders and non-responders were observed. Typically, in responders, full pain relief was observed at concentrations exceeding 100 ng ml(-1). The high plasma concentrations required for the anti-hyperalgesic effect relative to the receptor affinity are consistent with restricted transport of 5'dCPA to the site of action in the spinal cord and/or the brain.

The application of the principles of clinical drug development to pharmacological challenge tests of the serotonergic system

Pharmacological challenge tests of the serotonergic system have extensively been used during the past 20 years and new tests are in development. It is of crucial importance to standardize challenge tests to ascertain that observed variability is due to the state of the challenged system and not caused by variability of the test itself. This is even more important now that challenge tests increasingly are used in complex studies (e.g. in combination with neuroimaging and in large population studies with repeated tests over time). The Guideline for Good Clinical Practice may be of great help in the standardization of these tests. This is a recently developed guideline for pharmaceutical drug-development, which increasingly is used as a reference for all research in humans. To exemplify the possible usefulness of this approach, we apply it to meta-chlorophenylpiperazine, one of the most commonly used drugs in serotonergic challenge tests. We conclude that much can be learned from the development of this particular challenge. In the discussion, we address general issues that emerged from this review and their relevance to the development of future challenge tests.

Population pharmacokinetic-pharmacodynamic modelling of S 15535, a 5-HT(1A) receptor agonist, using a behavioural model in rats

The pharmacokinetic-pharmacodynamic relationship of S 15535 (1-(benzodioxan-5-yl) 4-(indan-2-yl)piperazine) and its active 5-hydroxy metabolite S 32784 (1-(benzodioxan-5-yl) 4-(5-hydroxyindan-2-yl)piperazine), and buspirone as a reference, were studied in male Wistar rats using a behavioural model of anxiety by determining the reduction in the number of fear-induced ultrasonic vocalisations. S 15535 and buspirone were administered p.o. and i.v. S 32784, present in man but not in rat, was administered i.v. The pharmacokinetics and pharmacokinetic-pharmacodynamic relationships were described using non-linear mixed effects modelling. The no-drug effect was constant and all compounds were active in the model, reducing ultrasonic vocalisations immediately after administration. The sigmoid E(max) model was used to describe the pharmacokinetic-pharmacodynamic relationships, with E(max) values of a 90% decrease in baseline ultrasonic vocalisations. Corrected for plasma protein binding, all compounds showed similar potency. The study shows that ultrasonic vocalisations can be considered a suitable endpoint for the anxiolytic effect when used in conjunction with non-linear mixed effects modelling to overcome the limited sampling and effect measurements.

Pharmacokinetic-pharmacodynamic modeling: why?

At present, pharmacokinetic-pharmacodynamic (PK-PD) modeling has emerged as a major tool in clinical pharmacology to optimize drug use by designing rational dosage forms and dosage regimes. Quantitative representation of the dose-concentration-response relationship should provide information for prediction of the level of response to a certain level of drug dose. Several mathematical approaches can be used to describe such relationships, depending on the single dose or the steady-state measurements carried out. With concentration and response data on-phase, basic models such as fixed-effect, linear, log-linear, E(MAX), and sigmoid E(MAX) can be sufficient. However, time-variant pharmacodynamic models (effect compartment, acute tolerance, sensitization, and indirect responses) can be required when kinetics and response are out-of-phase. To date, methodologies available for PK-PD analysis barely suppose the use of powerful computing resources. Some of these algorithms are able to generate individual estimates of parameters based on population analysis and Bayesian forecasting. Notwithstanding, attention must be paid to avoid overinterpreted data from mathematical models, so that reliability and clinical significance of estimated parameters will be valuable when underlying physiologic processes (disease, age, gender, etc.) are considered.