Population pharmacokinetic modelling of the enterohepatic recirculation of diclofenac and rofecoxib in rats.

BACKGROUND AND PURPOSE: Enterohepatic recirculation (EHC) is a common pharmacokinetic phenomenon that has been poorly modelled in animals. The presence of EHC leads to the appearance of multiple peaks in the concentration-time profile and increased exposure, which may have implications for drug effect and extrapolation across species. The aim of this investigation was to develop a population pharmacokinetic model for diclofenac and rofecoxib that describes EHC and to assess its consequence for the pharmacodynamics of both drugs. EXPERIMENTAL APPROACH: The pharmacokinetics of diclofenac and rofecoxib was characterized in male rats following intravenous, intraperitoneal and oral administration. Blood samples were collected at pre-defined time points after dosing to determine plasma concentrations over time. A parametric approach using nonlinear mixed effects modelling was applied to describe EHC, whilst simulations were used to evaluate its impact on PGE(2) inhibition. KEY RESULTS: For diclofenac, EHC was described by a compartmental model with periodic transfer rate and metabolite formation rate. For rofecoxib, EHC modelling required a conversion compartment with first-order recycling rate and lag time. Based on model predictions, EHC causes an increase of 95% in the systemic exposure to diclofenac and of 15% in the exposure to rofecoxib. In addition, EHC prolongs the inhibition of PGE(2) and increases the duration of the anti-inflammatory effect (24 h for rofecoxib 10 mg kg(-1)) without affecting maximum inhibition. CONCLUSIONS AND IMPLICATIONS: Our findings show the relevance of exploring EHC in a quantitative manner to accurately interpret pharmacodynamic findings in vivo, in particular when scaling across species.

Pharmacokinetic-pharmacodynamic correlations and biomarkers in the development of COX-2 inhibitors.

The mechanism by which COX inhibitors exert their analgesic effect is well established. However, data show no direct correlation between drug concentrations in plasma and the analgesic or adverse effects in chronic inflammatory conditions. This represents a major problem in the development of COX inhibitors, since it is difficult to predict the appropriate dosing regimen for the treatment of chronic inflammatory pain, based upon information from pre-clinical studies and eventually early clinical studies. The factors that determine response in inflammatory pain must be understood in order to make predictions about the time course of the analgesic effect. In this review the determinants of drug response and their variability will be discussed: physicochemical properties, pharmacokinetics (PK), pathophysiology and disease progression. From a mechanistic point of view, endogenous mediators of inflammation might be used as a biomarker for the analgesic effect and safety assessment. Such a biomarker can be an intermediate step between drug exposure and response. In addition, its concentration-effect relationship could be characterized by pharmacokinetic-pharmacodynamic (PK/PD) modelling. Indeed, recent investigations have shown that COX-2 inhibition, as determined by modelling of prostaglandin E2 (PGE2) levels in the whole blood assay in vitro can be used as a marker to predict drug effects (analgesia) in humans. A model-derived parameter, IC80, (total and unbound) was found to correlate directly with the analgesic plasma concentration of different COX inhibitors varying in enzyme selectivity. These findings indicate that PGE2 and thromboxane B2 inhibition can be used to predict and select efficacious doses in humans.

Mechanism-based modeling of the pharmacodynamic interaction of alphaxalone and midazolam in rats.

The objective of the present investigation was to characterize the pharmacodynamic interaction between the synthetic neuroactive steroid alphaxalone and the benzodiazepine midazolam. The time course of the electroencephalographic (EEG) effect (11.5-30 Hz) was determined in rats in conjunction with plasma concentrations. Alphaxalone was administered as a continuous intravenous infusion of 0, 1.2, 2.2, or 5.2 mg over 360 min. Midazolam was administered as a 5-min intravenous bolus infusion of 4 mg.kg-1. The pharmacokinetic profiles of both drugs were described by a two-compartment model. No pharmacokinetic interaction was observed. The EEG effect versus time profiles of midazolam and alphaxalone, when administered separately and in combination, were modeled on the basis of the recently proposed mechanism-based pharmacokinetic/pharmacodynamic model for GABAA receptor modulators, which contains separate expressions to describe the drug-receptor interaction and the stimulus-response relationship. The pharmacodynamic interaction between alphaxalone and midazolam was best characterized using an independent drug-drug interaction model without an expression for allosteric modulation of the effect of midazolam by alphaxalone. The final model contained an exponential expression to account for acute functional adaptation to the EEG effect upon continuous infusion of alphaxalone. The mechanism-based analysis showed that this functional adaptation is best explained by a change in the system-specific stimulus-response relationship, rather than the drug-receptor activation process. It is concluded that the pharmacodynamic interaction between alphaxalone and midazolam in vivo is best described using an independent interaction model without allosteric modulation.