Pharmacokinetic-pharmacodynamic modelling of meperidine in goats (II): Modelling

Pharmacokinetic/pharmacodynamic integration in drug development and dosage-regimen optimization for veterinary medicine

Pharmacokinetic (PK)/pharmacodynamic (PD) modeling is a scientific tool to help developers select a rational dosage regimen for confirmatory clinical testing. This article describes some of the limitations associated with traditional dose-titration designs (parallel and crossover designs) for determining an appropriate dosage regimen. It also explains how a PK/PD model integrates the PK model (describing the relationship between dose, systemic drug concentrations, and time) with the PD model (describing the relationship between systemic drug concentration and the effect vs time profile) and a statistical model (particularly, the intra- and interindividual variability of PK and/or PD origin). Of equal importance is the utility of these models for promoting rational drug selection on the basis of effectiveness and selectivity. PK/PD modeling can be executed using various approaches, such as direct versus indirect response models and parametric versus nonparametric models. PK/PD concepts can be applied to individual dose optimization. Examples of the application of PK/PD approaches in veterinary drug development are provided, with particular emphasis given to nonsteroidal anti-inflammatory drugs. The limits of PK/PD approaches include the development of appropriate models, the validity of surrogate endpoints, and the acceptance of these models in a regulatory environment.

Pharmacokinetics and pharmacodynamics of tolfenamic acid in calves

Pharmacokinetic/pharmacodynamic modelling was applied to a study in which tolfenamic acid was administered intravenously to calves at a dose rate of 2 mg kg-1. The drug had a shorter mean (SEM) elimination half-life (T1/2 beta) of 2.5 (0.95) hours and a larger volume of distribution (Vdarea) of 0.98 (0.28) litre kg-1 than other non-steroidal anti-inflammatory drugs. Its body clearance was high with a mean value of 0.30 (0.06) litre kg-1 h-1. It had inhibitory effects on inflammatory exudate PGE2 and beta-glucuronidase, serum TxB2 and bradykinin-induced swelling but it did not affect exudate LTB4 concentrations. Its mean EC50 values were lower for exudate PGE2, beta-glucuronidase and bradykinin-induced swelling inhibition (0.077 [0.018]; 0.040 [0.017] and 0.030 [0.020] microgram ml-1, respectively) than for serum TxB2 inhibition (0.137 (0.079) microgram ml-1). There were also differences in its equilibration halflife, which was short for the inhibition of serum TxB2, intermediate for exudate PGE2 and beta-glucuronidase and longer for bradykinin-induced swelling. These differences may be explained by the existence of three distribution compartments relating to the different sites of action of the drug.