Analysis of the action of idazoxan calls into question the reliability of the rat isolated small mesenteric artery assay as a predictor for alpha 1-adrenoceptor-mediated pressor activity

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.

Idazoxan effects upon contractile activity in the rat aorta are related to alpha adrenoceptors and L-type channels

Idazoxan is an alpha(2) adrenoceptor antagonist and alpha(1)/alpha(2) partial agonist which also blocks imidazoline receptors. Although idazoxan is widely used in pharmacological studies, its intrinsic vasoactive properties could bring about some limitations. Others have shown that in rat aorta contracted by phenylephrine idazoxan induces relaxation and that in rat small arteries it preferentially antagonizes the alpha(1)-mediated response. We further investigated this matter, using the rat aorta and focusing on the endothelium-independent effects and on L-type channels. In our study, idazoxan inhibited the contraction induced by phenylephrine, an effect which was stronger in the presence of endothelium, but did not affect the contractions induced by various other agents (high potassium, angiotensin II, prostaglandin F(2alpha)). This preferential inhibition was attenuated by 10(-4) m, but not by 10(-5) m yohimbine, and also reduced by 10(-2) m tetraethylammonium and blunted by 10(-4) m methoxyverapamil. In concentrations above 10(-5) m idazoxan induced weak contractions of the de-endothelized rings, which were prazosin- and methoxyverapamil-sensitive. Others have suggested that cyclic guanosine monophospate mediates the idazoxan-induced endothelium-dependent relaxation, but this is difficult to reconcile with our findings. Potassium efflux could play some role in the direct relaxing effect of idazoxan. The observed idazoxan effects appear as based on action upon alpha(1) receptors, but a direct interaction with L-type calcium channels could also be taken into consideration.