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In recent years, pharmacokinetic/pharmacodynamic (PK/PD) modeling has developed from an empirical descriptive discipline into a mechanistic science that can be applied at all stages of drug development. Mechanism-based PK/PD models differ from empirical descriptive models in that they contain specific expressions to characterize processes on the causal path between drug administration and effect. Mechanism-based PK/PD models have much improved properties for extrapolation and prediction. As such, they constitute a scientific basis for rational drug discovery and development. In this report, a novel classification of biomarkers is proposed. Within the context of mechanism-based PK/PD modeling, a biomarker is defined as a measure that characterizes, in a strictly quantitative manner, a process, which is on the causal path between drug administration and effect. The new classification system distinguishes seven types of biomarkers: type 0, genotype/phenotype determining drug response; type 1, concentration of drug or drug metabolite; type 2, molecular target occupancy; type 3, molecular target activation; type 4, physiological measures; type 5, pathophysiological measures; and type 6, clinical ratings. In this paper, the use of the new biomarker classification is discussed in the context of the application of mechanism-based PK/PD analysis in drug discovery and development.

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We have previously reported that endocytic sorting of ET A endothelin receptors to the recycling pathway is dependent on a signal residing in the cytoplasmic carboxyl-terminal region. The aim of the present work was to characterize the carboxylterminal recycling motif of the ET A receptor. Assay of truncation mutants of the ET A receptor with increasing deletions of the carboxyl-terminal tail revealed that amino acids 390 to 406 contained information critical for the ability of the receptor to recycle. This peptide sequence displayed significant sequence similarity to several protein segments confirmed by X-ray crystallography to adopt antiparallel ␤-strand structures (␤-finger). One of these segments was the ␤-finger motif of neuronal nitric-oxide synthase reported to function as an internal PDZ (postsynaptic density-95/disc-large/zona occludens) domainbinding ligand. Based on these findings, the three-dimensional structure of the recycling motif of ET A receptor was predicted to attain a ␤-finger conformation acting as an internal PDZ ligand. Site-directed mutagenesis at residues that would be crucial to the structural integrity of the putative ␤-finger conformation or PDZ ligand function prevented recycling of the ET A receptor. Analysis of more than 300 G protein-coupled receptors (GPCRs) identified 35 different human GPCRs with carboxylterminal sequence patterns that fulfilled the structural criteria of an internal PDZ ligand. Among these are several receptors reported to follow a recycling pathway. In conclusion, recycling of ET A receptor is mediated by a motif with the structural characteristics of an internal PDZ ligand. This structural motif may represent a more general principle of endocytic sorting of GPCRs.The physiological effects of the vasoactive peptide endothelin-1 (ET-1) are mediated by the ET A and ET B receptors, which belong to class A G protein-coupled receptors (GPCRs) (Yanagisawa et al., 1988). In the vasculature, ET A receptors residing on smooth muscle cells mediate prolonged vasoconstriction, whereas ET B receptors, which are located on the plasma membrane of endothelial cells, are primarily considered to cause NO-mediated vasodilation (Yanagisawa and Masaki, 1989). In addition, considerable evidence now also supports a role for the ET B receptor in clearance of plasma ET-1 from the circulation (Berthiaume et al., 2000;Opgenorth et al., 2000). We have previously shown that agonist-induced internalization of the two receptor subtypes depends on a mechanism involving G protein-coupled receptor kinase, arrestin, clathrin, and dynamin (Bremnes et al., 2000). After internalization, however, the two receptor subtypes are targeted to different intracellular fates. The ET A receptor follows the recycling pathway through the pericentriolar recycling compartment and subsequently reappears on the plasma membrane, whereas the ET B receptor is directed to lysosomes for degradation (Bremnes et al., 2000). In terms of physiological effects, rapid recycling of the ET A receptor may provide ba...