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In general, xenobiotic metabolizing enzymes (XMEs) are expressed in lower levels in the extrahepatic tissues than in the liver, making the former less relevant for the clearance of xenobiotics. Local metabolism, however, may lead to tissue-specific adverse responses, e.g. organ toxicities, allergies or cancer. This review summarizes the knowledge on the expression of phase I and phase II XMEs and transporters in extrahepatic tissues at the body's internal-external interfaces. In the lung, CYPs of families 1, 2, 3 and 4 and epoxide hydrolases are important phase I enzymes, while conjugation is less relevant. In skin, phase I-related enzymatic reactions are considered less relevant. Predominant skin XMEs are phase II enzymes, whereby glucuronosyltransferases (UGT) 1, glutathione- S-transferase (GST) and N-acetyltransferase (NAT) 1 are important for detoxification. The intestinal epithelium expresses many transporters and phase I XME with high levels of CYP3A4 and CYP3A5 and phase II metabolism is mainly related to UGT, NAT and Sulfotransferases (SULT). In the kidney, conjugation reactions and transporters play a major role for excretion processes. In the bladder, CYPs are relevant and among the phase II enzymes, NAT1 is involved in the activation of bladder carcinogens. Expression of XMEs is regulated by several mechanisms (nuclear receptors, epigenetic mechanisms, microRNAs). However, the understanding why XMEs are differently expressed in the various tissues is fragmentary. In contrast to the liver - where for most XMEs lower expression is demonstrated in early life - the XME ontogeny in the extrahepatic tissues remains to be investigated. [ABSTRACT FROM AUTHOR]

The linkage point between kinetics and dynamics of drugs and other foreign chemicals is at the active site of the target. The response is determined within certain limits by the concentration of the substance at the target and its mechanism of action. However, the concentration of the substance at the active site under in vivo conditions cannot be easily measured, and surrogates such as plasma concentration are used instead. The emergence of in vitro approaches to study some primary pharmacokinetic (PK) and pharmacodynamic (PD) processes in simpler experimental setups could provide ways to estimate the concentration at the active site, which then can be used in a more reliable extrapolation and prediction of in vivo behavior and action of the substance. Imaging and other novel analytical techniques are providing direct or indirect measurements of in vivo concentrations in tissues. In this review, processes such as transport, metabolism, etc. affecting the concentration of the substance at the active site are discussed, analytical possibilities to measure directly or estimate indirectly the concentration at the active site are illustrated, the overall design and performance of physiologically based PK-PD models are described, and some examples concerning blood-brain barrier, liver, and lungs are presented. In conclusion, several important recommendations are made: (1) In vitro systems should be characterized in a much more detailed way than has been done so far, especially with respect to unbound concentrations at the sites of disposition and action; (2) kinetics of the compound in the in vitro system have to be more thoroughly characterized; (3) local models encompassing the in vitro system should be developed and used; and (4) PBPK/PD models should incorporate these local models. [ABSTRACT FROM AUTHOR]

The new European REACH (Registration, Evaluation, Authorisation of Chemicals) legislation requires new information on chemicals, which is tiered according to production volume. The need for data promotes the use of (quantitative) structure-activity relationships ([Q]SARs) in order to meet the European political goal to protect the lives of animals. Within the preparation for REACH, the EU Commission set up REACH implementation projects (RIPs). They are aimed at giving guidance to industry and regulators on how to interpret the legislation. Within the framework of REACH, (Q)SARs are expected to have specific applications in several fields such as raising concerns for chemicals with low production volume for which no (eco)toxicological data have to be submitted, supporting the grouping of chemicals (family approach), and supporting the classification of chemicals as PBT (persistent/bioaccumulative/toxic) or vPvB (very persistent, very bioaccumulative) substances. Currently, the use of (Q)SARs in European regulatory decision making is limited. However, a new European tool (Toxtree) will be made available along with training courses to promote the use of (Q)SAR approaches under REACH. It is expected that the experimental data, to be submitted by industry and made publicly available under REACH, will provide opportunities to improve existing prediction tools and to develop new approaches for (Q)SAR analysis. In the political perspective, the tools will provide support as a rational basis for the intelligent design of new chemicals with sustainable (eco)toxicological properties. [ABSTRACT FROM AUTHOR]