ToxSci Advance Access originally published online on April 25, 2008
Toxicological Sciences 2008 104(1):1-3; doi:10.1093/toxsci/kfn075
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Beam Me Up Scotty: Incorporating Transporters in Physiologically Based Pharmacokinetic-Pharmacodynamic Modeling
Department of Discovery Toxicology, Bristol-Myers Squibb Co., Route 206, Province Line Road, Princeton, New Jersey 08543
For correspondence via e-mail: lois.lehman-mckeeman{at}bms.com. Fax: 609-252-7046
Received April 7, 2008; accepted April 8, 2008
Physiologically based pharmacokinetic (PBPK) models are important tools in toxicology that facilitate simulation of exposure-dose response relationships and contribute to extrapolating toxic exposures across species to inform or improve human risk assessment. Developing an accurate predictive model requires the integration of mathematics with biological processes, and a recent workshop noted that successful model development requires a multi-disciplinary team including modelers, statisticians, applied mathematicians, toxicologists, and risk assessors engaged in all aspects of evaluation and implementation (Barton et al., 2007
). Extending the application of these models to the evaluation of biologic effects is also an important avenue of research. In general, however, pharmacokinetic-pharmacodynamic relationships are more routinely determined for agents that produce a desired efficacy rather than an untoward effect. In the present issue of Toxicological Sciences, Lohitnavy et al. (2008) report the development of a PK/PD model for the environmental contaminant, PCB126. In this article, the kinetic data, including tissue dosimetry estimations, are positioned relative to the development of hepatic preneoplastic lesions, a precursor of hepatocellular tumors. Furthermore, in developing the model, the authors also assessed its prediction of tissue concentrations of PCB126 and determined that binding to multidrug resistance–associated protein-2 (Mrp2) was required to accurately simulate empirical data generated from several laboratories. The work is significant in that it provides toxicologists with a PBPK model of PCB126 that correctly describes tissue concentrations observed in animal studies and correlates target organ dosimetry with the potential for clonal growth and hepatocarcinogenicity. The model also provides evidence for the likely involvement of Mrp2 in the disposition of the compound, and most importantly, represents one of the first models to identify and directly incorporate the role of a xenobiotic transporter in a PBPK model. Although all of these attributes contribute to the significance and impact of the research, this highlight focuses on the implications of including a role for Mrp2 in the model.
The contribution of multidrug resistance–associated proteins to the disposition of xenobiotics was originally deduced from their role in mediating tumor resistance in which they function to increase efflux and thereby reduce the intracellular concentrations of cancer chemotherapeutic agents (Cole et al., 1992). In addition to their expression and induction in cancer cells, constitutive and inducible expression in normal tissues, including liver, kidney, intestine, and brain, has been described (Klaassen and Lu, 2008). In liver, Mrp2 is expressed on canalicular membranes where it plays an essential role in the biliary excretion of xenobiotics. Although it is generally considered to be an organic anion transporter with high affinity for glutathione, glucuronide, and sulfate conjugates, there is evidence that it also transports some highly lipophilic compounds as would be the case for PCB126. In the present work, the authors did not empirically demonstrate transport of PCB126 by Mrp2, but applied an existing three-dimensional quantitative structure-activity relationship model to evaluate its potential to bind to Mrp2 (Hirono et al., 2005). An important follow-up to the present data would be to confirm the transport of PCB126 by Mrp2 using membrane vesicles or other model transport systems and to empirically establish the kinetic parameters that define such transport.
PCB126 is known to interact with high affinity to the Ah receptor (AhR) leading to transcriptional effects on numerous gene targets. Most models developed for assessing toxicity parameters for polyaromatic hydrocarbons include effects on cytochrome P450 1A (CYP1A) enzymes, well-established targets of AhR. Previous models to predict hepatic concentrations of PCB126 included binding constants for interactions with both AhR and CYP1A2 (NTP, 2006). However, the present data suggest that saturable binding to these high-affinity sites leads to an increased role for Mrp2 and subsequent transporter-mediated biliary excretion. In purely mathematical terms, it could be reasoned that any model that includes a component describing biliary excretion has to some extent included a role for Mrp2 or other biliary transporters. However, the authors also suggest that increased hepatic levels of Mrp2 after partial hepatectomy contribute to the observed hepatic concentrations of PCB126 that are ultimately associated with preneoplastic changes. In its totality, the work underscores the complex, dynamic interplay between biotransformation (phase I and phase II) and xenobiotic transporters. Furthermore, the present work provides an illustrative foundation from which to develop additional models that include the contribution of xenobiotic transporters in determining tissue distribution, target organ dosimetry, and pharmacodynamic effects.
From the perspective of a "nonmodeler," there are several additional, relevant issues and questions raised by the work of Lohitnavy et al. First, Mrp2 is only one of the many xenobiotic transporters expressed in the liver, and as Mrp family proteins have overlapping substrate specificities, it is possible that other transporters, and in particular other Mrps, might contribute to the target organ dosimetry for PCB126. For example, Mrp3, expressed on hepatic basolateral membranes, is an important efflux transporter with affinity for many of the same organic anion substrates as Mrp2. In what is considered to be an important compensatory process, Mrp3 is highly upregulated in rats and humans that are deficient for Mrp2 (Konig et al., 1999; Ortiz et al., 1999). Accordingly, an important question for further evaluation concerns determining whether other transporters should be considered in ongoing model development for PCBs and whether PCB126 could also be transported by other proteins including Mrp3.
PCB126 has been shown to alter thyroid hormone homeostasis in rats, an effect that appears to be mediated at least in part through a mechanism involving increased hepatic clearance of thyroxine (T4). In addition to induction of cytochrome P450 enzymes, PCB126 induces the glucuronidation of T4 (Craft et al., 2002; Fisher et al., 2006) and enhances its excretion in bile, a process that is likely mediated by Mrp2. Fisher et al. (2006) showed a linear relationship between the amount of T4-glucuronide formed in liver and hepatic concentrations of PCB126. Although it is highly unlikely that concentrations of the lipophilic PCB are overwhelmed by hepatic levels of T4-glucuronide, differences in affinity for Mrp2 could contribute to competition or other interaction between these processes in the liver. The present data should provide some impetus for additional efforts to explore the role of biliary excretion of PCB126 not only on its own disposition but also on the disposition of endogenous compounds such as T4 and its metabolites.
A key contribution of PBPK models involves the ability to probe species differences in exposure that may contribute to differences in toxicity and the application of such data for human risk assessment. To that end, species differences in the regulation of Mrp2 by PCB126 have been reported, and the impact of such differences in determining hepatic concentrations of this compound and its potential for adverse effects is not known. As noted above, hepatic constitutive expression of Mrp3 is increased in Mrp2-deficient rats and humans as a compensatory mechanism for this loss of function. In contrast, in Mrp2-null mice, hepatic Mrp3 levels are unaffected, whereas Mrp4, another transporter localized to the basolateral membrane, is upregulated (Chu et al., 2006). In a parallel manner, the available data suggest that, although AhR agonists in general (and PCB126 in particular) induce Mrp2 in mice (Maher et al., 2005), rat Mrp2 is not similarly increased (Johnson and Klaassen, 2002). It should be noted that different dosages of PCB126 were administered to rats and mice in these studies, complicating a direct comparison between species. However, based on the available data, the PBPK model may not be fully applicable to extrapolation to other species but enables investigation into the contribution of possible differences in molecular regulation of transporters to species differences in disposition and toxicity.
As noted above, this work is one of the first papers to include a role for transporters in the hepatic disposition of a xenobiotic, although some PBPK models have included transporter function in evaluating other organs and processes (Merrill et al., 2005). Good research often generates as many new questions as those it answers, and the modeling efforts of Lohitnavy et al. are no exception to this paradigm. Moving forward, the research has provided a very useful tool for probing the disposition of PCB126, while simultaneously establishing the foundation for studying other structurally related compounds, and demonstrating the utility of incorporating xenobiotic transporters into PBPK models. Collectively, the work represents a movement—no, a "transport"—to enhancing the development of biologically relevant and accurate connections between pharmacokinetics and pharmacodynamics. Beam me up.
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