A toxicologist’s guide to biomarkers ofhepatic responseDE Amacher*Drug Safety Evaluation, MS 8274-1227, Pfizer Global Research and Development, Eastern Point Road, Groton,Connecticut 06340, USABiological markers (biomarkers) are used to recognize ,characterize an d monitor treatment-related responsesfollowing exposure to xenobiotics. Biomarkers serve thre eprimary applications in toxicology: 1) to confirm expo-sure to a deleterious agent, 2) to provide a system formonitoring individual susceptibility to a toxicant, and 3)to quantitatively assess deleterious effects of a toxicant toan organism or individual. Because the liver is a generaltarget for adverse effects of drugs and other chemicals,biomarkers of untoward hepatic response to xenobioticsare of particular interest to the pharmaceutical toxicolo-gist. General requirements for the latter category o fbiomarkers are sample availability, target organ specific-ity, sensitivity for the toxicity of interest, accessibility, arelatively short half-life, and available detection systems.Biomarkers that can be assayed in biological fluids fromboth human and animal subjects are particularly desir-able. Histologically, acute and subacute hepatic toxicitycommonly involves necrosis, steatosis, cholestasis, vascu-lar disorders, or multiple lesions. The purpose of thisreview is to summarize reported applications usingclinical analytes and biochemical indicators of hepaticdysfunction with emphasis on those that show promise ofsupplementing or improving upon standar d laboratoryprocedures. Liver function markers refer to peripheralindicators of hepatic synthetic and secretory activities,enterohepatic function, or perturbations of the hepaticuptake and clearance of circulating biomolecules. Liverinjury biomarkers include various peripheral proteinsreleased in response to a cellular damage or locally,proteins that are significantly altered within the liver.These include both circulating cytosolic, mitochondrial, orcanalicular membrane markers, and the up-regulationor depletion of radical scavengers, modulators, andstabilizers of intracellular damage. Subsequent recoveryfrom a toxic insult involves repair, regenerative, andproliferative responses that constitute the third class ofbiomarkers. Of these, protein markers fou nd either in sera,plasma, or urine either during or just prior to the earlymanifestation of histological hepatic lesions are of greatestinterest. Examples of a number of these markers, theirdocumented applications in human s or animals, andpotential advantages as well as limitations are presented.Human & Experimental Toxicology (2002) 21, 253 ± 262.Key words: biomarkers; hepatotoxicity; liver; reviewApplications in toxicologyBiomarkers serve three distinct toxicological applica-tions. The first involves the assessment of exposure toa potentially deleterious agent. This application isparticularly useful in environmental or occupationalstudies. In the simplest case, biomarkers of exposurecan include the substance of interest or its free orconjugated metabolites in biological fluids. Examplesinclude the formation of DNA adducts,1proteinadducts,2the induction of specific sequesteringproteins such as the heavy-metal-binding metallo-thionein,3or marked increases for certain seru msialoglycoconjugates during heavy metal toxicity.4Formation of a reactive metabolite or binding specif-icity is often required. Protein adducts are generallymore useful than DNA adducts because both cytosolicproteins and circulating proteins such as haemoglo-bin and seru m albumin are generally more accessibleto a wid e rang e of chemical reactants and, unlikenuclear DNA,2are not affected by repair mechanisms.Protein adducts tend to be stable and relatively long-lived in the body. Recent examples of occupationalmonitoring of peripheral fluids for specific bio-markers include the detection of haemoglobinadducts of some 1,3-butadiene metabolites,5plasma -glucuronidase of liver origin as a more sensitive andrapid biomarker of organophosphorous insecticidepoisoning compared to plasma acetylcholinesteraseinhibition,6and epichlorohydrin DNA adducts incirculating blood cells during occupational expo-sure.7The induction of hepatic dr ug-metabolizing© Arnold 2002 10.1191/0960327102ht247oa*Correspondence: David E. Amacher, Drug Safety Evaluation, MS8274-1227, Pfizer Global Research and Development, Eastern PointRoad, Groton, Connecticut 06340, USA.E-mail: [email protected] 13 August 2001; revised 28 February 2002; accepted 31March 2002Human & Experimental Toxicology (2002) 21, 253 ± 262www.hetjournal.comenzymes indicative of a particular clas s of xenobiot-ics, as for example, CYP4A induction following expo-sure to di-(2-ethyl-hexyl) phthalate can serve as asensitive indicator of exposure in male rats, whichare susceptible.8A second major biomarker application involvesmonitoring to gauge s usceptibility of a subject tothe toxicant, especially in genetically heterogeneouspopulations or species with a high incidence of ge-netic polymorphism. This category has its greatestutilization in clinical monitoring, either to selectrepresentative responders as participants for dru gtreatment or to monitor high-risk individuals in thepostmarketing phas e whe n genetic susceptibility totoxic injury has been clearly e stablished. A hi ghincidence of polymorphism in some human hepaticdrug-metabolizing enzymes, for example, can re-sult in substantial variation in susceptibility to thedeleterious effects of certain drugs extensively me-tabolized by the liver. Periodic clinical monitoringof conventional serum markers of hepatic injurysuch as the transaminases is a routine postmarket-ing recommendation for some chronic-use drugsincluding certain HMG-CoA reductase inhibitors,nonsteroidal anti-inflammatories, and antihyperten-sive agents.9A thir d application is the use of biomarkers toassess the potential of a therapeutic agent to act as atoxicant in the organism as part of the safety eval-uation process. Inherent in this application is thedemonstration of a mechanistic correlation betweenthe biomarker and the unintentional biological activ-ity or deleterious effect of the chemical in the liver.A well known example is the formation of the toxicreactive metabolite (N-acetyl-p-benzoquinoneimine)from acetaminophen when detoxification by conjuga-tion is overwhelmed,10leading to acute hepatocellularnecrosis and elevated levels of the serum biomar-ker, alanine
View Full Document
Unlocking...