Adverse drug reactions (ADRs) are a major complication of drug therapy and an impediment to drug develop- ment and clinical use after marketing. As a consequence of evidence of toxicity, 16 out of 548 (2.9%) new chemical entities that were approved for the US market between 1975 and 1999 were subsequently withdrawn from the market, and 56 out of 548 (10.2%) acquired a black box warning 1 . Excessive dose, drug accumulation and/or the formation of chemically reactive metabolites (CRMs) have been implicated in many off-target (including idi- osyncratic) ADRs. The organ that is most frequently affected by CRM- mediated ADRs is the liver. Drug-induced liver injury accounts for more than half the cases of acute liver fail- ure in the United States, and acetaminophen is respon- sible for 80% of drug-associated cases of liver failure 2 . Acetaminophen-induced hepatotoxicity is generally predictable from our understanding of its metabolism; however, many other drugs cause idiosyncratic drug- induced liver injury, which, although rare and unpre- dictable, can cause significant morbidity and mortality. Studies with model compounds and drugs — such as acetaminophen — have helped to define the roles that chemical stress and drug bioactivation have in the vari- ous biological outcomes that may be triggered by CRMs. These include effects on transcription factors and/or signalling protein-adaptation (cell defence), apoptosis, necrosis, inflammation and activation of the innate and adaptive immune systems 3 . In addition to their role in drug-induced liver injury, CRMs have been implicated in a number of off-target ADRs in humans; these ADRs have the clinical hall- marks of hypersensitivity reactions and may affect vari- ous systems in addition to the liver, skin and formed *MRC Centre for Drug Safety Science, Division of Molecular & Clinical Pharmacology, The Institute of Translational Medicine, University of Liverpool, Liverpool, L69 3GE, UK. Correspondence to B.K.P. or D.J.T e-mails: B.K.Park@liverpool. ac.uk; donald.tweedie@ boehringer-ingelheim.com doi:10.1038/nrd3408 Managing the challenge of chemically reactive metabolites in drug development B. Kevin Park*, Alan Boobis ‡ , Stephen Clarke § , Chris E. P. Goldring*, David Jones || , J. Gerry Kenna ¶ , Craig Lambert # , Hugh G. Laverty*, Dean J. Naisbitt*, Sidney Nelson**, Deborah A. Nicoll-Griffith ‡‡ , R. Scott Obach §§ , Philip Routledge |||| , Dennis A. Smith ¶¶ , Donald J. Tweedie ## , Nico Vermeulen***, Dominic P. Williams*, Ian D. Wilson ‡‡‡ and Thomas A. Baillie §§§ Abstract | The normal metabolism of drugs can generate metabolites that have intrinsic chemical reactivity towards cellular molecules, and therefore have the potential to alter biological function and initiate serious adverse drug reactions. Here, we present an assessment of the current approaches used for the evaluation of chemically reactive metabolites. We also describe how these approaches are being used within the pharmaceutical industry to assess and minimize the potential of drug candidates to cause toxicity. At early stages of drug discovery, iteration between medicinal chemistry and drug metabolism can eliminate perceived reactive metabolite-mediated chemical liabilities without compromising pharmacological activity or the need for extensive safety evaluation beyond standard practices. In the future, reactive metabolite evaluation may also be useful during clinical development for improving clinical risk assessment and risk management. Currently, there remains a huge gap in our understanding of the basic mechanisms that underlie chemical stress-mediated adverse reactions in humans. This Review summarizes our views on this complex topic, and includes insights into practices considered by the pharmaceutical industry. REVIEWS 292 | APRIL 2011 | VOLUME 10 www.nature.com/reviews/drugdisc © 2011 Macmillan Publishers Limited. All rights reserved