An Academic Perspective on Metabolite Toxicity

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An Academic Perspective on Metabolite Toxicity
the Safety Evaluation of Metabolites
Toxicology Forum Aspen, Colorado 12 July 2005

• Prof. F. P. Guengerich
• Vanderbilt University School of Medicine
• f.guengerich_at_vanderbilt.edu
• http//www.toxicology.mc.vanderbilt.edu/CenterInve
  st/guengerich
• http//isihighlycited.com/author.cgi?link1Browse
  link2Resultsid197

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Target/lead compounds
Newer approach to drug development (ca. 2000)
Efficacy selectivity testing
Design synthesis
Developability screening predictions
Toxicology Pharmaceutics ADME
Profile Pharmacokinetics Pharmacodynamics Cytoch
rome P450 inhibition Cytochrome P450
induction Permeability Transporter
interactions Intrinsic clearance Reaction
phenotyping Reactive metabolites
Candidate
Detailed physicochemical, ADME, safety workup
Clinical trials
(Slide concept thanks to Dr. W. G. Humphreys, BMS)
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Historical Phase I, III, III (R.
T. Williams, 1959)
Phase I oxidation, reduction, hydrolysis Phase
II conjugation Phase III transport (export)
Problems!
Phase I, II, III implies chronological order
(not valid) Hydrolysis more related to
conjugation than redox Conjugation can
activate Not necessarily making compounds more
polar
Josephy, Guengerich, Miners (2005) Drug Metab.
Rev., in press Phase 1 and Phase 2 Drug
Metabolism Terminology That We Should Phase
Out?
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Distinctions among metabolites

• Reactive intermediates
• Have to infer structures from trapped products)
• Cannot prepare may be able to make other
  precursors

• Stable
• Can isolate characterize
• Can synthesize (in principle) for additional
  testing

With both, would like to use test systems
(animals) that yield enough metabolism to get
similar exposure (AUC) to that expected in
humansif possible Questions What levels of
each of these are a problem? How should we treat
these two groups?
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Are stable metabolites really a problem?

• What we know
• Some drugs have alternate targets and yield
  unexpected pharmacology (off-target), even new
  drug uses (e.g. minoxidilgt Rogaine)
• Most metabolites have attenuated activity towards
  the same target (as the parent drug)
• Some metabolites have enhanced activity towards
  the same target (as the parent drug) (e.g.
  pro-drugs)
• Few good examples of a drug producing a
  metabolite that works on a different target
  (trichloroethylene gt chloral)

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Distinctions among metabolites

• Stable
• Can isolate characterize
• Can synthesize (in principle) for additional
  testing

• Reactive intermediates
• Have to infer structures from trapped products)
• Cannot prepare may be able to make other
  precursors

Structures of stable metabolites can be
elucidated with small amounts.
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Testosterone 1b-hydroxylation by human
P450 3A4 Application of LC-SPE-(MS)-NMR
J.A. Krauser, M.Voehler, L-H. Tseng, A.B.
Schefer, M. Godejohann, F. P. Guengerich Eur.
J. Biochem. 271, 3962, 2004
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Distinctions among metabolites

• Stable
• Can isolate characterize
• Can synthesize (in principle) for additional
  testing

• Reactive intermediates
• Have to infer structures from trapped products)
• Cannot prepare may be able to make other
  precursors

Structures of stable metabolites can be
elucidated with small amounts. Some testing can
be done in sensitive systems with small amounts
(e.g. bacterial genotoxicity). Preparing
large amounts of (stable) metabolites may be
challenging the route of administration may
not be useful in analysis of the roles in the
metabolism of the parent drugenter cells?
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Contexts of Drug Toxicity

• On-target toxicity (mechanisms-based) same
  receptor, wrong tissue (e.g. statins)
• Hypersensitivity immunological reactions (e.g.,
  penicillins)
• Off-target pharmacology (e.g., terfenadine hERG
  channel effects)
• Bioactivation to reactive intermediates (e.g.,
  acetaminophen)
• Idiosyncratic toxicities

Liebler Guengerich (2005) Nature Rev. Drug Disc
4, 410-420
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hERG channel receptor
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Contexts of Drug Toxicity

• On-target toxicity (mechanisms-based) same
  receptor, wrong tissue (e.g. statins)
• Hypersensitivity immunological reactions (e.g.,
  penicillins)
• Off-target pharmacology (e.g., terfenadine hERG
  channel effects)
• Bioactivation to reactive intermediates (e.g.,
  acetaminophen)
• Idiosyncratic toxicities

Liebler Guengerich (2005) Nature Rev. Drug Disc
4, 410-420
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Contexts of Drug Toxicity

• On-target toxicity (mechanisms-based) same
  receptor, wrong tissue (e.g. statins)
• Hypersensitivity immunological reactions (e.g.,
  penicillins)
• Off-target pharmacology (e.g., terfenadine hERG
  channel effects)
• Bioactivation to reactive intermediates (e.g.,
  acetaminophen)
• Idiosyncratic toxicities
• Simply rare events that dont fit into other
  categories

Liebler Guengerich (2005) Nature Rev. Drug Disc
4, 410-420
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Some recent review papers on mechanisms of
toxicity reactive metabolites ( idiosyncracies)

• Liebler Guengerich (2005) Nature Reviews, Drug
  Discov. 4, 410-420. Elucidating Mechanisms of
  Drug-induced Toxicity.
• B. K. Park et al. (2005) Annu. Rev. Pharmacol.
  Toxicol. 45, 177-202. The Role of Metabolic
  Activation in Drug-Induced Hepatotoxicity.
• Walgren et al. (2005) Crit. Rev. Toxicol. 35,
  325-361. Role of Metabolism in Drug-Induced
  Idiosyncratic Hepatoxicity.
• N. Kaplowitz (2005) Nature Rev. Drug Discov. 4,
  489-499. Idiosyncratic Drug Hepatotoxicity.
• J. Shenton et al. (2004) Chem.-Biol. Int. 150,
  53-70. Animal Moldels of idiosyncratic Drug
  Reactions.

FDA /CDER Guidance for Industry Safety Testing
of Drug Metabolites
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Some Principles of Reactive Species and Toxicity

• Basic reactions are governed by simple chemistry
• Xd Yd- XY
• electrophile nucleophile
• Free radical propagation
• The first product or most obvious chemical may
  not be the reactant
• Stability of reactive products
• A shorttime (t1/2 1 s) may be considerable
• Some reactive molecules are long-lived (t1/2 min
  to h)

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Some Principles of Reactive Species and Toxicity,
cont.

• In vitro systems are models
• Good for elucidating details
• Some results, not all, apply in vivo
• The dose is an (the) issue (Paracelsus)
• Covalent binding can be an index of toxicity
• Exceptions exist, even after considerations of
  dose
• Other issues
• Receptor-mediated events (signaling)
• Cell proliferation
• Immune responses
• Ability to repair damage

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Assay Development
2.
1.
3.
4.
Liebler Guengerich (2005) Nature Rev. Drug
Disc. 4, 410-420
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Newer approaches to early toxicity screening

• In silico
• Covalent binding screens
• Transcriptonomics
• Proteomics
• Metabonomics

Also see Evans et al. (2004) Chem. Res. Toxicol.
17, 3-16 Liebler Guengerich (2005) Nature
Rev. Drug Disc. 4, 410-420
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How do the problem drugs classify into categories
of mechanism, with our present knowledge?J.
L.Walgren, M. D. Mitchell, D. C. Thompson
(2005) Crit. Rev. Toxicol. 35, 325-361

• Drugs withdrawn 5 of 6 have reactive metabolites
• Benoxaprofen
• Iproniazid
• Nefazodone
• Tienilic acid
• Troglitazone
• (Bromfenac unclear)

• Drugs with Black Box warnings 8 of 15 have
  reactive metabolites
• Dacarbazine
• Dantrolene
• Felbamate
• Flutamide
• Isoniazid
• Ketoconazole
• Tolcapone
• Valproic acid
• (Reactive metabolites not reported for
  Acitretin, Bosentan, Gemtuzumab, Ozogamicin,
  Naltrexone, Nevirapine, Pemoline, Trovafloxacin)

Overall 62 involve metabolism reactive
products
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What fraction of toxicity problems are really due
to metabolism activation?

• The 62 mentioned previously is for drugs that
  got on the market.
• The fraction may be much lower (?) if one
  considers the drug candidates that fail before
  getting into or through clinical trials safety
  assessment.
• Not sure what this number is, suspect it is lower
  but an accurate estimate is probably impossible
  in that most of the data needed are proprietary.
• Suspect that a large fraction is due to
  off-target pharmacology or detrimental effects
  of attenuation of the main target itself
• In many cases the mechanisms may not be
  determined leads are simply scrapped.

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J. L.Walgren, M. D. Mitchell, D. C. Thompson
(2005) Crit. Rev. Toxicol. 35, 325-361

• Drugs with reactive metabolites that have
  warnings of precautions for hepatoxicity
• Acetaminophen
• Carbamazepine
• Clozapine
• Diclofenac
• Disulfiram
• Halothane
• Leflunomide
• Methyldopa
• Rifampin
• Tacrin
• Tamoxifen
• Terbinafine
• Ticlopidine
• Zileuton

• Drugs with reactive metabolites never approved
  in US
• Alpidem
• Amineptine
• Amodiaquine
• Cinchophen
• Dihydralazine
• Dilevaolo
• Ebrotidine
• Glafenine
• Ibufenac
• Isoxanine
• Niperotidien
• Perhexiline
• Pirprofen
• Tilbroquinol

With few exceptions , the drugs associated with
idiosyncratic hepatoxity were given at a daily
dose of 100 mg or higher.
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Summary

• A number of mechanisms can be involved in drug
  toxicity
• Most problems are associated with higher doses
  (some exceptions)
• How many good examples are there of stable
  metabolites causing problems?
• A significant fraction of drug problems seem to
  involve metabolism reactive metabolites,
  although whether this is the majority is still
  unclear
• We understand much about the chemistry of
  reactive metabolites biology prediction are
  issues
• At what level do we worry about a stable product?
  
• Do multiples (in test animals) really insure
  safety?
• Does the information really add to safety?

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Chem. Eng. News
See also Liebler Guengerich (2005) Nature Rev.
Drug Disc. 4, 410-420
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American Chemical Society

• Fall National Meeting
• Symposium series (every year) Divisions of Med.
  Chem. Chem. Toxicology
• Drug Safety Issues
• F. Guengerich N. Meanwell, organizers
• This year 31 Aug 2005
• Natl. ACS Meetg., Washington, D.C.

• Prospective Series Mechanisms of Chemical
  Toxicity Drug Safety Prediction
• F. Guengerich J. MacDonald, organizers
• 5-6 June 2006
• Washington, D.C. (Omni-Shoreham Hotel)
• Attendance 90 max.