The Role of Drug Metabolism Studies in
Optimizing Drug Candidates
Kenneth Santone, PhD
Bristol-Myers Squibb
Metabolism and Pharmacokinetics /
Pharmaceutical Candidate Optimization
ALTERNATE TITLE:
Why All the Chemist's Wonderful
Compounds Don't Become Drugs!
Our Focus
Unmet medical need
First in class
Best in class
Need for efficiency and
productivity enhancement
What are we faced with?
Industrialization of pharmaceutical research
– Unprecedented increase in identification of targets
– Corresponding increase in throughput of chemistry
– Blurring of traditional discovery-development interface
Focus and emphasis on “developability”
(early go/no go decisions)
Improve success rate
Reduce development timeline
– Necessity for increasing efficiency and productivity
Drug Discovery Paradigm Shift
‘Old’ Model ‘New’ Model
More informed
of Drug Discovery of Drug Discovery decision making
Hits Validated Hits during Lead
Optimization,
through quicker and
earlier evaluation of
Design Efficacy & Design PAT attributes
& Synthesis Selectivity Testing Efficacy & PAT
& Synthesis
Selectivity Screening &
Testing Predictions
Lead Candidates
Physicochemical, ADME Detailed Physicochemical,
& Tox Workup ADME & Tox Workup
Development Compound Development Compound
The Hand-off from Drug Discovery to Development:
The Top Ten Quotations We All Know and Love*
10. “The molecular weight? 850. Why? Is that a problem?”
9. “We’ll need eight different capsule strengths for Phase I.”
8. “The compound is very potent in the in vitro screen but does not work well in the
animal efficacy model.”
7. “Now that you mention it, our solutions were a little cloudy.”
6. “The compound is highly insoluble but Pharmaceutical Development will fix the
problem.”
5. “BMS-XXXXXX is a highly potent and selective inhibitor of (the target).
In mouse models, the optimal dose was 200 mg/kg.”
4. “Toxicity?! It’s not the drug; must be a metabolite unique to that animal species.”
3. “Animal bioavailability ranged from 65% to 60
a: metabolism rate in nmol/min.mg protein in rat liver microsomes
b: rat oral exposure studies at 0.1 mmol/kg
Issue
• Similar metabolism and in vitro activity profile but different in vivo
activity profile
• Apparent PK/PD disconnect
Solution
• Rapid in vitro metabolism and biological activity assays
Assessment of Active Metabolites
In vitro Activity of Liver Microsomal
Product in Cell Based Assay (IC50 (nM)) % parent
Compound
Parent 0 min 30 min remaining
incubation incubation
BMS-X 19 12 19 <1
BMS-Y 19 60 490 20
Structural identification of active metabolites
• MS/MS indicated presence of monohydroxylation
• NMR showed site of hydroxylation
Subsequent steps
• Monohydroxylated metabolite synthesized
• Activity and PK properties confirmed
Assessment of Reactive Metabolites
•A number of functional (chemical) elements have been
associated with problems in drug discovery leading to toxicity
Metabolic activation to reactive intermediates
Interference with metabolic processes
•Clinical manifestations include (preclinical measure)
Cellular (hepatic) necrosis (animal toxicity)
Idiosyncratic toxicity (glutathione adducts, protein
covalent binding, immunogenic response)
Drug-drug interactions (mechanism-dependent CYP
inhibition)
Examples of Reactive Metabolites
Furans
O O
O
CYP3A4
CYP3A4 O O O O (epoxidation)
O
(epoxidation)
O
OH
O
O
O OCH3 OH
Aflatoxin B 6',7'-dihydroxybergamottin
Furan substructure is associated with toxicity (eg. aflatoxin) and
with CYP inhibition (eg. bergamottin)
Examples of Reactive Metabolites
Thiophenes
O O
S S S
Nu
O O
CYP2C9 O
O S
H2N N
HO S
O
O
Cl Cl
Cl
Tienilic acid Tenidap
Thiophene substructure has been associated with several types of
toxicity (predominately hepatotoxicity). Other thiophene
containing drugs: ticlopidine, clopidigrel, raloxifene.
Examples of Reactive Metabolites
Anilines, Nitroaromatics
OH O
NO2 NH2 HN N
O O
O S
O
H2N S
N O
H N H2N NH2
Sulfamethoxazole Dapsone
Anilines are associated with a number of types of toxicity (eg. methemoglobinemia, skin
rashes, etc.). Nitroaromatics are primarily activated by initial reduction, often in the
gut, followed by N-oxidation.
Anilines of polycyclic aromatic systems are often potent mutagens and carcinogens (eg.,
naphthylamine, aminofluorene) through conjugation of the hydroxylamine and
subsequent loss of the conjugate to leave a nitrenium ion.
Examples of Reactive Metabolites
O
Amines, alkylamines N N
O
S
O
N
O O
N
Diltiazem
The metabolism of amines or alkylamines is generally related to time-
dependent inhibition of CYP enzymes, with the nitroso species forming a tight
complex with the heme iron, known as a MI complex. Other compounds that
undergo this type of transformation and inhibit CYPs are TAO, erythromycin
and verapamil
Examples of Reactive Metabolites
Quinone, Quinoid
O O
X = O, N, C
X X
O
HN O
O
O S
NH
HO
O
OH
Acetaminophen Troglitazone
Quinone-like compounds can exert their effects through direct
alkylation of nucleophiles or through redox cycling between their
oxidized and reduced forms
Examples of Reactive Metabolites
Acetylenes
O
N
OH OH
O O
Gestodene Mifepristone (RU 486)
Acetylenes have been found to be time-dependent inhibitors of
CYP enzymes.
Examples of Reactive Metabolites
Acyl glucuronidation formation
Direct reaction with nucleophiles
O O
OH OGluc
Amidori rearrangement, then reaction with nucleophiles
O
O
N OH
N OH
O
O
Cl
Zomipirac Tolmentin
Acyl glucuronides have been implicated in both direct hepatic
damage and idiosyncratic toxicities
Challenges and Opportunities
HTS screens for prediction of permeability, metabolic stability, metabolic
reactivity and DDI
– How are we using these data?
– Retrospective analysis on return of investment
– The numbers in gray zone!
– Secondary assays for better predictability
Application of animal PK/bioavailability data for lead optimization
– Adequacy of permeability and metabolic stability data
– Animals vs. humans: quantitative and qualitative differences in ADME
properties
Informed decision based on drug metabolism and pharmacokinetic data
– Low bioavailability vs. oral efficacy
– Role of metabolite(s), reactivity of metabolite(s)
– Protein binding
– In vitro- in vivo correlation in animals and extrapolation to humans
Issue of enzyme induction in humans
– In-vitro models and predictability
– False and real alarm from in-vivo animal data
Challenges and Opportunities
Use of biomarkers
– In-vivo biology, animals vs. humans
– Development and validation of assays
– Transfer from preclinical to clinical laboratories
– Biomarkers = Surrogate marker = Efficacy/Toxicity
– A balancing act of emerging science
The feedback loops
– To and from chemistry
– To and from biology
– To and from drug safety
– To and from pharmaceutics
– To and from clinical pharmacology
Volume of data
– Conversion of information into knowledge
– Timing and availability
A Focused Application of ADME Studies
• Active involvement earlier in the Discovery Process
• Timely guidance to Chemistry to select chemotypes with
desirable ADME properties
• Maximize informed decision making during Lead
Optimization
• Improved ability to predict human metabolism and
pharmacokinetics
• Stronger partnerships with Drug Discovery and all areas
of Pharmaceutical Development
Our Mission
To ensure that no development candidate
fails in the clinic due to an
unforeseen metabolic or
pharmacokinetic property
Acknowledgements
David Rodrigues and Griff Humphreys
Saeho Chong, Punit Marathe, Wen Chyi Shyu
and Mike Sinz
And finally ….