Metabolic Changes of Drugs

1
Metabolic Changes of Drugs
Books 1. Wilson and Gisvolds Textbook of
Organic Medicinal and Pharmaceutical Chemistry
11th ed. Lippincott, Williams Wilkins ed. 2.
Foyes Principles of Medicinal Chemistry
2
Introductory Concepts

• Biochemically speaking Metabolism means
  Catabolism (breaking down of substances)
  Anabolism (building up or synthesis of
  substances)
• But when we speak about drug metabolism, it is
  only catabolism
• That is drug metabolism is the break down of drug
  molecules
• So what is building the drug molecules? We use
  the word synthesis, then
• Drugs are synthesized in laboratory and thus is
  not an endogenous event
• Lipid soluble drugs require more metabolisms to
  become polar, ionizable and easily excretable
  which involve both phase I and phase II
  mechanisms.

3
What Roles are Played by Drug Metabolism?

• One of four pharmacokinetic parameters, i.e.,
  absorption, distribution, metabolism and
  excretion (ADME)
• Elimination of Drugs Metabolism and excretion
  together are elimination
• Excretion physically removes drugs from the body
• The major excretory organ is the kidney. The
  kidney is very good at excreting polar and
  ionized drugs without any major metabolism. The
  kidney is unable to excrete drugs with high LWPC
• In general, by metabolism drugs become more
  polar, ionizable and thus more water soluble to
  enhance elimination
• It also effect deactivation and thus detoxication
  or detoxification
• Many drugs are metabolically activated (Prodrugs)
• Sometimes drugs become more toxic and carcinogenic

4
Routes that result in the formation of inactive
metabolites are often referred to as
detoxification.
The metabolite may exhibit either a different
potency or duration of action or both to the
original drug.
5
Stereochemistry of Drug Metabolism
6
Sites of Drug Metabolism

• Liver Major site, well organized with all enzyme
  systems

The first-pass effect Following drugs are
metabolized extensively by first-pass effect
Isoproterenol, Lidocaine Meperidine, Morphine,
Pentazocine, Propoxyphene, Propranolol,
Nitroglycerin, Salicylamide

• Intestinal Mucosa The extra-hepatic metabolism,
  contains CYP3A4 isozyme
• Isoproterenol exhibit considerable sulphate
  conjugation in GI tract
• Levodopa, chlorpromazine and diethylstilbestrol
  are also reportedly metabolized in GI tract
• Esterases and lipases present in the intestine
  may be particularly important carrying out
  hydrolysis of many ester prodrugs
• Bacterial flora present in the intestine and
  colon reduce many azo and nitro drugs (e.g.,
  sulfasalazine)
• Intestinal b-glucuronidase can hydrolyze
  glucuronide conjugates excreted in the bile,
  thereby liberating the free drug or its
  metabolite for possible reabsorption
  (enterohepatic circulation or recycling)

7
Enzymes Involved in Drug Metabolism
CYP450, Hepatic microsomal flavin containing
monooxygenases (MFMO or FMO) Monoamine Oxidase
(MAO) and Hydrolases

• Cytochrome P450 system localized in the smooth
  endoplasmic reticulum.
• Cytochrome P450 is a Pigment that, with CO bound
  to the reduced form, absorbs maximally at 450nm
• Cytochromes are hemoproteins (heme-thiolate) that
  function to pass electrons by reversibly changing
  the oxidation state of the Fe in heme between the
  2 and 3 state and serves as an electron
  acceptordonor
• P450 is not a singular hemoprotein but rather a
  family of related hemoproteins. Over 1000 have
  been identified in nature with 50 functionally
  active in humans with broad substrate specificity

Simplified apoprotein portion
Heme portion with activated Oxygen
8
Cytochrome P450 Naming

• Before we had a thorough understanding of this
  enzyme system, the CYP450 enzymes were named
  based on their catalytic activity toward a
  specific substrate, e.g., aminopyrine
  N-demethylase now known as CYP2E1
• Currently, all P450s are named by starting with
  CYP (CYtochrome P450, N1, L, N2 - the first
  number is the family (gt40 homology), the letter
  is the subfamily (gt 55 homology), and the second
  number is the isoform. The majority of drug
  metabolism is by 10 isoforms of the CYP1, CYP2
  and CYP3 families in humans
• Major human forms of P450 Quantitatively, in the
  liver the percentages of total P450 protein are
  CYP3A4 28, CYP2Cx 20, CYP1A2 12, CYP2E1
  6, CYP2A6 4, CYP2D6 4
• By number of drugs metabolized the percentages
  are CYP3A4 35, CYP2D6 20, CYP2C8 and
  CYP2C9 17, CYP2C18 and CYP2C19 - 8 CYP 1A1
  and CYP1A2 -10, CYP2E1 4, CYP2B6 3

9
Few Important CYP450 Isozymes
10
(No Transcript)
11
Drug Interactions Metabolism

• The drug interactions depend upon
• the isoform(s) required by the drug in question,
• the isoforms altered by concomitant therapy,
• the type of enzyme alteration (induction or
  inhibition).

12
General Metabolic Pathways

• Oxidation
• Aromatic moieties
• Olefins
• Benzylic allylic C atoms and a-C of CO and CN
• At aliphatic and alicyclic C
• C-Heteroatom system
• C-N (N-dealkylation, N-oxide formation,
  N-hydroxylation)
• C-O (O-dealkylation)
• C-S (S-dealkylation, S-oxidation, desulfuration)
• Oxidation of alcohols and aldehydes
• Miscellaneous

• Hydrolytic Reactions
• Esters and amides
• Epoxides and arene oxides by epoxide hydrase

Phase II - Conjugation
Phase I - Functionalization
Drug Metabolism

• Reduction
• Aldehydes and ketones
• Nitro and azo
• Miscellaneous

• Glucuronic acid conjugation
• Sulfate Conjugation
• Glycine and other AA
• Glutathion or mercapturic acid
• Acetylation
• Methylation

13
Tetrahydrocannabinol (D1-THC) Metabolism
The metabolite is polar, ionisable and hydrophilic
14
Oxidative Reactions
15

• Hydroxylation is the primary reaction mediated by
  CYP450
• Hydroxylation can be followed by non-CYP450
  reactions including conjugation or oxidation to
  ketones or aldehydes, with aldehydes getting
  further oxidized to acids
• Hydroxylation of the carbon a to heteroatoms
  often lead to cleavage of the carbon heteroatom
  bond seen especially with N, O and S, results in
  N, S or Odealkylation.
• Must have an available hydrogen on atom that gets
  hydroxylated, this is important!!!

16
Aromatic Hydroxylation

• Mixed function oxidation of arenes to arenols via
  an epoxide intermediate arene oxide
• Major route of metabolism for drugs with phenyl
  ring
• Occurs primarily at para position
• Substituents attached to aromatic ring influence
  the hydroxylation
• Activated rings (with electron-rich substituents)
  are more susceptible while deactivated (with
  electron withdrawing groups, e.g., Cl, NR3,
  COOH, SO2NHR) are generally slow or resistant to
  hydroxylation

17
Amphetamine
Phenytoin
p-hydroxyphenytoin
Warfarin sodium
17-a-Ethinylestradiol
Propranolol
Phenylbutazone
Atorvastatin
18
Antihypertensive drug clonidine undergo little
aromatic hydroxylation and the uricosuric agent
probenecid has not been reported to undergo any
aromatic hydroxylation
Probenecid
Clonidine
Preferentially the more electron rich ring is
hydroxylated
Diazepam
Chlorpromazine
NIH Shift Novel Intramolecular Hydride shift
named after National Institute of Health where
the process was discovered. This is most
important detoxification reaction for arene oxides
19
Oxidation of olefinic bonds (also called alkenes)

• The second step may not occur if the epoxide is
  stable, usually it is more stable than arene
  oxide
• May be spontaneous and result in alkylation of
  endogenous molecules
• Susceptable to enzymatic hydration by epoxide
  hydrase to form trans-1,2-dihydrodiols (also
  called 1,2-diols or 1,2-dihydroxy compounds)
• Terminal alkenes may form alkylating agents
  following this pathway

Q. Any similarities or dissimilarities with
aromatic NIH Shift, Conjugation with
macromolecules?
20
Benzylic Carbon Hydroxylation

• Hydroxylate a carbon attached to a phenol group
  (aromatic ring)
• R1 and R2 can produce steric hindrance as they
  get larger and more branched
• So a methyl group is most likely to hydroxylate
• Primary alcohol metabolites are often oxidized
  further to aldehyde and carboxylic acids and
  secondary alcohols are converted to ketones by
  soluble alcohol and aldehyde dehydrogenase

Dicarboxylic acid is the major metabolite
21
Oxidation at Allylic Carbon Atoms
22
Pentazocine
23
Hydroxylation at C a to CO and CN
The benzodiazepines are classic examples with
both functionalities
The sedative hypnotic glutethimide possesses C a
to carbonyl function
24
Aliphatic hydroxylation

• Catalyzes hydroxylation of the ? and ?-1 carbons
  in aliphatic chains
• Generally need three or more unbranched carbons

Pentobarbital Metabolism
Ibuprofen Metabolism

25
Alicyclic (nonaromatic ring) Hydroxylation

• Cyclohexyl group is commonly present in many drug
  molecules
• The mixed function oxydase tend to hydroxylate at
  the 3 or 4 position of the ring
• Due to steric factors if position 4 is
  substituted it is harder to hydroxylate the
  molecules

Acetohexamide Metabolism
26
Oxidation Involving Carbon-Heteroatom Systems

• C-N, C-O and occasionally C-S
• Two basic types of biotransformation processes
• Hydroxylation of a-C attached directly to the
  heteroatom (N,O,S). The resulting intermediate is
  often unstable and decomposes with the cleavage
  of the C-X bond
• Oxidative N-, O-, and S-dealkylation as well as
  oxidative deamination reaction fall under this
  category
• Hydroxylation or oxidation of heteroatom (N, S
  only, e.g., N-hydroxylation, N-oxide formation,
  sulfoxide and sulfone formation)
• Metabolism of some N containing compounds are
  complicated by the fact that C or N hydroxylated
  products may undergo secondary reactions to form
  other, more complex metabolic products (e.g.,
  oxime, nitrone, nitroso, imino)

27
C-N systems

• Aliphatic (1o, 2o, 3o,) and alicyclic (2o and 3o)
  amines Aromatic and heterocyclic nitrogen
  compounds Amides
• Enzymes
• CYP mixed-function oxidases a-C hydroxylation
  and N-oxidation
• Amine oxidases or N-oxidases (non-CYP, NADPH
  dependent flavoprotein and require O)
  N-oxidation

• 3o Aliphatic and alicyclic amines are metabolized
  by oxidative N-dealkylation (CYP)
• Aliphatic 1o, 2o amines are susceptible to
  oxidative deamination, N-dealkylation and
  N-oxidation reactions
• Aromatic amines undergoes similar group of
  reactions as aliphatic amines, i.e., both
  N-dealkylation and N-oxidation

28
N-Dealkylation (Deamination)

• Deamination and N-dealkylation differ only in the
  point of reference If the drug is R1 or R2 then
  it is a deamination reaction and If the drug is
  R3 or R4 then it is an N-dealkylation
• In general, least sterically hindered carbon (a)
  will be hydroxylated first, then the next, etc.
  Thus the more substituent on this C, the slower
  it proceeds branching on the adjacent carbon
  slows it down, i.e. R1, R2 H is fastest.
• Any group containing an a-H may be removed, e.g.,
  allyl, benzyl. Quaternary carbon cannot be
  removed as contain no a-H
• The more substituents placed on the nitrogen the
  slower it proceeds (steric hindrance)
• The larger the substituents are the slower it
  proceeds (e.g. methyl vs. ethyl). In general,
  small alkyl groups like Me, Et and iPro are
  rapidly removed branching on these substituents
  slows it down even more

Imipramine N-Dealkylation
29
Alicyclic Amines Often Generate Lactams
30
3oAmine drugs
Tamoxifen
Disopyramide
Lidocaine
Diphenhydramine
Chlorpromazine
Benzphetamine
Brompheniramine
Alicyclic Amine drugs
Meperidine
Morphine
Dextromethorphan
31
2o 1o Amines
Generally, dealkylation of secondary amines
occurs before deamination. The rate of
deamination is easily influenced by steric
factors both on the a-C and on the N so it is
easier to deaminate a primary amine but much
harder for a tertiary amine.
32
Exceptions Some 2o and 3o amines can undergo
deamination directly without dealkylation.
33
N-Oxidation
Aromatic amines
1 amines
2 amines
3 amines
34

• The attack is on the unbonded electrons so 3o
  amines can be oxidized
• Generally, only occurs if nothing else can
  happen, so it is a rare reaction
• Performed by both amine oxidases and hepatic
  MFOs
• Good examples would include amines attached to
  quaternary carbons since they cannot be deaminated

Chlorphentermine N-Hydroxylation
Hydroxylamine
Nitroso
Nitro
Phentermine
Amantadine
35
Amides
C-N bond cleavage via a-C hydroxylation
(formation of carbinolamide) and N-hydroxylation
reactions
36
Oxidation involving C-O System (O-Dealkylation)

• Converts an ether to an alcohol plus a ketone or
  aldehyde
• Steric hindrance discussion similar to
  N-dealkylation

Trimethoprim O-Dealkylation
37
Codeine
Phenacetin
Indomethacin
Metoprolol
Prazosin

• One exception that appears to be a form of
  O-dealkylation is the oxidation of ethanol by
  CYP2E1
• In this case R3 is hydrogen instead of carbon to
  form the terminal alcohol rather than an ether
• The enzyme involved is CYP2E1 and has been
  historically referred to as the Microsomal
  Ethanol Oxidizing System (MEOS)

38
Oxidation involving C-S System

• S-Dealkylation
• Desulfuration
• S-Oxidation

Steric hindrance discussion similar to
N-dealkylation
39
(No Transcript)
40
(No Transcript)
41
Oxidative Dehalogenation

• Requires two halogens on carbon
• With three there is no hydrogen available to
  replace
• With one, the reaction generally wont proceed
• The intermediate acyl halide is very reactive

Q. What is Gray Baby Syndrome?
42
Hepatic Microsomal Flavin Containing
Monooxygenases (MFMO or FMO)

• Oxidize S and N functional groups
• Mechanism is different but end products are
  similar to those produced by S and N oxidation by
  CYP450
• FMOs do not work on primary amines
• FMOs will not oxidize substrates with more than
  a single charge
• FMOs will not oxidize polyvalent substrates

Cimetidine MFMO S-Oxidation
Q. What is the difference with MFO?
43
Non-Microsomal Oxidation Reactions

• Monoamine oxidase (outer membrane of
  mitochondria, flavin containing enzyme )
• Dehydrogenases (cytoplasm)
• Purine oxidation (Xanthene oxidase)

Monoamine oxidase

• Two MAOs have been identified MAOA and MAOB.
  Equal amounts are found in the liver, but the
  brain contains primarily MAOB MAOA is found in
  the adrenergic nerve endings
• MAOA shows preference for serotonin,
  catecholamines, and other monoamines with
  phenolic aromatic rings and MAOB prefers
  nonphenolic amines
• Metabolizes 1 and 2 amines N must be attached
  to a-carbon both C N must have at least one
  replaceable H atom. 2 amines are metabolized by
  MAO if the substituent is a methyl group
• bPhenylisopropylamines such as amphetamine and
  ephedrine are not metabolized by MAOs but are
  potent inhibitors of MAOs

44
Alcohol dehydrogenase
Aldehyde dehydrogenase
Metabolizes 1 and 2 alcohols and aldehydes
containing at least one H attached to a-C 1
alcohols typically go to the aldehyde then acid
2 alcohols are converted to ketone, which cannot
be further converted to the acid. The aldehyde is
converted back to an alcohol by alcohol (keto)
reductases (reversible), however, it goes forward
as the aldehyde is converted to carboxylic acid
3 alcohols and phenolic alcohols cannot be
oxidized by this enzyme No H attached to
adjacent carbon
Ethanol Metabolism
Purine oxidation
Molybdenum Containing
45
Reductive Reactions

• Bioreduction of CO (aldehyde and keton)
  generates alcohol (aldehyde ? 1o alcohol ketone
  ? 2o alcohol)
• Nitro and azo reductions lead to amino
  derivatives
• Reduction of N-oxides to their corresponding 3o
  amines and reduction of sulfoxides to sulfides
  are less frequent
• Reductive cleavage of disulfide (-S-S-) linkages
  and reduction of CC are minor pathways in drug
  metabolism
• Reductive dehalogenation is a minor reaction
  primarily differ from oxidative dehalogenation is
  that the adjacent carbon does not have to have a
  replaceable hydrogen and generally removes one
  halogen from a group of two or three

46
Reduction of Aldehydes Ketones

• CO moiety, esp. the ketone, is frequently
  encountered in drugs and additionally, ketones
  and aldehydes arise from deamination
• Ketones tend to be converted to alcohols which
  can then be glucuronidated. Aldehydes can also be
  converted to alcohols, but have the additional
  pathway of oxidation to carboxylic acids
• Reduction of ketones often leads to the creation
  of an asymmetric center and thus two
  stereoisomeric alcohols are possible
• Reduction of a, b unsaturated ketones found in
  steroidal drugs results not only in the reduction
  of the ketone but also of the CC
• Aldoketo oxidoreductases carry out bioreductions
  of aldehydes and ketones. Alcohol dehydrogenase
  is a NAD dependent oxidoreductase that oxidizes
  alcohols but in the presence of NADH or NADPH,
  the same enzyme can reduce carbonyl compounds to
  alcohols

47
Naloxone
Daunomycin
Naltrexone
48
(No Transcript)
49
Reduction of Nitro Azo Compounds
50

• R1 and R2 are almost always aromatic
• Usually only seen when the NO2 functional group
  is attached directly to an aromatic ring and are
  rare
• Nitro reduction is carried out by NADPH-dependent
  microsomal and soluble nitroreductases (hepatic)
• NADPH dependent multicomponent hepatic microsomal
  reductase system reduces the azo
• Bacterial reductases in intestine can reduce both
  nitro and azo

Dantrolene
Sulfasalazine
Clonazepam
51
Reduction of Sulfur Containing Compounds
Sulfoxide reduction (Cannot reduce a sulfone)
X
Sulfoxide
Sulfone
Disulfide reduction
Sulindac
52
Hydrolytic Reactions
Hydrolyzes (adds water to) esters and amides and
their isosteres the OH from water ends up on the
carboxylic acid (or its isostere) and the H in
the hydroxy or amine

• Enzymes Non-microsomal hydrolases however,
  amide hydrolysis appears to be mediated by liver
  microsomal amidases, esterases, and deacylases
• Electrophilicity of the carbonyl carbon, Nature
  of the heteroatom, substituents on the carbonyl
  carbon, and substituents on the heteroatom
  influnce the rate of hydrolysis
• In addition, Nucleophilicity of attacking
  species, Electronic charge, and Nature of
  nucleophile and its steric factors also influence
  the rate of hydrolysis

Table Naming carbonyl - heteroatom groups
53
The Reactions
Ester hydrolysis
Amide hydrolysis (slower)
Carbonate hydrolysis
Carbamate hydrolysis
Urea hydrolysis
Hydrazide hydrolysis
54
Drug Examples
Indomethacin
Prazosin
Lidocaine
55
Stereoselectivity of Hydrolysis

• Etomidate (Amidate, hypnotic) R-()-isomer is
  more rapidly hydrolyzed, but S-(-)-isomer is more
  rapidly hydroxylated.

56
The Concept of Prodrugs and Antedrugs

• Prodrug Need metabolic activation
• Antedrug Active drug that is quickly inactivated
  thereby minimizing systemic effects

57
Prodrugs and Related Terms

• Albert in 1958 coined the term prodrug to refer a
  pharmacologically inactive compound that is
  metabolically activated in the mammalian system
• Hard Drugs are not susceptible to metabolic or
  chemical transformation, have high lipid
  solubility and thus accumulation or high water
  solubility
• Celecoxib t1/2 10-12 h in humans t1/2 ca.
  680 h (Liver toxicity)
• Soft drugs are active compounds that after
  exerting its action undergo inactivation to give
  a nontoxic product. Indeed soft drugs are a group
  of modified compounds that are also designed to
  delivery the drugs in to the brain (the chemical
  delivery system). Bodor coined the term.

58
Basic Concepts of Prodrugs

• Carrier-linked prodrugs a pro-moiety is
  attached, which is not necessary for activity but
  may impart some desired property to the drug,
  such as increased lipid or water solubility, or
  site-directed delivery
• Advantages may include
• increased absorption
• alleviation of pain at the site of injection if
  the agent is given parenterally
• elimination of an unpleasant taste associated
  with the drug
• decreased toxicity
• decreased metabolic inactivation
• increased chemical stability
• prolonged or shortened action
• Bioprecursor prodrugs contain no pro-moiety but
  rather rely on metabolism to introduce the
  functionality necessary to create an active
  species

59
Prodrug Chloramphenicol Hemisuccinate Na Salt
Prodrug Prednisolon Hemisuccinate Sodium Salt

• Inactive as it is and activated by hydrolysis by
  plasma esterases to chloramphenicol/ prednisolon
• Increased water solubility for parenteral
  administration, which otherwise would precipitate
  and cause pain by damaging surrounding tissues

Prodrug Chloramphenicol Palmitate
Prodrug Clindamycin Palmitate

• Inactive as it is activated by hydrolysis by
  intestinal esterases to chloramphenicol/
  clindamycin
• Minimize their bitter taste and improve their
  palatability in pediatric liquid suspensions

60
Prodrug Carbenicillin Indanyl Ester

• Inactive as it is and activated by hydrolysis by
  plasma esterases to carbenicillin
• Lipophilic indanyl ester furnish improved oral
  bioavailability

61
Prodrugs of Functional Groups

• Carboxylic acids and alcohols Most common
• Amines and azo linkages Not been used much
• Carbonyl compounds Not found to be used widely

62
Carboxylic Acids and Alcohols
Converted to ester prodrugs which are often
hydrolyzed to active drug by different types of
esterase enzymes Ester hydrolase Lipase Choleste
rol esterase Acetylcholinesterase Carboxypeptidase
Cholinesterase Microflora in the
gut Manipulation of steric and electronic
properties of promoiety allows control of rate
and extent of hydrolysis
63
Advantage of Prodrug Formation I Increased
absorption of hydrophilic drugs by making less
hydrophilic or more lipophilic

• Prodrug of Epinephrine Dipivefrin
• More lipophilic, thus achieve higher intraocular
  concentration
• Hydrolysis occur in cornea, conjunctiva, and
  aqueous humor after ophthalmic application

64
Advantage of Prodrug Formation II Masking
unpleasant taste
Chloramphenicol palmitate and Clindamycin
palmitate has already been shown. Other drugs
include
N-Acetyl sulfisoxazole
Erythromycin estolate
Troleandomycin
65
Not all carboxylic esters hydrolyzed in vivo
where double ester approach is used
66
(No Transcript)
67
Advantage of prodrug formation III Increase
hydrophilicity and thus water solubility to apply
parenterally or also orally when compounds are
too lipophilic to formulate in liquid dosage form
68
(No Transcript)
69
Chemical Delivery System
The site specific delivery of drugs is an
important way of increasing drugs therapeutic
index. The knowledge of prodrug and drug
metabolism is used to concentrate drugs at its
target site thus minimizing the systemic
toxicity.
70
Antedrugs (Soft Drugs)
I stopped taking medicine as I prefer original
disease to side effects !!
Why ?
Because, Vioxxll treat pain but wholl treat
vioxx ??
71
Safety-Based Drug Withdrawals from U.S. Market
(2006-2007)
72
Why the Adverse Drug Reactions Occur?

• Because of unintended systemic actions in most
  therapeutic classes of drugs

To bring a drug from concept to market

• It takes about 10-15 years
• 897 millions to 1.7 billions
• Overall attrition rate 10,0001

73
What is Antedrug?
74
(No Transcript)
75
Chemical Approaches

• The Carboxylic Esters and Amides
• 20-Thioester Derivatives
• g-Butyrolactone Derivatives

76
(No Transcript)
77
Advantages of Antedrugs

• Localization of the drug effects
• Elimination of toxic metabolites, increasing the
  therapeutic index
• Avoidance of pharmacologically active metabolites
  that can lead to long-term effects
• Elimination of drug interactions resulting from
  metabolite inhibition of enzymes
• Simplification of PK problems caused by multiple
  active species

M.O.F.Khan, K.K.Park, H.J.Lee. Antedrugs An
Approach to Safer Drugs. Curr. Med. Chem.,
12(19), 2227-2239, 2005.
78
Phase II Drug Conjugation

• Attachment of small polar endogenous molecules
  such as glucuronic acid, sulfate and amino acids
  to Phase I metabolites or parent drugs
• Products are more water-soluble and easily
  excretable
• Attenuate pharmacological activity and thus
  toxicity
• Trapping highly electrophilic molecules with
  endogenous nucleophiles such as glutathione
  prevent damage to important macromolecules (DNA,
  RNA, proteins)
• Regarded as true detoxifying pathway (with few
  exceptions)
• In general, appropriate transferase enzymes
  activate the transferring group (glucuronate,
  sulphate, methyl, acetyl) in a coenzyme form

79
Glucuronic Acid Conjugation

• Glucuronidation is the most common conjugation
  pathway
• The coenzyme, UDP glucuronic acid is synthesized
  from the corresponding phosphate
• UDP-glucuronic acid contains D-glucuronic acid in
  the a-configuration at the anomeric center, but
  glucuronate conjugates are b-glycoside, meaning
  inversion of stereochemistry is involved in the
  glucuronidation
• Glucuronides are highly hydrophilic and water
  soluble
• UDP glucuronosyltransferase is closely associated
  with Cyp450 so that Phase I products of drugs are
  efficiently conjugated
• Four general classes of glucuronides O-, N-, S-,
  and C-
• Neonates have undeveloped liver
  UDP-glucuronosyltransferase activity, and may
  exhibit metabolic problem. For example,
  chloramphenicol (Chloroptic) leads neonates to
  gray baby syndrome
• Neonatal jaundice may be attributable to their
  inability to conjugate bilirubin with glucuronic
  acid

80
Formation of Glucuronide Conjugate
81
Types of Compounds Forming Glucuronides
82
Salicylic acid
Aryl acids
Fenoprofen
Arylalkyl acids
N-Glucuronides
Sulfonamides
7-Amino-5-nitroindazole
Arylamines
Sulfisoxazole
Alkylamines
3o Amines
Desipramine
Cyproheptadine
Meprobamate
Amides
83
S-Glucuronides
Methimazole
Sulfhydryl
Carbodithioic acid
Disulfirum (reduced form)
C-Glucuronides
Phenylbutazone
84
Sulfate Conjugation

• Occurs less frequently than does glucuronidation
  presumably due to fewer number of inorganic
  sulfates in mammals and fewer number of
  functional groups (phenols, alcohols, arylamines
  and N-hydroxy compounds)
• Three enzyme-catalyzed reactions are involved in
  sulfate conjugation

85
Sulfation of Drugs

• Phenolic sulfation predominates
• Phenolic O-glucuonidation competes favorably with
  sulfation due to limited sulfate availability
• Sulfate conjugates can be hydrolyzed back to the
  parent compound by various sulfatases
• Sulfoconjugation plays an important role in the
  hepatotoxicity and carcinogenecity of
  N-hydroxyarylamides
• In infants and young children where
  glucuronyltransferase activity is not well
  developed, have predominating O-sulfate
  conjugation
• Examples include a-methyldopa, albuterol,
  terbutaline, acetaminophen, phenacetin

a-Methyldopa
Terbutaline
Albuterol
86
Possible Mechanism of Phenacetin Toxicity
Electrophilic nitreneum
87
Amino Acid Conjugation

• The first mammalian drug metabolite isolated,
  hippuric acid, was the product of glycine
  conjugation of benzoic acid
• Amino acid conjugation of a variety of caroxylic
  acids, such as aromatic, arylacetic, and
  heterocyclic carboxylic acids leads to amide bond
  formation
• Glycine conjugates are the most common
• Taurine, arginine, asparagine, histidine, lysine,
  glutamate, aspartate, alanine, and serine
  conjugates have also been found

88
Mechanism of Amino Acid conjugation
89
Brompheniramine Metabolism
90
Glutathione Conjugation

• Glutathione is a tripeptide (Glu-Cys-Gly) found
  virtually in all mammalian tissues
• Its thiol functions as scavenger of harmful
  electrophilic parent drugs or their metabolites
• Examples include SN2 reaction, SNAr reaction, and
  Michael addition

91
SN2 Examples
92
SNAr Examples
93
Michael Addition
94
Mercapturic Acid Conjugates
95
Acetyl Conjugation

• Metabolism for drugs containing a primary amino
  group, (aliphatic and aromatic amines), amino
  acids, sulfonamides, hydrazines, and hydrazides
• The function of acetylation is to deactivate the
  drug, although N-acetylprocainamide is as potent
  as the parent antiarrhythmic drug procainamide
  (Procanbid) or more toxic than the parent drug,
  e.g., N-acetylisoniazid
• Acetylation is two-step, covalent catalytic
  process involving N-acetyl transferase

Genetic polymorphism in N-acetyltransferase
activity Multiple NAT2 alleles (NAT25, 6, 7,
and 14) have substantially decreased acetylation
activity and are common in Caucasians and
populations of African descent. In these groups,
most individuals carry at least one copy of a
slow acetylator allele, and less than 10 are
homozygous for the wild type (fast acetylator)
trait. The ratio of NAT2 activity is 7 in
Caucasians to 18 in the Chinese population.
96
Example of Acetylated Drugs
97
Fatty Acid and Cholesterol Conjugation

• Hydroxyl-containing drugs can undergo conjugation
  with a wide range of endogenous fatty acids such
  as saturated acids from C10 to C16 and
  unsaturated acids such as oleic and linoleic
  acids
• Cholesterol ester metabolites have been detected
  for drugs containing either an ester or a
  carboxylic acid

98
Methyl Conjugation

• Minor conjugation pathway, important in
  biosynthesis of epinephrine and melatonin in the
  catabolism of norepinephrine, dopamine,
  serotonin, and histamine and in modulating the
  activities of macromolecules (proteins and
  nucleic acids)
• Except for the formation of quarternary ammonium
  salts, methylation of an amine reduces the
  polarity and hydrophilicity of the substrates
• A variety of methyl transferase, such as COMT
  (catechol O-methyl transferase),
  phenol-O-methyltransferase, N-methyl transferase,
  S-methyltransferase etc are responsible for
  catalyzing the transfer of methyl group from SAM
  to RXH

99
Case Study
Case 2. Imagine yourself as a drug information
specialist at a poison control center. A
technician from the coroners office is
investigating a case and requires assistance in
identifying the possible sources of
benzodiazepines (BZDs) in the toxicology profile
of a particular corpse. The technician has
identified four distinct BZDs in this blood
sample. She believes that the major component is
diazepam (1) (72 of the identified BZDs) and
that the remaining three components are
metabolites (NOTE the assay identifies only
active compounds).
Q. What are the three structures of potential
ACTIVE metabolites for diazepam?
Assignment Due by this Friday
http//www-home.cr.duq.edu/harrold/basic_concepts
_index.html
100
Study Guide

• What Roles are Played by Drug Metabolism? Know
  with structural examples
• Role of stereochemistry in metabolism of drugs
  with example of warfarin, ibuprofen and itomidate
• What is first pass effect enterohepatic
  circulation? Why and how they occur? Drug
  examples
• Metabolisms in the intestinal mucosa
• CYP450, Hepatic microsomal flavin containing
  monooxygenases (MFMO or FMO) Monoamine Oxidase
  (MAO) and Hydrolases. Drugs metabolised by these
  enzymes and the active sites of these enzymes.
  Types of metabolic reaction catalyzed by these
  enzymes
• Specific CYP enzymes with the number of drugs
  they metabolize
• Few CYP family with their main functions
• Drug interaction basics related to metabolic
  enzymes

101
Study Guide Cont.

• Mechanism and routes of aromatic hydroxylation.
  The effects of electron donating and withdrawing
  groups in aromatic hydroxylation. Drug examples.
  What is NIH shift?
• Oxidation of olefins. Role of epoxide hydrolase.
  Can olefenic epoxide be converted to alcohol as
  in aromatic epoxide by NIH shift?
• What type of C in a drug molecule can not be
  hydroxylated?
• What is allylic and benzylic hydroxylation? Show
  drug examples.
• Show the drug examples where hydroxylation occur
  on Ca to CO and CN bonds
• Show the drug examples where hydroxylation occur
  at aliphatic and alicyclic carbon atoms. Which
  carbons are more easily hydroxylated?
• What is N-oxidatin and N-dealkylation. What
  enzymes are involved? How do you differentiate
  between N-dealkylation and deamination. Drug
  examples. What types of drugs generates lactams
  instead of causing dealkylation?
• What is the difference between mixed function
  oxidases and amine oxidases?

102
Study Guide Cont.

• What is the difference between ethanol oxidation
  and O-dealkylation?
• What is S-dealkylation, desulfuration and
  S-oxidation? Drug examples.
• How does steric factors influence S- O- and
  N-dealkylations?
• Oxidative dehalogenation with special example of
  chloramphenicol. Why chloramphenicol cause
  toxicity to the babies?
• What is MFMO and its active site? What types of
  functional groups are metabolized by this enzyme?
  Drug examples.
• MAO, dehydrogenases, xanthene oxidases and their
  functions with drug examples. Difference between
  MAO-A and MAO-B.
• Alcohol and aldehyde dehydrogenases, the
  coenzymes and the types of drugs they work on.
• Azo and nitro reductases, their coenzymes and the
  drugs they act on.

103
Study Guide Cont.

• Different types of hydrolytic enzymes. Compare
  rate of hydrolysis of esters, amides, carbonates
  and carbamates.
• What are prodrugs and antedrugs? What are the
  advantages? Examples.
• What are different types of conjugation
  reactions?
• The enzymes and substrates involved in
  glucuronidation, and sulfate conjugation.
• Why acetaminophen is toxic to neonates? Mechanism
  of phenacetin and acetaminophen toxicity.
• What types of drugs or metabolites may form
  glycin conjugates?
• What are different mechanisms involved in
  glutathione conjugation? What is mercapturic acid
  conjugate? Mercapturic acid conjugate of
  acetaminophen is a sign of its toxicity why?
• Mechanism of acetylation. What is slow and fast
  acetylator?
• What is COMT? What coenzymes is involved in its
  action? What types of drugs and/or
  neurotransmitters are metabolized by COMT?