Metabolism

1
Metabolism

• Elimination of drugs and foreign compounds
• Occurs primarily in liver hepatocytes
• Most organics are lipophillic
• Lipid soluble compounds reabsorbed (renal
  tubules)
• Metabolized to water soluble products for
  excretion
• Metabolites are inactive and relatively non-toxic
• Detoxification process
• Drug metabolites can be the source of toxic
  effects
• Parent drug inactive but metabolite is active
  (Prodrug)

2
General Metabolic Pathways

• Phase I reactions Functionalization
• Oxidation
• Reduction
• Hydrolytic reactions
• Purpose
• Introduction of polar functional groups in a
  molecules
• Hydroxyl groups
• Carboxylate groups
• Amino Groups
• Thiol Groups
• Increase a molecules polarity
• May not produce metabolites for excretion
• Does provide a site for phase II metabolism

3
General Metabolic Pathways

• Phase II reactions Conjugation
• Purpose
• Introduce highly polar conjugates
• Glucuronic acid
• Sulfate
• Detoxification
• Glycine or other Amino Acids (some solubility)
• Acetyl
• Methylations
• Glutathione
• Site of attachment often introduced in Phase I
• Hydroxyl
• Carboxylate
• Amino
• Conjugates are readily excreted in the urine
• increased water solubility relative to the parent
  compound

4
?1-Tetrahydrocannabinol (?9)

• Undergoes a series of metabolic steps prior to
  urine excretion

5
Factors that influence metabolism

• Age
• older people less efficient at metabolism
• Sex
• Linked to hormonal differences
• Heredity
• Genetic differences can influence amounts and
  efficiency of metabolic enzymes
• Disease states
• Liver, cardiac, kidney disease

6
Sites of Metabolism

• Liver
• Primary site!
• Highly perfused organ
• Rich in enzymes
• Acts on endogenous and exogenous compounds
• First pass effect!
• Extrahepatic metabolism sites
• Intestinal wall
• Sulfate conjugation
• Esterase and lipases - important in prodrug
  metabolism
• i.e. b-glucuronidase enzymes hydrolyze
  glucuronides for reabsorption
• Enterohepatic recirculation
• Bacterial flora
• Reduction of Aromatic nitro and azo compounds
• Lungs, kidney, placenta, brain, skin, adrenal
  glands
• Limited ability and largely unknown

7
First Pass Effect
8
First Pass Effect

• Green 1 g dose Red dots 2 g dose.
• Normally amount of drug present is no more than
  the enzymes can handle.
• Metabolism then proceeds in apparent first order
  fashion.
• However for some drugs the enzymes can not keep
  up.
• Saturation of first pass metabolism allows a
  higher fraction of drug to be absorbed.

9
First Pass Effect Example
Oxidative Deamination
Hydroxylation
Propanolol is a b-blocker commonly used in
patients with heart disease.
10
HELP!!!

• Things to keep in mind with metabolism
• Phase I or II?
• Look for a change
• Oxidation/Reduction/Hydrolysis vs.
  Functionalization
• What are the likely sites for the reaction?
• What is the responsible enzyme/system?
• Where is the reaction likely to occur?
• What is the purpose of the reaction?

11
Cytochrome P-450 Oxidative

• Addition of an oxygen atom or bond
• Most common process (Liver)
• General chemical equation is
• RH NADPH O2 H ? ROH NADP H2O
• Mixed function oxidases or monooxygenases located
  in the liver hepatocyte endoplasmic reticulum
• Require NADH or NADPH and O2 as cofactors
• Cytochrome P-450 or cytochrome b5 enzymes
• Heme proteins
• Iron containing porphyrin - binds O2
• Works on a large number of diverse compounds

12
What is a Heme Protein?
Cytochrome P-450, Hemoglobin, Myoglobin ALL
Heme Proteins!
13
Cytochrome P-450 Oxidative

• Structural diversity due to
• Nonspecificity
• Isozymes - multiple forms of an enzyme
• Enzymes are inducible by various chemicals
• Exposure increases the rate of enzyme production
• Enzymes isolated by disruption of the liver cells
• Endoplasmic reticulum - microsomes when disrupted
• Enzymes are membrane bound
• Explains why lipophilic drugs are processed
• Catalytic process ? heme binds O2

14
Cytochrome P-450 Oxidative

• Isozymes differ in protein structure
• Different amino acid sequences
• Produce different 3-D structures
• Drug bound to the protein portion
• Remember
• All activated oxygen chemistry occurs at the iron
  center heme with oxygen transfer to the protein
  bound substrate

15
Cytochrome P-450 Cycle
16
Nomenclature
Allyl
o
m
Vinyl
p
Benzyl
Aromatic
Alicyclic
Aliphatic
17
Oxidation Reactions

• Oxidation of aromatic rings
• Proceed via an intermediate arene oxide (epoxide)
• -OH always goes in the para- position
• If blocked, goes to the ortho- position
• Occurs most easily in Electron rich systems
• e- withdrawing groups slow or prevent oxidation

18
Oxidation Reactions

• Phase 1Oxidation Phase 2Conjugation

19
Oxidation Reactions

• Selectivity
• Oxidation occurs at the most e- rich carbon

20
Oxidation Reactions
21
Toxicology

• Metabolism leading to a carcinogen
• Oxidation product reacts with DNA

Benzoapyrene
22
Oxidation of CC (olefins)

• Similar to aromatic compounds
• Epoxide hydrases form trans 1,2-diols
• Subject to glutathione conjugation
• Most epoxides are unstable (some isolated)

23
Other Olefinic Oxidations
24
Oxidation at Benzylic Carbons

• Methyl groups primary substrate
• Methylene groups secondary groups

25
Benzylic Oxidations
26
Oxidation at Allylic Carbons

• Allylic carbon C adjacent to a CC
• A hydroxyl group is introduced
• Steric hinderance can influence this oxidation

27
Oxidation at Allylic Carbons
28
Oxidation C alpha to CO or CN

• An imine is a carbon double bonded to a nitrogen
  (CN)

29
Oxidation Saturated Carbons

• Occurs in drugs with straight or branched alkyl
  chains
• Oxidation at terminal methyl groups omega w
• Oxidation at the next to the last carbon omega
  1 (w-1)
• Initially inserts a hydroxyl
• Oxidation can continue - aldehydes, ketones or
  acid groups
• Also occurs in saturated alkyl rings

30
OxidationSaturated Carbons
31
Oxidation at alicyclic carbons
Position 3 or 4 of the cyclohexyl ring
32
Oxidation Carbon-Heteroatom

• Hydroxylation or oxidations of heteroatom
• Cytochrome P-450 mixed function oxidase enzymes
• N-hydroxylation and N-oxidations are preformed
  also by amine oxidases (N-oxidases)
• NADPH-dependent flavoproteins require O2
• DO NOT contain ferric ion-heme centers

33
Oxidation Carbon-Heteroatom

• Oxidation of carbon attached to O, S, N
• Also called N-, O-, and S- dealkylation Rx

Tertiary aliphatic and alicyclic amines
34
Tertiary aliphatic and alicyclic amines
This is called an Oxidative dealkylation
35
Tertiary aliphatic and alicyclic amines
More examples of Oxidative dealkylation
36
Oxidation Primary Secondary Amines

• Secondary amines subject to N-dealkylations,
  oxidative deamination, and N-oxidations
• N-dealkylation proceeds similar to tertiary amines

37
Oxidation Primary Secondary Amines
If secondary amine contains methyl or similar
group ?nitrone
This is an aside and nitrones are not a common
metabolic product
38
Oxidation Primary Secondary Amines

• Primary amines
• N-oxidation occurs when a-carbon oxidation cant
• tertiary carbon
• This limitation is NOT an absolute (next slide)

39
Oxidation Primary Secondary Amines
Remember N-hydroxy compounds are unstable!
40
Aromatic amines/Heterocyclic N-compounds

• Like C and N oxidation for primary alkylamines
• Tertiary aromatic amines N-dealkylations,
  N-oxides
• Secondary aromatic amines N-dealkylations,
  N-hydroxylations
• Two above are relatively rare in drugs
• Primary aromatic amines are widespread,
  generated by
• reduction of nitro groups
• reductive cleavage of azo compounds
• hydrolysis of aromatic amides
• Primary aromatic amines ? N-hydroxyl Nitroso
  compounds
• Simple example Aniline

41
Oxidation of Nitrogen Heterocycles
42
Oxidation of Amides
Hydroxylation alpha to the amine nitrogen
43
Oxidation of Amides
Hydroxylation alpha to the amine nitrogen
44
Oxidation of C-O systems
Drugs that O-dealkylate indomethacin,
prazosin, metoprolol, trimethoprim
45
Oxidation of C-S bonds

• S-dealkylations, desulfurizations and
  S-oxidations
• S-dealkylation involves a-carbon hydroxylation
• Not often observed
• Few S-containing drugs
• S-oxidations competition
• Desulfurization CS conversion to CO

46
Sulfur oxidations
47
Alcohols and Aldehyde Oxidation
Catalyzed by alcohol dehydrogenases (liver)
NADP may also be a cofactor
Other oxidative pathways

• Oxidative dehalogenations

48
Oxidative aromatization or dehydrogenation
Other Oxidations
Aromatase Inhibitors used to treat breast
cancer in post menopausal women
49
Reductive Reactions

• Occasionally convert carbonyl, nitro and azo
  groups
• Carbonyl reductions generate alcohols
• Nitro and Azo reductions generate amines
• Alcohols and amines ?Phase II facilitate
  elimination
• Reductions of N-oxides and sulfoxides to sulfides
  are rare
• Reduction of S-S disulfide linkages occur (minor
  pathways)
• Reduction of ketones ? secondary alcohols
• Reduction of aldehydes ? primary alcohols (rare)
• Reactions mediated by Aldo-Keto reductase enzymes
• Liver and kidney, use NADPH
• Example Liver alcohol dehydrogenase
• Acts both as oxidizing and reducing enzyme
  depending on availability of either (NAD or
  NADP) or (NADH or NADPH)

50
Reductive reactions cont
Bioreduction of ketones is often stereoselective
51
Nitro and Azo Reduction
NADPH dependent microsomal and nitro-reductase
enzymes Bacterial reductases play a role in
enterohepatic recirculation of nitro or azo
containing drugs
52
Nitro and Azo Reduction
53
Misc. Reductions
54
Hydrolysis of Esters and Amides

• Catalyzed by widely distributed hydrolytic
  enzymes
• Esters ? alcohols, phenols and carboxylic acids
• Non-specific esterases (liver, kidney, and
  intestine)
• Plasma pseudocholinesterases also participate
• Amides ? amines and carboxylic acids
• Liver microsomal amidases, esterases and
  deacylases
• Hydrolysis of esters ? major metabolic pathway
  for ester drugs
• Why? Ease of hydrolysis

55
Hydrolysis of Esters and Amides

• Amides are more difficult to hydrolyze than
  esters

56
Hydrolysis of Esters and Amides

• Many prodrugs are esters
• Many new biotech. drugs are recombinant human
  peptide drugs and hormones
• Examples
• Human insulin
• Growth hormone
• Prolactin
• Various carboxypeptidases, aminopeptidases and
  other proteases hydrolyze these drugs and
  hormones
• ß-Glucuronidases hydrolyze sugars from cardiac
  glycosides
• Also phosphatases, sulfatases, epoxide hydrases

57
Phase II Conjugation Rx

• Purpose Attach polar, ionizable small molecule
• Result Sufficient water solubility for kidney
  excretion
• Typically glucuronic acid, sulfate, amino acids
• Glutathione (GSH) conjugation occurs to combine
  with chemically reactive compounds to prevent
  their reacting with DNA, RNA and other proteins
  (Detoxification)
• Other Phase II conjugations
• Terminate or attenuate drug pharmacology
• Methylation (attachment of a methyl group)
• Acetylation (attachment of an acetyl group)
• Phase II feature
• Conjugating group is activated in the form of a
  coenzyme and involves a transferase enzyme

58
Glucuronic acid conjugation

• Glucuronic acid attachment is common
• Derived from D-glucose
• Carboxylic acid group pKa 3.2 so nearly 100
  ionized
• Occur at hydroxyl groups, carboxylic acid
  groups, amino groups, thiol groups and rarely
  carbon atoms
• UDP-glucuronyl transferase enzyme mediates this
  process
• Liver, lung, kidney, skin, brain and intestine)
• Stereospecific process ß - orientation product

59
Oxygen glucuronides

• Attachment sites are hydroxyls
• Alcohols, phenols, enols, N-hydroxyls, acids
• Most common phenolic and alcohol sites
• Di-glucuronic acid conjugates very rare
• Oxygen site often from Phase I
• Enol example

60
Oxygen glucuronides cont
Alcohol hydroxyl example
Phenol hydroxyl example
61
Oxygen glucuronides cont
Carboxylate hydroxyl example

N-hydroxyl example
62
Sulfate Conjugation

• Occurs primarily with phenols
• Rarely alcohols, aromatic amines, and N-hydroxyl
  compounds
• Catalyzed by sulfotransferases
• liver, kidney and intestine
• Pool of sulfate available is limited
• Leads to inactive water-soluble metabolites
• Glucuronate conjugation often more competitive
  process

sulfotransferase
AMP-sulfate
63
Sulfate Conjugation
64
Amino Acid Conjugation

• Amino acids are conjugated with carboxylic acids
• Occurs in the mitochondria of liver and kidney
  cells
• Conjugation in humans mostly glycine and
  L-glutamine
• Conjugation is limited by amino acid pool
  available
• Conjugates excreted in the urine via kidney
  (occasionally bile)
• The carboxylic acid to be conjugated
• Activated to form an active thioester from
  coenzyme A and ATP
• Amino acid N-acyl-transferase enzymes mediate
  reaction

65
Conjugation with glycine
66
Glutathione (GSH) Conjugation

• DETOXIFICATION of electrophiles!
• Electrophilic chemicals cause
• Tissue necrosis
• Carcinogenicity
• Mutagenicity
• Teratogenicity
• The thiol (SH group) ties up potent electrophiles
  
• GSH is in most tissues
• GSH conjugates are NOT typically excreted
  further metabolized
• Cyplasmic enzyme mediator glutathione
  S-transferase
• Liver and kidney
• No preactivation needed
• Requirements sufficient electrophilicity
• SH nucleophilic displacement
• SH addition to an activated double bond (Michael
  additions)

67
Glutathione (GSH) Conjugation
68
GSH Nucleophilic Displacement
69
GSH vs. Epoxides
Glutathione addition to electrophilic activated
double bonds Michael addition nucleophilic
addition to an a,b unsaturated carbonyl
compound
70
GSH Michael Addition

• May occur with a,b unsaturated aldehydes,
  ester, nitriles etc.
• Steric hinderence of the double bond prevents
  Michael addition!

71
Acetylation

• Important for drugs with primary amino groups
• Generally, metabolites are nontoxic and inactive
• Acetylation does NOT increase water solubility
• Detoxification or termination of drug activity
• Acetylation rarely leads to increased activity or
  toxicity
• Acetyl group is provided by Acetyl-CoA cofactor
  and mediated by N-acetyltransferase enzymes
• Primary site hepatic reticuloendothelial cells
• Also lung, spleen, gastric mucosa, RBC, and
  lymphocytes

72
N-Acetylation of Drugs
73
Metabolism via Acetylation

• Acetylation with Acetyl-CoA is either fast or
  slow
• Genetic differences in N-acetyltransferase
  activity
• Most Eskimo and Asian people are FAST acetylators
• Egyptians and some Western European SLOW
  acetylators
• Intermediate blends among other ethnic groups
• SLOW acetylators more likely to show toxicity or
  adverse reactions to drugs
• FAST acetylators more likely to show an
  inadequate therapeutic response to standard doses
  of drugs

74
Metabolism via Acetylation

• Example Isoniazid used for tuberculosis
• SLOW t1/2 140-200 minutes
• Higher plasma accumulation and higher cure rate
• More adverse side effects and drug-drug
  interactions
• Example of drug interaction phenytoin use with
  isoniazid
• Isoniazid inhibits phenytoin metabolism leading
  to accumulation of high and toxic plasma levels
  of phenytoin
• Fast t1/2 45-80 minutes
• Lower plasma accumulation and lower cure rate
• More associated liver damage and hepatitis with
  isoniazid due to the more rapid formation of more
  acetylhydrazine

75
Metabolism via Methylation

• S-Adenosylmethionine (SAM) Methyltransferase
• Key for biosynthesis of many compounds
• Important in the inactivation of physiologically
  active biogenic amines ? neurotransmitters
• norepinephrine, dopamine, serotonin, histamine
• Minor pathway in the metabolism of drugs

76
Metabolism via Methylation

• Methylation does NOT increase water solubility
• Most methylated products are inactive
• Important methyl transferase enzymes involved
• Catechol-O-methyl transferase (COMT)
• Phenol-O-methyltransferase
• Nonspecific N-methyl transferase
• S-methyltransferase
• Catechol-O-methyl transferase (COMT)
• Widely distributed in tissues especially the
  liver and kidney
• Responsible for inactivation of dopamine
  norepinephrine

77
Methylation examples
Resorcinols (1,3 hydroxyl) are NOT substrates for
COMT
78
Methylation examples