Biotransformation Xenobiotic metabolism

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BiotransformationXenobiotic metabolism

• Essentials of Toxicology

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Biotransformation

• Biotransformation means chemical alteration of
  chemicals such as nutrients, amino acids, toxins,
  xenobiotics or drugs in the body. It is also
  needed to render nonpolar compounds polar so that
  they are not reabsorbed in renal tubules and are
  excreted.
• Biotransformation may results into-
• Active Inactive form
• Active Active or
  toxic form
• Inactive Active form
• Unexcretable Excretable form
  

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Biotransformation
Results of biotransformation
Drug or Poison
biotransformed Drug or Poison

• In general -
• nonsynthetic precede synthetic reactions
• nonsynthetic reactions can produce active
  metabolites
• synthetic reactions produce inactive metabolites

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Biotransformation
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Biotransformation
Why is Biotransformation necessary? Most drugs
are excreted by the kidneys For renal
excretion drugs should have small molecular
mass be polar in nature should be fully
ionized at body pH Most drugs are complex and
do not have these properties and thus have to be
broken down to simpler products.
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Biotransformation
Ctd

• Pharmacologically active organic molecules tend
  to-
• Be highly lipophilic remain unionized or
  partially ionized at physiologic PH.
• Thus readily pass across biological barriers
  membranes
• Strongly bound to plasma proteins
• Such substances are not readily filtered at the
  glomerulus.
• Their lipophilicity also facilitates to be
  reabsorbed through lipophilic renal tubular
  membranes.
• This property also stops them from getting
  eliminated
• They have to be converted to simpler
  hydrophilic compounds so that they are eliminated
  and their action is terminated.

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Biotransformation
Ctd
Biotransformation can also result in
bioactivation, which involves the production of
reactive metabolites that are more toxic,
mutagenic, or carcinogenic than their parent
compound(s). Drugs may converted to- less
toxic materials more toxic materials
materials with different type of effect or
toxicity Beside these, biotransformation is
called a biochemical defense mechanism as it
handles different xenobiotics, drugs, toxicants,
body wastes (hemoglobin) or other unwanted
substances to those we get exposed.
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Biotransformation

• Where do biotransformations occur?
• Liver is the principal organ of drug metabolism
  although every tissue has some ability to
  metabolize drugs.
• Other tissues that display considerable activity
  include the GIT, the lungs, the skin, and the
  kidneys.
• Following oral administration, many drugs (e.g.
  isoproterenol,morphine) absorbed intact from the
  small intestine and transported first via the
  portal system to the liver, where they undergo
  extensive metabolism ( first-pass metabolism).
• Some orally administered drugs (e.g.clonazepam,
  chlorpromazine) are extensively metabolized in
  the intestine than in the liver.

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Biotransformation
Ctd

• Thus intestinal metabolism may contribute to the
  overall first-pass effect.
• First pass effects may so greatly limit the
  bioavailability of orally administered drugs.
• The lower gut harbors intestinal microorganisms
  that are capable of many biotransformation
  reactions.
• Although drug biotransformation in vivo can occur
  by spontaneous, noncatalyzed chemical reactions,
  the vast majority are catalyzed by specific
  cellular enzymes.At the cellular level, these
  enzymes may be located in the
• i) Endoplasmic reticulum ii) mitochondria iii)
  cytosol iii) lysosomes iv) even the nuclear
  envelope or v) plasma membrane.

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Biotransformation

• Water soluble xenobiotics are easier to eliminate
  in urine, feces but not exhalation as t1/2 is
  low.
• Lipophilic barbiturates such as thiopental
  phenobarbital would have half-lives greater than
  100 years if they were not converted to
  water-soluble compounds.
• Multiple enzymes (families)
• Constitutively expressed
• Inducible
• Broad specificity
• Polymorphic
• Stereo-isomer specificity

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Biotransformation
Relatively harmless
Potentially toxic xenobiotic
Metabolic activation
Detoxification
Inactive metabolite
Reactive intermediate
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Converting lipophilic to water soluble compounds
Lipophilic (non-polar)
Xenobiotic
Phase I - Activation
Reactive intermediate
Phase II - Conjugation
Conjugate
Water soluble (polar)
Excretion
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Phase I

• introduction of functional group
• hydrophilicity increases slightly
• may inactivate or activate original compound
• major player is CYP or mixed function oxygenase
  (MFO) system in conjunction with NAD(P)H
• location of reactions is smooth endoplasmic
  reticulum

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Phase II

• conjugation with endogenous molecules
• (GSH, glycine, cystein, glucuronic acid)
• hydrophilicity increases substantially
• neutralization of active metabolic intermediates
• facilitation of elimination
• location of reactions is cytoplasm

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Phase I reactions

• Oxidation
• Hydroxylation (addition of -OH group)
• N- and O- Dealkylation (removal of -CH side
  chains)
• Deamination (removal of -NH side chains)
• Epoxidation (formation of epoxides)
• Oxygen addition (sulfoxidation, N-oxidation)
• Hydrogen removal
• Reduction
• Hydrogen addition (unsaturated bonds to
  saturated)
• Donor molecules include GSH, FAD, NAD(P)H
• Oxygen removal
• Hydrolysis
• Splitting of C-N-C (amide) and C-O-C (ester) bonds

epoxide
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Biotransformation

• Activation of xenobiotics is a key element
• (e.g. benzene, vinyl chloride)
• Reactive intermediates include epoxides and free
  radical species (unpaired electrons) that are
  short-lived and hence highly reactive
• Protection is provided by
• endogenous antioxidant substances, e.g. GSH
• vitamins C and E
• antioxidant enzymes
• Antioxidant molecules are oxidized in the process
  but have the capacity to regenerate the reduced
  form from the oxidized - NAD(P)H is a key player

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Cytochrome P450 (CYP) Mixed Function Oxidases
(MFO)

• Located in many tissues but highly in liver ER
• Human 16 gene families
• CYP 1,2,3 perform drug metabolism
• gt48 genes sequenced
• Key forms CYP1A2, CYP2C9, CYP2C19, CYP2D6,
  CYP2E1, and CYP3A4
• Highly inducible
• Alcohol CYP2E1
• Barbiturates CYP2B

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CYPs are the major enzymes involved in drug
metabolism, accounting for 75 of the total
metabolism.Most drugs undergo deactivation by
CYPs, either directly or by facilitated excretion
from the body. Also, many substances are
bioactivated by CYPs to form their active
compounds.
Proportion of drugs metabolized by different CYPs
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Figure CYP450 Reaction Sequence
OH
NADPH H
e-
OH
CYP450 reductase
NADPH H

O
..
H
e-
O2
H2O
O21-
O2
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Oxidation of vinyl chloride to an epoxide
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Metabolic enzymes

1. Microsomal
2. CYP450 monooxygenases
3. Flavin monooxygenase
4. Non-microsomal
5. Alcohol dehydrogenase
6. Aldehyde dehydrogenase
7. Monoamine and diamine oxidases
8. Both
9. Esterases and Amidases
10. Prostaglandin synthase
11. Peroxidases

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Cooxidation of acetaminophen by prostaglandin
endoperoxide synthetase
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Hydrolysis of esters and amides
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Hydrolysis of organophosphates
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Hydrolysis of epoxides
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Stereoselective hydroxylation
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Metabolism of benzo(a)pyrene to 9,10
epoxide Potent mutagen that binds DNA