Biotransformation
Xenobiotic metabolism
“Essentials of Toxicology”

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

Biotransformation
Results of biotransformation active active
Drug or Poison biotransformed
Drug or Poison inactive inactive more potent less potent
TOXIC
In general -
• nonsynthetic precede synthetic reactions
• nonsynthetic reactions can produce active metabolites
• synthetic reactions produce inactive metabolites

Biotransformation
H
3
CO
Codeine
ACTIVE narcotic analgesic
H
N CH
3
H
Morphine
ACTIVE (more potent) narcotic analgesic
H
N CH
3
H
O
OH HO
O
OH
Acetylsalicylic Acid
ACTIVE analgesic
CO
2
H
OCCH
3
O
Salicylic acid
ACTIVE analgesic
CO
2
H
OH
Methanol
ACTIVE CNS depressant
CH
3
OH
Formaldehyde
TOXIC (

)
HCH
O
Formic Acid
TOXIC (

)
HCOH
O

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.

Biotransformation
Pharmacologically active organic molecules tend to-
•
Be highly lipophilic & remain unionized or partially ionized at physiologic P H .
• 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.
Ctd…

Biotransformation
• 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).
Ctd…
• 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.

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.

Biotransformation
• 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.
Ctd…

Biotransformation
• Water soluble xenobiotics are easier to eliminate in urine, feces but not exhalation as t
1/2 is low.
Lipophilic barbiturates such as thiopental & phenobarbital would have halflives 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

Biotransformation
Potentially toxic xenobiotic
Detoxification
Inactive metabolite
Relatively harmless
Metabolic activation
Reactive intermediate

Converting lipophilic to water soluble compounds
Lipophilic
Xenobiotic (non-polar)
Phase I - Activation
Reactive intermediate
Phase II - Conjugation
Conjugate
Water soluble
(polar)
Excretion

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

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

 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
C
O
C epoxide
 Reduction

Hydrogen addition (unsaturated bonds to saturated)

Donor molecules include GSH, FAD, NAD(P)H

Oxygen removal

Hydrolysis
C O

Splitting of C-N-C (amide) and C-O-C (ester) bonds

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

• Located in many tissues but highly in liver ER
• Human: 16 gene families
• CYP 1,2,3 perform drug metabolism
• >48 genes sequenced
• Key forms: CYP1A2, CYP2C9, CYP2C19,
CYP2D6, CYP2E1, and CYP3A4
• Highly inducible
– Alcohol
CYP2E1
– Barbiturates CYP2B

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

Figure CYP450 Reaction
Sequence
OH
DRUG DRUG
CYP450
Fe
3+
CYP450
Fe
3+
DRUG
OH
CYP450 reductase
O
CYP450
Fe
3+
DRUG
H +
H
2
O
NADPH + H + e -
CYP450
Fe
2+
DRUG
O
2
1-
NADPH + H +
O
CYP450
Fe
2+
2 e -
CYP450
Fe
3+
CYP450
Fe
2+
DRUG
DRUG
DRUG
O
2

Oxidation of vinyl chloride to an epoxide

Metabolic enzymes
1.
Microsomal:
1.
CYP450 monooxygenases
2.
Flavin monooxygenase
2.
Non-microsomal
1.
Alcohol dehydrogenase
2.
Aldehyde dehydrogenase
3.
Monoamine and diamine oxidases
3.
Both
1.
Esterases and Amidases
2.
Prostaglandin synthase
3.
Peroxidases

Cooxidation of acetaminophen by prostaglandin endoperoxide synthetase

Hydrolysis of esters and amides

Hydrolysis of organophosphates

Hydrolysis of epoxides

Stereoselective hydroxylation

Metabolism of benzo(a)pyrene to
9,10 epoxide:
Potent mutagen that binds DNA