BIOTRANSFORMATION Flashcards
WHY DRUG TRANSFORMATION NECESSARY?
• Pharmacologically active organic molecules tend to
be lipophilic and they remain unionized or only
partially ionized at physiologic pH; these are readily
reabsorbed from the glomerular filtrate in the
nephron.
• Certain lipophilic compounds are often strongly
bound to plasma proteins and may not be readily
filtered at the glomerulus.
• Metabolic products are often less
pharmacodynamically active than the parent drug
and may even be inactive.
• However, some biotransformation products have
enhanced activity or toxic properties.
• Therefore, lipid-soluble agents are first metabolized
into more polar (hydrophilic) substances in the liver
via two general sets of reactions, called phase 1
and phase 2.
Phase 1 reactions
usually convert the parent drug to
a more polar metabolite by introducing or
unmasking a functional group (–OH, –NH2, –SH)
If phase 1 metabolites are sufficiently polar
they may be readily excreted.
However, many phase 1 products are not eliminated
rapidly and undergo a subsequent reaction in which
an endogenous substrate such as
glucuronic acid,
sulfuric acid, acetic acid, or an amino acid combines
with the newly incorporated functional group to form
a highly polar conjugate.
hydrazide moiety of isoniazid is
known to form an
N-acetyl conjugate in a phase 2
reaction
Thus, phase 2 reactions may actually precede
phase 1 reactions.
Phase 2 reaction,
this facilitates elimination of the
drug and it can be in the form of conjugation, sulfation, acetylation or methylation.
• Phase 1 involves
Oxidation-Reduction (REDOX
Reaction)
Phase 2 involves
transferases or conjugation
This conjugation reaction usually requires the
substrate to have Oxygen, Nitrogen or Sulfur that
serve as acceptor sites for the hydrophilic moiety,
such as glutathione, glucuronic acid, sulfate or
acetyl
: a drug may undergo Phase 1 via
oxidation, addition of Oxygen. And that Oxygen
would serve as a substrate, where in this component
would be added in replace to the functional group.
What is the importance of biotransformation?
For elimination
What will happen if the drug is not metabolized?
Increase risk for toxicity because the drug
remains in the blood. So, after absorption, it will
be distributed and then metabolize for it to be
excreted.
Phenytoin is highly lipophilic, via Phase 1 reaction
acted upon by
CYT p450
Phenoytin Reaction
converted, and then addition of a functional
group, forming 4-hydroxy-phenytoin. Then after
Phase 1, this phenytoin would be slightly soluble in
water. It would further undergo Phase 2 reaction by
glucuronidation, addition of UGT, where in your
hydroxyl group serves as the substrate. This
metabolite is more water soluble and it can be easily
excreted in the kidney
WHERE DO DRUG BIOTRANSFORMATION
OCCUR?
Liver is the principal organ of drug
metabolism.
If phase 1 metabolites are sufficiently polar
they
may be readily excreted.
The small intestine plays an important role in metabolism since drug that are orally administered absorbed by the GUT and taken to the liver through
the portal vein
FPM
xenobiotic metabolizing enzyme
located
in the epithelial cells of the GI are also responsible
for the initial metabolic process of most oral
medication. So, this should be consider as the initial
site of drug metabolism
Drugs that undergo Substantial FPM
Asprin Glyceral Trinitrate Isosorbide dinitrate Levodopa Lidocaine Metoprolol Morphine Propanolol Salbutamol Verapmil
are more extensively
metabolized in the intestine than in the liver
clonazepam, chlorpromazine, cyclosporine
undergo significant (~50%) intestinal metabolism.
midazolam
First-pass effects may limit the bioavailability of
orally administered drugs (eg., lidocaine) so greatly
that
alternative routes of administration must be used to achieve therapeutically effective blood levels.
Furthermore, the lower gut harbors intestinal microorganisms that are capable of many
biotransformation reactions
Drugs may be metabolized by gastric acid
Penicillin
Digestive enzumes metabolism
Polypeptides like insulin
enzymes in the wall of the
intestine
sympathomimetic catecholamines
RER are used for
protein
synthesis
SER are rich in
enzymes responsible for
oxidative drug
metabolism and this is where your Cytochrome P450
enzyme or your mixed function oxidases are located.
PHASE 1 REACTIONS
: Oxygenase, addition of
functional group, they can be accomplished by
Cytochrome P450, FMO as well as epoxide
hydroxylases.
phase 1 reaction are three enzymes
CytP450
In this oxidation reaction process, 2 microsomal
enzymes play a key role
First is your NADPH
(nicotinamide adenine dinucleotide phosphate
dehydrogenase) and 1 mole of this enzyme donates
an electron to the flavoprotein P450 reductase.
(contains 1 mole of FMN/Flavin mononucleotide and
FAD/Flavin adenine dinucleotide).
The second microsomal enzyme is a hemoprotein
called your Cytochrome P450
which serves as your
terminal oxidase. So, in its reduced form, in ferrous
form (Fe²⁺) it binds carbon monoxide to give a
complex that absorb light at approximately at 450nm.
Hence, the name Cytochrome P450.
Nomenclature:
o The family name is indicated by the Arabic
number that follows CYP, and the capital
letter designates the subfamily, for example, CYP3A
o A second number indicates the specific
isozyme, as in CYP3A4
CYP3A4
Responsible for the metabolism of over 50% of the prescription drugs metabolized by the liver.
most common
drugs are
CYP1A2, 2A6, 2B6, 2C9, 2D6, 2E1,
3A4.
g efficacy
and there is adverse effect. For example, CYP2D6
has mentioned to exhibit
. So, if
the is genetic polymorphism, the risk for drug-drug
interaction also increases.
PHASE 1 REACTIONS: CYTOCHROME P450 (CYP)
ENZYME INDUCTION
This results in increased biotransformation of drugs
and can lead to significant decreases in plasma
concentrations of drugs metabolized by these CYP
isozymes, with concurrent loss of pharmacologic
effect
f drug red is metabolized by CYP3A4 and
the drug yellow is also metabolized by CYP3A4. If
drug yellow induced the metabolism of drug red, will
there be an increase or decrease in the metabolism of the drug
red?
Increase in the metabolism of drug red.
What would happen to the effect of drug
red?
Decrease in effect. Since faster
metabolism of drug red. Drug yellow induces or
accelerates the metabolism of drug red.
Question: If you are given or prescribed with 2 drugs
and 1 drug induces the metabolism of the other drug.
How would you now address the situation to obtain
the desirable effect? Would you increase the dosage
of drug yellow or increase the dosage of drug red? Or
decrease dosage of drug yellow or decrease dosage
of drug red?
You can increase the dose of
drug red or decrease yellow or better is to change
drug yellow
Maximal effects usually occur after
7 to 14 days and
would require equal or longer time to dissipate after
the enzyme inducer is stopped
CYTOCHROME P450 (CYP) ENZYME INDUCTION
When a drug such as atorvastatin
(used to decrease your blood cholesterol;
mechanism: HMG-COA reductase inhibitor). So, your
atorvastatin here serves as the ligand, it enters the
cell and it can bind to nuclear receptor such as your
PXR, then PXR forms a complex with your RXR
(Retinoid X Receptor), and it binds to DNA upstream
of target genes. So, this binding would recruit your
coactivator which binds to your TATA box and
activates transcription by RNA polymerase.
Aromatic HC
binds the ligand-binding domain of
soluble protein, AROMATIC HC RECEPTOR (AHR)
● Complex is transported to the nucleus by AHR
nuclear translocator (PROMOTES GENE
TRANSCRIPTION
Aryl Hydrocarbon AHR
Omeparazole
CAR (Androstane)
Phenobarbital
PXR
Rifapmin
FXR
Bile acid
PPAR
Fibrates
RAR
All trans retinoic acid
RXR
9-cis Retinoic acid
Induction
it would accelerate the
metabolism by inducing the synthesis of your
Cytochrome P450, reducing the effect of the ligand.
The most common form of inhibition
through
competition for the same isozyme.
drug red and then drug yellow,
wherein they are metabolized by the same CYP450,
but drug yellow is an inhibitor. So, what would be the
effect on the metabolism of your drug red
? It would
be enzyme inhibition.
if there is decrease in the
metabolism of drug red, what would happen to its
biologic effect?
An increase in biological effect, It can increase the risk for toxicity.
How would you address the problem in enzyme
inhibition wherein there is increased risk for toxicity?
Decrease the dose of drug red or you can change
drug yellow, as we are preventing drug-drug
interaction, to prevent toxicity
most common is your CYP3A4
50% are metabolized by it
CYP1A2, has
has substrate of
acetaminophen, with inducers of lansoprazole,
primidone, rifampin. Acetaminophen effect is not that
significant unless you give a toxic dose.
carbamazepine and rifampin wherein
they are metabolized by your CYP2C8
if both drugs
are given where rifampin is an inducer, so meaning
rifampin would facilitate or accelerate the metabolism
of your carbamazepine, decreasing its effect but if
carbamazepine is given together with your
gemfibrozil (inhibitor), metabolized by the same CYP,
it would inhibit the metabolism of carbamazepine,
therefore the effect of carbamazepine is increased
when you give it concomitantly with gemfibrozil.
● 16-year-old patient sought to consult due to a cough
of 2 weeks duration.
● He is known asthmatic. On PE, RR 45 with subcostal
retractions and wheezes and rales on both lungs.
With an impression of atypical pneumonia and
Bronchial asthma in acute exacerbation, Prednisone
and Salbutamol nebulization was given.
● Which of the following antimicrobials would be of
benefit for this patient?
a) Amoxicillin
b) Cefuroxime
c) Azithromycin
d) Clarithromycin
C Azithromycin
Drugs given were Prednisone and salbutamol,
Case is chlamydia
Azithromycin and Clarithromycin have
coverage for atypical microorganisms such as mycoplasma and chlamydia.
Prednisone is
metabolized by CYP3A4 then clarithromycin is an
inhibitor, considering drug-drug interaction,
prednisone concentration in the body would be
increased therefore increasing its effect and toxicity.
So, among these drugs, better to give azithromycin
as it has no drug interaction with prednisone nor
salbutamol.
PHASE 2 REACTIONS
Parent drugs or their phase 1 metabolites that contain
suitable chemical groups often undergo coupling or
conjugation reactions with an endogenous substance
to yield drug conjugates.
• These reactions are synthetic, anabolic and involving
conjugations. Phase 2 reactions also occurred in the
liver. So, if your phase 1 is adding your functional
group which serves as a substrate where in the Phase
2 reactions, you conjugate the drug or you couple it to
yield drug conjugate.
most reactions are via
UGT make table of phase 1 and phase 2 rxn tables.
GLUCURONIDATION
Glucuronide formation involves the formation of a
high-energy phosphate compound, uridine
diphosphate glucuronic acid (UDPGA) → from which
glucuronic acid is transferred to an electron-rich atom
(N, O or S) on the substrate, forming an amide, ester
or thiol bond.
UDP-glucuronyl
transferase.
This is an example of
glucuronidation reaction, via the addition of glucuronide
SULFATION
Sulfotransferases (SULTs) are cytosolic and conjugate sulfate derived from 3ʹ-phosphoadenosine5ʹ-phosphosulfate (PAPS) to the hydroxyl and, less
frequently, amine groups of aromatic and aliphatic compounds.
• SULTs play an important role in normal human homeostasis.
SULT2B1b
predominant form expressed in skin,
carrying out the catalysis of cholesterol.
SULT2A1
highly expressed in the fetal adrenal
gland.
SULT1A3 is
highly selective for catecholamines
• SULT1E1
estrogen.
GLUTATHIONE CONJUGATION
• The substrate in the reaction is a tripeptide
glutathione which is synthesized from glutamic acid,
cysteine, and glycine
Glutathione exist in cell as oxidized or reduced form
and the ratio is critica
maintaining cellular
environment in reduced state.
The glutathione-S-transferases (GSTs) catalyze the
transfer of glutathione to reactive electrophile
a function that serves to protect cellular
macromolecules from interacting with electrophiles
that contain electrophilic heteroatoms (-O, -N, and -S)
and in turn protects the cellular environment from
damage.
ACETYLATION
There are two genes for N-acetylation, NAT1 and NAT2
Example of a drug that undergoes N-acetylation
Isoniazid
If the subject or the patient has a slow acetylator
gene,
decrease metabolism of isoniazid by Nacetylation, thereby increase in the risk for adverse
effect.
hydralazine
which contains the
hydrazine group, undergoes N-acetylation, so if there
is genetic polymorphism, and the patient is a slow
acetylator type, and then you give hydralazine, which
is used in treating hypertension, it can aggravate
hypotension, tachycardia, and headache. But if the
patient is a rapid acetylator, there is decreased in the
effect of hydralazine as well as decreased side effect
Acetaminophen, an analgesic antipyretic drug, is
quite safe in therapeutic doses (1.2 g/d for an adult).
It normally undergoes glucuronidation and sulfation to
the corresponding conjugates, which together make
up 95% of the total excreted metabolites. The
alternative P450-dependent GSH conjugation
pathway accounts for the remaining 5%. When
acetaminophen intake far exceeds therapeutic doses
Acetaminophen metabolism via Phase 2, 95% would
undergo
glucuronidation as well as sulfation. Only 5%
is cytochrome P450 dependent GSH conjugate
pathway? At therapeutic dose, acetaminophen would
undergo this pathway and sulfation pathway
there is toxic dose for acetaminophen
toxic dose is a single injection of 7.5-10g or acute
injection of 12g of acetaminophen, and for children
ingestion of more that 150mg/kg, so glucuronidation
reaction as well as sulfation reaction would be
saturated, so they cannot afford the excess
acetaminophen, there will be increase in the
metabolism via cytochrome P450 enzyme.
treatment of acetaminophen toxicity
NAC, within 6-8 hours
GENETIC FACTORS
A true genetic polymorphism is defined as the
occurrence of a variant allele of a gene at a population
frequency of ≥ 1%, resulting in altered expression or
functional activity of the gene product, or both
PHASE 1 ENZYME POLYMORPHISM
It involves FOM, hydroxylases, as well as
cytochrome P450.
Poor Metabolizer (PM)
faulty expression of the P450 protein due to either defective mRNA splicing or protein folding, resulting in little or no isoform-catalyzed drug metabolism. o meaning, the Phase 1 reaction is prolonged secondary to messenger RNA defect
Ultrarapid Metabolizer (UM)
o most common in Ethiopians and Saudi Arabians, populations that display it in up to one-third of individuals o require twofold to threefold higher daily doses o has fast drug metabolism o for fast metabolizers or ultrarapid metabolizers, to attain the therapeutic the effect, increase the dose by two times or three times
PHASE 2 ENZYME POLYMORPHISM
. PHASE 2 ENZYME POLYMORPHISM • The problem can be in conjugation, acetylation, sulfation, methylation • The commonly described is your acetylation enzyme polymorphism Phase 2. • So, they can be categorized as Slow Acetylator or Rapid Acetylator
SLOW ACETYLATOR PHENOTYPE
The defect in slow acetylators (of isoniazid and similar
amines) appears to be caused by the synthesis of
less of the NAT2 enzyme rather than of an abnormal
form of it.
• For Slow Acetylator Phenotype, mostly, their
polymorphism affects NAT2 than NAT1.
• 50% of blacks and whites in the USA, more frequently
in Europeans living in high northern latitudes
• Associated with a higher incidence of isoniazidinduced peripheral neuritis, drug-induced
autoimmune disorders, and bicyclic aromatic amineinduced bladder cancer
Isoniazid is metabolized by a Phase II through Nacetylation. If the patient has the slow acetylator
phenotype, metabolism of your isoniazid is
Slow
if there is
an increase in the effect of Isoniazid, it can increase
the adverse effects of Isoniazid such as
Peripheral Neuritis
Hydralazine would also
undergo acetylation? If the patient has slow acetylator
phenotype, it can aggravate the hypotensive effect as
well as
tachycardia caused by your Hydralazine
Sulfonamide, which also undergoes acetylation.
It can aggravate the side effect of your Sulfonamide
hypersensitivity reaction leading to druginduced autoimmune disorder.
For Slow Acetylators,
f the drug is really needed,
you can decrease the dosage or prolong the
frequency of interval of administration of the drug
to reduce the risk for adverse effects.
RAPID ACETYLATOR PHENOTYPE
Isoniazid, so decreased biologic effects of Isoniazid.
To achieve the therapeutic effects of Isoniazid to a
patient classified as Rapid Acetylator, you increase
the dosage.
Charcoal-broiled foods ®
induce CYP1A enzymes
Grapefruit juice ®
inhibit the CYP3A metabolism of
co-administered drug substrate
Cigarette smoker ®
metabolize some drugs more
rapidly than nonsmokers because of enzyme
induction
Industrial workers exposed to some pesticides ®
metabolize drug more rapidly
Extremes of ages
increased susceptibility to
pharmacologic or toxic activity of drugs because of the decrease in metabolism
eonates and children have limited
ability for xenobiotic biotransformation because:
At birth, the level of your cytochrome P450 is
diminished, and then as their age increases, the level
of cytochrome P450 also increases. However, the
effect and biologic activity of cytochrome P450 are
not that mature.
Slower metabolism
Reduced activity of metabolic enzymes
o Reduced availability of essential endogenous
cofactors
DRUG-DRUG INTERACTIONS DURING
METABOLISM
Overlapping substrate specificities by the CYPs is
one of the underlying reasons for the predominance
of drug-drug interactions.
• So, when 2 co-administered drugs are both
metabolized by your single cytochrome P450, they
compete to bind to the enzyme active site. So, this
drug can either induce or inhibit the metabolism of the
same drug. So, this refers to your drug-drug
interaction leading to the cause of your adverse
effect.
An inducer may enhance not only the
metabolism of the other drug but also its own
metabolism
Atrovastatin
Atorvastatin itself would induce the
synthesis of the cytochrome P450 enzyme
CYP3A4
Cimetidine (H2 blocker). Which is
prescribed for Acid Peptic Disease. So, if Cimetidine
is given concomitantly with Warfarin (anti-coagulant).
Since Cimetidine is an enzyme inhibitor, there will be
decreased metabolism of Warfarin. S
So, there will be
an increase in effect of Warfarin, meaning, an
increased risk for bleeding.
INTERACTIONS BETWEEN DRUGS &
ENDOGENOUS COMPOUNDS
steep dose-response curve or a narrow
margin of safety, potentiation of its therapeutic
and toxic effects may result
. DISEASES AFFECTING DRUG METABOLISM
• Liver. Acute or chronic diseases that affect liver
architecture or function markedly affect hepatic
metabolism of some drugs. Isn’t it the primary organ
for biotransformation is your liver?
o Alcoholic hepatitis
o Active or inactive hepatitis
o Biliary cirrhosis
o Acute viral or drug-induced hepatitis
o Impair hepatic drug-metabolizing enzymes
(microsomal oxid
