DETAILS OF GENETIC POLYMORPHISM IN DRUG METABOLISM : CYP450 ISOENZYMES

1. GENETIC POLYMORPHISM
IN DRUG METABOLISM:
CYP450 ISOENZYMES
RAJNANDINI SINGHA
PHARMD

2.  Genetic polymorphism are variation in gene
sequence that occurs in at least 1% of the general
population resulting in multiple alleles or variant of
a gene sequence.

3. GP
IM PM EM
UM

4. Poor metabolizers:
Two defective alleles
(ex: CYP2D6*4/*5 and CYP2D6*4/*4)
 Combination of alleles including one resulting in no enzyme
(ex: CYP2D6*5 and CYP2D6*4 deletion)
Intermediate metabolizers
Heterozygous –having only one wild type allele and one
defective allele
Normal metabolizers
 Carry wild type alleles (ex: CYP2D6*1/*3).
 Wild type alleles encode genes for normal enzyme function

5. Extensive metabolizers
Carry one wild type and one amplified gene
 ex: CYP2D6*1/*2, CYP2D6*A/*1a, and CYP2D6*1A/*5
Ultra-rapid metabolizers
Carry two or more copies of amplified gene
 ex: CYP2D6*2/*3

7. Polymorphisms affect drug interactions by altering
the effect of inhibitors and inducers on the enzyme
results in an exaggerated effect or minimal effect
on the substrate
 Inhibitor: An enzyme inhibitor is a molecule, which
binds to enzymes and decreases their activity.
 Inducer: An enzyme inducer is a type of drug that
increases the metabolic activity of an enzyme either
by binding to the enzyme and activating it, or by
increasing the expression of the gene coding for the
enzyme.

8.  Extensive Metabolizers -Inhibitors
◦ Extensive metabolizer -----level of substrate
drug is normally low due to rapid metabolism by
the enzyme. An inhibitor to the enzyme will
inhibit the extensive metabolism and cause
significant elevations in the substrate drug.
◦ Effect of inhibitors is much greater in an EM
inc.level of substrate levels

9.  Poor Metabolizers -Inhibitors
◦ In a poor metabolizer, the level of substrate drug
remains high because the metabolism of the
substrate is much less than normal. When an
inhibitor is added, the additional inhibition of
metabolism is not much greater than is already
occurring in the PM.
◦ The effect of inhibitor is less in a PM than in
normal metabolizers.
◦ The drug interaction might not occur.

10. Extensive Metabolizers -Inducers
 Level of substrate drug is lower than in a normal
metabolizer due to rapid metabolism.
 The addition of an inducer does not cause a greater
difference in the level of substrate because the
metabolism is already increased greatly.
 The drug interaction might not occur.

11. Poor Metabolizers -Inducers
 Level of substrate drug is higher than expected in
normal metabolizer because of the lower
metabolism of substrate.
 The addition of inducer will cause a signification
increase in the metabolism of the substrate much
lower level of substrate than expected in a normal
metabolizer.
 Drug interaction may occur to a greater extent.
 Drug interaction may result in substrate levels
similar to those of normal metabolizers.

12. **NOTE**
 The effect of inhibitor is great in EMs than in PMs.
 The effect of inducer is greater in PMs than in EMs.

13. Genetic polymorphism
Pharmacokinetics
Transporter
Plasma protien binding
Metabolism
Pharmacodyanamics
Receptors
Ion channel
Enzyme
Immune molecules

14. Genetic variation
Deletion Insertion SNP

15. Polymorphism
Consequences
Truncated
proteins
Unstable
proteins
Protien
expressions
Altered proteins

18.  The polymorphic P450 (CYP) enzyme super family is the most
important system involved in the biotransformation of many
endogenous and exogenous substances including drugs,
toxins, and carcinogens.
 Genotyping for CYP polymorphisms provides important
genetic information that help to understand the effects of
xenobiotics on human body.
 For drug metabolism, the most important polymorphisms are
those of the genes coding for CYP2C9, CYP2C19, CYP2D6,
and CYP3A4/5, which can result in therapeutic failure or
severe adverse reactions

19.  Most extensively studied polymorphic drug metabolizing enzyme
 These drugs include Antidepressants, Anti arrhythmics, Beta-
adrenergic antagonist and Opiods.
 Population(10% caucasian,1%asian ,19%africans) with PM Phenotype Of
CYP2D6 result in increase plasma conc. of the parent drug due to
decreased metabolic clearance.
 Debrisoquine---marked hypotension as plasma conc. increases.
◦ Impaired ability to hydroxylate, and therefore, inactivate
debrisoquin
◦ 5-10% subjects have relative deficiency in ability to oxidize
debrisoquin
◦ Also have impaired ability to metabolize the antiarrhythmic and
oxytocic drug .
◦ PM lower urinary concentration, higher plasma concentrations.
 Tricyclic antidepressant with PM Phenotype increase plasma
concentration and potentiates CNS Depression.
◦ PM METABOLIZERS have two null alleles, which do not code for
functional CYP2D6 due to frame shift, a splicing defect, gene
depletion.

20.  The UM (Caucasian population1%-10%,saudi
Arabians 20% and Ethiopians 29%)have high rates
of CYP2D6 enzyme activity resulting in low plasma
concentration of drugs with lower efficacy.
 Active drugs like the tricyclic anti depressant such
as AMITRIPTYLINE may require doses several fold
higher than standard doses to achieve therapeutic
activity .
 Codeine is converted to morphine by CYP2D6 –o-
demethylation reaction to provide analgesic effects
and morphine associated toxicity.

21.  Accounts for approximately 18% of the CYP content in the
liver
 Catalyzes roughly 20% of the CYP-mediated metabolism of
drugs
 CYP2C19
◦ These are highly polymorphic drug metabolizing enzyme .
◦ Polymorphisms in CYP2C19 results in variable drug
response to Clopidrogrel and several antidepressants.
◦ The PM phenotype often results in non functional null
alleles.(CYP2C19*2- Splicing defect and CYP2C19*3-
Frameshit).
◦ The CYP2C19 PM Phenotype results in lack of efficacy for
the ant platelet prodrug clopdrogrel.
◦ The deficiency of CYP2C19 activity may result of adverse
cardiovascular outcomes because the PM does not activate
clopidrogrel.

22.  The PM Phenotype results in higher plasma concentration,
larger AUC values, greater efficacy in lowering gastric PH than
EM .
 Also catalyzes the metabolism of several proton pump
inhibitors (i.e. omeprazole), diazepam, thalidomide, and some
barbiturates

23.  Diazepam is demethylated by CYP2C19
 Half-life of the desmethyldiazepam metabolite
is also longer in CYP2C19 poor metabolizers.
 High frequency in Asian population.
 Diazepam induced toxicity may occur as a
result of slower metabolism –careful dosing in
Asian population.

24.  Major CYP2C subfamily member in the liver
 Primarily responsible for the oxidative metabolism
of important compounds –warfarin, phenytoin,
tolbutamide, glipizide, losartan, etc.
 6 different polymorphisms –CYP2C9*1, *2, *3, *4,
*5, *6
 CYP2C9*1 –wild type allele, CYP2C9*2-*6 –variants
 Variants *2 and *3 alleles are common in
Caucasians (≈35%)
 CYP2C9*2 and *3 alleles associated with significant
reduction in the metabolism and clearance of
selected CYP2C9 substrates.
 CYP2C9 is a major contributor to the metabolism
of the narrow therapeutic index blood thinner
,warfarin.

25.  Polymorphisms linked to both toxicity and
dosage requirements for optimal
anticoagulation with warfarin.
 When the patient has got these two
polymorphism, the dose of warfarin is needed
to be clinically relevant as the drug clearance is
reduce.
◦ *2 and *3 variants –higher risk of acute bleeding
complications than patients with *1 wild type
genotype.
◦ Require 15-30% lower maintenance dose of warfarin
to achieve target INR

26.  Metabolism of antineoplastic agent fluorouracil.
 In the 1980s, fatal CNS toxicity developed in
several patients after treatment with standard
doses fluorouracil.
◦ Patients had inherited deficiency of
dihyropyrimidinedehydrogenase.
◦ Severe fluorouracil toxicity occurs when DPD
activity < 100 mmol/min/mg protein.
◦ Heterozygous individuals do not exhibit no
phenotype until challenged with fluorouracil.

27.  CYP3A subfamily plays a critical role in the
metabolism of more drugs than any other
phase I enzyme.
◦ Expressed in liver and small intestine
◦ Contribute to oral absorption, first-pass, and
systemic metabolism

28.  More than 30 SNPs have been identified for CYP3A4
gene
 Unlike other P450s, there is no evidence for deleted
or null allele for CYP3A4.
◦ The most common variant in CYP3A4, CYP3A4*1B
is an transition in the promoter region referred to
as the nifedipine response element.
◦ One study shows that this variant may be
associated with a slower clearance of NEFIDIPINE.
◦ This is a rather controversial finding.

29.  Polymorphically expressed in adults in about 10-
20% in Caucasians, 33% in Japanese, and 55% in
African Americans.
 The variable CYP3A5*3 is a result of improper
mRNA splicing and reduced translation of
functional protein.
 CYP3A5 has been linked to Tacrolimus dose
requirements to maintain adequate
immunosuppressant in solid organ transplant
patients

33.  N-Acetyltansferase (NAT) is a polymorphic enzymes through
phenotypic observation of fast or slow acetylators of the anti-
tuberculosis drug, isoniazid.
 There are two different human genes, NAT1 and NAT2, that code
for functional NAT activity.
 Patients that are slow metabolizers of isoniazid exhibit increased
blood level of the drug, which result in an increased incidence of
neurotoxicity.
 Individuals are slow or rapid acetylators-
◦ Slow acetylation: Japanese (10%), Chinese (20%), Caucasians (60%)
◦ 27 unique NAT2 alleles identified
◦ NAT2*4 is the wild type allele.
◦ Slow metabolizer are associated with an increased risk of lupus
erythematous.
◦ with fast metabolizers,there can also be an increased toxicity of
topoisomerase ii inhibitirs, amonafide which is associated with a higher
incidence of MYELOSUPRESSION

34.  Involved in degradation of thiopurine drugs including 6-
mercaptopurine and Azathioprine.
 TPMT Polymorphism account for about one third of the variable
responses to MP and Azathioprine.
 At least 28 allelic variants in coding and splicing region of
TPMT have been identified.
 Most of null phenotypes being associated with TPMT*2 ,
TPMT*3A, TPMT*3B alleles.
 Results non synonymous mutation leads to production of
unstable enzyme and reduced activity.
 Loss of TPMT function results in accumulation of MP leading to
increased risk for leukopenia.

35.  Super family of phase II drug metabolizing enzyme.
 Produce glucuronidation metabolites through
conjugation reactions.
 Drug metabolism is catalyzed almost exclusively by
UGT1 and UGT2.
 At least 200 alleles for UGT1 and UGT2 gene families
have been reported causing changes in enzymatic
activity or expression levels that may contribute to
individual variations in drug response.
 Potential effect of variable activity of UGT is dependent
on relationship between parent drug and metabolite.
 While most UGT metabolites are inactive, examples of
activation including morphine metabolism to the active
6-glucuronide metabolite and various carboxylic acid
metabolism to reactive, potentially toxic,
acylglucuronides.