28629 L6 - genetic variability in phase I and II drug metabolising enzymes

intro to pharmacogenetics:

• Long known that there are differences in drug metabolism – between species, between strains of animals, between individuals

• Pharmacogenetics is the study of the genetic basis of variations in drug metabolism

• Polymorphisms are changes in DNA sequence in drug metabolising genes

• Different “forms” of the same gene are called alleles

• Allelic variation is common in human populations

• Important factor in inter-individual differences in drug metabolism along with environmental factors like diet – enzyme inducers, enzyme inhibitors

• Variation affects pharmacokinetics of drugs

Effects of polymorphisms on drug metabolism:

• No effect

• Reduce efficacy

• Increase activity – associated with toxicity

CYP2D6 polymorphisms:

Around 1000 polymorphisms with multiple outcomes - CYP 2D6*4 most prevalent (decreased)

• normal

• increased

• decreased

• no activity

• Poor Metabolizers (PM) (little or no function):

  • slower drug metabolism

  • higher peak plasma levels (3–4×)

  • higher area under the curve (AUC, >4×)

  • higher risk of adverse drug reaction

  • no response from certain prodrugs e.g. codeine

• Ultra-rapid Metabolizers (UM) (multiple 2D6 copies)

  • Lower peak plasma levels and AUC

  • Little drug response at ordinary dosage

Examples of drugs metabolised by CYP2D6:

CYP 2D6 different drugs different outcomes:

• rapid metabolisers - produce more of the toxic product

• poor metabolisers - produce less of the active product

maybe check this in the recording

Tamoxifen metabolism and CYP2D6:

• Tamoxifen is the major adjuvant therapy for ER-positive breast cancers in premenopausal woman

• Metabolism of TAM to the active metabolites 4OH-TAM and Endoxifen mainly undertaken by CYP2D6

Does CYP2D6 genotype affect clinical outcome in women taking tamoxifen?

• FDA-approved drug label does not discuss genetic testing for CYP2D6

• National Comprehensive Cancer Network does not recommend CYP2D6 testing as a tool to determine endocrine treatment

• Dutch Pharmacogenetics Working Group has made recommendations for tamoxifen therapy based on CYP2D6 genotypes

CYP2C9 and warfarin:

CYP2C9 is polymorphic in the human population:

• Around 800 known polymorphisms

• CYP2C19*2, 3 and 4 all have reduced activity towards S-Warfarin

• Increased abnormal bleeding in patients with variant CYP2C9 alleles (*2 - 4*)

• The FDA-approved drug label for warfarin states: “that CYP2C9 and VKORC1 genotype information, when available, can assist in the selection of the initial dose of warfarin”

phase II polymorphisms:

• noted in majority of phase II enzymes

• tend to be of lesser clinical/toxicological significance

• e.g. 

  • N-acetylation

  • glucuronidation

  • sulfonation

  • glutathione conjugation

• polymorphisms can occur in both the enzyme + in enzymes involved in production of a cofactor e.g. PAPS, UDP-glucuronic acid

N-acetyl transferases (NATs):

• 2 known functional isoforms of NAT in humans (NAT1 and NAT2)

• Individuals can be phenotyped based on activity towards “probe drugs” e.g. sulfamethazine, isoniazid

• About 1/3 of the population have a “slow” acetylator phenotype

• Autosomal recessive trait

Many NAT2 alleles exist: at least 60

NAT2 role in toxicity of aryl amines:

activation of carcinogen arylamines:

Case study 4: UGT1A1

At least 16 functional isoforms of UGT in 2 subfamilies:

UGT1 family – 10 isoforms

UGT2 family – 6 isoforms

Numerous polymorphisms have been identified including more than 100 variants of UGT1A1

UGT1A1*28 and sensitivity to ironotecan

Effect of UGT1A1*28 on pharmacokinetics of ironotecan and incidence of neutropenia:

The FDA-approved drug label for irinotecan states that “when administered as a single-agent, a reduction in the starting dose by at least one level of irinotecan hydrochloride injection should be considered for patients known to be homozygous for the UGT1A1*28 allele” 

Case study 5: GSTs

• Major important polymorphisms are in GSTM family of genes

• GSTM1, GSTM3 and GSTM4 are known to be polymorphic.

• GSTM1*0 is a common deletion variant:

• Ethnic Group   Frequency

  Pacific Islander/Malay   62 – 100%

  European   35 – 62%

  Asian   32 – 53%

  Hispanic   40 – 53%

  African   23 – 41%

• GSTM1*0: increased risk of asthma, sensitivity to some chemicals?

Associations between GSTM1 null genotype and lung cancer:

Interaction between SNPs in CYP1A1 and GSTM1 null genotype and levels of BaP adducts:

conclusions:

• Almost all drug metabolism and drug transporter genes are polymorphic

• These polymorphisms can greatly influence drug metabolism

• Partly responsible for inter-individual variation in pharmacokinetics and drug toxicity

• None of these polymorphism exist in isolation

• Multiple polymorphisms are very likely to interact – need to consider the whole pathway

• Also need to consider environmental factors e.g. diet