Lecture 3 Flashcards
What is an early example of pharmacogenetics in clinical practice?
Butyrylcholinesterase (BCHE)
Succinylcholine muscle relaxants developed in the 1950’s and are still widely used for rapid short term skeletal muscle paralysis - eg intubation. However, some patients experience life-threatening prolonged apnea – due to genetic variation in the BCHE gene.
- Kalow et al (1956) developed phenotypic assays (the dibucaine number) to detect “Atypical” or the A variant, assay still used today.
- Subsequently, genetic basis of the Atypical (A) and Kalow (K) variants defined. More than 60 variants now described, mostly very rare and associated with the Silent (S) or deficient phenotype
- Phenotype and genotype are used to characterise patients, warning cards issued where BCHE activity is low, cascade testing is done in at risk family members
- Note: testing is completed after toxicity has occurred thus not a true pharmacogenetic test.
Silent (S) phenotype in genetics does not have an effect, what does this mean in pharmacogenetics?
Enzyme inactive and thus completely deficient.
What are three aims of pharmacogenetic testing?
Benefit patient – reduced morbidity and mortality
Optimise use of proven first line therapies
Cost saving to the NHS by reducing adverse drug reactions and hence hospital admissions
Why is pharmacogenetics important?
With ageing population, number of those diagnosed with cancer increases and thereby need effect therapies that cause little toxicity.
What are examples of pharmacogenetics variation?
- Coding region single nucleotide polymorphisms (SNPs) and indels which change an amino acid sequence.
- Variants affecting splicing – new exons created, exon skipping
- Repeat variation in regulatory elements.
- Gene duplications and deletions.
- May be multiple rare variants within a single gene
Normal genetic polymorphism is only revealed when?
There is a drug challenge
In what scenario does one not need to be aware of what the cause of a variant is?
When the Markers are in disequilibrium with a causative variant
What is used when exploring pharmacogenetic associations?
What does this require and what becomes mores useful?
ADME
Absorption of drug- often poorly understood
Distribution of drug- First pass metabolism – gut and liver, uptake by cells
Metabolism of drug- Complex -may be incompletely understood
Excretion of drug- Conjugation reactions, solute carriers (SLCs).
Requires a good understanding of how the drug is metabolised, often knowledge is incomplete. Thus WGS approaches to investigating associations between genetic variation and drug metabolism become more useful.
The same enzymes and pathways that clear dietary and environmental toxins also work on drugs.
What is an example
Xenobiotics- chemicals derived from diet and overall environment, and are potentially harmful. Enzymes evolved to remove them.
Xenobiotic metabolism historically classified as Phase I and II, converting hydrophobic lypophilic compounds into derivatives that can easily be excreted in urine or the bile
Phase I enzymes carry out oxidation, reduction, hydrolytic reactions – usually drug inactivation, may activate pro-drugs.
Phase II enzymes – conjugation reactions at site of Phase I leading to increased water solubility and elimination
This is a “convenient” classification of metabolism with many exceptions
Phase 1 metabolism involves which pathways?
Clue: Huge, ostriches, run
Hydrolysis
Oxidation
Reduction
Phase 2 metabolism involve which pathways?
Getting, animals, sugar, guides, summer
Glucuronidation
Acetylation
S-methylation
Glutathion conjugation
Sulfo conjugation
Phase 2 metabolism aim to increase what?
Molecular weight to aid excretion through biliary tract or urine
Phase 1 metabolism mostly involve what enzymes?
Cytochrome P450 (CYP)
How many Cytochrome P450 (CYP) genes exist?
How does these genes differ?
57 functional genes, 58 pseudogenes
CYPs 1-3: 22 different isoforms, high degree of polymorphism
• CYPs 4-51: metabolism of mostly endogenous compounds, little polymorphism.
What are pseudogenes?
Inactive copies of the gene
What is the nomenclature for CYP2E1?
CYP- Abbreviation for mammalian cytochrome P450
2- Gene family
E- Sub family
1- Individual enzyme
How are CYP families 1-3 characterised?
22 different isoforms, high degree of polymorphism
How are CYP families 4-51 characterised?
Metabolism of mostly endogenous compounds, little polymorphism.
What are the CYP phenotypes?
Clue: Puppies, immediately, eat, ultrasounds
Poor metabolisers – two defective alleles
Intermediate metabolisers – heterozygous for
a defective allele
Extensive metabolizers – two functional alleles
Ultra-rapid metabolisers >2 copies or promoter variants
Multiple CYPS can metabolise the same drug
True or false
Give an example
True
Caffeine
There is a strong association between CYP2C8 and what drug?
Is this clinically useful?
CYP2C8 and ibuprofen plasma levels. If CYP2C8 33 homozygous maintain drug levels for considerable length of time
Not clinically useful, self medicate- individuals will not be tested prior to use of this
What serious side effect does Ibuprofen cause?
What was found?
Is this introduced into clinical practise?
Gastroduodenal bleeding
CYP2C9*3 genotyping may identify subgroups of persons who potentially are at increased risk of gastroduodenal bleeding when treated with NSAIDs.
Further studies are needed before genotyping is introduced into clinical practice..
In clinical practice what is focused upon?
Focus on drugs with serious toxicity or a clear therapeutic benefit in optimising therapy
What enzyme subfamily are the most important drug-metabolizing enzymes in liver and gut, and amount to ~30 and 70% of total CYP content?
CYP3A5*3